Turning and Boring on a Lathe Online Reprint Chapter 2 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 II

LATHE TURNING TOOLS AND CUTTING SPEEDS

Notwithstanding the fact that a great variety of work can be done in the lathe, the number of turning tools required is comparatively small. Fig. 1 shows the forms of tools that are used principally, and typical examples of the application of these various tools are indicated in Fig. 2. The reference letters used in these two illustrations correspond for tools of the same type, and both views should be referred to in connection with the following description.

Fig. 1. Set of Lathe Turning Tools for General Work

Fig. 2. Views illustrating Use of Various Types of Lathe Tools

Turning Tools for General Work.—The tool shown at A is the form generally used for rough turning, that is for taking deep cuts when considerable metal has to be removed. At B a tool of the same type is shown, having a bent end which enables it to be used close up to a shoulder or surface s that might come in contact with the tool-rest if the straight form were employed. Tool C, which has a straight cutting end, is used on certain classes of work for taking light finishing cuts, with a coarse feed. This type of tool has a flat or straight cutting edge at the end, and will leave a smooth finish even though the feed is coarse, provided the cutting edge is set parallel with the tool's travel so as to avoid ridges. Broad-nosed tools and wide feeds are better adapted for finishing cast iron than steel. When turning steel, if the work is at all flexible, a broad tool tends to gouge into it and for this reason round-nosed tools and finer feeds are generally necessary. A little experience in turning will teach more on this point than a whole chapter on the subject.

The side-tools shown at D and E are for facing the ends of shafts, collars, etc. The first tool is known as a right side-tool because it operates on the right end or side of a shaft or collar, whereas the left side-tool E is used on the opposite side, as shown in Fig. 2. Side-tools are also bent to the right or left because the cutting edge of a straight tool cannot always be located properly for facing certain surfaces. A bent right side-tool is shown at F. A form of tool that is frequently used is shown at G; this is known as a parting tool and is used for severing pieces and for cutting grooves, squaring corners, etc. The same type of tool having a bent end is shown at H (Fig. 2) severing a piece held in the chuck. Work that is held between centers should not be entirely severed with a parting tool unless a steadyrest is placed between the tool and faceplate, as otherwise the tool may be broken by the springing of the work just before the piece is cut in two. It should be noted that the sides of this tool slope inward back of the cutting edge to provide clearance when cutting in a narrow groove.

At I a thread tool is shown for cutting a U. S. standard thread. This thread is the form most commonly used in this country at the present time. A tool for cutting a square thread is shown at J. This is shaped very much like a parting tool except that the cutting end is inclined slightly to correspond with the helix angle of the thread, as explained in Chapter IV, which contains descriptions of different thread forms and methods of cutting them. Internal thread tools are shown at K and L for cutting U. S. standard and square threads in holes. It will be seen that these tools are somewhat like boring tools excepting the ends which are shaped to correspond with the thread which they are intended to cut.

A tool for turning brass is shown at M. Brass tools intended for general work are drawn out quite thin and they are given a narrow rounded point. The top of the brass tool is usually ground flat or without slope as otherwise it tends to gouge into the work, especially if the latter is at all flexible. The end of a brass tool is sometimes ground with a straight cutting edge for turning large rigid work, such as brass pump linings, etc., so that a coarse feed can be used without leaving a rough surface. The tools at N and O are for boring or finishing drilled or cored holes. Two sizes are shown, which are intended for small and large holes, respectively.

The different tools referred to in the foregoing might be called the standard types because they are the ones generally used, and as Fig. 2 indicates, they make it possible to turn an almost endless variety of forms. Occasionally some special form of tool is needed for doing odd jobs, having, perhaps, an end bent differently or a cutting edge shaped to some particular form. Tools of the latter type, which are known as “form tools,” are sometimes used for finishing surfaces that are either convex, concave, or irregular in shape. The cutting edges of these tools are carefully filed or ground to the required shape, and the form given the tool is reproduced in the part turned. Ornamental or other irregular surfaces can be finished very neatly by the use of such tools. It is very difficult, of course, to turn convex or concave surfaces with a regular tool; in fact, it would not be possible to form a true spherical surface, for instance, without special equipment, because the tool could not be moved along a true curve by simply using the longitudinal and cross feeds. Form tools should be sharpened by grinding entirely on the top surface, as any grinding on the end or flank would alter the shape of the tool.

