But in the case of a tool holder, or of a chaser holder, the tool may be ground on the top face, and adjusted for height by any suitable means, the top of the holder serving as a guide to set the tool by.
[Ill.u.s.tration: Fig. 986.]
The line of the cutting edge of the tool must, to obtain correct results, be presented to the work in the same manner as it was presented to the gauge to which its angles were ground, so that if the tool were in position in the tool post, and the gauge were applied, it would point to the axis of the lathe centre, for if this is not the case the thread cut will not be of correct angle or depth. Thus, in Figs. 986 and 987 the tool T would cut threads too shallow, although placed at the correct height, because the cutting edges are at an angle to the radial lines C C.
[Ill.u.s.tration: Fig. 987.]
It becomes obvious, then, that it is improper to set the height of a screw-cutting tool by means of any tool elevating or setting-device that throws it out of the horizontal position. To enable the correct setting of threading tools, and to avoid having to grind the angles correct to gauge every time the tool requires sharpening, various kinds of tool holders have been designed by means of which the tool may be ground on the top face, and set at correct height and in the proper plane.
[Ill.u.s.tration: Fig. 988.]
[Ill.u.s.tration: Fig. 989.]
To facilitate grinding the tools to a correct angle, the gauge shown in Fig. 988 is employed, the various notches being for the pitches of thread for which they are respectively marked, but, the edge of the gauge being circular, does not afford much guide to the eye in grinding the angles equal from the sides of the body of the tool; hence the form of gauge shown in Fig. 989 is preferable, because the tool can be so ground that the edge of the gauge stands parallel with the side of the tool steel, so that the tool will, when in correct position, point straight to the work axis. To insure correctness in setting the tool, it may then be set with a square S in Fig. 990, held firmly with its back against the side of the tool, which may be adjusted in the tool post until the blade B comes fair with the work.
[Ill.u.s.tration: Fig. 990.]
Another method of setting the tool is with a gauge as in Fig. 991, which sets it true with the angle independent of whether the angle is true with the side of the tool or not. In Fig. 992 is a form of gauge that will serve to grind the tool by to correct angle, and also to set it in the lathe by the angles, independent of the side of the tool.
The same gauge may be used for setting internal threading tools by first facing the work quite true and then applying the gauge as in Fig. 993.
[Ill.u.s.tration: Fig. 991.]
[Ill.u.s.tration: Fig. 992.]
[Ill.u.s.tration: Fig. 993.]
By reason of the comparatively sharp points of thread-cutting tools, they are more readily dulled than the rounder pointed ordinary lathe tool, and by reason of their cutting edges extending along a greater length of the work, and therefore causing it to spring or bend more from the strain of the cut, they cannot be employed to take such heavy cuts as ordinary tools. Hence, in all thread cutting, it is necessary to turn the work down to the finished diameter before using the threading tool, so that the thread will be finished when it is cut to the proper depth.
To test that depth on a piece of work having a United States standard, or a sharp [V]-thread, a gauge such as shown in Fig. 994 may be used, consisting of a piece of sheet steel about 1/50 inch thick, having a single tooth formed correct for the s.p.a.ce of the thread, so that the edge of the gauge will meet the tops of the thread when the s.p.a.ce is cut to admit the tooth on the gauge; the most accurate method of producing such a gauge having been described in the remarks upon screw threads.
[Ill.u.s.tration: Fig. 994.]
If the tool is known to be ground to the correct angle and is set properly, the gauge for depth may be dispensed with by turning the body of the work to correct diameter, and also turning a small part, as a in Fig. 995, down to the correct diameter for the bottom of the thread, so that when the tool point meets A the thread will be cut to correct depth.
[Ill.u.s.tration: Fig. 995.]
Figs. 996 and 997 represent a method of cutting a round top and bottom, or any other form of thread, by means of a single-pointed circular cutting tool, which is mounted on a holder. On the circ.u.mference of the cutter is cut a single thread, and a piece is cut out at E to form a cutting edge. To cut a right-hand thread on the work, a left-hand one must be cut on the cutter, so as to make its thread slant in the proper direction. The tool is sharpened by grinding the top face, and moved on the holding pin to set it to the proper height or in position to enable it to cut. A top view of the tool and holder is shown in figure 997.
[Ill.u.s.tration: Fig. 996.]
It is obvious that two gaps may be cut in the wheel or cutter so as to provide two cutting edges, one of which may be used for roughing, and the other for finishing cuts.
[Ill.u.s.tration: Fig. 997.]
In roughing out coa.r.s.e threads, a single-pointed tool, formed as in Fig.
998, and set considerably above the centre as shown, may be used to great advantage. It will carry a heavy cut and throw off a cutting but very little curved; hence but little power is absorbed in bending the cutting. To preserve the cutting edge, the point of the tool should be slightly rounded. Such a tool, however, requires to be rigidly held, and requires experience to use it to the best advantage.
[Ill.u.s.tration: Fig. 998.]
An English tool holder for a single-pointed tool for cutting coa.r.s.e pitch threads, such as square threads, is shown in Fig. 999. The stem of the holder is cylindrical, and is held between two clamping pieces, while the short piece of steel used as a tool (which is thinnest at the bottom, so as to provide for the clearance without grinding it) is clamped in a swiveled post, so that it may be set at the angle sideways required for the particular pitch of thread to be cut, as is shown in the end view.
[Ill.u.s.tration: Fig. 999.]
[Ill.u.s.tration: Fig. 1000.]
[Ill.u.s.tration: Fig. 1001.]
