Machinery's Handbook, 31st Edition
2188
THREAD ROLLING
1 1 ∕ 8 inch(15.875–28.575 mm), and 150 to 175 per minute in machine screw sizes from No. 10 (.190 inch) to No. 6 (.138 inch). In the case of heat-treated alloy steels in the usual hardness range of 26 to 32 RC (Rockwell C scale) , the production may be 30 or 40 per minute or less. With a cylindrical die type of machine, which is designed primarily for precision work and hard metals, 10 to 30 parts per minute are common production rates, the amount depending upon the hardness of material and allowable rate of die penetration per work revolution. These production rates are intended as a general guide only. The diameters of rolled threads usually range from the smallest machine screw sizes up to 1 or 1 1 ∕ 2 inches (25.4 or 38.1 mm), depending upon the type and size of machine. Precision Thread Rolling.— Both flat and cylindrical dies are used in aeronautical and other plants for precision work. With accurate dies and blank diameters held to close lim its, it is practicable to produce rolled threads for American Standard Class 3 and Class 4 fits. The blank sizing may be by centerless grinding or by means of a die in conjunction with the heading operations. The blank should be round, and, as a general rule, the diame ter tolerance should not exceed 1 ∕ 2 to 2 ∕ 3 the pitch diameter tolerance. The blank diameter should range from the correct size (which is close to the pitch diameter, but should be determined by actual trial), down to the allowable minimum, the tolerance being minus to insure a correct pitch diameter, even though the major diameter may vary slightly. Preci sion thread rolling has become an important method of threading alloy steel studs and other threaded parts, especially in aeronautical work where precision and high-fatigue resistance are required. Micrometer screws are also an outstanding example of precision thread rolling. This process has also been applied in tap making, although it is the general practice to finish rolled taps by grinding when the Class 3 and Class 4 fits are required. Steels for Thread Rolling.— 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 RC, with tensile strengths varying from 130,000 to 150,000 psi (896–1034 MPa). While harder materials might be rolled, grind - ing is more practicable when the hardness exceeds 40 RC. Thread rolling is applicable not only to a wide range of steels but for nonferrous materials, especially if there is difficulty in cutting due to “tearing” the threads. Diameter of Blank for Thread Rolling.— The diameter of the screw blank or cylindrical part upon which a thread is to be rolled should be less than the outside screw diameter by an amount that will just compensate for the metal that is displaced and raised above the original surface by the rolling process. The increase in diameter is approximately equal to the depth of one thread. While there are rules and formulas for determining blank diameters, it may be necessary to make slight changes in the calculated size in order to secure a well-formed thread. Blank diameter should be verified by trial, especially when rolling accurate screw threads. Some stock offers greater resistance to displacement than other stock, owing to greater hardness or tenacity of the metal. The following figures may prove useful in establishing trial sizes. The blank diameters for screws varying from 1 ∕ 4 to 1 ∕ 2 are from 0.002 to 0.0025 inch(50.8–63.5 μ m) larger than the pitch diameter, and for screws varying from 1 ∕ 2 to 1 inch (12.7–25.4 mm) or larger, the blank diameters are from 0.0025 to .003 inch (63.5–76.2 μ m) larger than the pitch diameter. Blanks which are slightly less than the pitch diameter are intended for bolts, screws, etc., which are to have a comparatively free fit. Blanks for this class of work may vary from 0.002 to 0.003 inch (50.08–76.2 μ m) less than the pitch diameter for screw thread sizes varying from 1 ∕ 4 to 1 ∕ 2 inch (6.35–12.7 μ m), and from 0.003 to 0.005 inch (76.2–127 μ m) less than the pitch diameter for sizes above 1 ∕ 2 inch. If the screw threads are smaller than 1 ∕ 4 inch, the blanks are usually from 0.001 to 0.0015 inch (25.4–38.1 μ m) less than the pitch diameter for ordinary grades of work.
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