Tool-holders with Inserted Cutters.—All of the tools shown in Fig. 1 are forged from the bar, and when the cutting ends have been ground down considerably it is necessary to forge a new end. To eliminate the expense of this continual dressing of tools and also to effect a great reduction in the amount of tool steel required, tool-holders having small inserted cutters are used in many shops. A tool-holder of this type, for outside turning, is shown in Fig. 3. The cutter C is held in a fixed position by the set-screw shown, and it is sharpened, principally, by grinding the end, except when it is desired to give the top of the cutter a different slope from that due to its angular position. Another inserted-cutter turning tool is shown in Fig. 4, which is a heavy type intended for roughing. The cutter in this case has teeth on the rear side engaging with corresponding teeth cut in the clamping block which is tightened by a set-screw on the side opposite that shown. With this arrangement, the cutter can be adjusted upward as the top is ground away.

A parting tool of the inserted blade type is shown in Fig. 5. The blade B is clamped by screw S and also by the spring of the holder when the latter is clamped in the toolpost. The blade can, of course, be moved outward when necessary. Fig. 6 shows a boring tool consisting of a holder H, a bar B that can be clamped in any position, and an inserted cutter C. With this type of boring tool, the bar can be extended beyond the holder just far enough to reach through the hole to be bored, which makes the tool very rigid. A thread tool of the holder type is shown in Fig. 7. The angular edge of the cutter C is accurately ground by the manufacturers, so that the tool is sharpened by simply grinding it flat on the top. As the top is ground away, the cutter is raised by turning screw S, which can also be used for setting the tool to the proper height.

The Position of Turning Tools.—The production of accurate lathe work depends partly on the condition of the lathe used and also on the care and judgment exercised by the man operating it. Even though a lathe is properly adjusted and in good condition otherwise, errors are often made which are due to other causes which should be carefully avoided. If the turning tool is clamped so that the cutting end extends too far from the supporting block, the downward spring of the tool, owing to the thrust of the cut, sometimes results in spoiled work, especially when an attempt is made to turn close to the finished size by taking a heavy roughing cut. Suppose the end of a cylindrical part is first reduced for a short distance by taking several trial cuts until the diameter d, Fig. 8, is slightly above the finished size and the power feed is then engaged. When the tool begins to take the full depth e of the cut, the point, which ordinarily would be set a little above the center, tends to spring downward into the work, and if there were considerable springing action, the part would probably be turned below the finished size, the increased reduction beginning at the point where the full cut started.

This springing action, as far as the tool is concerned, can be practically eliminated by locating the tool so that the distance A between the tool-block and cutting end, or the “overhang,” is as short as possible. Even though the tool has little overhang it may tilt downward because the toolslide is loose on its ways, and for this reason the slide should have a snug adjustment that will permit an easy movement without unnecessary play. The toolslides of all lathes are provided with gibs which can be adjusted by screws to compensate for wear, or to secure a more rigid bearing.

When roughing cuts are to be taken, the tool should be located so that any change in its position which might be caused by the pressure of the cut will not spoil the work. This point is illustrated at A in Fig. 9. Suppose the end of a rod has been reduced by taking a number of trial cuts, until it is ¹/₃₂ inch above the finished size. If the power feed is then engaged with the tool clamped in an oblique position, as shown, when the full cut is encountered at c, the tool, unless very tightly clamped, may be shifted backward by the lateral thrust of the cut, as indicated by the dotted lines. The point will then begin turning smaller than the finished size and the work will be spoiled. To prevent any change of position, it is good practice, especially when roughing, to clamp the tool square with the surface being turned, or in other words, at right angles to its direction of movement. Occasionally, however, there is a decided advantage in having the tool set at an angle. For example, if it is held about as shown at B, when turning the flange casting C, the surfaces s and s₁ can be finished without changing the tool's position. Cylindrical and radial surfaces are often turned in this way in order to avoid shifting the tool, especially when machining parts in quantity.

