Workpiece Material Effects

​We are often asked about the workpiece material and how it affects the stability lobe diagram. I like to use a basketball analogy. If I drop a ball from eye level on a concrete floor it will bounce fairly high. If I drop the same ball from the same height on a carpeted floor the ball will not bounce as high. The carpet absorbs some of the energy of the dropping ball. Think of the floor as the workpiece material of a milling operation. Let’s use a multi-material tool, like a conventional 4 flute 30-degree helix carbide endmill in a toolholder. In steel, a tooth of the endmill impacts the workpiece and causes the tool to deflect. In a soft material like aluminum, some of the tooth impact’s energy is absorbed and it deflects less.  Now this is where it gets a little counter-intuitive. The speeds (lower axis) of the stable lobes DO NOT CHANGE with the material. After all, the beam (tool/toolholder/spindle) didn’t change. However, the height of the lobes (left axis) are REDUCED as the material gets harder. Why? Like the ball on concrete, the greater the height of the bounce the longer it takes for next tooth to present itself to the workpiece, therefore the force, or depth of cut, has to be reduced to reduce the bounce.