Modern Machine-Shop Practice Part 90

[Ill.u.s.tration: Fig. 1310.]

It is obvious that the sc.r.a.per may be given any required shape to meet the work, Fig. 1310 representing a sc.r.a.per of this kind; but it must in this case be fed endways only to its cut, if the work is to be cut to fit the sc.r.a.per.

[Ill.u.s.tration: Fig. 1311.]

[Ill.u.s.tration: Fig. 1312.]

In Fig. 1311 is shown a half-round sc.r.a.per, which is shown in Fig. 1312 in position to sc.r.a.pe out a bore or hole. This tool is made by grinding the flat face and the two edges of a worn-out half-round smooth file, and is used to ease out bores that fit too tightly. The cutting edges are carefully oilstoned, and the work revolved at a very quick feed.

[Ill.u.s.tration: Fig. 1313.]

[Ill.u.s.tration: Fig. 1314.]

[Ill.u.s.tration: Fig. 1315.]

When a number of small pieces of duplicate form are to be turned by hand, a great deal of measuring may be saved and the work very much expedited by means of the device shown in Fig. 1313. It consists of a tool stock or holder, the middle of which, denoted by A, is square, and contains three or four square slots, with a set-screw to each slot to hold different turning tools. Each end of the stock is turned parallel, as denoted by B, C. In Figs. 1313 and 1314, D, E, and F are the tools, and G, H, are the set-screws.

Fig. 1315 represents top and side view of a plate, of which there must be two, one to fasten on the headstock and one on the tailstock of the lathe, as shown in Fig. 1316. In Fig. 1317 the manner of using the tool is shown, similar letters of reference denoting similar parts in all the figures.

The plates P P are bolted by screws to the headblock H and the tailstock T of the lathe. The tool holder is placed so that the cylindrical ends B, C, rest on the ends of these plates, and in the angles P' P'. The cutting tool D is sustained, as shown, upon the lathe rest R. In use the operator holds the stock A in his hands in the most convenient manner, using the tool E as a handle when there is a tool in the position of E.

The cutting point of the tool is pressed up to the work W, and the feed is carried along by hand. It is obvious, however, that when the perimeters of A B meet the shoulders O O, Fig. 1315, of the plates P P, the tool cannot approach any nearer to the diametrical centre of the work; hence the diameter to which the tool will turn is determined by the distance of the shoulder O of the plate P from the centre of the lathe centres, as shown in Fig. 1316 by the line L. In carrying the cut along it is also obvious that the lateral travel of the stock or holder must end when the end of the square part A comes against the side face of either of the plates. In the engraving we have shown the tool D cutting a groove in the work W, while the shoulder of the holder is against the plate fastened to the lathe tailstock T; and so long as the operator, in each case, keeps the shoulder against that plate, the grooves upon each piece of work will be cut in the same position, for it will be observed that the position in the length of the work performed by each tool is determined by the distance of the cutting part of each tool from the end of the square part A of the tool holder. All that is necessary, then, is to adjust each tool so that it projects the proper distance to turn the requisite diameter and stands the required distance from the shoulders of the square to cut to the desired length, and when once set error cannot occur.

[Ill.u.s.tration: Fig. 1316.]

This plain description of the device, however, does not convey an adequate idea of its importance. Suppose, for example, that it is required to turn a number of duplicate pieces, each with a certain taper: all that is necessary is to adjust the plates P in their distances from the lathe centres. If the large end of the taper on the work is required to stand nearest the lathe headstock A, the plate P on the headstock must be moved until its shoulder O is farther from the lathe centre. If, however, the work requires to be made parallel, the plates P must be set the same distance for the axial line of the centres. If it be desired to have a parallel and a taper in proximity upon the same piece of work, the tool must have one of its cylindrical ends taper and use it upon the taper part of the work.

[Ill.u.s.tration: Fig. 1317.]

In Fig. 1317 the tool D is shown cutting a square groove. The tool at F serves to turn the parallel part X, and the tool E would cut the [V]-shaped groove I.

All kinds of irregular work may be turned by varying the parallelism and form of the cylindrical ends B C; but in this event the shoulders O O, Fig. 1315, should be made [V]-shaped and hardened to prevent them from rapid wear.

[Ill.u.s.tration: Fig. 1318.]

[Ill.u.s.tration: Fig. 1319.]

SCREW CUTTING WITH HAND TOOLS.--Screw threads are cut by hand in the lathe with chasers, of which there are two kinds, the outside and the inside chaser. In Fig. 1319 is shown an outside, or male, and in Fig.