Tool Grinding.—In the grinding of lathe tools there are three things of importance to be considered: First, the cutting edge of the tool (as viewed from the top) needs to be given a certain shape; second, there must be a sufficient amount of clearance for the cutting edge; and third, tools, with certain exceptions, are ground with a backward slope or a side slope, or with a combination of these two slopes on that part against which the chip bears when the tool is in use.

In Fig. 10 a few of the different types of tools which are used in connection with lathe work are shown. This illustration also indicates the meaning of the various terms used in tool grinding. As shown, the clearance of the tool is represented by the angle α, the back slope is represented by the angle β, and the side slope by the angle γ. The angle δ for a tool without side slope is known as the lip angle or the angle of keenness. When, however, the tool has both back and side slopes, this lip angle would more properly be the angle between the flank f and the top of the tool, measured diagonally along a line z—z. It will be seen that the lines A—B and A—C from which the angles of clearance and back slope are measured are parallel with the top and sides of the tool shank, respectively. For lathe tools, however, these lines are not necessarily located in this way when the tool is in use, as the height of the tool point with relation to the work center determines the position of these lines, so that the effective angles of back slope, clearance and keenness are changed as the tool point is lowered or raised. The way the position of the tool affects these angles will be explained later.

While tools must, of necessity, be varied considerably in shape to adapt them to various purposes, there are certain underlying principles governing their shape which apply generally; so in what follows we shall not attempt to explain in detail just what the form of each tool used on the lathe should be, as it is more important to understand how the cutting action of the tool and its efficiency is affected when it is improperly ground.When the principle is understood, the grinding of tools of various types and shapes is comparatively easy.

Shape or Contour of Cutting Edge.—In the first place we shall consider the shape or contour of the cutting edge of the tool as viewed from the top, and then take up the question of clearance and slope, the different elements being considered separately to avoid confusion. The contour of the cutting edge depends primarily upon the purpose for which the tool is intended. For example, the tool A, in Fig. 11, where a plan view of a number of different lathe tools is shown, has a very different shape from that of, say, tool D, as the first tool is used for rough turning, while tool D is intended for cutting grooves or severing a turned part. Similarly, tool E is V-shaped because it is used for cutting V-threads. Tools A, B and C, however, are regular turning tools; that is, they are all intended for turning plain cylindrical surfaces, but the contour of the cutting edges varies considerably, as shown. In this case it is the characteristics of the work and the cut that are the factors which determine the shape. To illustrate, tool A is of a shape suitable for rough-turning large and rigid work, while tool B is adapted for smaller and more flexible parts. The first tool is well shaped for roughing because experiments have shown that a cutting edge of a large radius is capable of higher cutting speed than could be used with a tool like B, which has a smaller point. This increase in the cutting speed is due to the fact that the tool A removes a thinner chip for a given feed than tool B; therefore, the speed may be increased without injuring the cutting edge to the same extent. If, however, tool A were to be used for turning a long and flexible part, chattering might result; consequently, a tool B having a point with a smaller radius would be preferable, if not absolutely necessary.

The character of the work also affects the shape of tools. The tool shown at C is used for taking light finishing cuts with a wide feed. Obviously, if the straight or flat part of the cutting edge is in line with the travel of the tool, the cut will be smooth and free from ridges, even though the feed is coarse, and by using a coarse feed the cut is taken in less time; but such a tool cannot be used on work that is not rigid, as chattering would result. Therefore, a smaller cutting point and a reduced feed would have to be employed. Tools with broad flat cutting edges and coarse feeds are often used for taking finishing cuts in cast iron, as this metal offers less resistance to cutting than steel, and is less conducive to chattering.

