Part 3 (2/2)
=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, princ.i.p.ally, 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.
[Ill.u.s.tration: Fig. 5. Parting Tool with Inserted Blade]
[Ill.u.s.tration: Fig. 6. Boring Tool with Inserted Cutter and Adjustable Bar]
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.
[Ill.u.s.tration: Fig. 7. Threading Tool]
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.
[Ill.u.s.tration: Fig. 8. To avoid springing, Overhang A of Tool should not be Excessive]
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 ill.u.s.trated 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 s.h.i.+fted 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 f.l.a.n.g.e casting _C_, the surfaces _s_ and _s_{1}_ can be finished without changing the tool's position. Cylindrical and radial surfaces are often turned in this way in order to avoid s.h.i.+fting the tool, especially when machining parts in quant.i.ty.
=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.
[Ill.u.s.tration: Fig. 9. (A) The Way in which Tool is sometimes displaced by Thrust of Cut, when set at an Angle. (B) Tool Set for Finis.h.i.+ng both Cylindrical and Radial Surfaces]
In Fig. 10 a few of the different types of tools which are used in connection with lathe work are shown. This ill.u.s.tration also indicates the meaning of the various terms used in tool grinding. As shown, the clearance of the tool is represented by the angle [alpha], the back slope is represented by the angle [beta], and the side slope by the angle [gamma]. The angle [delta] 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.
[Ill.u.s.tration: Fig. 10. Ill.u.s.tration showing the Meaning of Terms used in Tool Grinding as applied to Tools of Different Types]
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.
[Ill.u.s.tration: 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 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 ill.u.s.trate, 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 finis.h.i.+ng 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 coa.r.s.e, and by using a coa.r.s.e 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 coa.r.s.e feeds are often used for taking finis.h.i.+ng 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 ill.u.s.trated 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 [alpha] between its cutting edges must equal the angle between the sides of a V-thread, or 60 degrees. The tool ill.u.s.trated 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 finis.h.i.+ng 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 cla.s.ses of work, such as thread tools, or those for turning bra.s.s 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 [alpha] 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.
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