Measuring and managing drawbar forces

Figure 1: Measured FRFs for a CAT 50 holder in a spindle with 12kN drawbar force (left) and 32kN drawbar force (right).

The metal-removal rate that can be achieved while milling depends largely on the dynamic characteristics of the machining system—including toolholder and cutting tool—as seen at the tool tip.

In particular, the dynamic stiffness as characterized by the frequency response function (FRF) sets limits on machining performance. The dynamic characteristics as seen at the tool tip are often strongly affected by the characteristics of the connection between the toolholder and the spindle.

Considerable effort has been devoted recently to the computation and measurement of the static stiffness, dynamic stiffness, accuracy, repeatability, torque capacity and maximum speeds of various toolholder/spindle interfaces.

Intuitively, it would seem the best connection would be the stiffest one. If that were the case, the surface finish should be as smooth as possible, meaning that a polished, shiny surface would be preferable. A close fit between the toolholder and the spindle would also be desirable, as would a high drawbar force. However, intuition is often wrong, and measurements clearly indicate that those interface conditions can be detrimental to cutting performance.

The reason that intuition can fail is we often think statically. Static stiffness is an easy-to-understand concept. The higher the static stiffness, the smaller the tool deflection in response to an applied static force.

However, when milling, the cutting force is not static. It is variable because the teeth enter and leave the cut as the tool rotates, and because as the tool vibrates it changes the thickness of the chip. Therefore, the cutting performance depends on the dynamic stiffness (a combination of static stiffness and damping), and the relationship is usually expressed in a FRF.