Modern Machine-Shop Practice Part 74

1083. The reamer is fed end-ways into the work at a cutting speed of about 15 to 18 feet per minute.

[Ill.u.s.tration: Fig. 1081.]

[Ill.u.s.tration: Fig. 1082.]

The main considerations in determining the form of a reamer are as follows:--

1. The number of its cutting edges.

2. The s.p.a.cing of the teeth.

3. The angles of the faces forming the cutting edges.

4. Its maintenance to standard diameter.

[Ill.u.s.tration: Fig. 1083.]

As to the first, it is obvious that the greater the number of cutting edges the more lines of contact there are to steady it on the walls of the hole; but in any case there should be more than three teeth, for if three teeth are used, and one of them is either relieved of its cut or takes an excess of cut by reason of imperfections in the roundness of the hole, the other two are similarly affected and the hole is thus made out of round.

An even number of teeth will not work so steadily as an odd one, for the following reasons.

In Fig. 1084 is represented a reamer having 6 teeth and each of these teeth has a tooth opposite to it; hence, if the hole is out of round two teeth only will operate to enlarge its smallest diameter. In Fig. 1085 is a reamer having 7 teeth, and it will be seen that if any one tooth cuts there will be two teeth on the opposite side of the reamer that must also cut; hence, there are three lines of contact to steady the reamer instead of two only as in the case of the 6 teeth. An even number of teeth, however, may be made to operate more steadily by s.p.a.cing the teeth irregularly, and thus causing three teeth to operate if the hole is out of round. Thus, in Fig. 1086 the teeth are s.p.a.ced irregularly, and it will be seen that as no two teeth are exactly opposite, if a tooth on one side takes a cut there must be two on the opposite side that will also cut. The objection to irregular s.p.a.cing is that the diameter of the reamer cannot be measured by calipers. Another method of obtaining steadiness, however, is to make the flutes and the cutting edges spiral instead of parallel to the axis, but in this case the spiral must be left-handed, as in Fig. 1087, or else the cutting edges acting on the principle of a screw thread will force the reamer forward, causing it to feed too rapidly to its cut. If, however, a reamer have considerable degree of taper, it may be given right-hand flutes, which will a.s.sist in feeding it.

[Ill.u.s.tration: Fig. 1084.]

[Ill.u.s.tration: Fig. 1085.]

[Ill.u.s.tration: Fig. 1086.]

[Ill.u.s.tration: Fig. 1087.]

Referring to the second, the s.p.a.cing of the teeth must be determined to a great extent by the size of the reamer, and the facility afforded by that size to grind the cutting edges to sharpen them.

[Ill.u.s.tration: Fig. 1088.]

The method employed to grind a reamer is shown in Fig. 1088, in which is shown a rapidly-revolving emery-wheel, above the reamer, and also a gauge against which the front face of each tooth is held while its top or circ.u.mferential face is being sharpened. The reamer is held true to its axis and is pushed end-ways beneath the revolving emery-wheel. In order that the wheel may leave the right-hand or cutting edge the highest (as it must be to enable it to cut), the axis of the emery-wheel must be on the left hand of that of the reamer, and the s.p.a.cing of the teeth must be such that the periphery of the emery-wheel will escape tooth B, for otherwise it would grind away its cutting edge. It is obvious, however, that the less the diameter of the emery-wheel the closer the teeth may be s.p.a.ced; but there is an objection to this, inasmuch as that the top of the tooth is naturally ground to the curvature of the wheel, as is shown in Fig. 1089, in which two different-sized emery-wheels are represented operating on the same diameter of reamer. The cutting edge of A has the most clearance, and is therefore the weakest and least durable; hence it is desirable to employ as large a wheel as the s.p.a.cing of the teeth will allow, there being at least four teeth, and preferably six, on small reamers, and their number increasing with the diameter of the reamer.

[Ill.u.s.tration: Fig. 1089.]

It would appear that this defect might be remedied by placing the emery-wheel parallel to the teeth as in Fig. 1090; but if this were done, the wear of the emery-wheel would cause the formation of a shoulder at S in the figure, which would round off the cutting edge of the tooth. This, however, might be overcome by giving the emery-wheel enough end motion to cause it to cross and recross the width of the top facet; or the reamer R may be presented to the wheel W at an angle to the plane of wheel rotation, as in Fig. 1091, which would leave a straight instead of a curved facet, and, therefore, a stronger and more durable cutting edge.

