Machinery's Handbook, 31st Edition
2194 THREAD GRINDING It is not practicable to use this form of wheel on thread pitches where the root is less than 0.007 inch (177.8 μ m) wide, because of difficulties in wheel dressing. When this method can be applied, it is the fastest means known of producing threads in hardened materials. It is not recommended, however, that thread gages, taps, and work of this character be ground with multi-ribbed wheels. The single-ribbed wheel has a definite field for accurate, small-lot production. It is necessary, in multi-ribbed grinding, to use more horsepower than is required for single-ribbed wheel grinding. Coarse threads, in particular, may require a wheel motor with two or three times more horsepower than would be necessary for grinding with a single-ribbed wheel. Alternate-Ribbed Wheel for Fine Pitches.— The spacing of ribs on this type of wheel (Fig. 3 ) equals twice the pitch, so that during the first revolution every other thread groove section is being ground; consequently, about two and one-half work revolutions are re - quired for grinding a complete thread, but the better distribution of cooling oil and result ing increase in work speeds makes this wheel very efficient. This alternate-type of wheel is adapted for grinding threads of fine pitch. Since these wheels cannot be tipped to the helix angle of the thread, they are not recommended for anything closer than Class 3 fits. The “three-ribbed” wheels referred to in a previous paragraph are also made in the alter - nate type for the finer pitches. Grinding Threads “from the Solid”.— The process of forming threads entirely by grinding, or without preliminary cutting, is applied both in the manufacture of certain classes of threaded parts and also in the production of precision tools, such as taps and thread gages. For example, in airplane engine manufacture, certain parts are heat treated and then the threads are ground “from the solid,” thus eliminating distortion. Minute cracks are sometimes found at the roots of threads that were cut and then hardened, or ground from the solid. Steel threads of coarse pitch that are to be surface hardened, may be rough threaded by cutting, then hardened and finally corrected by grinding. Many ground thread taps are produced by grinding from the solid after heat treatment. Hardening high- speed steel taps before the thread is formed will ensure there are no narrow or delicate crests to interfere with the application of the high temperature required for uniform hard ness and the best steel structure. Number of Wheel Passes.— The number of cuts or passes for grinding from the solid de - pends upon the type of wheel and accuracy required. In general, threads of 12 or 14 per inch and finer may be ground in one pass of a single-edge wheel unless the “unwrapped” thread length is much greater than normal. Unwrapped length = pitch circumference × total number of thread turns, approximately. For example, a thread gage 1 1 ∕ 4 inches long with 24 threads per inch would have an unwrapped length equal to 30 × pitch circumfer - ence. (If more convenient, outside circumference may be used instead of pitch circumfer - ence.) Assume that there are 6 or 7 feet of unwrapped length on a screw thread having 12 threads per inch. In this case, one pass might be sufficient for a Class 3 fit, whereas two passes might be recommended for a Class 4 fit. When two passes are required, too deep a roughing cut may break down the narrow edge of the wheel. To prevent this, try a roughing cut depth equal to about two-thirds the total thread depth, thus leaving one-third for the finishing cut. Wheel and Work Rotation.— When a screw thread, on the side being ground, is moving upward or against the grinding wheel rotation, less heat is generated and the grinding operation is more efficient than when wheel and work are moving in the same direction on the grinding side; however, to avoid running a machine idle during its return stroke, many screw threads are ground during both the forward and return traversing movements, by reversing the work rotation at the end of the forward stroke. For this reason, thread grinders generally are equipped so that both forward and return work speeds may be changed; they may also be designed to accelerate the return movement when grinding in one direction only.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online