ROBERT GIBSON GRISWOLD

The two threads in principal use in this country are the sharp " V" thread and the Sellers or U. S. Standard thread. The former, shown in Fig. 1, is the one most readily cut. It has many disadvantages, however, chief among them being its liability to injury owing to the very sharp points. When these are battered it becomes very difficult to run the nut on and stripping may result from forcing.

Fig. 1.

In cutting the " V " thread in the lathe, the work is first swung on the centers. The thread tool is then ground so that the two sides of the point form an angle of 60°. This is generally tested and determined by using a thread gauge, as shown in Fig. 2.

When properly ground and the edge properly stoned for smooth cutting, the tool is set in the tool-post by aid of the thread-gauge, as shown in Fig. 3. One edge is placed in contact with the work, and the tool is then adjusted so that the point exactly fits the "V" which insures the proper angle between the sides of the thread and the axis of the work.

The point of the tool is now brought into contact with the surface to be cut and the thread-stop on the cross-slide set. A light cut is then run along for the proper length of thread, and the number of threads per inch counted to determine if the gears have been properly set.

In many lathes it is necessary to reverse the lathe to return the tool to the beginning of the thread, but some of the later models are being provided with an attachment operated by a reversing lever placed in the apron. This renders unnecessary the reversal of the lathe and the tool will catch the thread at any position. In the ordinary lathe this is almost impossible since, if the tool is removed from the cut and returned to the start by hand after releasing the nut, the exact position on the lead screw for closing the nut may or may not be readily found. And besides it is almost as quick to reverse the lathe and let the lead-screw carry the tool back.

A very important caution is to always back the tool out from its cut so that it will not drag on the return. If this is not done the point will invariably be broken off, as it has no support on the top side. While the tool is being returned the gauge is set so that the next cut will be slightly deeper. Never attempt a very heavy cut with this tool as the point is not strong enough. As it is, the most delicate portion of the tool is exposed to the hardest work.

The proper depth of cut may be readily determined, as the tops of the threads will become perfectly sharp, but it is better to try a nut on the thread and thus ascertain the fit. At the root, diameter may be measured with a pair of calipers made for the purpose.

A beginner may experience some difficulty in withdrawing the tool at the end of the cut without snapping off the point. If possible, it is much better to cut a groove in the piece at the end of the threaded portion into which the tool runs at the end of its cut. This is shown in Fig. 4. This in no way weakens the piece, as the root diameter of the thread and the groove are the same.

In very large threads, and especially with square and acme threads, the thread groove often terminates in a hole drilled in the piece as in Fig. 5, which gives the tool an excellent chance to finish a good thread and hack out.

There are several lubricants used in screw cutting, but lard oil is quite as good as any. Solutions of soda and soap are often used with good effect, and are cheap.

Fig. 7.

A thread tool is seldom allowed much top rake, owing to difficulties in setting, and its cut is thus necessarily of a scraping nature. A keen edge is necessary at all times to prevent making a rough thread. Sometimes a tool, such as shown in Fig. 9, is used where a compound rest is fitted. The point is ground to a 65° angle, and the slide is set so as to feed the tool to the work at an angle of 30°. All the cutting is thus done with one side, similar to a cutting-off tool. With care a very good thread may be made, but one side is like" ly to show the path of the tool point and must be cut smooth with a final chip.

The U. S. Standard thread or Sellers thread is a modification of the sbarp "V " and is sbown in Fig. 7.

The fact that the outer point of the sharp "V" thread had little strength, was liable to injury, and also that the sharp corners at the root of the thread had a tendency to weaken the bolt, led to the adoption by the U. S. Government of the Sellers system, sometimes spoken of as the Franklin Institute standard. As shown in Fig. 7, the top of the thread is flattened and the bottom of the groove is filled in parallel to the axis by an amount equal to one-eighth of the pitch of the thread, or the slant side of the original "V". The advantages of this thread are at once obvious. It is less liable to injury and the bolt is much stronger since the cross-sectional area is greater than that provided by the sharp "V " thread. The bearing area of the thread is lessened somewhat, but that is seldom a matter of importance in this thread.

The tool for cutting this thread is set in exactly the same manner as that used for the "V" thread, but in grinding it the point is flattened by the correct amount. Every different thread requires a special tool, owing to the varying widths of flats. In the case of sharp "V " thread one tool would cut any one of the many sizes.

The Whitworth thread, shown in Fig. 10, is the English standard and seldom seen the United States. The included angle is 55° and the top of the thread is rounded. The bottom of the groove is also rounded, and in this respect it resembles the U. S. thread.

With all threads made with angular sides there is a tendency to split the nut when a strain is put upon the bolt and to obviate this the trapezoidal thread is used in many instances where great loads are to be sustained. The breech blocks of the modern United States Navy guns are mostly fitted with this thread. It is shown in sections in Fig. 8, and is merely aV-thread turned a little to one side so that the load will he taken on the flat side of the thread. It has this advantage over " V " and square threads. There is no tendency to split the nut under load in the former case, and the resistance to stripping at the base of the thread is about twice that of the square thread. Its principal drawback lies in its inadaptability to resisting strains in both directions, owing to the great bursting pressure that would be placed upon the nut.

Fig.10.

The square thread, shown in Fig. 8 is principally used for such purposes as lifting screws in jacks and machines and almost universally for the lead screws of lathes. It presents a flat wearing surface in either direction of resistance, but as stated above, has only half the strength of a "V" thread in shearing resistance. In cutting a square thread in the lathe, the tool must be of such angular shape as to follow in the groove cut. It frequently occurs that more than one thread will be cut on the screws, in which case we have double, triple and even quadruple threads. The pitches of the threads in these cases are just two, three or four times the single pitch, and one thread is cut between the turns of the others. These multiple pitches are mostly used where a considerable movement of the nut is required per revolution of the screw. The chief drawback to the square thread is its cost. Aside from the care required in its cutting, it necessitates very careful fitting, which is a tedious operation.

The Acme thread, shown in Fig. 9, has been designed to overcome some of the objections to the square thread and incorporate some of the good features of the "V" thread. The included angle between the sides is 29°, and the threads partake somewhat of the general cross section of the U. S. standard thread. The nut bursting tendency is not so great as the 60° "V " thread, and the shearing area at the hase of the thread is greater than that of a square thread of like pitch. Owing to the taper of the thread it is less difficult to make a good fit.

In cutting threads on pieces of comparatively small diameter and considerable length, a back rest must be used to prevent the thrust of the tool springing the work and making the root diameter of the thread larger in the middle than at the ends.