Turning and Boring on a Lathe Online Reprint Chapter 3 This a complete book, published in 1914, divided into chapters on how to use a metal lathe, covering all turning and boring operations.

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Turning and Boring by Franklin D. Jones Published by Industrial Press 1914 A special treatise for machinists students in industrial and engineering schools, and apprentices on turning and boring methods including modern practice with engine lathes, vertical, and horizontal boring machines. Shop Amazon.com

CHAPTER 3

[1]

TAPER TURNING—SPECIAL OPERATIONS—FITTING

It is often necessary, in connection with lathe work, to turn parts tapering instead of straight or cylindrical. If the work is mounted between the centers, one method of turning a taper is to set the tailstock center out of alignment with the headstock center. When both of these centers are in line, the movement of the tool is parallel to the axis of the work and, consequently, a cylindrical surface is produced; but if the tailstock h₁ is set out of alignment, as shown in Fig. 1, the work will then be turned tapering as the tool is traversed from a to b, because the axis x—x is at an angle with the movement of the tool. Furthermore the amount of taper or the difference between the diameters at the ends for a given length, will depend on how much center h₁ is set over from the central position.

Fig. 1. Taper Turning by the Offset-center Method

The amount of taper is usually given on drawings in inches per foot, or the difference in the diameter at points twelve inches apart. For example, the taper of the piece shown at A, Fig. 2, is 1 inch per foot, as the length of the tapering surface is just twelve inches and the difference between the diameters at the ends is 1 inch. The conical roller shown at B has a total length of 9 inches and a tapering surface 6 inches long, and in this case the taper per foot is also 1 inch, there being a difference of ¹/₂ inch in a length of 6 inches or 1 inch in twice that length. When the taper per foot is known, the amount that the tailstock center should be set over for turning that taper can easily be estimated, but it should be remembered that the setting obtained in this way is not absolutely correct, and is only intended to locate the center approximately. When a taper needs to be at all accurate, it is tested with a gage, or by other means, after taking a trial cut, as will be explained later, and the tailstock center is readjusted accordingly. There are also more accurate methods of setting the center, than by figuring the amount of offset, but as the latter is often convenient this will be referred to first.

Fig. 2. Examples of Taper Work

Setting Tailstock Center for Taper Turning.—Suppose the tailstock center is to be set for turning part C, Fig. 2, to a taper of approximately 1 inch per foot. In this case the center would simply be moved toward the front of the machine ¹/₂ inch, or one-half the required taper per foot, because the total length of the work happens to be just 12 inches. This setting, however, would not be correct for all work requiring a taper of 1 inch per foot, as the adjustment depends not only on the amount of the taper but on the total length of the piece.

Fig. 3. Detail View of Lathe Tailstock

For example, the taper roller B has a taper of 1 inch per foot, but the center, in this case, would be offset less than one-half the taper per foot, because the total length is only 9 inches. For lengths longer or shorter than twelve inches, the taper per inch should be found first; this is then multiplied by the total length of the work (not the length of the taper) which gives the taper for that length, and one-half this taper is the amount to set over the center. For example, the taper per inch of part B equals 1 inch divided by 12 = ¹/₁₂ inch. The total length of 9 inches multiplied by ¹/₁₂ inch = ³/₄ inch, and ¹/₂ of ³/₄ = ³/₈, which is the distance that the tailstock center should be offset. In this example if the taper per foot were not known, and only the diameters of the large and small ends of the tapered part were given, the difference between these diameters should first be found (2¹/₂ - 2 = ¹/₂); this difference should then be divided by the length of the taper (¹/₂ ÷ 6 = ¹/₁₂ inch) to obtain the taper per inch. The taper per inch times the total length represents what the taper would be if it extended throughout the entire length, and one-half of this equals the offset, which is ³/₈ inch.

Fig. 4. Taper Plug and Gage

Example of Taper Turning.—As a practical example of taper turning let us assume that the piece A, Fig. 4, which has been centered and rough-turned as shown, is to be made into a taper plug, as indicated at B, to fit a ring gage as at C. If the required taper is 1¹/₂ inch per foot and the total length is 8 inches, the tailstock center would be offset ¹/₂ inch.

To adjust the tailstock, the nuts N (Fig. 3) are first loosened and then the upper part A is shifted sidewise by turning screw S. Scales are provided on some tailstocks for measuring the amount of this adjustment; if there is no scale, draw a line across the movable and stationary parts A and B, when the tailstock is set for straight turning. The movement of the upper line in relation to the lower will then show the offset, which can be measured with a scale.

When the adjustment has been made, nuts N are tightened and the part to be turned, with a dog attached, is placed between the centers the same as for straight turning. The taper end is then reduced by turning, but before it is near the finished size, the work is removed and the taper tested by inserting it in the gage. If it is much out, this can be felt, as the end that is too small can be shaken in the hole. Suppose the plug did not taper enough and only the small end came into contact with the gage, as shown somewhat exaggerated at D; in that case the center would be shifted a little more towards the front, whereas if the taper were too steep, the adjustment would, of course, be in the opposite direction. A light cut would then be taken, to be followed by another test. If the plug should fit the gage so well that there was no perceptible shake, it could be tested more closely as follows: Draw three or four chalk lines along the tapering surface, place the work in the gage and turn it a few times. The chalk marks will then show whether the taper of the plug corresponds to that of the gage; for example, if the taper is too great, the marks will be rubbed out on the large end, but if the taper is correct, the lines throughout their length will be partially erased.

Another and more accurate method of testing tapers is to apply a thin coat of Prussian-blue to one-half of the tapering surface, in a lengthwise direction. The work is then inserted in the hole or gage and turned to mark the bearing. If the taper is correct, the bearing marks will be evenly distributed, whereas if the taper is incorrect, they will appear at one end. Tapering pieces that have to be driven tightly into a hole, such as a piston-rod, can be tested by the location of the bearing marks produced by actual contact.

After the taper is found to be correct, the plug is reduced in size until it just enters the gage as at C. The final cut should leave it slightly above the required size, so that a smooth surface can be obtained by filing. It should be mentioned that on work of this kind, especially if great accuracy is required, the final finish is often obtained by grinding in a regular grinding machine, instead of by filing. When this method is employed, a lathe is used merely to rough-turn the part close to size.

Fig. 5. Setting Work for Taper Turning by use of Caliper Gage

When the amount that the tailstock center should be offset is determined by calculating, as in the foregoing example, it is usually necessary to make slight changes afterward, and the work should be tested before it is too near the finished size so that in case one or more trial cuts are necessary, there will be material enough to permit this. When there are a number of tapered pieces to be turned to the same taper, the adjustment of the tailstock center will have to be changed unless the total length of each piece and the depth of the center holes are the same in each case.

Fig. 6. Side View showing Relative Positions of Gage and Work

Setting the Tailstock Center with a Caliper Tool.—Another method of setting the tailstock center for taper turning is illustrated in Fig. 5. The end of an engine piston-rod is to be made tapering as at A and to dimensions a, b, c and d. It is first turned with the centers in line as at B. The end d is reduced to diameter b up to the beginning of the taper and it is then turned to diameter a as far as the taper part c extends. The tailstock center is next set over by guess and a caliper tool is clamped in the toolpost. This tool, a side view of which is shown in Fig. 6, has a pointer p that is free to swing about pivot r, which should be set to about the same height as the center of the work. The tailstock center is adjusted until this pointer just touches the work when in the positions shown by the full and dotted lines at C, Fig. 5; that is, until the pointer makes contact at the beginning and end of the taper part. The travel of the carriage will then be parallel to a line x—x, representing the taper; consequently, if a tool is started at the small end, as shown by the dotted lines at D, with the nose just grazing the work, it will also just graze it when fed to the extreme left as shown. Of course, if the taper were at all steep, more than one cut would be taken.