The shape of a tool (as viewed from the top) which is intended for a more specific purpose than regular turning, can be largely determined by simply considering the tool under working conditions. This point may be illustrated by the parting tool D which, as previously stated, is used for cutting grooves, squaring corners, etc. Evidently this tool should be widest at the cutting edge; that is, the sides d should have a slight amount of clearance so that they will not bind as the tool is fed into a groove. As the tool at E is for cutting a V-thread, the angle α between its cutting edges must equal the angle between the sides of a V-thread, or 60 degrees. The tool illustrated at F is for cutting inside square threads. In this case the width w should be made equal to one-half the pitch of the thread (or slightly greater to provide clearance for the screw), and the sides should be given a slight amount of side clearance, the same as with the parting tool D. So we see that the outline of the tool, as viewed from the top, must conform to and be governed by its use.

Direction of Top Slope for Turning Tools.—Aside from the question of the shape of the cutting edge as viewed from the top, there remains to be determined the amount of clearance that the tool shall have, and also the slope (and its direction) of the top of the tool. By the top is meant that surface against which the chip bears while it is being severed. It may be stated, in a general way, that the direction in which the top of the tool should slope should be away from what is to be the working part of the cutting edge. For example, the working edge of a roughing tool A (Fig. 11), which is used for heavy cuts, would be, practically speaking, between points a and b, or, in other words, most of the work would be done by this part of the cutting edge; therefore the top should slope back from this part of the edge. Obviously, a tool ground in this way will have both a back and a side slope.

When most of the work is done on the point or nose of the tool, as, for example, with the lathe finishing tool C which takes light cuts, the slope should be straight back from the point or cutting edge a—b. As the side tool shown in Fig. 10 does its cutting along the edge a—b, the top is given a slope back from this edge as shown in the end view. This point should be remembered, for when the top slopes in the right direction, less power is required for cutting. Tools for certain classes of work, such as thread tools, or those for turning brass or chilled iron, are ground flat on top, that is, without back or side slope.

Clearance for the Cutting Edge.—In order that the cutting edge may work without interference, it must have clearance; that is, the flank f (Fig. 10) must be ground to a certain angle α so that it will not rub against the work and prevent the cutting edge from entering the metal. This clearance should be just enough to permit the tool to cut freely. A clearance angle of eight or ten degrees is about right for lathe turning tools.

The back slope of a tool is measured from a line A—B which is parallel to the shank, and the clearance angle, from a line A—C at right angles to line A—B. These lines do not, however, always occupy this position with relation to the tool shank when the tool is in use. As shown to the left in Fig. 12, the base line A—B for a turning tool in use intersects with the point of the tool and center of the work, while the line A—C remains at right angles to the first. It will be seen, then, that by raising the tool, as shown to the right, the effective clearance angle α will be diminished, whereas lowering it, as shown by the dotted lines, will have the opposite effect.

A turning tool for brass or other soft metal, particularly where considerable hand manipulation is required, could advantageously have a clearance of twelve or fourteen degrees, as it would then be easier to feed the tool into the metal; but, generally speaking, the clearance for turning tools should be just enough to permit them to cut freely. Excessive clearance weakens the cutting edge and may cause it to crumble under the pressure of the cut.

Angle of Tool-point and Amount of Top Slope.—The lip angle or the angle of keenness δ (Fig. 10) is another important consideration in connection with tool grinding, for it is upon this angle that the efficiency of the tool largely depends. By referring to the illustration it will be seen that this angle is governed by the clearance and the slope β, and as the clearance remains practically the same, it is the slope which is varied to meet different conditions. Now, the amount of slope a tool should have depends on the work for which it is intended. If, for example, a turning tool is to be used for roughing medium or soft steel, it should have a back slope of about eight degrees and a side slope ranging from fourteen to twenty degrees, while a tool for cutting very hard steel should have a back slope of about five degrees and a side slope of nine degrees.

