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. 1shows the forms of tools that are used principally, and typical examples of the application of these various tools are indicated inFig. 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.
Set of Lathe Turning Tools for General WorkFig. 1. Set of Lathe Turning Tools for General Work
Set of Lathe Turning Tools for General Work
Fig. 1. Set of Lathe Turning Tools for General Work
Views illustrating Use of Various Types of Lathe ToolsFig. 2. Views illustrating Use of Various Types of Lathe Tools
Views illustrating Use of Various Types of Lathe Tools
Fig. 2. Views illustrating Use of Various Types of Lathe Tools
Turning Tools for General Work.—The tool shown atAis the form generally used for rough turning, that is for taking deep cuts when considerable metal has to be removed. AtBa tool of the same type is shown, having a bent end which enables it to be used close up to a shoulder or surfacesthat might come in contact with the tool-rest if the straight form were employed. ToolC, 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 atDandEare 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-toolEis used on the opposite side, as shown inFig. 2. Side-tools are also bent to the right or left becausethe cutting edge of a straight tool cannot always be located properly for facing certain surfaces. A bent right side-tool is shown atF. A form of tool that is frequently used is shown atG; 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 atH(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 isplaced 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 slopeinward back of the cutting edge to provide clearance when cutting in a narrow groove.
AtIa 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 atJ. 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 inChapter IV, which contains descriptions of different thread forms and methods of cutting them. Internal thread tools are shown atKandLfor 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 atM. 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 atNandOare 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 asFig. 2indicates, they make it possible to turn an almostendless 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.
Turning Tool with Inserted CutterFig. 3. Turning Tool with Inserted Cutter
Fig. 3. Turning Tool with Inserted Cutter
Heavy Inserted-cutter Turning ToolFig. 4. Heavy Inserted-cutter Turning Tool
Fig. 4. Heavy Inserted-cutter Turning 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 inFig. 3. The cutterCis 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 ofthe cutter a different slope from that due to its angular position. Another inserted-cutter turning tool is shown inFig. 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.
Parting Tool with Inserted BladeFig. 5. Parting Tool with Inserted Blade
Fig. 5. Parting Tool with Inserted Blade
Boring Tool with Inserted Cutter and Adjustable BarFig. 6. Boring Tool with Inserted Cutter and Adjustable Bar
Fig. 6. Boring Tool with Inserted Cutter and Adjustable Bar
Threading ToolFig. 7. Threading Tool
Fig. 7. Threading Tool
A parting tool of the inserted blade type is shown inFig. 5. The bladeBis clamped by screwSand 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. 6shows a boring tool consisting of a holderH, a barBthat can be clamped in any position, and an inserted cutterC. 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 inFig. 7. The angular edge of the cutterCis accuratelyground 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 screwS, which can also be used for setting the tool to the proper height.
To Avoid springing, Overhang A of Tool should not be ExcessiveFig. 8. To avoid springing, Overhang A of Tool should not be Excessive
To Avoid springing, Overhang A of Tool should not be Excessive
Fig. 8. To avoid springing, Overhang A of Tool should not be Excessive
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 diameterd,Fig. 8, is slightly above the finished size and the power feed is then engaged. When the tool begins to take the full deptheof 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 distanceAbetween 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.
Tool DisplacementFig. 9. (A) The Way in which Tool is sometimes displaced by Thrust of Cut, when set at an Angle.(B) Tool Set for Finishing both Cylindrical and Radial Surfaces
Tool Displacement
Fig. 9. (A) The Way in which Tool is sometimes displaced by Thrust of Cut, when set at an Angle.(B) Tool Set for Finishing both Cylindrical and Radial Surfaces
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 atAinFig. 9. Suppose the end of a rod has been reduced by taking a number of trial cuts, until it is1/32inch 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 atc, 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 havingthe tool set at an angle. For example, if it is held about as shown atB, when turning the flange castingC, the surfacessands1can 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.