Fig. 7. Obtaining Tailstock Center Adjustment by use of Square

If these various operations are carefully performed, a fairly accurate taper can be produced. The straight end d is reduced to size after the tail-center is set back to the central position. Some mechanics turn notches or grooves at the beginning and end of the tapering part, having diameters equal to the largest and smallest part of the taper; the work is then set by these grooves with a caliper tool. The advantage of the first method is that most of the metal is removed while the centers are in alignment.

Setting the Tailstock Center with a Square.—Still another method of adjusting the tailstock for taper turning, which is very simple and eliminates all figuring, is as follows: The part to be made tapering is first turned cylindrical or straight for 3 or 4 inches of its length, after the ends have been properly centered and faced square. The work is then removed and the tailstock is shifted along the bed until the distance a—b between the extreme points of the centers is exactly 1 foot. The center is next offset a distance b—c equal to one-half the required taper per foot, after which a parallel strip D, having true sides, is clamped in the toolpost. Part D is then set at right angles to a line passing from one center point to the other. This can be done conveniently by holding a 1-foot square (preferably with a sliding head) against one side of D and adjusting the latter in the toolpost until edge E of the square blade is exactly in line with both center points. After part D is set, it should be clamped carefully to prevent changing the position. The angle between the side of D and an imaginary line which is perpendicular to axis a—b is now equal to one-half the angle of the required taper.

Fig. 8. Second Step in Adjusting Tailstock Center by use of Square

The axis of the part to be turned should be set parallel with line E, which can be done by setting the cylindrical surface which was previously finished, at right angles to the side of D. In order to do this the work is first placed between centers, the tailstock being shifted along the bed if necessary; the tail-center is then adjusted laterally until the finished cylindrical surface is square with the side of D. A small try-square can be used for testing the position of the work, as indicated in Fig. 8. If the length of the work is less than 1 foot, it will be necessary to move the center toward the rear of the machine, and if the length is greater than 1 foot, the adjustment is, of course, in the opposite direction.

The Taper Attachment.—Turning tapers by setting over the tailstock center has some objectionable features. When the lathe centers are not in alignment, as when set for taper turning, they bear unevenly in the work centers because the axis of the work is at an angle with them; this causes the work centers to wear unevenly and results in inaccuracy. Furthermore, the adjustment of the tailstock center must be changed when turning duplicate tapers, unless the length of each piece and the depth of the center holes are the same. To overcome these objections, many modern lathes are equipped with a special device for turning tapers, known as a taper attachment, which permits the lathe centers to be kept in alignment, as for cylindrical turning, and enables more accurate work to be done.

Fig. 9. A Lathe Taper Attachment

Taper attachments, like lathes, vary some in their construction, but all operate on the same principle. An improved form of taper attachment is illustrated in Figs. 9 and 10. Fig. 9 shows a plan view of a lathe carriage with an attachment fitted to it, and Fig. 10 a sectional view. This attachment has an arm A on which is mounted a slide S that can be turned about a central pivot by adjusting screw D. The arm A is supported by, and is free to slide on, a bracket B (see also sectional view) that is fastened to the carriage, and on one end of the arm there is a clamp C that is attached to the lathe bed when turning tapers. On the slide S there is a shoe F that is connected to bar E which passes beneath the toolslide. The rear end of the cross-feed screw is connected to this bar, and the latter is clamped to the toolslide when the attachment is in use.

Fig. 10. Sectional View of Taper Attachment

When a taper is to be turned, the carriage is moved opposite the taper part and clamp C is fastened to the bed; this holds arm A and slide S stationary so that the carriage, with bracket B and shoe F, can be moved with relation to the slide. If this slide S is set at an angle, as shown, the shoe as it moves along causes the toolslide and tool to move in or out, but if the slide is set parallel to the carriage travel, the toolslide remains stationary. Now if the tool, as it feeds lengthwise of the work, is also gradually moved crosswise, it will turn a taper, and as this crosswise movement is caused by the angularity of slide S, different tapers are obtained by setting the slide to different positions.

By means of a graduated scale G at the end of slide S, the taper that will be obtained for any angular position of the slide is shown. On some attachments there are two sets of graduations, one giving the taper in inches per foot and the other in degrees. While tapers are ordinarily given in inches per foot on drawings, sometimes the taper is given in degrees instead. The attachment is set for turning tapers by adjusting slide S until pointer p is opposite the division or fractional part of a division representing the taper. The whole divisions on the scale represent taper in inches per foot, and by means of the sub-divisions, the slide can be set for turning fractional parts of an inch per foot. When slide S is properly set, it is clamped to arm A by the nuts N. Bar E is also clamped to the toolslide by bolt H, as previously stated. The attachment is disconnected for straight turning by simply loosening clamp C and the bolt H.

Fig. 11. Lathe with Taper Attachment arranged for Boring Taper Hole in Engine Piston

Application of Taper Attachment.—Practical examples of lathe work, which illustrate the use of the taper attachment, are shown in Figs. 11 and 12. Fig. 11 shows how a taper hole is bored in an engine piston-head, preparatory to reaming. The casting must be held either in a chuck C or on a faceplate if too large for the chuck. The side of the casting (after it has been “chucked”) should run true, and also the circumference, unless the cored hole for the rod is considerably out of center, in which case the work should be shifted to divide the error. The side of the casting for a short space around the hole is faced true with a round nose turning tool, after which the rough-cored hole is bored with an ordinary boring tool t, and then it is finished with a reamer to exactly the right size and taper.

Fig. 12. Taper Attachment Set for Turning Taper End of Piston-rod

This particular taper attachment is set to whatever taper is given on the drawing, by loosening nuts N and turning slide S until pointer P is opposite that division on the scale which represents the taper. The attachment is then ready, after bolt H and nuts N are tightened, and clamp C is fastened to the lathe bed. The hole is bored just as though it were straight, and as the carriage advances, the tool is gradually moved inward by the attachment. If the lathe did not have a taper attachment, the taper hole could be bored by using the compound rest.

The hole should be bored slightly less than the finish size to allow for reaming. When a reamer is used in the lathe, the outer end is supported by the tailstock center and should have a deep center-hole. The lathe is run very slowly for reaming and the reamer is fed into the work by feeding out the tailstock spindle. The reamer can be kept from revolving, either by attaching a heavy dog to the end or, if the end is squared, by the use of a wrench long enough to rest against the lathe carriage. A common method is to clamp a dog to the reamer shank, and then place the tool-rest beneath it to prevent rotation. If the shank of a tool is clamped to the toolpost so that the dog rests against it, the reamer will be prevented from slipping off the center as it tends to do; with this arrangement, the carriage is gradually moved along as the tailstock spindle is fed outward. Some reamers are provided with stop-collars which come against the finished side of the casting when the hole has been reamed to size.

After the reaming operation, the casting is removed from the chuck and a taper mandrel is driven into the hole for turning the outside of the piston. This mandrel should run true on its centers, as otherwise the outside surface of the piston will not be true with the bored hole. The driving dog, especially for large work of this kind, should be heavy and stiff, because light flexible clamps or dogs vibrate and frequently cause chattering. For such heavy work it is also preferable to drive at two points on opposite sides of the faceplate, but the driving pins should be carefully adjusted to secure a uniform bearing on both sides.

The foregoing method of machining a piston is one that would ordinarily be followed when using a standard engine lathe, and it would, perhaps, be as economical as any if only one piston were being made; but where such work is done in large quantities, time could be saved by proceeding in a different way. For example, the boring and reaming operation could be performed much faster in a turret lathe, which is a type designed for just such work, but a turret lathe cannot be used for as great a variety of turning operations as a lathe of the regular type. There are also many other classes of work that can be turned more quickly in special types of machines, but as more or less time is required for arranging these special machines and often special tools have to be made, the ordinary lathe is frequently indispensable when only a few parts are needed; in addition, it is better adapted to some turning operations than any other machine.

Fig. 12 illustrates how a taper attachment would be used for turning the taper fitting for the crosshead end of an engine piston-rod. Even though this taper corresponds to the taper of the hole in the piston, slide S would have to be reset to the corresponding division on the opposite side of the central zero mark, because the taper of the hole decreased in size during the boring operation, whereas the rod is smallest at the beginning of the cut, so that the tool must move outward rather than inward as it advances. The taper part is turned practically the same as a cylindrical part; that is, the power feed is used and, as the carriage moves along the bed, the tool is gradually moved outward by the taper attachment.