The reason for decreasing the slope and thus increasing the lip angle for harder metals is to give the necessary increased strength to the cutting edge to prevent it from crumbling under the pressure of the cut. The tool illustrated at A, Fig. 13, is much stronger than it would be if ground as shown at B, as the former is more blunt. If a tool ground as at A, however, were used for cutting very soft steel, there would be a greater chip pressure on the top and, consequently, a greater resistance to cutting, than if a keener tool had been employed; furthermore the cutting speed would have to be lower, which is of even greater importance than the chip pressure; therefore, the lip angle, as a general rule, should be as small as possible without weakening the tool so that it cannot do the required work. In order to secure a strong and well-supported cutting edge, tools used for turning very hard metal, such as chilled rolls, etc., are ground with practically no slope and with very little clearance. Brass tools, while given considerable clearance, as previously stated, are ground flat on top or without slope; this is not done, however, to give strength to the cutting edge, but rather to prevent the tool from gouging into the work, which it is likely to do if the part being turned is at all flexible and the tool has top slope.

Experiments conducted by Mr. F. W. Taylor to determine the most efficient form for lathe roughing tools showed that the nearer the lip angle approached sixty-one degrees, the higher the cutting speed. This, however, does not apply to tools for turning cast iron, as the latter will work more efficiently with a lip angle of about sixty-eight degrees. This is doubtless because the chip pressure, when turning cast iron, comes closer to the cutting edge which should, therefore, be more blunt to withstand the abrasive action and heat. Of course, the foregoing remarks concerning lip angles apply more particularly to tools used for roughing.

Grinding a Lathe Tool.—The way a turning tool is held while the top surface is being ground is shown to the left in Fig. 14. By inclining the tool with the wheel face, it will be seen that both the back and side slopes may be ground at the same time. When grinding the flank of the tool it should be held on the tool-rest of the emery wheel or grindstone, as shown by the view to the right. In order to form a curved cutting edge, the tool is turned about the face of the stone while it is being ground. This rotary movement can be effected by supporting the inner end of the tool with one hand while the shank is moved to and fro with the other.

Often a tool which has been ground properly in the first place is greatly misshapen after it has been sharpened a few times. This is usually the result of attempts on the part of the workman to re-sharpen it hurriedly; for example, it is easier to secure a sharp edge on the turning tool shown to the left in Fig. 12, by grinding the flank as indicated by the dotted line, than by grinding the entire flank. The clearance is, however, reduced and the lip angle changed.

There is great danger when grinding a tool of burning it or drawing the temper from the fine cutting edge, and, aside from the actual shape of the cutting end, this is the most important point in connection with tool grinding. If a tool is pressed hard against an emery or other abrasive wheel, even though the latter has a copious supply of water, the temper will sometimes be drawn. When grinding a flat surface, to avoid burning, the tool should frequently be withdrawn from the stone so that the cooling water (a copious supply of which should be provided) can reach the surface being ground. A moderate pressure should also be applied, as it is better to spend an extra minute or two in grinding than to ruin the tool by burning, in an attempt to sharpen it quickly. Of course, what has been said about burning applies more particularly to carbon steel, but even self-hardening steels are not improved by being over-heated at the stone. In some shops, tools are ground to the theoretically correct shape in special machines instead of by hand. The sharpened tools are then kept in the tool-room and are given out as they are needed.

Cutting Speeds and Feeds.—The term cutting speed as applied to turning operations is the speed in feet per minute of the surface being turned, or, practically speaking, it is equivalent to the length of a chip, in feet, which would be turned in one minute. The term cutting speed should not be confused with revolutions per minute, because the cutting speed depends not only upon the speed of the work but also upon its diameter. The feed of a tool is the amount it moves across the surface being turned for each revolution; that is, when turning a cylindrical piece, the feed is the amount that the tool moves sidewise for each revolution of the work. Evidently the time required for turning is governed largely by the cutting speed, the feed, and the depth of the cut; therefore, these elements should be carefully considered.