Illustration showing the Meaning of Terms used in Tool Grinding as applied to Tools of Different TypesFig. 10. Illustration showing the Meaning of Terms usedin Tool Grinding as applied to Tools of Different Types
Illustration showing the Meaning of Terms used in Tool Grinding as applied to Tools of Different Types
Fig. 10. Illustration showing the Meaning of Terms usedin Tool Grinding as applied to Tools of Different Types
InFig. 10a 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 angleα, the back slope is represented by the angleβ, and the side slope by the angleγ. The angleδfor 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 flankfand the top of the tool, measured diagonally along a linez—z. It will be seen that the linesA—BandA—Cfrom 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 theeffectiveangles 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.
Plan View of Lathe Turning and Threading ToolsFig. 11. Plan View of Lathe Turning and Threading Tools
Plan View of Lathe Turning and Threading Tools
Fig. 11. Plan View of Lathe Turning and Threading Tools
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 toolA, inFig. 11, where a plan view of a number of different lathe tools is shown, has a very different shape from that of, say, toolD, as the first tool is used for rough turning, while toolDis intended for cutting grooves or severing a turned part. Similarly, toolEis V-shaped because it is used for cutting V-threads. ToolsA,BandC, 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, toolAis of a shape suitable for rough-turning large and rigid work, while toolBis 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 likeB, which has a smaller point. This increasein the cutting speed is due to the fact that the toolAremoves a thinner chip for a given feed than toolB; therefore, the speed may be increased without injuring the cutting edge to the same extent. If, however, toolAwere to be used for turning a long and flexible part, chattering might result; consequently, a toolBhaving 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 atCis 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 toolDwhich, as previously stated, is used for cutting grooves, squaring corners, etc. Evidently this tool should be widest at the cutting edge; that is, the sidesdshould have a slight amount of clearance so that they will not bind as the tool is fed into a groove. As the tool atEis for cutting a V-thread, the angleαbetween its cutting edges must equal the angle between the sides of a V-thread, or 60 degrees. The tool illustrated atFis for cutting inside square threads. In this case the widthwshould 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 toolD. 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 theworking partof the cutting edge. For example, the working edge of a roughing toolA(Fig. 11), which is used for heavy cuts, would be, practically speaking, between pointsaandb, 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 toolCwhich takes light cuts, the slope should be straight back from the point or cutting edgea—b. As the side tool shown inFig. 10does its cutting along the edgea—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 flankf(Fig. 10) must be ground to a certain angleαso 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.
Illustrations showing how Effective Angles of Slope and Clearance change as Tool is raised or loweredFig. 12. Illustrations showing how Effective Angles of Slopeand Clearance change as Tool is raised or lowered
Illustrations showing how Effective Angles of Slope and Clearance change as Tool is raised or lowered
Fig. 12. Illustrations showing how Effective Angles of Slopeand Clearance change as Tool is raised or lowered
The back slope of a tool is measured from a lineA—Bwhich is parallel to the shank, and the clearance angle, from a lineA—Cat right angles to lineA—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 inFig. 12, the base lineA—Bfor a turning tool in use intersects with the point of the tool and center of the work, while the lineA—Cremains at right angles to the first. It will be seen, then, that by raising the tool, as shown to the right, theeffectiveclearance angleαwill 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 keennessδ(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 slopeβ, 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, whilea tool for cutting very hard steel should have a back slope of about five degrees and a side slope of nine degrees.
Blunt Tool for Turning Hard Steel, Tool-point Ground to give KeennessFig. 13. (A) Blunt Tool for Turning Hard Steel.(B) Tool-point Ground to give Keenness
Fig. 13. (A) Blunt Tool for Turning Hard Steel.(B) Tool-point Ground to give Keenness
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 atA,Fig. 13, is much stronger than it would be if ground as shown atB, as the former is more blunt. If a tool ground as atA, 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 thenearer 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 the Top and Flank of a Turning ToolFig. 14. Grinding the Top and Flank of a Turning Tool
Fig. 14. Grinding the Top and Flank of a Turning Tool
Grinding a Lathe Tool.—The way a turning tool is held while the top surface is being ground is shown to the left inFig. 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 securea sharp edge on the turning tool shown to the left inFig. 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.