Limit the bending moment: General Industry Coverage

All images courtesy of S. Smith

Figure 1: Static load is applied through a hydraulic cylinder at the right side and the deflection is measured using capacitance gages during toolholder tests for stiffness, damping and capability to support bending moment loads.

Bending moment is an important consideration in machine tool spindles because it causes the tool to separate from the toolholder or the toolholder to separate from the spindle.

A moment is a force multiplied by the distance between that force and the point of interest measured in foot-pounds or newton-meters. A bending moment exists in a mechanical structure when an applied moment causes that structure to bend. All tool/spindle connections have bending moment limits, beyond which the connection may break or the toolholder pulls out of the spindle.

Let’s look, for example, at milling a slot in aluminum with a 2-flute tool at a chip load of 0.2mm per tooth and an axial DOC of 5mm. The maximum force seen by the tool is about 850 N. If the distance from the spindle face is 150mm, then the toolholder supports a bending moment of about 127 Nm. This number will increase if there are more teeth simultaneously cutting, the material has a higher specific power, the axial DOC is larger, the chip load is higher or the distance from the tool tip to the spindle face increases.

Figure 2: Bending moment vs. rotation and drawbar force.