Modern Machine-Shop Practice Part 70

[Ill.u.s.tration: Fig. 1011.]

Since, however, it is impracticable to measure male threads at the root, it becomes a problem as to the proper size of hole to bore for any given diameter and pitch of thread. This, however, may be done by the following rules:--

To find the diameter at the roots or bottom of the thread of United States standard threads:

Rule.--Diameter - (1.299 pitch) = diameter at root.

Example.--What is the diameter at the root of a United States standard thread measuring an inch in diameter at the top of the thread and having an 8 pitch?

Here 1.299 8 = .162375.

/ 1.000000 Then 1 - .162375 | .162375 | = .8376.

| -------- | .837635 /

For the sharp [V]-thread the following rule is employed:

Rule.--Diameter - (1.73205 pitch) = diameter at root.

Example.--What is the diameter at the root of a sharp [V]-thread of 8 pitch, and measuring 1 inch diameter at the top of the thread?

Here 1.73205 8 = .21650.

/ 1.0000 Then 1 - .2165 | .2165 | = .7835.

| ------ | .7835 /

For cutting square threads the cla.s.s of tool shown in Fig. 1013 is employed, being made wider at the cutting point C than at B or at D, so that the cutting may be done by the edge C, and the sides _a_ may clear, which is necessary to reduce the length of cutting edge and prevent an undue pressure of cut from springing the work.

[Ill.u.s.tration: Fig. 1012.]

The sides of the tool from _a_ to B must be inclined to the body of the tool steel, as shown in Fig. 1014, the degree of the inclination depending upon the pitch of thread to be cut. It may be determined, however, by the means shown in Fig. 1015.

[Ill.u.s.tration: Fig. 1013.]

[Ill.u.s.tration: Fig. 1014.]

[Ill.u.s.tration: Fig. 1015.]

Draw the line A, and at a right angle to it line B, whose length must equal the circ.u.mference of the thread to be cut and measured at its root. On the line A set off from B the pitch of thread to be cut as at C, then draw the diagonal D, which will represent the angle of the bottom of the thread to the work axis, and the angle of the tool sides must be sufficiently greater to give the necessary clearance. The width of the point C of the tool should be made sufficiently less than the width of the thread groove to permit of the sides of the thread being pinched (after the thread is cut to depth) with a tool such as was shown in Fig. 968.

[Ill.u.s.tration: Fig. 1016.]

For coa.r.s.er pitches the thread is cut as shown in Fig. 1016. The tool is made one-half the width of the thread groove, and a groove, _a_, _a_, _a_, is cut on the work. The tool is then moved laterally and a second cut as at B B is taken, this second cut being shown in the engraving to have progressed as far as C only for clearness of ill.u.s.tration. When the thread has in this manner been cut to its proper depth, the side tools are introduced to finish the sides of the thread. If the thread is a shallow one each side may be finished at one cut by a side tool ground and set very true; but in the case of a deep one the tool may be made to cut at and wear its end only, and after taking a cut, the tool fed in and another cut taken, and so on until, having begun at the top of the thread, the tool operated or fed, after each traverse, by the cross feed, finally reaches the bottom of the thread. If a very fine or small amount of cut is taken, both sides of the thread may in this way be finished together, the tool being made to the exact proper width.

[Ill.u.s.tration: Fig. 1017.]

[Ill.u.s.tration: Fig. 1018.]

When used on wrought iron the tool is sometimes given top rake, which greatly facilitates the operation, as the tool will then take a heavier as well as a cleaner cut.

[Ill.u.s.tration: Fig. 1019.]

After the first thread cut is taken along the work, it is usual to remove it from the lathe and drill, at the point where it is desired that the thread shall terminate, a hole equal in diameter to the width of the thread groove, and in depth to the depth of the thread. This affords relief to the cutting tool at the end of the cut, enables the thread to end abruptly, and leaves a neat finish.

[Ill.u.s.tration: Fig. 1020.]

On account of the broad cutting edge on a screw-cutting tool, the lathe is always run at a slower speed than it would be on the same diameter of work using an ordinary turning tool. After the tool is set to just clear the diameter of the work it is moved (for a right-hand thread) past the end of the work at the dead centre, and a cut is put on by operating the cross-feed screw. The feed nut is then engaged with the feed screw and the tool takes its cut as far along the work as the thread is to be, when the tool is rapidly withdrawn from the work and the lathe carriage traversed back again, ready to take another cut. If, however, the thread to be cut runs close up to a shoulder, head, or collar, the lathe may be run slower as the tool approaches that shoulder by operating the belt shipper and moving the overhead belt partly off the tight pulley and on to the loose one, or the lathe may be stopped when the tool is near the shoulder and the belt pulled by hand.

An excellent method of finishing square threads after having cut them in the lathe to very nearly the finished dimensions is with an adjustable die in a suitable stock, such as in Figs. 1017 and 1018, in which S is a stock having handle H, and containing a die D, secured by a cap C, pivoted at P. To adjust the size of the die, two screws, _a_ and _b_, are used, _a_ pa.s.sing through the top half of the die and threading into the half below the split, while _b_ threads into the lower half and abuts against the face of the split in the die, so that, by adjusting these two screws, the wear may be taken up and the size maintained standard. This device is used to take a very light finishing cut only, and is found to answer very well, because it obviates the necessity of fine measurement in finishing the thread. The die D is seated in a recess at the top and at the bottom so as to prevent it moving sideways and coming out.