[Ill.u.s.tration: Fig. 1090.]

[Ill.u.s.tration: Fig. 1091.]

Another method of accomplis.h.i.+ng the same object would be to mount the emery-wheel as in Fig. 1092, using its side face, which might be recessed on the side, leaving an annular ring of sufficient diameter to pa.s.s clear across the tooth, and thus prevent a shoulder from forming on the side face of the wheel.

Yet another method is to use an emery-wheel bevelled on its edge, and mount it as in Fig. 1093, in which case it would be preferable to make the bevel face narrow enough that all parts would cross the facet of the tooth.

[Ill.u.s.tration: Fig. 1092.]

[Ill.u.s.tration: Fig. 1093.]

Referring to the third, viz., the angles of the faces forming the cutting edges, it is found that the front faces, as A and B in Fig.

1094, should be a radial line, for if given rake as at C, the tooth will spring off the fulcrum at point E in the direction of D, and cause the reamer to cut a hole of larger diameter than itself, an action that is found to occur to some extent even where the front face is a radial line. As this spring augments with any increase of cut-pressure, it is obvious that if a number of holes are to be reamed to the same diameter it is essential that the reamer take the same depth of cut in each, so that the tooth spring may be equal in each case. This may be accomplished to a great extent by using two reamers, one for equalizing the diameters of the holes, and the other for the final finis.h.i.+ng. The clearance at the top of the teeth is obviously governed by the position of the reamer with relation to the wheel, and the diameter of the wheel, being less in proportion as the reamer is placed farther beneath the wheel, and the wheel diameter is increased. In some forms of reamer the teeth are formed by circular flutes, such as at H in Fig. 1094, and but three flutes are used. This leaves the teeth so strong and broad at the base that the teeth are not so liable to spring; but, on the other hand, the clearance is much more difficult to produce and to grind in the resharpening.

[Ill.u.s.tration: Fig. 1094.]

[Ill.u.s.tration: Fig. 1095.]

[Ill.u.s.tration: Fig. 1096.]

As to the maintenance of the reamer to standard diameter, it is a matter of great importance, for the following reasons: The great advantage of the standard reamer is to enable holes to be made and pieces to be turned to fit in them without requiring any particular piece to be fitted to some particular hole, and in order to accomplish this it is necessary that all the holes and all the pieces be exactly alike in diameter. But the cutting edges of the reamer begin to wear--and the reamer diameter, therefore, to reduce--from the very first hole that it reams, and it is only a question of time when the holes will become too small for the turned pieces to enter or fit properly. In all pieces that are made a sliding or a working fit, as it is termed when one piece moves upon the other, there must be allowed a certain lat.i.tude of wear before the one piece must be renewed.

One course is to make the reamer when new enough larger than the proper size to bore the holes as much larger as this limit of wear, and to restore it to size when it has worn down so that the holes fit too tightly to the pieces that fit them. But this plan has the great disadvantage that the pieces generally require to have other cutting operations performed on them after the reaming, and to hold them for these operations it is necessary to insert in them tightly-fitting plugs, or arbors, as they are termed. If, therefore, the holes are not of equal diameter the arbor must be fitted to the holes, whereas the arbor should be to standard diameter to save the necessity of fitting, which would be almost as costly as fitting each turned piece to its own hole. It follows, therefore, that the holes and arbors should both be made to a certain standard, and the only way to do this is to so construct the reamer that it may be readily adjusted to size by moving its teeth.

It is obvious that a reamer must, to produce parallel holes, be held axially true with the holes, or else be given liberty to adjust itself true. Fig. 1095 shows a method of accomplis.h.i.+ng this object. The reamer is made to have a slight freedom or play in the sleeve, being 1/32 inch smaller, and the hole for the pin is also made large so that the reamer may adjust itself for alignment.

For short holes the sh.e.l.l reamer shown in Fig. 1096 may be employed. Its bore is coned so that it will have sufficient friction upon its driving arbor to prevent its coming off; when it is to be withdrawn from the work it is provided with two slots into which fit corresponding lugs on the driving arbor. Fig. 1097 shows the Morse Twist Drill and Machine Company's arbor.

[Ill.u.s.tration: Fig. 1097.]

[Ill.u.s.tration: Fig. 1098.]