Cutting Speeds and Feeds for Turning Tools[1]

Average Cutting Speeds for Turning.—The cutting speed is governed principally by the hardness of the metal to be turned; the kind of steel of which the turning tool is made; the shape of the tool and its heat-treatment; the feed and depth of cut; whether or not a cooling lubricant is used on the tool; the power of the lathe and also its construction; hence it is impossible to give any definite rule for determining either the speed, feed, or depth of cut, because these must be varied to suit existing conditions. A general idea of the speeds used in ordinary machine shop practice may be obtained from the following figures:

Ordinary machine steel is generally turned at a speed varying between 45 and 65 feet per minute. For ordinary gray cast iron, the speed usually varies from 40 to 50 feet per minute; for annealed tool steel, from 25 to 35 feet per minute; for soft yellow brass, from 150 to 200 feet per minute; for hard bronze, from 35 to 80 feet per minute, the speed depending upon the composition of the alloy. While these speeds correspond closely to general practice, they can be exceeded for many machining operations.

The most economical speeds for a given feed and depth of cut, as determined by the experiments conducted by Mr. F. W. Taylor, are given in the table, “Cutting Speeds and Feeds for Turning Tools.” The speeds given in this table represent results obtained with tools made of a good grade of high-speed steel properly heat-treated and correctly ground. It will be noted that the cutting speed is much slower for cast iron than for steel. Cast iron is cut with less pressure or resistance than soft steel, but the slower speed required for cast iron is probably due to the fact that the pressure of the chip is concentrated closer to the cutting edge, combined with the fact that cast iron wears the tool faster than steel. The speeds given are higher than those ordinarily used, and, in many cases, a slower rate would be necessary to prevent chattering or because of some other limiting condition.

Factors which limit the Cutting Speed.—It is the durability of the turning tool or the length of time that it will turn effectively without grinding, that limits the cutting speed; and the hardness of the metal being turned combined with the quality of the tool are the two factors which largely govern the time that a tool can be used before grinding is necessary. The cutting speed for very soft steel or cast iron can be three or four times faster than the speed for hard steel or hard castings, but whether the material is hard or soft, the kind and quality of the tool used must also be considered, as the speed for a tool made of ordinary carbon steel will have to be much slower than for a tool made of modern “high-speed” steel.

When the cutting speed is too high, even though high-speed steel is used, the point of the tool is softened to such an extent by the heat resulting from the pressure and friction of the chip, that the cutting edge is ruined in too short a time. On the other hand, when the speed is too slow, the heat generated is so slight as to have little effect and the tool point is dulled by being slowly worn or ground away by the action of the chip. While a tool operating at such a low speed can be used a comparatively long time without re-sharpening, this advantage is more than offset by the fact that too much time is required for removing a given amount of metal when the work is revolving so slowly.

Generally speaking, the speed should be such that a fair amount of work can be done before the tool requires re-grinding. Evidently, it would not pay to grind a tool every few minutes in order to maintain a high cutting speed; neither would it be economical to use a very slow speed and waste considerable time in turning, just to save the few minutes required for grinding. For example, if a number of roughing cuts had to be taken over a heavy rod or shaft, time might be saved by running at such a speed that the tool would have to be sharpened (or be replaced by a tool previously sharpened) when it had traversed half-way across the work; that is, the time required for sharpening or changing the tool would be short as compared with the gain effected by the higher work speed. On the other hand, it might be more economical to run a little slower and take a continuous cut across the work with one tool.

The experiments of Mr. Taylor led to the conclusion that, as a rule, it is not economical to use roughing tools at a speed so slow as to cause them to last more than 1¹/₂ hour without being re-ground; hence the speeds given in the table previously referred to are based upon this length of time between grindings. Sometimes the work speed cannot be as high as the tool will permit, because of the chattering that often results when the lathe is old and not massive enough to absorb the vibrations, or when there is unnecessary play in the working parts. The shape of the tool used also affects the work speed, and as there are so many things to be considered, the proper cutting speed is best determined by experiment.