If the rod is being fitted directly to the crosshead (as is usually the case), the approximate size of the small end of the taper could be determined by calipering, the calipers being set to the size of the hole at a distance from the shoulder or face side of the crosshead, equal to the length of the taper fitting on the rod. If the crosshead were bored originally to fit a standard plug gage, the taper on the rod could be turned with reference to this gage, but, whatever the method, the taper should be tested before turning too close to the finished size. The test is made by removing the rod from the lathe and driving it tightly into the crosshead. This shows how near the taper is to size, and when the rod is driven out, the bearing marks show whether the taper is exactly right or not. If the rod could be driven in until the shoulder is, say, ¹/₈ inch from the crosshead face, it would then be near enough to finish to size by filing. When filing, the lathe is run much faster than for turning, and most of the filing should be done where the bearing marks are the heaviest, to distribute the bearing throughout the length of the taper. Care should be taken when driving the rod in or out, to protect the center-holes in the ends by using a “soft” hammer or holding a piece of soft metal against the driving end.

After the crosshead end is finished, the rod is reversed in the lathe for turning the piston end. The dog is clamped to the finished end, preferably over a piece of sheet copper to prevent the surface from being marred. When turning this end, either the piston reamer or the finished hole in the piston can be calipered. The size and angle of the taper are tested by driving the rod into the piston, and the end should be fitted so that by driving tightly, the shoulder will just come up against the finished face of the piston. When the taper is finished, the attachment is disengaged and a finishing cut is taken over the body of the rod, unless it is to be finished by grinding, which is the modern and most economical method.

Fig. 13. Tool Point should be in same Horizontal Plane as Axis of Work for Taper Turning

Height of Tool when Turning Tapers.—The cutting edge of the tool, when turning tapers, should be at the same height as the center or axis of the work, whether an attachment is used or not. The importance of this will be apparent by referring to Fig. 13. To turn the taper shown, the tool T would be moved back a distance x (assuming that an attachment is used) while traversing the length l. As an illustration, if the tool could be placed as high as point a, the setting of the attachment remaining as before, the tool would again move back a distance x, while traversing a distance l, but the large end would be under-sized (as shown by the dotted line) if the diameters of the small ends were the same in each case. Of course, if the tool point were only slightly above or below the center, the resulting error would also be small. The tool can easily be set central by comparing the height of the cutting edge at the point of the tool with one of the lathe centers before placing the work in the lathe.

Fig. 14. Plan View showing Method of Turning a Taper with the Compound Rest

Taper Turning with the Compound Rest.—The amount of taper that can be turned by setting over the tailstock center and by the taper attachment is limited, as the centers can only be offset a certain distance, and the slide S (Fig. 9) of the attachment cannot be swiveled beyond a certain position. For steep tapers, the compound rest E is swiveled to the required angle and used as indicated in Fig. 14, which shows a plan view of a rest set for turning the valve V. This compound rest is an upper slide mounted on the lower or main cross-slide D, and it can be turned to any angular position so that the tool, which ordinarily is moved either lengthwise or crosswise of the bed, can be fed at an angle. The base of the compound rest is graduated in degrees and the position of these graduations shows to what angle the upper slide is set. Suppose the seat of valve V is to be turned to an angle of 45 degrees with the axis or center, as shown on the drawing at A, Fig. 15. To set the compound rest, nuts n on either side, which hold it rigidly to the lower slide, are first loosened and the slide is then turned until the 45-degree graduation is exactly opposite the zero line; the slide is then tightened in this position. A cut is next taken across the valve by operating handle w and feeding the tool in the direction of the arrow.

Fig. 15. Example of Taper Work Turned by using Compound Rest

In this particular instance the compound rest is set to the same angle given on the drawing, but this is not always the case. If the draftsman had given the included angle of 90 degrees, as shown at B, which would be another way of expressing it, the setting of the compound rest would, of course, be the same as before, or to 45 degrees, but the number of degrees marked on the drawing does not correspond with the angle to which the rest must be set. As another illustration, suppose the valve were to be turned to an angle of 30 degrees with the axis as shown at C. In this case the compound rest would not be set to 30 degrees but to 60 degrees, because in order to turn the work to an angle of 30 degrees, the rest must be 60 degrees from its zero position, as shown. From this it will be seen that the number of degrees marked on the drawing does not necessarily correspond to the angle to which the rest must be set, as the graduations on the rest show the number of degrees that it is moved from its zero position, which corresponds to the line a—b. The angle to which the compound rest should be set can be found, when the drawing is marked as at A or C, by subtracting the angle given from 90 degrees. When the included angle is given, as at B, subtract one-half the included angle from 90 degrees to obtain the required setting. Of course, when using a compound rest, the lathe centers are set in line as for straight turning, as otherwise the angle will be incorrect.

Rules for Figuring Tapers

Fig. 16. Disk Gage for Accurate Measurement of Angles and Tapers

Accurate Measurement of Angles and Tapers.—When great accuracy is required in the measurement of angles, or when originating tapers, disks are commonly used. The principle of the disk method of taper measurement is that if two disks of unequal diameters are placed either in contact or a certain distance apart, lines tangent to their peripheries will represent an angle or taper, the degree of which depends upon the diameters of the two disks and the distance between them. The gage shown in Fig. 16, which is a form commonly used for originating tapers or measuring angles accurately, is set by means of disks. This gage consists of two adjustable straight-edges A and A₁, which are in contact with disks B and B₁. The angle α or the taper between the straight-edges depends, of course, upon the diameters of the disks and the center distance C, and as these three dimensions can be measured accurately, it is possible to set the gage to a given angle within very close limits. Moreover, if a record of the three dimensions is kept, the exact setting of the gage can be reproduced quickly at any time. The following rules may be used for adjusting a gage of this type.

To Find Center Distance for a Given Taper.—When the taper, in inches per foot, is given, to determine center distance C. Rule: Divide the taper by 24 and find the angle corresponding to the quotient in a table of tangents; then find the sine corresponding to this angle and divide the difference between the disk diameters by twice the sine.

Example: Gage is to be set to ³/₄ inch per foot, and disk diameters are 1.25 and 1.5 inch, respectively. Find the required center distance for the disks.

The angle whose tangent is 0.03125 equals 1 degree 47.4 minutes;

To Find Center Distance for a Given Angle.—When straight-edges must be set to a given angle α, to determine center distance C between disks of known diameter. Rule: Find the sine of half the angle α in a table of sines; divide the difference between the disk diameters by double this sine.

Example: If an angle α of 20 degrees is required, and the disks are 1 and 3 inches in diameter, respectively, find the required center distance C.

To Find Angle for Given Taper per Foot.—When the taper in inches per foot is known, and the corresponding angle α is required. Rule: Divide the taper in inches per foot by 24; find the angle corresponding to the quotient, in a table of tangents, and double this angle.

Example: What angle α is equivalent to a taper of 1¹/₂ inch per foot?

The angle whose tangent is 0.0625 equals 3 degrees 35 minutes, nearly; then, 3 deg. 35 min. × 2 = 7 deg. 10 min.

To Find Angle for Given Disk Dimensions.—When the diameters of the large and small disks and the center distance are given, to determine the angle α. Rule: Divide the difference between the disk diameters by twice the center distance; find the angle corresponding to the quotient, in a table of sines, and double the angle.

Example: If the disk diameters are 1 and 1.5 inch, respectively, and the center distance is 5 inches, find the included angle α.

The angle whose sine is 0.05 equals 2 degrees 52 minutes; then, 2 deg. 52 min. × 2 = 5 deg. 44 min. = angle α.

