Machinery's Handbook, 31st Edition
THREAD ROLLING 2189 Thread Rolling in Automatic Screw Machines.— Screw threads are sometimes rolled in automatic screw machines and turret lathes when the thread is behind a shoulder so that it cannot be cut with a die. In such cases, the advantage of rolling the thread is that a second operation is avoided. A circular roll is used for rolling threads in screw machines. The roll may be presented to the work either in a tangential direction or radially, either method producing a satisfactory thread. In the former case, the roll gradually comes into contact with the periphery of the work and completes the thread as it passes across the surface to be threaded. When the roll is held in a radial position, it is simply forced against one side until a complete thread is formed. The method of applying the roll may depend upon the relation between the threading operation and other machining operations. Thread rolling in automatic screw machines is generally applied only to brass and other relatively soft metals, owing to the difficulty of rolling threads in steel. Thread rolls made of chrome- nickel steel containing from 0.15 to 0.20 percent of carbon have given fairly good results, however, when applied to steel. A 3 percent nickel steel containing about 0.12 percent carbon has also proved satisfactory for threading brass. Factors Governing the Diameter of Thread Rolling.— The threading roll used in screw ma- chines may be about the same diameter as the screw thread, but for sizes smaller than, say, 3 ∕ 4 inch (19.05 mm), the roll diameter is some multiple of the thread diameter minus a slight amount to obtain a better rolling action. When the diameters of the thread and roll are practically the same, a single-threaded roll is used to form a single thread on the screw. If the diameter of the roll is made double that of the screw, in order to avoid using a small roll, then the roll must have a double thread. If the thread roll is three times the size of the screw thread, a triple thread is used, and so on. These multiple threads are necessary when the roll diameter is some multiple of the work, in order to obtain corresponding helix angles on the roll and work. 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 shown at the end of this paragraph. A description of the terms used in the formula is given as follows: D = pitch diameter of threading roll, 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: D N d T T 2 = − − a k Example: Find, by using above formula, the pitch diameter of a double-thread roll for rolling a 1 ∕ 2 -inch American standard screw thread. Pitch diameter d = 0.4500 inch and thread depth T = 0.0499 inch. . . . . D 2 04500 2 0 0499 0 0499 0 8001 inch = − − = a k Kind of Thread on Roll and Its Shape.— The thread (or threads) on the roll should be left hand for rolling a right-hand thread, and vice versa. The roll should be wide enough to overlap the part to be threaded, provided there are clearance spaces at the ends, which should be formed if possible. The thread on the roll should be sharp on top for rolling an American (National) standard form of thread, so that less pressure will be required to displace the metal when rolling the thread. The bottom of the thread groove on the roll may also be left sharp or it may have a flat. If the bottom is sharp, the roll is sunk only far enough into the blank to form a thread having a flat top, assuming that the thread is the American form. The number of threads on the roll (whether double, triple, quadruple, etc.) is selected, as a rule, so that the diameter of the thread roll will be somewhere between 1 1 ∕ 4 and 2 1 ∕ 4 inches (31.75–57.15 mm). In making a thread roll, the ends are beveled at an
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