The rose reamer, or rose bit, has its cutting edges on the end only, as shown in Fig. 1098, the grooves being to supply lubricating material (as oil or water) only, and, as a result, will bore a more parallel hole than the ordinary reamer in cases in which the reamer has liberty to move sideways, from looseness in the mechanism driving it. Furthermore, when the work is composed of two parts, the outer one, through which the reamer must pa.s.s before it meets the inner one, guides the reamer without becoming enlarged by reason of the reamer having cutting edges, which is especially advantageous when the inner hole requires to be made true with the outer one, or in cases where a piece has two holes with a s.p.a.ce between them, and one hole requires to be made true with the other, and both require to be made to the same diameter as the reamer.

Fig. 1099 represents the Morse Twist Drill Company's sh.e.l.l rose reamer for short holes, corresponding in principle to the solid rose reamer, but fitting to an arbor for the same purposes as the sh.e.l.l reamer.

Instead of having upon a reamer a flat tooth top to provide clearance, very accurate and smooth work may be produced by letting the back of the tooth, as A in Fig. 1100, proceed in a straight line to B, leaving the reamer, when soft, too large, so that after hardening it may be ground by an emery-wheel to size; and the clearance may be given by simply oilstoning the top of each tooth lengthwise, the oilstone marks barely effacing the emery marks at the cutting edge and removing slightly more as the back of the tooth is approached from the cutting edge. This produces cutting edges that are very easily fed to the cut, which must obviously, however, be a light one, as should always be the case for finis.h.i.+ng, so that the wear of the teeth may be a minimum, and the reamer may therefore maintain its standard diameter as long as possible.

[Ill.u.s.tration: Fig. 1099.]

[Ill.u.s.tration: Fig. 1100.]

When a solid reamer has worn below its required diameter, the same may be restored by upsetting the teeth with a set chisel, by driving it against the front face; and in determining the proper diameter for a reamer for work to be made to gauge under the interchangeable system the following considerations occur.

Obviously the diameter of a reamer reduces as it wears; hence there must be determined a limit to which the reamer may wear before being restored to its original diameter. Suppose that this limit be determined as 1/1000 inch, then as the reamer wears less in diameter the bolts to fit the holes it reams must also be made less as the reamer wear proceeds, or otherwise they will not enter the reamed holes. But it is to be observed that while the reamer wears smaller, the standard gauges to which the pins or bolts are turned wear larger, and the wear is here again in a direction to prevent the work from fitting together. It is better then to make the reamer when new too large to the amount that has been determined upon as the limit of wear, so that when the work begins to go together too tight, the reamer requires resharpening and restoring.

[Ill.u.s.tration: Fig. 1101.]

A still better plan, however, is to use reamers adjustable for diameter, so that the wear may be taken up, and also the reamer sharpened, without being softened, which always deteriorates the quality of the steel.

Reamers that are too small to be made adjustable for size by a combination of parts may be constructed as in Fig. 1101, in which the reamer is drilled and threaded, and countersunk at the end to receive a taper-headed screw S, which may be screwed in to expand the reamer, which contains three longitudinal splits to allow it to open. To cause S to become locked in its adjusted position a plug screw P is inserted for the end of S to abut against. It is obvious that in this form the reamer is expanded most at the end.

Fig. 1102 represents a single-tooth adjustable reamer, in which the body A is ground to the standard diameter, and the wear of the cutter C is taken up by placing paper beneath the cutter. In this case the reamer cannot, by reason of the wear of the cutting edge, ream too small, because the body A forms a gauge of the smallest diameter to which the reamer will cut. The cutter may, however, be set up to the limit allowed for wear of cutting edge, which for work to fit should not be more than 1/5000 inch.

[Ill.u.s.tration: Fig. 1102.]

An adjustable reamer designed and used by the author for holes not less than 1-1/2 inches in diameter, is shown in Fig. 1103, in which A represents the body of the reamer containing dovetail grooves tapered in depth with the least depth at the entering end. The grooves receive cutters B, having gib heads. C is a ring or washer interposed between the gib heads of the cutter and the face or shoulder of A, the cutters being locked against that face by a nut and a washer E. By varying the thickness of C, the cutters are locked in a different position in the length of the grooves, whose taper depth therefore causes the cutters to vary in diameter. Suppose, for example, that with a given thickness of washer C, the cutters are adjusted in diameter so as to produce a hole a tight working fit to a plug turned to a 2-inch standard gauge: a slightly thinner washer may be used, setting the cutters so as to bore a hole an easy working fit to the plug; or a slightly thicker washer may be employed so as to produce a hole a driving fit to the same plug.