Page 57 - Demo

55MACHINING CALCULATIONSCoefficients x and y depend on machining operations, machined material, cutting geometry and cutting material etc. In practice, using averaged values of x and y enables quite acceptable results. Therefore, after calculating tangential force Ft, the main component of the total cutting force, the other components can be easily found from equation (8.5).In estimating tangential force Ft, a method that is based on specific cutting force values is reasonable. The actual specific cutting force kc is the force that is needed to remove the material chip area of 1 mm%u00b2 (.0016 in%u00b2), which has an average chip thickness referred to as hm.kc=kc1%u00d7hm-mc (8.6)where kc1 is the specific cutting force to remove the material chip area of 1 mm%u00b2 (.0016 in%u00b2) with 1 mm (.004 in) thickness.mc is the chip thickness factor that reflects the dependence of kc on kc1with changing the actual chip thickness when compared to 1 mm%u00b2 (.0016 in%u00b2).kc1 and mc are characteristics of the machined material based on test results. The analysis of empirical data has enabled specifying these characteristics as averaged values for all groups of engineering materials. In various sources of machining data, kc1 relates to cutting material with a tool bearing a zero rake angle (%u03b3=0%u00b0). If the actual tool rake significantly differs from zero, the above equation may be corrected in the following way:kc=kc1%u00d7hm-mc%u00d7(1-%u03b3/100) (8.7)Knowing the specific force and the crosssection area of the cut material layer A, one can easily determine the tangential force.Example: A 90-degree milling cutter machines a square shoulder that has cross-sectional dimensions 4 mm%u00d79.5 mm (.16%u201d%u00d7.38%u201d). The workpiece material is annealed high alloyed steel AISI H13 (DIN W.-Nr. 1.2344). The rake angle of the cutter is 10%u00b0. We need to find the tangential cutting force if 0.1 mm average chip thickness (.004%u201d chip load) is maintained.According to the Materials and Grade section in ISCAR%u2019s Milling Line Catalog, the machined material relates to a material group, which features kc1 =2450 N/mm%u00b2 (355 ksi) and mc=0.23.From equation (8.7) actual cutting force kc=2450%u00d70.1-0.23%u00d7 (1-0.1) = 3745 N/mm%u00b2 (543166 psi or ~543 ksi).Tangential cutting force Ft%u2019= 3745%u00d74%u00d79.5 = 142310 N= 142.3 kN, and Ft%u2019%u2019= 543166%u00d70.16%u00d70.3 8= 33024 lbf (~33 klbf).Avoid Misunderstandings: Specific Cutting ForceIn technical literature, the actual specific cutting force may be designated by kc1, and specific cutting force to remove a material chip area of 1 mm%u00b2 (.0016 in%u00b2) with .1 mm (.004 in) thickness by kc1.1. The number %u201c1%u201d that follows index %u201cc%u201drelates to 1 mm2 (.0016 in2) chip area, and addition %u201c1.1%u201d highlights %u201c1 mm2 (.0016 in2) chip area with .1 mm (.004 in) thickness%u201d. Not to be confused!