Fig. 17. Setting Center Mark in Line with Axis of Lathe Spindle by use of Test Indicator

Use of the Center Indicator.—The center test indicator is used for setting a center-punch mark, the position of which corresponds with the center or axis of the hole to be bored, in alignment with the axis of the lathe spindle. To illustrate, if two holes are to be bored, say 5 inches apart, small punch marks having that center-to-center distance would be laid out as accurately as possible. One of these marks would then be set central with the lathe spindle by using a center test indicator as shown in Fig. 17. This indicator has a pointer A the end of which is conical and enters the punch mark. The pointer is held by shank B which is fastened in the toolpost. The joint C by means of which the pointer is held to the shank is universal; that is, it allows the pointer to move in any direction. Now when the part being tested is rotated by running the lathe, if the center-punch mark is not in line with the axes of the lathe spindle, obviously the outer end of pointer A will vibrate, and as joint C is quite close to the inner end, a very slight error in the location of the center-punch mark will cause a perceptible movement of the outer end, as indicated by the dotted lines. When the work has been adjusted until the pointer remains practically stationary, the punch mark is central, and the hole is bored. The other center-punch mark is then set in the same way for boring the second hole. The accuracy of this method depends, of course, upon the location of the center-punch marks. A still more accurate way of setting parts for boring holes to a given center-to-center distance is described in the following:

Locating Work by the Button Method.—Among the different methods employed by machinists and toolmakers for accurately locating work such as jigs, etc., on the faceplate of a lathe, the one most commonly used is known as the button method. This scheme is so named because cylindrical bushings or buttons are attached to the work in positions corresponding to the holes to be bored, after which they are used in locating the work. These buttons, which are ordinarily about ¹/₂ inch in diameter, are ground and lapped to the same size and the ends squared. The diameter should, preferably, be such that the radius can be determined easily, and the hole through the center should be about ¹/₈ inch larger than the retaining screw, so that the button can be shifted.

Fig. 18. Jig-plate with Buttons attached, ready for Boring

As an illustration of the practical application of the button method, we shall consider, briefly, the way the holes would be accurately machined in the jig-plate in Fig. 18. First the centers of the seven holes should be laid off approximately correct by the usual methods, after which small holes should be drilled and tapped for the clamping screws S. After the buttons B are clamped lightly in place, they are all set in correct relation with each other and with the jig-plate. The proper location of the buttons is very important as their positions largely determine the accuracy of the work. A definite method of procedure that would be applicable in all cases cannot, of course, be given, as the nature of the work as well as the tools available make it necessary to employ different methods.

In this particular case, the three buttons a, b and c should be set first, beginning with the one in the center. As this central hole must be 2.30 and 2.65 inches from the finished sides A and A₁, respectively, the work is first placed on an accurate surface-plate as shown; by resting it first on one of these sides and then on the other, and measuring with a vernier height gage, the central button can be accurately set. The buttons a and c are also set to the correct height from side A₁ by using the height gage, and in proper relation to the central button by using a micrometer or a vernier caliper and measuring the over-all dimension x. When measuring in this way, the diameter of one button would be deducted to obtain the correct center-to-center distance. After buttons a, b and c are set equidistant from side A₁ and in proper relation to each other, the remaining buttons should be set radially from the central button b and the right distance apart. By having two micrometers or gages, one set for the radial dimension x and the other for the chordal distance y, the work may be done in a comparatively short time.

Fig. 19. Setting a Button True Preparatory to Boring, by use of Test Indicator

After the buttons have been tightened, all measurements should be carefully checked; the work is then mounted on the faceplate of the lathe, and one of the buttons, say b, is set true by the use of a test indicator as shown in Fig. 19. When the end of this indicator (which is one of a number of types on the market) is brought into contact with the revolving button, the vibration of the pointer I shows how much the button runs out of true. When the pointer remains practically stationary, thus showing that the button runs true, the latter should be removed. The hole is then drilled nearly to the required size, after which it is bored to the finish diameter. In a similar[ manner the other buttons are indicated and the holes bored, one at a time. It is evident that if each button is correctly located and set perfectly true in the lathe, the various holes will be located at the required center-to-center dimensions within very close limits.

Fig. 20. Testing Concentricity of Button with Dial Gage

Fig. 21. Drilling a Bushing Hole

Fig. 20 shows how one of the buttons attached to a plate in which three holes are to be bored is set true or concentric. The particular indicator illustrated is of the dial type, any error in the location of the button being shown by a hand over a dial having graduations representing thousandths of an inch. Fig. 21 shows how the hole is drilled after the button is removed. It will be noted that the drill is held in a chuck, the taper shank of which fits into the tailstock spindle, this being the method of holding small drills. After drilling, the hole is bored as shown in Fig. 22. The boring tool should have a keen edge to avoid springing, and if the work when clamped in position, throws the faceplate out of balance, it is advisable to restore the balance, before boring, by the use of a counter-weight, because the lathe can be rotated quite rapidly when boring such a small hole.

Fig. 22. Boring a Bushing Hole

Fig. 23. Example of Work illustrating Accumulation of Errors

When doing precision work of this kind, the degree of accuracy will depend upon the instruments used, the judgment and skill of the workman and the care exercised. A good general rule to follow when locating bushings or buttons is to use the method which is the most direct and which requires the least number of measurements. As an illustration of how errors may accumulate, let us assume that seven holes are to be bored in the jig-plate shown in Fig. 23, so that they are the same distance from each other and in a straight line. The buttons may be brought into alignment by the use of a straight-edge, and to simplify matters, it will be taken for granted that they have been ground and lapped to the same size. If the diameter of the buttons is first determined by measuring with a micrometer, and then this diameter is deducted from the center distance x, the difference will be the distance y between adjacent buttons. Now if a temporary gage is made to length y, all the buttons can be set practically the same distance apart, the error between any two adjacent ones being very slight. If, however, the total length z over the end buttons is measured by some accurate means, the chances are that this distance will not equal six times dimension x plus the diameter of one button, as it should, because even a very slight error in the gage for distance y would gradually accumulate as each button was set. If a micrometer were available that would span two of the buttons, the measurements could be taken direct and greater accuracy would doubtless be obtained. On work of this kind where there are a number of holes that need to have accurate over-all dimensions, the long measurements should first be taken when setting the buttons, providing, of course, there are proper facilities for so doing, and then the short ones. For example, the end buttons in this case should first be set, then the central one and finally those for the sub-divisions.

Fig. 24. Special Arbor for Turning Eccentrics

Eccentric Turning.—When one cylindrical surface must be turned eccentric to another, as when turning the eccentric of a steam engine, an arbor having two sets of centers is commonly used, as shown in Fig. 24. The distance x between the centers must equal one-half the total “throw” or stroke of the eccentric. The hub of the eccentric is turned upon the centers a—a, and the tongued eccentric surface, upon the offset centers, as indicated by the illustration. Sometimes eccentrics are turned while held upon special fixtures attached to the faceplate.

When making an eccentric arbor, the offset center in each end should be laid out upon radial lines which can be drawn across the arbor ends by means of a surface gage. Each center is then drilled and reamed to the same radius x as near as possible. The uniformity of the distance x at each end is then tested by placing the mandrel upon the offset centers and rotating it, by hand, with a dial indicator in contact at first one end and then the other. The amount of offset can also be tested either by measuring from the point of a tool held in the toolpost, or by setting the tool to just graze the mandrel at extreme inner and outer positions, and noting the movement of the cross-slide by referring to the dial gage of the cross-feed screw.

