Setting tool length

It’s common sense that to get the highest possible metal-removal rate when milling, the user should simply select the shortest tool that is geometrically capable of making the desired part feature. The tool should be as short as possible so that it is as stiff as possible. That’s because a stiff tool deflects and chatters less.

There is just one problem with that line of thought: it’s wrong. The reason is the highest achievable mrr depends not just on the stiffness, but also on the maximum spindle speed, damping and natural frequency of the toolholder-and-spindle system.

Courtesy of All Figures: Dr. Matt Davies, University of North Carolina at Charlotte

Figure 1: Stability lobe diagram for an 11.8mm-dia., 104mm-long, 2-flute, solid-carbide endmill in a 20,000-rpm spindle.

Figure 2: Stability lobe diagram for an 11.8mm-dia., 118mm-long, 2-flute, solid-carbide endmill in the same spindle and toolholder as Figure 1.

For most tools, the phenomenon that limits mrr is not available spindle power or the tool’s strength or wear rate, but rather the onset of chatter. For a given tool, toolholder and spindle, some spindle speeds are more stable against chatter and some less so. This is often expressed in a stability lobe diagram.

Figure 1 shows a stability lobe diagram for a long, slender endmill. It extends 104mm from the end of the holder for a length-to-diameter ratio of about 8.8:1. The shaded parts of the diagram represent chatter, and the white portions represent stable milling. While the spindle has a 20,000-rpm maximum speed, the stability lobe diagram was calculated to 30,000 rpm. At the 20,000-rpm top speed, the tool can only take an axial DOC of less than 0.1mm in a slot without experiencing chatter. This represents a low-power cut.