Rules for Calculating Cutting Speeds.—The number of revolutions required to give any desired cutting speed can be found by multiplying the cutting speed, in feet per minute, by 12 and dividing the product by the circumference of the work in inches. Expressing this as a formula we have

in which

For example if a cutting speed of 60 feet per minute is wanted and the diameter of the work is 5 inches, the required speed would be found as follows:

If the diameter is simply multiplied by 3 and the fractional part is omitted, the calculation can easily be made, and the result will be close enough for practical purposes. In case the cutting speed, for a given number of revolutions and diameter, is wanted, the following formula can be used:

Machinists who operate lathes do not know, ordinarily, what cutting speeds, in feet per minute, are used for different classes of work, but are guided entirely by past experience.

Feed of Tool and Depth of Cut.—The amount of feed and depth of cut also vary like the cutting speed, for different conditions. When turning soft machine steel the feed under ordinary conditions would vary between ¹/₃₂ and ¹/₁₆ inch per revolution. For turning soft cast iron the feed might be increased to from ¹/₁₆ to ¹/₈ inch per revolution. These feeds apply to fairly deep roughing cuts. Coarser feeds might be used in many cases especially when turning large rigid parts in a powerful lathe. The depth of a roughing cut in machine steel might vary from ¹/₈ to ³/₈ inch, and in cast iron from ³/₁₆ to ¹/₂ inch. These figures are intended simply to give the reader a general idea of feeds and cuts that are feasible under average conditions.

Ordinarily coarser feeds and a greater depth of cut can be used for cast iron than for soft steel, because cast iron offers less resistance to turning, but in any case, with a given depth of cut, metal can be removed more quickly by using a coarse feed and the necessary slower speed, than by using a fine feed and the higher speed which is possible when the feed is reduced. When the turning operation is simply to remove metal, the feed should be coarse, and the cut as deep as practicable. Sometimes the cut must be comparatively light, either because the work is too fragile and springy to withstand the strain of a heavy cut, or the lathe has not sufficient pulling power. The difficulty with light slender work is that a heavy cut may cause the part being turned to bend under the strain, thus causing the tool to gouge in, which would probably result in spoiling the work. Steadyrests can often be used to prevent flexible parts from springing, as previously explained, but there are many kinds of light work to which the steadyrest cannot be applied to advantage.

The amount of feed to use for a finishing cut might, properly, be either fine or coarse. Ordinarily, fine feeds are used for finishing steel, especially if the work is at all flexible, whereas finishing cuts in cast iron are often accompanied by a coarse feed. Fig. 15 illustrates the feeds that are often used when turning cast iron. The view to the left shows a deep roughing cut and the one to the right, a finishing cut. By using a broad flat cutting edge set parallel to the tool's travel, and a coarse feed for finishing, a smooth cut can be taken in a comparatively short time. Castings which are close to the finished size in the rough can often be finished to advantage by taking a single cut with a broad tool, provided the work is sufficiently rigid. It is not always practicable to use these broad tools and coarse feeds, as they sometimes cause chattering, and when used on steel, a broad tool tends to gouge or “dig in” unless the part being turned is rigid. Heavy steel parts, however, are sometimes finished in this way. The modern method of finishing many steel parts is to simply rough them out in a lathe to within, say, ¹/₃₂ inch of the required diameter and take the finishing cut in a cylindrical grinding machine.

Effect of Lubricant on Cutting Speed.—When turning iron or steel a higher cutting speed can be used, if a stream of soda water or other cooling lubricant falls upon the chip at the point where it is being removed by the tool. In fact, experiments have shown that the cutting speed, when using a large stream of cooling water and a high-speed steel tool, can be about 40 percent higher than when turning dry or without a cooling lubricant. For ordinary carbon steel tools, the gain was about 25 per cent. The most satisfactory results were obtained from a stream falling at a rather slow velocity but in large volume. The gain in cutting speed, by the use of soda water or other suitable fluids, was found to be practically the same for all qualities of steel from the softest to the hardest.