Fig. 25. Turning an Engine Crank-pin in an Ordinary Lathe

Turning a Crankshaft in a Lathe.—Another example of eccentric turning is shown in Fig. 25. The operation is that of turning the crank-pin of an engine crankshaft, in an ordinary lathe. The main shaft is first rough-turned while the forging revolves upon its centers C and C₁ and the ends are turned to fit closely the center-arms A and A₁. After the sides B and B₁ of the crank webs have been rough-faced, the center-arms are attached to the ends of the shaft as shown in the illustration. These arms have centers at D and D₁ (located at the required crank radius) which should be aligned with the rough pin, when attaching the arms, and it is advisable to insert braces E between the arms and crank to take the thrust of the lathe centers. With the forging supported in this way, the crank-pin and inner sides of the webs are turned and faced, the work revolving about the axis of the pin. The turning tools must extend beyond the tool-holder far enough to allow the crank to clear as it swings around. Owing to this overhang, the tool should be as heavy as possible to make it rigid and it is necessary to take comparatively light cuts and proceed rather cautiously. After finishing the crank-pin and inside of the crank, the center-arms are removed and the main body of the shaft and the sides B and B₁ are finished. This method of turning crankshafts is often used in general repair shops, etc., especially where new shafts do not have to be turned very often. It is slow and inefficient, however, and where crankshafts are frequently turned, special machines or attachments are used.

Fig. 26. LeBlond Lathe with Special Equipment for Crankshaft Turning

Special Crankshaft Lathe.—A lathe having special equipment for rough-turning gas engine crankshaft pins is shown in Fig. 26. This lathe is a heavy-duty type built by the R. K. LeBlond Machine Tool Co. It is equipped with special adjustable headstock and tailstock fixtures designed to take crankshafts having strokes up to about 6 inches. The tools are held in a three-tool turret type of toolpost and there are individual cross-stops for each tool. This lathe also has a roller steadyrest for supporting the crankshaft; automatic stops for the longitudinal feed, and a pump for supplying cutting lubricant. The headstock fixture is carried on a faceplate mounted on the spindle and so arranged as to be adjustable for cranks of different throw. When the proper adjustment for a given throw has been made, the slide is secured by four T-bolts. A graduated scale and adjusting screw permit of accurate adjustments.

The revolving fixture is accurately indexed for locating different crank-pins in line with the lathe centers, by a hardened steel plunger in the slide which engages with hardened bushings in the fixture. The index is so divided that the fixture may be rotated 120 or 180 degrees, making it adjustable for 2-, 4- and 6-throw cranks. After indexing, the fixture is clamped by two T-bolts which engage a circular T-slot. The revolving fixture is equipped with removable split bushings which can be replaced to fit the line bearings of different sized crankshafts. The work is driven by a V-shaped dovetail piece having a hand-nut adjustment, which also centers the pin by the cheek or web. The crank is held in position by a hinged clamp on the fixture. The tailstock fixture is also adjustable and it is mounted on a spindle which revolves in a bushing in the tailstock barrel. The adjustment is obtained in the same manner as on the headstock fixture, and removable split bushings as well as a hinged clamp are also employed.

The method of chucking a four-throw crank is as follows: The two fixtures are brought into alignment by two locking pins. One of these is located in the head and enters a bushing in the large faceplate and the other is in the tailstock and engages the tailstock fixture. The crankshaft is delivered to the machine with the line bearings rough-turned and it is clamped by the hinged clamp previously referred to and centered by the V-shaped driver. The locking pins for both fixtures are then withdrawn and the machine is ready to turn two of the pins. After these have been machined, the fixtures are again aligned by the locking pins, the two T-bolts of the headstock fixture and the hinged clamp at the tailstock are released, the indexing plunger is withdrawn and the headstock fixture and crank are turned 180 degrees or until the index plunger drops into place. The crank is then clamped at the tailstock end and the revolving fixture is secured by the two T-bolts previously referred to. After the locking pins are withdrawn, the lathe is ready to turn the two opposite pins.

Fig. 27. Diagrams showing Arrangements of Tools on LeBlond Lathe

Operation of Special Crankshaft Lathe.—The total equipment of this machine (see Fig. 27) is carried on a three-tool turret tool-block. The method of turning a crankshaft is as follows: A round-nosed turning tool is first fed into a cross stop as illustrated in the plan view at A, which gives the proper diameter. The feed is then engaged and the tool feeds across the pin until the automatic stop lever engages the first stop, which throws out the feed automatically. The carriage is then moved against a positive stop by means of the handwheel. The roller back-rest is next adjusted against the work by the cross-feed handwheel operating through a telescopic screw, and the filleting tools are brought into position as at B. These are run in against a stop, removing the part left by the turning tool and giving the pin the proper width and fillets of the correct radius. If the crankshaft has straight webs which must be finished, two tools seen at b are used for facing the webs to the correct width. During these last two operations, the crank is supported by the roller back-rest, thus eliminating any tendency of the work to spring.

After one pin is finished in the manner described, the back-rest is moved out of the way, the automatic stop lever raised, the carriage shifted to the next pin, and the operation repeated. The tools are held in position on the turret by studs, and they can be moved and other tools quickly substituted for pins of different widths. This machine is used for rough-turning the pins close to the required size, the finishing operation being done in a grinder. It should be mentioned, in passing, that many crankshafts, especially the lighter designs used in agricultural machinery, etc., are not turned at all but are ground from the rough.

Fig. 28. (A) Spherical Turning with Compound Rest.
(B) Concave Turning

Spherical Turning.—Occasionally it may be necessary to turn a spherical surface in the lathe. Sketch A, Fig. 28, shows how a small ball-shaped end can be turned on a piece held in a chuck. The lathe carriage is adjusted so that the pin around which the compound rest swivels is directly under the center a. The bolts which hold the swivel are slightly loosened to allow the top slide to be turned, as indicated by the dotted lines; this causes the tool point to move in an arc about center a, and a spherical surface is turned. Light cuts must be taken as otherwise it would be difficult to turn the slide around by hand.

Sketch B illustrates how a concave surface can be turned. The cross-slide is adjusted until swivel pin is in line with the lathe centers, and the carriage is moved along the bed until the horizontal distance between center b of the swivel, and the face of the work, equals the desired radius of the concave surface. The turning is then done by swinging the compound rest as indicated by the dotted lines. The slide can be turned more evenly by using the tailstock center to force it around. A projecting bar is clamped across the end of the slide at d, to act as a lever, and a centered bar is placed between this lever and the tailstock center; then by screwing out the tailstock spindle, the slide is turned about pivot b. The alignment between the swivel pin and the lathe centers can be tested by taking a trial cut; if the swivel pin is too far forward, the tool will not touch the turned surface if moved past center c, and if the pin is too far back, the tool will cut in on the rear side.

Fig. 29. Spherical Turning Attachment for Engine Lathe

Spherical Turning Attachments.—When spherical turning must be done repeatedly, special attachments are sometimes used. Fig. 29 shows an attachment applied to a lathe for turning the spherical ends of ball-and-socket joints. The height or radius of the cutting tool and, consequently, the diameter of the turned ball, is regulated by adjusting screw A. The tool is swung around in an arc, by turning handle B which revolves a worm meshing with an enclosed worm-wheel. As will be seen, the work is held in a special chuck, owing to its irregular shape.

Fig. 30. Attachment for Turning Spherical End of Gasoline Engine Piston

Another spherical turning attachment is shown in Fig. 30. This is used for machining the ends of gasoline engine pistons. The cross-slide has bolted to it a bar A carrying a roller which is pressed against a forming plate B by a heavy spring C. The forming plate B, which is attached to a cross-piece fastened to the ways of the lathe bed, is curved to correspond with the radius required on the piston end, and when the tool is fed laterally by moving the cross-slide, it follows the curve of plate B. The piston is held in a special hollow chuck which locates it in a central position and holds it rigidly.

In connection with lathe work, special attachments and tools are often used, especially when considerable work of one class must be turned; however, if a certain part is required in large quantities, it is usually more economical to use some semi-automatic or automatic turning machine, especially designed for repetition work.

Fig. 31. Front and Rear Tools used for Roughing

Turning with Front and Rear Tools.—In ordinary engine lathe practice, one tool is used at a time, but some lathes are equipped with tool-holders at the front and rear of the carriage so that two tools can be used simultaneously. Fig. 31 shows a detail view of a lathe in which front and rear tools are being used. These tools are of the inserted cutter type and the one at the rear is inverted, as the rotary movement of the work is, of course, upward on the rear side. This particular lathe was designed for taking heavy roughing cuts and has considerable driving power.

