Chip formation and choice of insert geometry
Breaking the chips
Chip control is one of the key factors in turning, and there are three principle chip breaking alternatives:
– self breaking, for example cast iron
– against the tool
– against the workpiece
Factors that have an influence on chip breaking are the:
- Insert geometry
- Nose radius, re
- Entering angle, kr
- Cutting depth, ap
- Feed, fn
- Cutting speed, vc
Turning geometries can be divided into three basic styles that are optimized for finishing, medium and roughing operations. The diagram shows the working area for each geometry, based on acceptable chip breaking, in relation to feed and depth of cut.
Roughing – R
High depth of cut and feed rate combinations. Operations requiring the highest edge security.
Medium – M
Medium operations to light roughing. Wide range of depth of cut and feed rate combinations.
Finishing – F
Operations at light depths of cut and low feed rates. Operations requiring low cutting forces.
Example of chip breaking for a -PM geometry
Chip breaking test of a CNMG 12 04 08-PM insert at different cutting depths and feeds. The chip breaking within the marked area is classified as good, and the results are transferred into a diagram.
Insert geometries for different workpiece materials
Many insert geometries are optimized for a certain workpiece material type, i.e. PF, PM, PR for turning of steels, MF, MM, MR for stainless steels, and KF, KM, KR for turning of cast irons, etc. Other geometries, like WMX, WF, WM, WR, are suitable for both steel, stainless and cast iron.