Turning and Boring Part 3

[Ill.u.s.tration: Fig. 46. Boring a Cylinder Lining in an Ordinary Engine Lathe]

Cylindrical parts attached to the carriage can also be bored by using a plain solid bar mounted between the centers. The bar must be provided with a cutter for small holes or a tool-head for larger diameters (preferably holding two or more tools) and the boring is done by feeding the carriage along the bed by using the regular power feed of the lathe. A symmetrically shaped casting like a bushing or lining is often held upon wooden blocks bolted across the carriage. These are first cut away to form a circular seat of the required radius, by using the boring-bar and a special tool having a thin curved edge. The casting is then clamped upon these blocks by the use of straps and bolts, and if the curved seats were cut to the correct radius, the work will be located concentric with the boring-bar. When using a boring-bar of this type, the bar must be long enough to allow the part being bored to feed from one side of the cutter-head to the other, the cutter-head being approximately in a central location.

[Ill.u.s.tration: Fig. 47. Method of Setting Circle on Work Concentric with Lathe Spindle]

=Boring Holes to a Given Center Distance.=--In connection with faceplate work, it is often necessary to bore two or more holes at a given distance apart. The best method of doing this may depend upon the accuracy required. For ordinary work sometimes two or more circles _A_ and _B_ (Fig. 47) are drawn upon the part to be bored, in the position for the holes; the piece is then clamped to the faceplate and one of the circles is centered with the lathe spindle by testing it with a pointer C held in the toolpost; that is, when the pointer follows the circle as the work is turned, evidently the circle is concentric with the spindle.

The hole is then drilled and bored. The other circle is then centered in the same way for boring the second hole. As will be seen, the accuracy of this method depends first, upon the accuracy with which the circles were laid out, and second; upon the care taken in setting them concentric. For a more accurate way of locating parts for boring, see "Use of Center Indicator" and "Locating Work by the b.u.t.ton Method."

=Turning Bra.s.s, Bronze and Copper.=--When turning soft yellow bra.s.s, a tool should be used having very little or no slope or rake on the top surface against which the chip bears, and for plain cylindrical turning, the point of the tool is drawn out quite thin and rounded, by grinding, to a radius of about 1/8 or 3/16 inch. If a tool having very much top slope is used for bra.s.s, there is danger of its gouging into the metal, especially if the part being turned is at all flexible. The clearance angle of a bra.s.s tool is usually about 12 or 14 degrees, which is 3 or 4 degrees greater than the clearance for steel turning tools. Most bra.s.s is easily turned, as compared with steel, and for that reason this increase in clearance is desirable, because it facilitates feeding the tool into the metal, especially when the carriage and cross-slide movements are being controlled by hand as when turning irregular shapes.

The speed for turning soft bra.s.s is much higher than for steel, being ordinarily between 150 and 200 feet per minute. When turning phosphor, tobin or other tough bronze compositions, the tool should be ground with rake the same as for turning steel, and lard oil is sometimes used as a lubricant. The cutting speed for bronzes varies from 35 or 40 to 80 feet per minute, owing to the difference in the composition of bronze alloys.

Turning tools for copper are ground with a little more top rake than is given steel turning tools, and the point should be slightly rounded. It is important to have a keen edge, and a grindstone is recommended for sharpening copper turning tools. Milk is generally considered the best lubricant to use when turning copper. The speed can be nearly as fast as for bra.s.s.

=Machining Aluminum.=--Tools for turning aluminum should have acute cutting angles. After rough-grinding the tool, it is advisable to finish sharpening the cutting edge on a grindstone or with an oilstone for fine work, as a keen edge is very essential. High speeds and comparatively light cuts are recommended. The princ.i.p.al difficulty in the machining of aluminum and aluminum alloys is caused by the clogging of the chips, especially when using such tools as counterbores and milling cutters.

This difficulty can be avoided largely by using the right kind of cutting lubricant. Soap-water and kerosene are commonly employed. The latter enables a fine finish to be obtained, provided the cutting tool is properly ground.

The following information on this subject represents the experience of the Brown-Lipe Gear Co., where aluminum parts are machined in large quant.i.ties: For finishing bored holes, a bar equipped with cutters has been found more practicable than reamers. The cutters used for machining 4-inch holes have a clearance of from 20 to 22 degrees and no rake or slope on the front faces against which the chips bear. The roughing cutters for this work have a rather sharp nose, being ground on the point to a radius of about 3/32 inch, but for securing a smooth surface, the finishing tools are rounded to a radius of about 3/4 inch. The cutting speed, as well as the feed, for machining aluminum is from 50 to 60 per cent faster than the speeds and feeds for cast iron. The lubricant used by this company is composed of one part "aqualine" and 20 parts water. This lubricant not only gives a smooth finish but preserves a keen cutting edge and enables tools to be used much longer without grinding. Formerly, a lubricant composed of one part of high-grade lard oil and one part of kerosene was used. This mixture costs approximately 30 cents per gallon, whereas the aqualine and water mixture now being used costs less than 4 cents per gallon, and has proved more effective than the lubricant formerly employed.

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 princ.i.p.ally, and typical examples of the application of these various tools are indicated in Fig.

2. The reference letters used in these two ill.u.s.trations correspond for tools of the same type, and both views should be referred to in connection with the following description.

=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 cla.s.ses of work for taking light finishing cuts, with a coa.r.s.e 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 coa.r.s.e, 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.

[Ill.u.s.tration: Fig. 1. Set of Lathe Turning Tools for General Work]

[Ill.u.s.tration: Fig. 2. Views ill.u.s.trating Use of Various Types of Lathe Tools]

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.

[Ill.u.s.tration: Fig. 3. Turning Tool with Inserted Cutter]

A tool for turning bra.s.s is shown at _M_. Bra.s.s tools intended for general work are drawn out quite thin and they are given a narrow rounded point. The top of the bra.s.s 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 bra.s.s tool is sometimes ground with a straight cutting edge for turning large rigid work, such as bra.s.s pump linings, etc., so that a coa.r.s.e 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.

[Ill.u.s.tration: 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 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 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 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 shifting 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 Finishing 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 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 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 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 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 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 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.