1318 an inside, or female chaser. The width of a chaser should be sufficient to give at least four teeth, and for the finer thread pitches it is better to have six or eight teeth, the number increasing as the pitch is finer, and the length of the work will permit. The leading tooth should be a full one, or otherwise it will break off, and if in cutting up the chaser a half or less than a full tooth is formed it should be ground off. The tooth points should not be in a plane at a right angle to the chaser length, but slightly diagonal thereto, as in Fig. 1319, so that the front edge of the chaser will clear a bolt head or shoulder, and permit the leading tooth to pa.s.s clear up to the head without fear of the front edge of the steel meeting the shoulder.

[Ill.u.s.tration: Fig. 1320.]

[Ill.u.s.tration: Fig. 1321.]

[Ill.u.s.tration: Fig. 1322.]

The method of producing a chaser from a hob is shown in Fig. 1320, in which H is a hob, which is a piece of steel threaded and serrated, as shown, to give cutting edges to act, as the hob rotates, upon the chaser C. If the chaser is cut while held in a constant horizontal plane, its teeth will have the same curvature as the hob, or, in other words, they will fit its circ.u.mference. Suppose that the chaser, being cut up by the hob and then hardened, is applied to a piece of work of the same diameter as the hob and held in the same vertical plane, as in Fig.

1320, it is obvious that, there being no clearance, the teeth cannot cut. Or, suppose it be applied to a piece of work of smaller diameter, as in Fig. 1324, it cannot cut unless its position be lowered, as in Fig. 1322, or else it must be elevated, as in Fig. 1323. In either case the angle of the thread cut will be different from the angle of the sides of the chaser teeth, and the thread will be of improper depth.

Thus, on referring to Fig. 1321, it will be seen that the chaser C has a tooth depth corresponding to that on the work W along the horizontal dotted line E only, because the true depth of thread on the work is its depth measured along a radial line, as line F or G, and the chaser teeth are, at the cutting edge, of a different angle. This becomes more apparent if we suppose the chaser thickness to be extended up to the dotted line H, and compare that part of its length that lies within the two circles I J, representing the top and bottom of the thread, with the length of radial line G, that lies within these circles. If, then, the chaser be lowered, to enable it to act, it will cut a thread whose sides will be of more acute angle than are the sides of the chaser teeth or of the hob from which it was cut. The same effect is caused by using a chaser upon a larger diameter of work than that of the hob from which the chaser was cut, because the increased curvature of the chaser teeth acts to give the teeth less contact with the work, as is shown in Fig.

1325, for the teeth cannot cut without either the lower corners A of the teeth being forced into the metal, or else the chaser being tilted to relieve them of contact. To obviate these difficulties and enable a chaser to be used upon various diameters of work, it is, while being cut up by the hob, moved continuously up and down, as denoted in Fig. 1326, by A and B, which represent two positions of the chaser. The amount of this movement is sufficient to make the chaser teeth more straight in their lengths, and to give them a certain amount of clearance, an example of the form of chaser thus produced being shown in Fig. 1327, applied to two different diameters of work, as denoted by the circle A and segment of a circle B, C representing the chaser.

[Ill.u.s.tration: Fig. 1323.]

[Ill.u.s.tration: Fig. 1324.]

[Ill.u.s.tration: Fig. 1325.]

[Ill.u.s.tration: Fig. 1326.]

To obtain the most correct results with such a chaser, it must be applied to the work in such a way that it has as little clearance as will barely enable it to cut, because it follows from what has been said with reference to single-pointed threading tools that to whatever amount the chaser has clearance, a corresponding error of thread angle and depth is induced. In hand use, therefore, it does not matter at what height the chaser is applied so long as it is elevated sufficiently to barely enable it to cut.

[Ill.u.s.tration: Fig. 1327.]

[Ill.u.s.tration: Fig. 1328.]

After the chaser is cut on the hob, its edges, as at C, and the corner, as at D, in Fig. 1328, should be rounded off, so that they may not catch in any burr which the heel of the hand tools may leave on the surface of the hand rest.

[Ill.u.s.tration: Fig. 1329.]

For roughing out the threads on wrought iron or steel the top face should be hollowed out, as shown in Fig. 1328, which will enable the chaser to cut very freely. For use on cast iron the top face should be straight, as shown in Fig. 1328 at A, while for use on soft metal, as bra.s.s, the top face must be ground off, as shown in Fig. 1329.

[Ill.u.s.tration: Fig. 1330.]