The part shown in this illustration is a chrome-nickel steel bar which is being roughed out to form a milling machine spindle. It is necessary to reduce the diameter of the bar from 5⁷/₁₆ inches to 3³/₄ inches for a length of 27 inches, because of a collar on one end. This reduction is made in one passage of the two tools, with a feed of ¹/₃₂ inch per revolution and a speed of 60 revolutions per minute. The use of two tools for such heavy roughing cuts is desirable, especially when the parts are required in large quantities, because the thrust of the cut on one side, which tends to deflect the work, is counteracted by the thrust on the opposite side.

Sometimes special tool-holders are made for the lathe, so that more than one tool can be used for turning different surfaces or diameters at the same time, the tools being set in the proper relation to each other. The advantage of this method has resulted in the design of a special lathe for multiple-tool turning.

Fig. 32. Lo-swing Lathe for Multiple Turning

A Multiple-tool Lathe.—The lathe shown in Fig. 32 (which is built by the Fitchburg Machine Works and is known as the Lo-swing) is designed especially for turning shafts, pins and forgings not exceeding 3¹/₂ inches in diameter. It has two carriages A and B which, in conjunction with special tool-holders, make it possible to turn several different diameters simultaneously. At the front of this lathe there is an automatic stop-rod C for disengaging the feed when the tools have turned a surface to the required length. This stop-rod carries adjustable stops D which are set to correspond with shoulders, etc., on the work. The rod itself is also adjustable axially, so that the tools, which are usually arranged in groups of two or more (depending upon the nature of the work), can be disengaged at a point nearer or farther from the headstock as may be required, owing to a variation in the depth of center holes. For example, if it were necessary to feed a group of tools farther toward the headstock after they had been automatically disengaged, the entire rod with its stops would be adjusted the required amount in that direction.

The gage G, which is attached to a swinging arm, is used to set the stop bar with reference to a shoulder near the end of the work, when it is necessary to finish other parts to a given distance from such a shoulder or other surface. The use of this gage will be explained more fully later. Cooling lubricant for the tools is supplied through the tubes E. The lathe shown in the illustration is arranged for turning Krupp steel bars. A rough bar and also one that has been turned may be seen to the right. The plain cylindrical bar is turned to five different diameters, by groups of tools held on both carriages.

Fig. 33. Plan View showing Method of driving Steering Knuckle and Arrangement of Tools

Examples of Multiple Turning.—Figs. 33 and 34 show how a Lo-swing lathe is used for turning the steering knuckle of an automobile. Four tools are used in this case, three cylindrical surfaces and one tapering surface being turned at the same time. For this job, the four tools are mounted on one carriage. The taper part is turned by the second tool from the headstock, which is caused to feed outward as the carriage advances by a taper attachment. This tool is held in a special holder and bears against a templet at the rear, which is tapered to correspond with the taper to be turned. This templet is attached to a bar which, in turn, is fastened to a stationary bracket seen to the extreme left in Fig. 33. This part is finished in two operations, the tool setting being identical for each operation, except for diameter adjustments. As the illustrations show, three of the four tools employed are used for straight turning on different diameters, while the fourth finishes the taper.

Fig. 34. Plan View showing Method of driving Steering Knuckle and Arrangement of Tools

These pieces, which are rough drop forgings, are first reduced to the approximate size. When it becomes necessary to grind the tools, they are reset and those parts which have been roughed out are turned to the finished size. The average time for the first operation, which includes starting, stopping, turning and replacing the piece, is one minute, while for the second operation with the finer feed, an average time of two minutes is required. The work is driven by sleeve S, which fits over the spindle and is held in position by the regular driver, as shown. This sleeve is notched to fit the knuckle, so that the latter can easily and quickly be replaced when finished.

One of the interesting features of this job lies in the method of locating the shoulders on each knuckle, at the same distance from the hole H which is drilled previously, and which receives the bolt on which the knuckle swivels when assembled in a car. As soon as the knuckle has been placed between the centers, a close-fitting plug P (Fig. 33) is inserted in this hole and the indicator arm with its attached gage or caliper G is swung up to the position shown. The stop-rod on which the stops have been previously set for the correct distance between the shoulders is next adjusted axially until the gage G just touches the plug P. The indicator is then swung out of the way, and the piece turned. If the next knuckle were centered, say, deeper than the previous one which would, of course, cause it to be located nearer the headstock, obviously all the shoulders would be located farther from the finished hole, provided the position of the stops remained the same as before. In such a case their position would, however, be changed by shifting the stop-rod until the gage G again touched the plug thus locating all the stops with reference to the hole. As the adjustment of the stop-rod changes the position of the taper templet as well as the stops, it is evident that both the shoulders and the taper are finished the same distance from the hole in each case. The connection of the bracket (to which the templet arm is attached) with the stop-rod is clearly shown in Fig. 33. This bracket can either be locked to the ways or adjusted to slide when the stop-rod is moved.

Fig. 35. First and Second Operations on Automobile Transmission Shaft—Lo-swing Lathe

The part illustrated in Fig. 35 is an automobile transmission shaft. In this particular case, cylindrical, tapering and spherical surfaces are turned. The upper view shows, diagrammatically, the arrangement of the tools and work for the first operation. After the shaft is “spotted” at A for the steadyrest, the straight part C and the collar B are sized with tools S and R which are mounted on the left-hand carriage. A concave groove is then cut in collar B by tool R, after which spherical end D is formed by a special attachment mounted on the right-hand carriage. This attachment is the same, in principle, as the regular taper-turning attachment, the substitution of a circular templet T for the straight kind used on taper work being the only practical difference.

After the surfaces mentioned have been finished on a number of pieces, the work is reversed and the tools changed as shown by the lower view. The first step in the second operation is to turn the body E of the shaft with the tool T on the left-hand carriage. The taper F and the straight part G are then finished, which completes the turning. It will be noted that in setting up the machine for this second operation, it is arranged for taper turning by simply replacing the circular templet with the straight one shown. When this taper attachment is not in use, the swiveling arm M, which is attached to a bracket, is swung out of the way.

The method of driving this shaft is worthy of note. A dog having two driving arms each of which bears against a pin N that passes through a hole in the spindle is used. As the ends of this pin, against which the dog bears, are beveled in opposite directions, the pin turns in its hole when the dog makes contact with it and automatically adjusts itself against the two driving members of the dog. The advantage of driving by a two-tailed dog, as most mechanics know, is in equalizing the tendency to spring slender parts while they are being turned.

Fig. 36. Axle End turned in One Traverse of the Five Tools shown

In Fig. 36 another turning operation on a lathe of this type is shown, the work in this case being a rear axle for a motor truck. The turning of this part is a good example of that class of work where the rapid removal of metal is the important feature. As the engraving shows, the stock, prior to turning, is 3¹/₂ inches in diameter and it is reduced to a minimum diameter of 1¹/₁₆ inch. This metal is turned off with one traverse of the carriage or by one passage of the five tools, and the weight of the chips removed from each end of the axle is approximately 12 pounds. The time required for the actual turning is about 9 minutes, while the total time for the operation, which includes placing the heavy piece in the machine, turning, and removing the work from the lathe, is 12 minutes. The axle revolves, while being turned, at 110 revolutions per minute and a feed equivalent to 1 inch of tool travel to 60 revolutions of the work is used. It will be noticed that the taper attachment is also employed on this part, the taper being turned by the second tool from the left. As the axle is equipped with roller bearings, it was found desirable to finish the bearing part by a separate operation; therefore, in the operation shown the axle is simply roughed down rather close to the finished dimensions, leaving enough material for a light finishing cut.

Knurling in the Lathe.—Knurling is done either to provide a rough surface which can be firmly gripped by the hand or for producing an ornamental effect. The handles of gages and other tools are often knurled, and the thumb-screws used on instruments, etc., usually have knurled edges. A knurled surface consists of a series of small ridges or diamond-shaped projections, and is produced in the lathe by the use of a tool similar to the one shown in Fig. 37, this being one of several different designs in common use. The knurling is done by two knurls A and B having teeth or ridges which incline to the right on one knurl and to the left on the opposite knurl, as shown by the end view. When these two knurls are pressed against the work as the latter revolves, one knurl forms a series of left-hand ridges and the other knurl right-hand ridges, which cross and form the diamond-shaped knurling which is generally used.

