Machinery's Handbook, 31st Edition
THREAD GRINDING 2195 Wheel Speeds.— Wheel speeds should always be limited to the maximum specified on the wheel by the manufacturer. According to the American National Standard Safety Code, resinoid and vitrified wheels are limited to 12,000 surface feet per minute (3657 m/min); however, according to Norton Co., the most efficient speeds are from 9,000 to 10,000 (2743–3048 m/min) for resinoid wheels and 7,500 to 9,500 (2286–2896 m/min) for vitrified wheels. Only tested wheels recommended by the wheel manufacturer should be used. After a suitable surface speed has been established, it should be maintained by increasing the rpm of the wheel, as the latter is reduced in diameter by wear. Since thread grinding wheels work close to the limit of their stock-removing capacity, some adjustment of the wheel or work speed may be required to get the best results. If the wheel speed is too slow for a given job and excessive heat is generated, try an increase in speed, assuming that such increase is within the safety limits. If the wheel is too soft and the edge wears excessively, again an increase in wheel speed will give the effect of a harder wheel and result in better form-retaining qualities. Work Speeds.— The work speed usually ranges from 3 to 10 fpm(0.9–3.0 m/min). In grinding with a comparatively heavy feed, and a mininum number of passes, the speed may not exceed 2 1 ∕ 2 or 3 fpm(0.76–0.9 m/min). If very light feeds are employed, as in grinding hardened high-speed steel, the work speed may be much higher than 3 fpm (0.9 m/min) and should be determined by test. If excessive heat is generated by removing stock too rapidly, a work speed reduction is one remedy. If a wheel is working below its normal capacity, an increase in work speed would prevent dulling of the grains and reduce the tendency to heat or “burn” the work. An increase in work speed and reduction in feed may also be employed to prevent burning while grinding hardened steel. Truing Grinding Wheels.— Thread grinding wheels are trued to maintain both the re- quired thread form and an efficient grinding surface. Thread grinders ordinarily are equipped with precision truing devices which function automatically. One type automati cally dresses the wheel and also compensates for the slight amount removed in dressing, thus automatically maintaining size control of the work. While truing the wheel, a small amount of grinding oil should be used to reduce diamond wear. Light truing cuts are advisable, especially in truing resinoid wheels which may be deflected by excessive truing pressure. A master former for controlling the path followed by the truing diamond may re - quire a modified profile to prevent distortion of the thread form, especially when the lead angles are comparatively large. Such modification usually is not required for 60-degree threads when the pitches for a given diameter are standard because then the resulting lead angles are less than 4 1 ∕ 2 degrees. In grinding Acme threads or 29-degree worm threads having lead angles greater than 4 or 5 degrees, modified formers may be required to pre - vent a bulge in the thread profile. The highest point of this bulge is approximately at the pitch line. A bulge of about 0.001 inch (25.4 μ m) may be within allowable limits on some commercial worms but precision worms for gear hobbers, etc., require straight flanks in the axial plane. Crushing Method: Thread grinding wheels are also dressed or formed by the crushing method, which is used in connection with some types of thread grinding machines. When this method is used, the annular ridge or ridges on the wheel are formed by a hardened steel cylindrical dresser or crusher. The crusher has a series of smooth annular ridges which are shaped and spaced like the thread that is to be ground. During the wheel dressing operation, the crusher is positively driven instead of the grinding wheel, and the ridges on the wheel face are formed by the rotating crusher being forced inward. Wheel Hardness or Grade.— Wheel hardness or grade selection is based upon a compro mise between efficient cutting and durability of the grinding edge. Grade selection de - pends on the bond and the character of the work. The following general recommendations are based upon Norton grading. Vitrified wheels usually range from J to M, and resinoid wheels from R to U. For heat- treated screws or studs and the Unified Standard Thread, try the following. For 8 to
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