LATHE TOOL HOLDERS FOR OUTSIDE TOOLS.--When a lathe cutting tool is made from a rectangular bar of steel it requires to be forged to bring it to the required shape at the cutting end, and to avoid this labor, and at the same time attain some other advantages which will be referred to presently, various forms of tool holders are employed.

These holders fasten in the tool post, or tool clamp, and carry short tools, which, from their shapes and the manner in which they are presented to the work, require no forging, and maintain their shapes while requiring a minimum of grinding.

Fig. 1019 represents a side view of Woodbridge's tool holder at work in the lathe, and Fig. 1020 is a view of the same set at an angle to the tool rest. Fig. 1021 is an end view of the tool and holder removed from the lathe.

The tool seat A is at an angle of about 4 degrees to the base of holder (a greater degree being shown in the cut for clearness of ill.u.s.tration), so that the side J of the tool will stand at an angle and have clearance without requiring such clearance to be produced by grinding. The seat B of the cap C upon the tool is curved, so that the cap will bind the middle of the tool and escape the edges, besides binding the tool fair upon its seat A. The top face is formed at the angle necessary for free and clean cutting, and the tools are, when the cutting edge is provided at one end only, hardened for half their length.

The holder, and therefore the tool, may obviously be swung at any chosen angle of the work or to suit the requirements.

[Ill.u.s.tration: Fig. 1021.]

[Ill.u.s.tration: Fig. 1022.]

[Ill.u.s.tration: Fig. 1023.]

Fig. 1022 shows a right and left-hand diamond-point tool in position in the holder with the cap removed, the cutting edge being at G, the angle of the top face being from F to E. The tool, it will be observed from the dotted line, is supported close up to its cutting corner.

[Ill.u.s.tration: Fig. 1024.]

Fig. 1023 shows a right and left-hand side tool in position, the dotted line showing that it is supported as close to the cutting edge D as the nature of facing work will permit. When left-hand tools are used the holder is turned end for end, so as to support the tools in the same manner as for right-hand ones, and for this purpose it is that the holder is beveled off at each end.

By grinding both ends of one tool, however, to the necessary shape and angle, one tool may be made to serve for both right and left, the tool holder being simply reversed end for end in the tool post. There are, however, furnished with each holder a right and left-hand diamond point and a right and left-hand side tool, each being hardened for half its full length.

It is obvious, however, that there is no front rake to the tool, and that it therefore derives its keenness from the amount of side rake, which may be regulated to suit the conditions.

When tool holders of this cla.s.s are employed, the end face only of the tool requires grinding to resharpen the cutting edges; hence the area of metal requiring to be ground is much less than that on forged tools, and therefore the grinding occupies less time; and if the workman grinds the tools, he is enabled to run more lathes and not keep them idle so long while grinding the tool. Or if the tools are kept ground in stock (about 200 of the tools or cutters serving to run 24 lathes a week) the workman has but to slip in a new tool as the old one becomes dull, no adjustment for height being necessary as in the forged tool.

When the tool requires to be set to an exact position, as in the case of screw cutting, it is desirable that the tool holder be so constructed that the tool may be removed therefrom and replaced without disturbing the position of the tool holder in the tool post or tool clamp; and means must therefore be provided for securing the tool to the holder independently of the tool post or clamp screw. Fig. 1024 represents a tool holder possessing these features: H is the holder provided with a clamp C, secured by a screw B, T representing the tool, which is in this case a chaser, having teeth down the full length of its front face; K is a key or feather fast in the holder H, and fitting into a groove provided in the side of the tool. The vertical angle of this feather obviously determines the angle of clearance at which the tool shall stand to the work.

The Pratt and Whitney Company, who are the manufacturers of this holder, make this angle of clearance 15 degrees. The height of the tool in the holder is adjusted by the screw S, which has journal bearing in the holder, and threads to the end edge of the tool.

Now it is obvious that the holder H, once being set to its proper position in the tool post, the tool T may be removed from and replaced in the exact same position, both in the holder and with reference to the work.

In Fig. 1025, for example, is a top view of the holder with a single-pointed threading tool T in place. W represents a piece of work supposed to be in the lathe, and G a tool-setting gauge; and it is obvious that, if the holder is not moved, the tool T may be removed, ground up, and replaced with the a.s.surance that it will stand in the exact same position as before, producing the exact same effect upon the work, providing that the height is maintained equal, and the tool is not altered in shape by the grinding. To maintain the height equal, all that is necessary is to have the upper face (H, Fig. 1024) of the holder horizontally level and in line with the line of centres of the lathe, and to set the top face of the tool level with that of the holder. In sharpening the tool the top face only is ground; hence the angles are not altered.

Fig. 1026 represents the holder with a tool in position to true up a lathe centre, the angle of the tool holder to the line of centres being the same as in Fig. 1025; and Fig. 1027 represents various forms of tools for curves. All these serve to ill.u.s.trate the advantages of such a tool holder.

[Ill.u.s.tration: Fig. 1025.]