Machinery's Handbook, 31st Edition
1268 GRINDING FEEDS AND SPEEDS Example 5: Find the cutting force when ECT = 0.00017 mm, the cutting edge length (width of cut) CEL is 10 mm, and K c =150000 N/mm 2 . The chip area is ECT 3 CEL = 0.0017 mm 2 . For K c =150000, the cutting force is 0.0017 3 150000 = 255 Newton. Another difference compared to turning is the influence of the negative rake angles, illustrated by the ratio of F H / F C , where F H is the normal force and F C the tangential grinding force acting in the wheel speed direction. F H is much larger than the grinding cutting force, generally F H / F C ratio is approximately 2 to 4. An example is shown in Fig. 9, where F H / F C is plotted versus ECT at wheel speeds between 1000 and 6000 m/min, under the same conditions as in Fig. 8.
2.1
V = 6120 V = 4320 V = 2183 V = 1000
2
1.9
0.00001
0.0001
0.001
ECT, mm
Fig. 9. F H /F C versus ECT ; Cutting Speed Plotted In both Fig. 8 and Fig. 9, it is apparent that both K c and F H / F C attain maximum values for given small values of ECT , in this case approximately ECT = 0.00005 mm. This fact illustrates that forces and wheel-life are closely linked; for example, wheel speed has a maximum for constant wheel-life at approximately the same values of ECT shown in the two graphs (compare with the trends illustrated in Fig. 2a, Fig. 2b, Fig. 2c, and Fig. 3). As a matter of fact, force relationships obey the same type of relationships as those of wheel-life. Colding’s force relationship uses the same 5 constants as the tool life equation, but requires values for the specific cutting force at ECT = 0.001 and an additional constant, obtained by a special data base generator. This requires more elaborate laboratory tests, or better, the design of a special test and follow-up program for parts running in the ordinary production. Grinding Data Selection Including Wheel Life The first estimate of machine settings is based on dividing work materials into 10 groups, based on grindability, as given in Table 1. Compositions of these work materials are found in the Handbook in the section STANDARD STEELS starting on page 393. Grinding wheel nomenclature is described in American National Standard Grinding Wheel Markings starting on page 1282. The wheel compositions are selected according to the grade recommendations in the section The Selection of Grinding Wheels starting on page 1283 . Grinding fluid recommendations are given in Cutting Fluids for Machining starting on page 1246. Note: Maximum wheel speeds should always be checked using the safety standards in the section Safe Operating Speeds starting on page 1312, because the recommendations will sometimes lead to speeds above safety levels. The material in this section is based on the use of a typical standard wheel composition such as 51-A-46-L-5-V-23, with wheel grade (wheel hardness) = L or above, and mesh (grit size) = 46 or above.
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