The difficulty of adjusting the height of threading tools that are ground on their top faces to sharpen them is obviated in a very satisfactory manner by the tool holder patented by the Pratt and Whitney Company, and represented in Figs. 1000 and 1001. A is the body of the holder, C is the tool clamp, and B the set screw for C; D is a pin fast in A and projecting into C to adjust it square upon A. The threading tool G has a groove H, into which the projection E fits, so that the tool is held accurately in position. F is the screw which adjusts the height of the tool, being threaded into A and partly into G, as is shown at I. The holder once being set in correct position, the threading tool may be removed for grinding, and reset with accuracy. The face K of the holder is made at 30 to the front or leading face of the holder, so that the stem or body of the holder will be at an angle and out of the way of the work driver.
[Ill.u.s.tration: Fig. 1002.]
If a chaser instead of a single-pointed tool be used to cut a thread, the thread requires to be gauged for its full diameter only, because both the angles of the thread sides and the thread depth are determined by the chaser itself. Chasers are also preferable to a single-pointed tool when the work does not require to be cut to an exact diameter, nor to have a fully developed thread clear up to a shoulder; but when such is the case a single-pointed tool is preferable, because if the leading tooth should happen to run against the shoulder the whole of the teeth dig into the work, and more damage is done to it than with a single-pointed tool. When the thread does not run up to a shoulder, or in cases where the thread may be permitted to run gradually out, and, again, where the thread is upon a part of enlarged diameter, a chaser may have its efficiency increased in two ways, the first of which is shown in Fig. 1002. When the chaser is set and formed as at A in the figure, the leading tooth takes all the cut, and the following tooth will only cut as it is permitted to do so from the wear of the leading bolt. This causes the tooth to wear, but the teeth may be caused to each take a proportion of the cut by chamfering them as at B in the figure, which will relieve the front tooth of a great part of its duty and let the following teeth perform duty, and thus preserve the sharpness of the cutting edges. We are limited in the degree of chamfer that may be given to the teeth, first, because as the cutting edge is broader and the strain of the cut is greater it causes the tool to spring or bend more under the cut pressure; and secondly, because if the tool be given many teeth in order to lengthen the chamfer, then the pitch is altered to a greater extent by reason of the expansion which accompanies the hardening of the chaser.
[Ill.u.s.tration: Fig. 1003.]
[Ill.u.s.tration: Fig. 1004.]
A chaser thus chamfered may be set square in the tool post by placing a scale against the work as at S in Fig. 1003, and setting the bottoms of the chaser teeth fair with the outer edge of the scale as in the figure.
The second method of increasing the efficiency of a chaser is to grind the top face at an angle as from A to B in Fig. 1004, and set it so that the last tooth B is at or a little above the work axis D. This causes the last tooth B to stand sufficiently nearer the work axis than the other teeth to enable it to take a light sc.r.a.ping cut, producing a smooth cut, because the duty on the last tooth being light it preserves its cutting edge, and therefore its form.
Chasers are often in shops, doing general work, formed in one piece in the same way as an ordinary tool, but it is preferable to use short chasers and secure them in holders.
[Ill.u.s.tration: Fig. 1005.]
Figs. 1005 and 1006 show a convenient form of holder, the chaser A being accurately fitted into a recess in the holder D, so that it may be set square in the holder without requiring to be adjusted to come fair with the thread grooves after having been ground to resharpen it. The short chasers are held by the clamp B, which has at C a projection fitting into a recess in the holder to cause the clamp to adjust itself fairly.
In setting a chaser to correct position in a tool post the points of the teeth may be set to the surface of the work as in Fig. 1007, or if the thread is partly produced and the lathe has a compound slide rest, the tool may be set to the tops of the thread as in Fig. 1008, and then brought into position to meet the thread grooves by operating the slide rest.
[Ill.u.s.tration: Fig. 1006.]
[Ill.u.s.tration: Fig. 1007.]
[Ill.u.s.tration: Fig. 1008.]
It is obvious that the height and position of a chaser require to be as accurately set as a single-pointed tool, but it is more difficult to set it because it can only be sharpened by grinding the top face, and this alters the height at each grinding.
Thus, suppose that when new its teeth are of correct height, when the bottom face I, Fig. 1009, lies upon the rest R, the face H being in line with the centre B B of the work, then as face H is ground the tool must be lifted to adjust its height. On account, however, of the curve of the teeth it is very difficult to find when the chaser is in the exact proper position, which in an ordinary chaser will be when it has just sufficient clearance to enable it to cut, as is explained with reference to cutting up chasers and using them by hand.
[Ill.u.s.tration: Fig. 1009.]
To obviate these difficulties, an excellent form of chaser holder is shown in Figs. 1010 and 1011. Its top face C being made of such a height that when the holder rests on the surface of the slide rest and is in the tool box, C will stand horizontally level with the horizontal centre of the work, as denoted by the horizontal line D E; then the tool proper may have long teeth as denoted by A, and the surface of the teeth may always be brought up level with the top surface of the tool holder as tested with a straight-edge. This is a ready and accurate mode of adjustment. A top view of the tool holder is shown in Fig. 1011, in which A is the tool holder, B the threading tool, with a clamp to hold B, and a screw to tighten the clamp.
[Ill.u.s.tration: Fig. 1010.]
It may now be pointed out that a common sharp [V]-chaser may be used to cut a United States standard thread by simply grinding off the necessary flats at the points of the teeth, because when the chaser has entered the work to the proper depth it will leave the necessary flat places at the top of the thread, as is shown in Fig. 1012.
In cutting internal, inside, or female threads (these terms being synonymous) the diameter of the bore or hole requires to be made of the diameter of the male thread _at the root_.