 | New
Concepts in Milling Handbook A practical approach and illustrated guide to milling cutter selection and use <Table of Contents © 1973 Niagara Cutter Inc. |
| New
Concepts in Milling Handbook A practical approach and illustrated guide to milling cutter selection and use <Table of Contents © 1973 Niagara Cutter Inc. |
Basic Cutter Styles and Their Application - ( «Back )
( Which Cutter to Choose?) - screen [ 1 | 2 | 3 ]
CHIP FORMATION
Extensive research has produced a wealth of useful information pertaining to chip formation. The present theory states that the chip is developed in the shearing process by plastic flow. That is, at the shear plane angle (Figure 65) the material ahead of the cutting edge of the tool is severely compressed resulting in temperatures high enough to allow plastic flow.Atoms in the steel (continuous chip Figure 66) or cast iron (segmental chip Figure 67 ) under high pressure and friction slide one another in the shear zone. The high plastic shearing strains they are subjected to generate high temperatures which affect tool life.
Mild steels may be severely hardened by the cutting process because of the high shearing strains they are subjected to. Where built-up edge occurs, (Figure 68) the fragments that escape under the flank of the cutter, and are left on the finished work surface can be as hard as the cutting tool material.
This accounts for the damaging effect built-up edge has on high speed steel and carbide. Even in the absence of built-up edge, mild steels will crater carbide and HSS where a continuous chip under load exists.
Approximately 60 percent of the heat generated comes from "A" in the shear plane zone (Figure 69), 30 percent from the chip tool interface in zone "B" and 10 percent from the radial pressure on the flank of the cutter in zone "C".
The hardness produced on a continuous chip by the shearing strain is directly proportionate to the strain. Maximum tool life can be attained only when the shearing strain is kept to a minimum.
A small shear plane angle (Figure 70) produces a short thick chip and considerable heat. A large shear plane angle (Figure 71 ) produces a thin, long chip and minimal heat. The object then is to deform the material being machined as freely as possible to minimize heat and pressure for maximum tool life.