Cast iron is usually turned dry or without a cutting lubricant. Experiments, however, made to determine the effect of applying a heavy stream of cooling water to a tool turning cast iron, showed the following results: Cutting speed without water, 47 feet per minute; cutting speed with a heavy stream of water, nearly 54 feet per minute; increase in speed, 15 per cent. The dirt caused by mixing the fine cast-iron turnings with a cutting lubricant is an objectionable feature which, in the opinion of many, more than offsets the increase in cutting speed that might be obtained.

Turret lathes and automatic turning machines are equipped with a pump and piping for supplying cooling lubricant to the tools in a continuous stream. Engine lathes used for general work, however, are rarely provided with such equipment and a lubricant, when used, is often supplied by a can mounted at the rear of the carriage, having a spout which extends above the tool. Owing to the inconvenience in using a lubricant on an engine lathe, steel, as well as cast iron, is often turned dry especially when the work is small and the cuts light and comparatively short.

Lubricants Used for Turning.—A good grade of lard oil is an excellent lubricant for use when turning steel or wrought iron and it is extensively used on automatic screw machines, especially those which operate on comparatively small work. For some classes of work, especially when high-cutting speeds are used, lard oil is not as satisfactory as soda water or some of the commercial lubricants, because the oil is more sluggish and does not penetrate to the cutting point with sufficient rapidity. Many lubricants which are cheaper than oil are extensively used on “automatics” for general machining operations. These usually consist of a mixture of sal-soda (carbonate of soda) and water, to which is added some ingredient such as lard oil or soft soap to thicken or give body to the lubricant.

A cheap lubricant for turning, milling, etc., and one that has been extensively used, is made in the following proportions: 1 pound of sal-soda, 1 quart of lard oil, 1 quart of soft soap, and enough water to make 10 or 12 gallons. This mixture is boiled for one-half hour, preferably by passing a steam coil through it. If the solution should have an objectionable odor, this can be eliminated by adding 2 pounds of unslaked lime. The soap and soda in this solution improve the lubricating quality and also prevent the surfaces from rusting. For turning and threading operations, plain milling, deep-hole drilling, etc., a mixture of equal parts of lard oil and paraffin oil will be found very satisfactory, the paraffin being added to lessen the expense.

Brass or bronze is usually machined dry, although lard oil is sometimes used for automatic screw machine work. Babbitt metal is also worked dry, ordinarily, although kerosene or turpentine is sometimes used when boring or reaming. If babbitt is bored dry, balls of metal tend to form on the tool point and score the work. Milk is generally considered the best lubricant for machining copper. A mixture of lard oil and turpentine is also used for copper. For aluminum, the following lubricants can be used: Kerosene, a mixture of kerosene and gasoline, soap-water, or “aqualine” one part, water 20 parts.

Lard Oil as a Cutting Lubricant.—After being used for a considerable time, lard oil seems to lose some of its good qualities as a cooling compound. There are several reasons for this: Some manufacturers use the same oil over and over again on different materials, such as brass, steel, etc. This is objectionable, for when lard oil has been used on brass it is practically impossible to get the fine dust separated from it in a centrifugal separator. When this impure oil is used on steel, especially where high-speed steels are employed, it does not give satisfactory results, owing to the fact that when the cutting tool becomes dull, the small brass particles “freeze” to the cutting tool and thus produce rough work. The best results are obtained from lard oil by keeping it thin, and by using it on the same materials—that is, not transferring the oil from a machine in which brass is being cut to one where it would be employed on steel. If the oil is always used on the same class of material, it will not lose any of its good qualities.

Prime lard oil is nearly colorless, having a pale yellow or greenish tinge. The solidifying point and other characteristics of the oil depend upon the temperature at which it was expressed, winter-pressed lard oil containing less solid constituents of the lard than that expressed in warm weather. The specific gravity should not exceed 0.916; it is sometimes increased by adulterants, such as cotton-seed and maize oils.

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