You Must Play all the Angles! Part 3
Cutting Tool Angles, Functions and Importance!
Part 3 of 3 - Clearance or Relief Angle
As we stated three weeks ago, when we began this blog all tools have three angles; lead, rake and relief or clearance. This week we complete the three-part series discussing the importance and function of the relief or clearance angle. First, we must review a few basics of metal cutting. Cutting tools especially carbide ones like to cut. They do not like to rub. Rubbing is for racing not for carbide. What two things does it take to make a chip; heat and pressure. Where do you want the heat to go; into the chip. Where do you not want the heat to go; into the tool.
While heat and pressure are required to make chips heat and pressure directed into the cutting tool causes wear. That is where the clearance or relief angle comes into the picture. The textbook definition of clearance or relief angle is; the inclination of the surface of the tool below the cutting edge and adjacent to the rake angle of the cutting edge. The clearance or relief angle of the tool prevents rubbing and allows the cutting edge to be fed into the workpiece material. A tool with no clearance or relief will not cut.
Now just like with the rake angle we use different clearance angles based on the material being machined. The more ductile a material the more the material has a tendency of springing out of the way as the cutting edge passes over it and springing back into its previous position once the cutting edge has passed. This spring back causes rubbing, excess heat and in many cases the rubbing negatively impacts the surface quality of the part. So, the more ductile a material the greater the clearance or relief angle must be.
But before we go any further remember the “Yin and Yang” rule of machining; for every good thing, there must be a bad thing. As we increase the clearance or relief angle to prevent rubbing we weaken the strength of the cutting edge. Remember cutting forces are always perpendicular to the cutting edge and carbide likes to be compressed. When we increase the clearance or relief angle we are removing material from the area of the tool right below the cutting edge thus reducing the bulk strength. The attached table provides a starting point based on general material types. Always consult the manufacturer's technical guide for recommendations for your specific material and application.
In summary, all cutting tools have three angles; lead, rake and relief. The tools lead angle controls the direction of the cutting forces and the chip thickness. The tools rake angle determines the strength of the cutting edge, the power consumed and the direction of chip flow. Last but not least, the clearance angle prevents rubbing and burnishing and allows the cutting edge to bite into the material being machined.