Thread Rolling

What is Thread Rolling?

The thread-rolling process is applied where bolts, screws, studs, threaded rods, etc. are required. Screw threads that are within the range of the rolling process may be produced more rapidly by this method than in any other way. The rolled thread, due to the cold-working action of the dies, is 10 - 20 % stronger than a cut or ground thread, and the increase may be much higher when tested for fatigue resistance.

Another advantage of the rolling process is that no stock is wasted in forming the thread, and the surface of a rolled thread is harder than that of a cut thread, thus increasing wear resistance.
In a thread-rolling machine, the unthreaded screw, bolt or other "blank" is placed between dies having thread-shaped ridges which sink into the blank. By displacing the metal, form a thread of the required shape and pitch.

What steels are used?

Steels vary from soft low-carbon types for ordinary screws and bolts, to nickel, nickel-chromium and molybdenum steels for aircraft studs, bolts, etc. or for any work requiring exceptional strength and fatigue resistance.

Typical SAE alloy steels are No. 2330, 3135, 3140, 4027, 4042, 4640 and 6160. The hardness of these steels after heat-treatment usually ranges from 26 to 32 Rockwell C, with tensile strengths varying from 130,000 to 150,000 pounds per square inch.

Thread rolling is applicable not only to a wide range of steels but for non-ferrous materials, especially if there is difficulty in cutting due to "tearing" the threads.

Determining diameter of threading roll:

The pitch diameter of a threading roll having a single thread is slightly less than the pitch diameter of the screw thread to be rolled, and in the case of multiple-thread rolls, the pitch diameter is not an exact multiple of the screw thread pitch diameter but is also reduced somewhat.
The amount of reduction recommended by one screw machine manufacturer is given by the formula:
D = N (d - (T / 2) ) - T where D = pitch diameter, d = pitch diameter of screw thread, N = number of single threads or "starts" on the roll (this number is selected with reference to diameter of roll desired.) & T = single depth of thread.

Considerations of Thread Rolling:

The shape of the work and the character of the operations necessary to produce it might be governed by the following points:

Diameter of the part to be threaded.
Location of the part to be threaded.
Length of the part to be threaded.
Relation that the thread rolling operation bears to the other operations.
Shape of the part to be threaded, whether straight, tapered or otherwise.
Method of applying the support.

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