Modern Machine-Shop Practice Part 75

Three or more washers may thus be used for every standard size, their thickness varying to suit the nature of the fit required.

[Ill.u.s.tration: Fig. 1103.]

It will be noted that it is mentioned that three _or more_ washers may be used, and this occurs because a diameter of fit that would be a driving fit for a hole of one length would be too tight for a driving fit of a much longer hole, the friction of course increasing with the length of hole, because of the increase of bearing area.

For large sizes, a reamer of this description is an excellent tool, because if it be required to guide the reamer by means of a plain cylindrical shank, a washer, or sleeve, having a bore to fit the shank at the termination of the thread, may be used, but such a reamer is not suitable for small diameters, because of the reduction of shank necessary to provide for the nut and thread.

Reamers for roughing out taper holes may be made with steps, as in Fig.

1104, which is taken from _The American Machinist_, there being a cutting edge where each step meets a flute. Such a reamer may be used to enlarge parallel holes, or to rough out taper ones, and the flutes (if not to be used for bra.s.s work) may be spiral, as in the figure. The end step being guided by the hole serves as a guide to the first cutting edge; the second step serves as a guide for the cutting edge that follows it, and so on.

[Ill.u.s.tration: Fig. 1104.]

[Ill.u.s.tration: Fig. 1105.]

The steps are best turned a trifle larger, say 1/1000 inch larger, at the cutting end. Half-round taper reamers, such as shown in Fig. 1105, are used for finishing holes. The flat face is cut down, leaving rather more than a half circle; the clearance being filed or ground on the cutting side so as to enable the reamer to cut, and extending from the cutting edge to nearly half-way to the bottom of the reamer.

For holes, however, that are large enough to admit a tool of sufficient strength, the single-pointed boring tool produces the most true work.

Bra.s.s finishers use square taper reamers, which produce upon bra.s.s more true work than the half-round reamer.

[Ill.u.s.tration: Fig. 1106.]

[Ill.u.s.tration: Fig. 1107.]

For reaming the bores of rifles, a square reamer, such as shown in Fig.

1106, is employed; the edges A B are the cutting ones, the edges C D being rounded off; E is a piece of wood, beneath which slips of paper are placed to restore the size as the wear proceeds. The entering end of the reamer is slightly tapered. On account of the extreme length of this reamer in proportion to its diameter, it is fed to its cut by being pulled instead of pushed as is usually the case, the pull placing the rod of the reamer under tension and thus stiffening it; the line of pull is of course true with the axis of the rifle bore. The reamer is revolved at high speed and freely supplied with oil.

[Ill.u.s.tration: Fig. 1108.]

By means of the slips of paper successive cuts and minute increases of diameter may be taken with the same reamer.

Fig. 1107 represents a cla.s.s of rose bit employed to reduce pins to a uniform diameter, and face off the shoulder under the head, or it may be used to cut a recess round a pin, or to cut a recess and leave a pin.

For making a recess round a hole, or, in other words, for cutting a flat-bottom countersink, a facing countersink, Fig. 1108, may be used, its cutting edges being at A, B, C, &c. The clearance is given at the ends of the teeth only, being shown from B to D. The pin P steadies the tool, and is made a working fit to the hole in the work. Or if too small, a ferrule may be placed upon it, thus increasing the capacity of the tool. When a tool of this kind is to be used on iron, steel, or copper, and not upon bra.s.s, the front face of the teeth may be given rake by cutting the grooves at an angle, as in Fig. 1109.

BORING TOOLS FOR LATHE WORK.--The princ.i.p.al object in forming a boring tool to be held in a slide rest is to have the body of the tool as large as can be conveniently got into the size of the hole to be bored; hence the cutting edge should not stand above the level of the top of the steel. By this means the tool will be as stiff as possible, and less liable to spring away from its cut, as boring tools are apt to do, especially when the cut or hole is a long one.

It is so difficult a matter to bore a long hole parallel with a long boring tool that cutters of various forms are usually preferred, and these will be described hereafter.

[Ill.u.s.tration: Fig. 1109.]

[Ill.u.s.tration: Fig. 1110.]

The boring tool is, upon cast iron and bra.s.s, exceedingly liable to chatter, but this may always be avoided by making the angles forming the cutting edge less acute: thus, in Fig. 1110 are three boring tools, A, B, C, operating in a piece of work D. Now the lateral pressure of a cut is exerted upon the tool at a right angle to the length of the cutting edge; hence (in addition to the vertical pressure) the lateral pressure of the tool A will be in the direction of the dotted line and arrow A, that on B in the direction of dotted line and arrow B, and that on C in the direction of dotted line and arrow C; hence the pressure of the cut would tend to force A towards the centre of the hole and off or away from its cut, B back from its cut, and C deeper into its cut. Now as the cut proceeds, the tool edge dulls, hence it would appear that a compromise between C and B would be the most desirable, as giving to the tool enough of the tendency to deepen its cut to compensate for the tendency to spring away from its cut, as the cutting edge dulls (which it does from the moment the cut begins). This is quite practicable in tools to be used on wrought iron, as shown in Fig. 1111, which represents the most desirable form.

In this form the part of the cutting edge performing duty under a deep cut will be mainly in front of the tool, but in light cuts the cutting edge would be farther back, where it is more nearly parallel to the line of the work bore, and will hence cut smoother.

