Patent GB-534786-A (GB)
Title: Improvements in or relating to a cutter for removing a layer of metal from a workpiece
IPC: B23C5/20
Filed: 1939-12-13
Abstract: 534,786. Milling-cutters. INGERSOLL MILL ING MACHINE CO. Dec. 13, 1939, No. 32168. Convention date, Dec. 19, 1938. [Class 83 (iii)] A face-milling cutter has a number of blades 20 secured in or formed integrally with a cutter body 15, the blades having end edges 24 defining a plane cutting face perpendicular to the cutting axis 16 and side edges 25 defining a frusto-conical cutting face, each of the end edges having its inner end 24a located ahead of its outer end and each of the side edges having an effective shear angle es, Fig. 5, of more than 15 degrees. An ordinary facemill, Fig. 15, has main cutting edges 2 which have a rake r, Fig. 16, and shear angle s, Fig. 15. If the corner of the blade is bevelled off at an angle b, Fig. 17, the edge 10 so produced becomes the main cutting edge and the rake and shear angles of the edge 10 are altered in a manner determined by Fig. 13, whence it appears that for a bevel angle of 30 degrees a cutter having apparent shear and rake angles each of 10 degrees (point 11) has the angles of the main edges modified by the bevelling to an effective rake of 17 degrees and an effective shear of -4 degrees. According to the invention the blades are so positioned that the apparent rake angle is of negative sign and gives an effective rake angle of positive sign and desired magnitude while the apparent shear angle is also such that when modified by the bevelling of the cutter the desired effective shear angle is obtained. The shear angle may be substantially greater than that required for efficient cutting in order to reduce end thrust on the cutter, and to facilitate chip flow. In order to give a negative rake angle to the edge 23 which would be the main cutting edge in the absence of bevel each blade is set so that the inner end 24a of the finishing edge 24 leads the outer end 24b of such edge, Fig. 3. The apparent shear angle of the edge 22 is of the same size as usual as seen in Fig. 1. In the cutter shown the apparent rake angle is -20 degrees and the apparent shear angle 20 degrees. With a bevel of 30 degrees it is seen from the diagram Fig. 13 (point 12) that there results an effective rake angle of 8 degrees and an effective shear angle of 29 degrees. Thus in constructing a cutter in accordance with the invention the desired effective rake and shear angles are decided in accordance with the metal to be milled. The bevel angle being also known, the apparent angles are determined from a diagram similar to Fig. 13 and the blade slots in the cutter body are located in accordance with such apparent angles. In order to facilitate the flow of chips from the cutter, the effective shear angle of the bevel edge is such that the chips are directed away from the cutter axis, Fig. 9, and the inner end of the edge leads the outer end. The effective shear angle may be from 25 degrees-29 degrees instead of 15 degrees as is usual with this type of cutter. For firm support of the blades, the latter do not project beyond the periphery 17 of the cutter body, also the end surface and the adjacent conical surface of the cutter body extend into close proximity with the end and bevel surfaces respectively of the blades. Considering the forces acting on a cutter blade, Fig. 10, the force R perpendicular to the blade edge is equal to KT where K is a constant depending on the material being cut and T is the tangential force. The force KT gives a component KT cos b in the direction of the cutter axis 16. Owing to the effective shear angle there is a component force along the blade of T sin es giving a component C = T sin es sin b acting axially of the cutter in opposition to the force E. Equating the axial forces, it is seen that there will be no resultant axial force on the cutter if sin es=K cot b. If K=.3 there is zero axial force with an effective shear angle of 30 degrees if the bevel angle is 30 degrees. Thus by increasing the shear angle smaller bevel angles than are usually practicable may be employed without unduly increasing the axial thrust.