If the surface to be knurled is wider than the knurls, the power feed of the lathe should be engaged and the knurling tool be traversed back and forth until the diamond-shaped projections are well formed. To prevent forming a double set of projections, feed the knurl in with considerable pressure at the start, then partially relieve the pressure before engaging the power feed. Use oil when knurling.

The knurls commonly used for lathe work have spiral teeth and ordinarily there are three classes, known as coarse, medium and fine. The medium pitch is generally used. The teeth of coarse knurls have a spiral angle of 36 degrees and the pitch of the knurled cut (measured parallel to the axis of the work) should be about 8 per inch. For medium knurls, the spiral angle is 29¹/₂ degrees and the pitch, measured as before, is 12 per inch. For fine knurls, the spiral angle is 25³/₄ degrees and the pitch 20 per inch. The knurls should be about ³/₄ inch in diameter and ³/₈ inch wide. When made to these dimensions, coarse knurls have 34 teeth; medium, 50 teeth; and fine knurls, 80 teeth.

The particular tool illustrated in Fig. 37 has three pairs of knurls of coarse, medium and fine pitch. These are mounted in a revolving holder which not only serves to locate the required set of knurls in the working position, but enables each knurl to bear against the surface with equal pressure. Concave knurls are sometimes used for knurling rounded edges on screw heads, etc.

Relieving Attachment.—Some lathes, particularly those used in toolrooms, are provided with relieving attachments which are used for “backing off” the teeth of milling cutters, taps, hobs, etc. If a milling cutter of special shape is to be made, the cutter blank is first turned to the required form with a special tool having a cutting edge that corresponds with the shape or profile of the cutter to be made. The blank is then fluted or gashed to form the teeth, after which the tops of the teeth are relieved or backed off to provide clearance for the cutting edges. The forming tool used for turning the blank is set to match the turned surface, and the teeth are backed off as the result of a reciprocating action imparted to the toolslide by the relieving attachment. The motion of the toolslide is so adjusted that the tool will meet the front of each tooth and the return movement begin promptly after the tool leaves the back end of the tooth.

Fig. 38. Hendey Relieving Attachment applied to a Lathe

These attachments differ somewhat in their construction and arrangement but the principle of their operation is similar. Fig. 38 shows a Hendey relieving attachment applied to a lathe. A bracket carrying the gearing A through which the attachment is driven is mounted upon the main gear box of the lathe, and the special slide B, which is used when relieving, is placed on the cross-slide after removing the regular compound rest. The gears at A are changed to suit the number of flutes or gashes in the cutter, tap or whatever is to be relieved. If we assume that the work is a formed milling cutter having nine teeth, then with this particular attachment, a gear having 90 teeth would be placed on the “stud” and a 40-tooth gear on the cam-shaft, the two gears being connected by a 60-tooth intermediate gear. With this combination of gearing, the toolslide would move in and out nine times for each revolution of the work, so that the tool could back off the top of each tooth. (The gearing to use for various numbers of flutes is shown by an index plate on the attachment.) The amount of relief is varied to suit the work being done, by means of a toothed coupling which makes it possible to change the relative position between the eccentric which actuates the toolslide and the cam lever, thereby lengthening or shortening the reciprocating travel of the tool.

Fig. 39. Relieving a Formed Cutter

Application of Relieving Attachment.—Some typical examples of the kind of work for which the relieving attachment is used are shown in Figs. 39 to 42, inclusive. Fig. 39 shows how a formed milling cutter is relieved. The toolslide is set at right angles to the axis of the work, and the tool moves in as each tooth passes, and out while crossing the spaces or flutes between the teeth. As the result of this movement, the tops of the teeth are backed off eccentrically but the form or shape is the same from the front to the back of the tooth; hence, a cutter that has been relieved in this way can be ground repeatedly without changing the profile of the teeth, provided the faces are ground so as to lie in a radial plane.

When relieving, the cutting speed should be much less than when turning in order to give the toolslide time to operate properly. A maximum of 180 teeth per minute is recommended, and, if wide forming tools are used, it might be advisable to reduce the speed so low that only 8 teeth per minute would be relieved. It is also essential to use a tool having a keen edge, and the toolslide should work freely but be closely adjusted to the dovetail of the lower slide. Before beginning to back off the teeth, it is a good plan to color the work either by heating it or dipping into a strong solution of copper sulphate. This will enable one to see plainly the cutting action of the tool in order to stop relieving at the proper time.

Fig. 40. Relieving Side of Angular Milling Cutter

Fig. 40 shows a method of relieving the teeth of an angular cutter. For an operation of this kind the toolslide is swiveled around at right angles to the side that is to be relieved. By the use of an additional universal joint and bearing to permit the toolslide to be swung to a 90-degree angle, the teeth of counterbores, etc., can be relieved on the ends. When the attachment is used for relieving inside work, such as hollow mills and threading dies, the eccentric which controls the travel of the toolslide is set so that the relieving movement is away from the axis of the cutter instead of toward it. This change is made by the toothed coupling previously referred to, which connects the cam lever and oscillating shaft, the latter being turned beyond the zero mark in a clockwise direction as far as is necessary to obtain the desired amount of travel. For internal work it is also necessary to change the position of the opposing spring of the toolslide, so that it will press against the end of the slide and prevent the tool from jumping into the work.

Fig. 41. Relieving a Right-hand Tap

Fig. 41 shows how a right-hand tap is relieved. The ordinary practice is to first set the tool the same as for cutting a thread. The motion of the toolslide is then adjusted so that the tool on the forward stroke will meet the front of each tooth, and start back as soon as the tool leaves the end of the land or top of the tooth. Taps having a left-hand thread can be relieved by two different methods. With the first method the cut starts at the cutting edge of each tooth, and ends at the “heel,” the tool moving in toward the center of the work. With the second method, the cut begins at the heel and discontinues at the cutting edge, the tool being drawn away from the work during the cut. When using the first method the tap must be placed with the point toward the headstock, the shank end being supported by the tailstock center. This is done by providing an extension or blank end at the point of the tap long enough to hold the driving dog. With the second method, the tap is held between centers the same as one having a right-hand thread, but the travel of the toolslide is set the same as for inside relief.

Fig. 42. Relieving a Hob having Spiral Flutes

Relieving Hobs or Taps Having Spiral Flutes.—With this attachment, taps or hobs having “spiral” or helical flutes can also be relieved. (A spiral flute is preferable to one that is parallel to the axis, because with the former the tool has cutting edges which are square with the teeth; this is of especial importance when the lead of the hob or tap thread is considerable.) When relieving work having spiral flutes (as illustrated in Fig. 42), the lead of the spiral and the gears necessary to drive the attachment are first determined. After the attachment is geared for the number of flutes and to compensate for the spiral, the lead-screw is engaged and the backing-off operation is performed the same as though the flutes were straight. The carriage should not be disengaged from the lead-screw after starting the cut, the tool being returned by reversing the lathe.

When gearing the attachment for relieving a tap or hob having spiral flutes, the gears are not selected for the actual number of flutes around the circumference but for a somewhat larger number which depends upon the lead of the hob thread and the lead of the spiral flutes. Let us assume that a hob has 6 spiral flutes and that the attachment is geared for that number. The result would be that as the tool advanced along the thread, it would not keep “in step” with the teeth because the faces of the teeth lie along a spiral (or helix which is the correct name for this curve); in other words, the tool would soon be moving in too late to begin cutting at the proper time, and to compensate for this, the attachment is geared so that the tool will make a greater number of strokes per revolution of the work than the actual number of flutes around the circumference.