[Ill.u.s.tration: Fig. 1111.]

[Ill.u.s.tration: Fig. 1112.]

Where a boring tool is intended for light cuts only on wrought iron it may have all, or nearly all, its rake at the top, as shown in Fig. 1112, from _a_ to B representing the cut, and C the tool.

Under ordinary conditions that in the form of tool shown in Fig.

1113[15] is best for bra.s.s work, the face A being horizontal or slightly depressed towards the point. Boring tools require very little bottom rake, and the cutting points should be as rounded as they can be made without chattering. On wrought iron the top rake may be as much as is consistent with strength, and water should be freely applied to the cut.

For cast iron the best form of tool is that shown in Fig. 1114, the edge A being parallel with the bore of the hole, and the feed being a coa.r.s.e one, taking a very light cut when finishing.

[15] From "The Complete Practical Machinist."

[Ill.u.s.tration: Fig. 1113.]

[Ill.u.s.tration: Fig. 1114.]

In cases, however, where the tool point requires to cut up to a sharp corner, the form of tool shown in Fig. 1115 (which represents a top and end view) may be used. Its end face C is at an obtuse angle to the length of the tool, so that on pa.s.sing up a bore and meeting a radial face the point only will meet that face. This angle, however, gives to the tool a keenness that will cause chattering on bra.s.s work unless the top face be bevelled to the tool body, as is A to B in the figure.

[Ill.u.s.tration: Fig. 1115.]

It frequently happens in boring cast iron that the skin or the surface of the metal is very hard, rapidly dulling the tool and forcing it away from its cut, unless the cut is deep enough to allow the point of the tool to cut beneath it, as shown in Fig. 1116, in which the hardness is supposed to extend from the bore to the dotted line.

In this case a tool formed as at C is employed, the point cutting in advance of the rest of the tool, and entering the soft metal beneath the hard metal; the hard metal will then break away in lumps or pieces, without requiring to be absolutely cut into chips or turnings, because of being undercut, as shown at B.

The cross slider or tool rest of a lathe should be adjusted to closely fit the cross slide of the lathe if true and parallel work is to be bored, because any lost motion that may exist in the slide is multiplied by the length the tool stands out from the tool post. Thus the centre of motion of the rest if it has play, as at B, Fig. 1117, and the direction of motion at the tool point, will be an arc of a circle of which B is the centre, the bend of the tool from the pressure of the cut will have its point of least motion or fulcrum at A; hence, both tend to cause the tool point to dip and spring unequally under the varying cut pressure that may arise from hard or soft places in the metal, and from inequalities in the cut depth.

[Ill.u.s.tration: Fig. 1116.]

The pressure of the cut increases as the tool point loses its sharpness, and this makes sufficient difference for the amount of tool spring in light boring tools or in long holes to cause the tool to bore a larger hole at the beginning than it does at the end of its feed traverse; or, in other words, to bore a taper hole, whose largest end is that at which the cut was started. If, therefore, the cut is traversed from the front to the back of the hole the latter will be of the smallest diameter at the back, and conversely if the cut proceeds from the back to the front of the hole the front will be of smallest diameter. The amount of the taper so caused (or in other words the error from parallelism) will obviously increase with the length of the hole.

To obviate this taper, the slide of the rest should for the finishing cut be set up firmly, and the tool after being sharpened should take a finishing cut through the hole, and then let traverse back, which can be done providing that care be taken not to bore the hole too large.

[Ill.u.s.tration: Fig. 1117.]

A boring tool will take a smoother cut and chatter less if the final cut be from the back to the front of the hole, and for the following reasons: When the tool is fed in, the strain or pressure of the cut is in a direction to partly compress and partly bend the steel which is being pushed to its cut, but when it is fed in the opposite direction it is pulled to its cut and the strain is in a direction to stretch the steel, and this the tool is more capable of resisting, hence it does not so readily vibrate to cause chattering.

In consequence, however, of the liability of a boring tool to spring away from its cut, it is far preferable to finish holes with standard cutters, reamers, or bits, in which case the boring tool may be employed to rough out and true up the hole, leaving a _fine_ cut for the finishing cutter or bit, so as to wear its cutting edge as little as possible. To further attain this latter object, the cutter or bit should be used at a slow cutting speed and with a coa.r.s.e feed.

[Ill.u.s.tration: Fig. 1118.]

[Ill.u.s.tration: Fig. 1119.]

If cutters or bits are not at hand, tool holders are desirable, and the forms of these depend upon the nature, or rather the diameter, of the hole to be bored. In all cases, however, the best results will be obtained when the diameter of the tool holder is as near that of the hole to be bored as will give it clearance. This occurs on account of the rigidity of the holder being greater than that of the tool.

[Ill.u.s.tration: Fig. 1120.]

[Ill.u.s.tration: Fig. 1121.]

For large work tool holders are desirable, in that the tools, being short, are easier to forge, to handle, and to grind.

For example, a tool holder of a cross section of two inches square may contain a tool whose cross section is 1 by 3/4 inch, in which case it is necessary to forge, grind, &c., the small tool only, whereas in the absence of the holder the tool would require to be of a cross section equal to that of the holder to obtain an equal degree of rigidity.