The Pratt and Whitney Co. cut up chasers by the following method: In place of a hob, a milling cutter is made, having concentric rings instead of a thread. The cutters are revolved on a milling machine in the ordinary manner. The chaser is fastened in a chuck fixed on the milling machine table, and stands at an angle of 15. It is traversed beneath the milling cutter, and thus cut up with teeth whose lengths are at a right angle to the top and bottom faces of the chaser; hence the planes of the length of the teeth are not in the same plane as that of the grooves of the thread to be cut. Thus, let _a_, _b_, _c_, and _d_, Fig. 1330, represent the planes of the thread on the work, and _e_, _f_, _g_, _h_, will be the planes of the lengths of the chaser teeth.

The chaser, however, is given 15 of bottom rake or clearance, and this causes the sides of the chaser teeth to clear the sides of the thread.

[Ill.u.s.tration: Fig. 1331.]

Now, suppose the top face A, Fig. 1331, of the chaser to be parallel with the face of the tool steel, and to lie truly horizontal and in the same plane as the centre of the work. This clearance will cause the thread cut by the chaser to be deeper than the natural depth of the chaser teeth. Thus, in Fig. 1331 is shown a chaser (with increased clearance to ill.u.s.trate the point desired), the natural depth of whose thread is represented by the line F, but it is shown on the section of work that the thread cut by the tool will be of the depth of the line D, which is greater than the length or depth of F, as may be more clearly observed by making a line E, which, being parallel to A, is equal in length to D, but longer than F. Hence, the clearance causes the chaser under these conditions to cut a thread of the same pitch, but deeper than the grooves of the hub, and this would alter the angles of the thread. This, however, is taken into account in forming the angles of the thread upon the milling cutter, and, therefore, of the chaser, which are such that with the tool set level with the work centre, the thread cut will be of correct angle, notwithstanding the clearance given to the teeth.

[Ill.u.s.tration: Fig. 1332.]

In order to enable the cutting of an inside chaser from a hub, it requires to be bent as in Fig. 1332, in which H is the hub, R the lathe rest, and C the chaser. After the chaser is cut, it has to be straightened out, as shown in Fig. 1318, in which is represented a washer being threaded and shown in section; C is the chaser and R the lathe rest, while P is a pin sometimes let into the lathe rest to act as a fulcrum for the back of the chaser to force it to its cut, the handle end of the chaser being pressed inwards.

[Ill.u.s.tration: Fig. 1333.]

[Ill.u.s.tration: Fig. 1334.]

When an inside chaser is cut from a hub (which is the usual method) or male thread, its teeth slant the same as does the male thread on the side of the hub on which it is cut, and in an opposite direction to that of the thread on the other side of the hub. Thus, in Fig. 1333, H is the hub, C the chaser, and R the lathe rest. The slope of the chaser-teeth is shown by the dotted line B. Now, the slant of the thread on the half circ.u.mference of the hub not shown or seen in the cut will be in an opposite direction, and in turning the chaser over from the position in which it is cut (Fig. 1333) to the position in which it is used (Fig.

1334), and applying it from a male to a female thread, we reverse the direction with relation to the work in which the chaser-teeth slant; or, in other words, whereas the teeth of the chaser should lie as shown in Fig. 1334 at A A, they actually lie as denoted in that figure by the dotted line B B. As a consequence, the chaser has to be tilted over enough to cause the sides of the chaser-teeth to clear the sides of the thread being cut, which, as they lie at opposite angles, is sufficient to cause the female thread cut by the chaser to be perceptibly shallower than the chaser-teeth, for reasons which have been explained with reference to Fig. 1321. It may be noted however, that an inside chaser cannot well be used with rake, hence the tilting in this case makes the thread shallower instead of deeper.

To obviate these difficulties the hub for cutting a right-hand inside chaser should have a left-hand thread upon it, and _per contra_, an inside chaser for cutting a left-hand thread should be cut from a hub having a right-hand thread.

The method of starting an outside thread upon wrought iron or steel to cut it up with a chaser is as follows:--

[Ill.u.s.tration: Fig. 1335.]

The work is turned up to the required diameter, and the [V]-tool shown in Fig. 1335 is applied; the lathe is run at a quick speed, and the heel of the tool is pressed firmly to the face of the lathe rest, the handle of the tool must be revolved from right to left at the same time as it is moved laterally from the left to the right, the movement being similar to that already described for the graver, save that it must be performed more rapidly. It is in fact the relative quickness with which these combined movements are performed which will determine the pitch of the thread. The appearance of the work after striking the thread will be as shown in Fig. 1336, A being the work, and B a fine groove cut upon it by the [V]-tool.