With this attachment, the two gears listed on the index plate for the actual number of flutes are selected, and then two compensating gears are added, thus forming a compound train of gearing. The ratio R of these compensating gears is determined as follows:

For example, if a hob has a pitch circumference of 3.25, a single thread of 0.75 inch lead, and 6 spiral flutes, what compensating gears would be required?

The lead L of the spiral flutes is first determined by dividing the square of the circumference C of the hob at the pitch line by the lead l of the hob thread. Thus lead L = C² ÷ l, or, in this case, L = 3.25² ÷ 0.75 = 14 inches, approximately. Then r = 14 ÷ 0.75 = 18²/₃. Inserting these values in the formula for ratio R,

Hence, the compensating gears will have 56 and 59 teeth, respectively, the latter being the driver. As the gears for 6 flutes listed on the regular index plate are, stud-gear 60 teeth, cam-shaft gear 40 teeth, the entire train of gears would be as follows: Gear on stud, 60; driven intermediate gear, 56; driving intermediate gear, 59; cam-shaft gear, 40. It will be understood that the position of the driving gears or the driven gears can be transposed without affecting the ratio.

Classes of Fits Used in Machine Construction.—In assembling machine parts it is necessary to have some members fit together tightly, whereas other parts such as shafts, etc., must be free to move or revolve with relation to each other. The accuracy required for a fitting varies for different classes of work. A shaft that revolves in its bearing must be slightly smaller than the bearing so that there will be room for a film of lubricant. A crank-pin that must be forced into the crank-disk is made a little larger in diameter than the hole, to secure a tight fit. When a very accurate fitting between two cylindrical parts that must be assembled without pressure is required, the diameter of the inner member is made as close to the diameter of the outer member as is possible. In ordinary machine construction, five classes of fits are used, viz; running fit, push fit, driving fit, forced fit and shrinkage fit. The running fit, as the name implies, is employed when parts must rotate; the push fit is not sufficiently free to rotate; the other classes referred to are used for assembling parts that must be held in fixed positions.

Forced Fits.—This is the term used when a pin, shaft or other cylindrical part is forced into a hole of slightly smaller diameter, by the use of a hydraulic press or other means. As a rule, forced fits are restricted to parts of small and medium size, while shrinkage fits have no such limitations and are especially applicable when a maximum “grip” is desired, or when (as in the construction of ordnance) accurate results as to the intensity of stresses produced in the parts united are required. The proper allowance for a forced fit depends upon the mass of metal surrounding the hole, the size of the work, the kind and quality of the material of which the parts are composed and the smoothness and accuracy of the pin and bore. When a pin or other part is pressed into a hole a second time, the allowance for a given tonnage should be diminished somewhat because the surface of the bore is smoother and the metal more compact. The pressure required in assembling a forced fit will also vary for cast hubs of the same size, if they are not uniform in hardness. Then there is the personal factor which is much in evidence in work of this kind; hence, data and formulas for forced fit allowances must be general in their application.

Allowances for Different Classes of Fits

(Newall Engineering Co.)

[1] Tolerance is provided for holes, which ordinary standard reamers can produce, in two grades, Classes A and B, the selection of which is a question for the user's decision and dependent upon the quality of the work required; some prefer to use Class A as working limits and Class B as inspection limits.

[2] Running fits, which are the most commonly required, are divided into three grades: Class X for engine and other work where easy fits are wanted; Class Y for high speeds and good average machine work; Class Z for fine tool work.

Allowance for Forced Fits.—The allowance per inch of diameter usually ranges from 0.001 inch to 0.0025 inch, 0.0015 being a fair average. Ordinarily, the allowance per inch decreases as the diameter increases; thus the total allowance for a diameter of 2 inches might be 0.004 inch, whereas for a diameter of 8 inches the total allowance might not be over 0.009 or 0.010 inch. In some shops the allowance is made practically the same for all diameters, the increased surface area of the larger sizes giving sufficient increase in pressure. The parts to be assembled by forced fits are usually made cylindrical, although sometimes they are slightly tapered. The advantages of the taper form are that the possibility of abrasion of the fitted surfaces is reduced; that less pressure is required in assembling; and that the parts are more readily separated when renewal is required. On the other hand, the taper fit is less reliable, because if it loosens, the entire fit is free with but little axial movement. Some lubricant, such as white lead and lard oil mixed to the consistency of paint, should be applied to the pin and bore before assembling, to reduce the tendency of abrasion.

Pressure for Forced Fits.—The pressure required for assembling cylindrical parts depends not only upon the allowance for the fit, but also upon the area of the fitted surfaces, the pressure increasing in proportion to the distance that the inner member is forced in. The approximate ultimate pressure in pounds can be determined by the use of the following formula in conjunction with the accompanying table of “Pressure Factors.”

Pressure Factors

Assuming that A = area of fitted surface; a = total allowance in inches; P = ultimate pressure required, in tons; F = pressure factor based upon assumption that the diameter of the hub is twice the diameter of the bore, that the shaft is of machine steel, and the hub of cast iron, then,

Example:—What will be the approximate pressure required for forcing a 4-inch machine steel shaft having an allowance of 0.0085 inch into a cast-iron hub 6 inches long?

A = 4 × 3.1416 × 6 = 75.39 square inches;

F, for a diameter of 4 inches, = 115 (see table of “Pressure Factors”). Then,

P = (75.39 × 0.0085 × 115)/2 = 37 tons, approximately.

Allowance for Given Pressure.—By transposing the preceding formula, the approximate allowance for a required ultimate tonnage can be determined. Thus, a = 2P ÷ AF. The average ultimate pressure in tons commonly used ranges from 7 to 10 times the diameter in inches. Assuming that the diameter of a machine steel shaft is 4 inches and an ultimate pressure of about 30 tons is desired for forcing it into a cast-iron hub having a length of 5¹/₂ inches, what should be the allowance?

A = 4 × 3.1416 × 5¹/₂ = 69 square inches,

F, for a diameter of 4 inches, = 115. Then,

Shrinkage Fits.—When heat is applied to a piece of metal, such as iron or steel, as is commonly known, a certain amount of expansion takes place which increases as the temperature is increased, and also varies somewhat with different kinds of metal, copper and brass expanding more for a given increase in temperature than iron and steel. When any part which has been expanded by the application of heat is cooled, it contracts and resumes its original size. This expansive property of metals has been taken advantage of by mechanics in assembling various machine details. A cylindrical part which is to be held in position by a shrinkage fit is first turned a few thousandths of an inch larger than the hole; the diameter of the latter is then increased by heating, and after the part is inserted, the heated outer member is cooled, causing it to grip the pin or shaft with tremendous pressure.

General practice seems to favor a smaller allowance for shrinkage fits than for forced fits, although in many shops the allowances are practically the same in each case, and for some classes of work, shrinkage allowances exceed those for forced fits. In any case, the shrinkage allowance varies to a great extent with the form and construction of the part which has to be shrunk into place. The thickness or amount of metal around the hole is the most important factor. The way in which the metal is distributed also has an influence on the results. Shrinkage allowances for locomotive driving wheel tires adopted by the American Railway Master Mechanics Association are as follows:

Whether parts are to be assembled by forced or shrinkage fits depends upon conditions. For example, to press a driving wheel tire over its wheel center, without heating, would ordinarily be a rather awkward and difficult job. On the other hand, pins, etc., are easily and quickly forced into place with a hydraulic press and there is the additional advantage of knowing the exact pressure required in assembling, whereas there is more or less uncertainty connected with a shrinkage fit, unless the stresses are calculated. Tests to determine the difference in the quality of shrinkage and forced fits showed that the resistance of a shrinkage fit to slippage was, for an axial pull, 3.66 times greater than that of a forced fit, and in rotation or torsion, 3.2 times greater. In each comparative test, the dimensions and allowances were the same.

The most important point to consider when calculating shrinkage fits is the stress in the hub at the bore, which depends chiefly upon the shrinkage allowance. If the allowance is excessive, the elastic limit of the material will be exceeded and permanent set will occur, or, in extreme cases, the ultimate strength of the metal will be exceeded and the hub will burst.

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