The role of drawbar force in machining

The drawbar in a machine tool spindle supplies the force that keeps the tool and toolholder assembly in the spindle even as centrifugal forces tend to open the spindle nose as the spindle rotates. In addition, the drawbar must supply enough force to hold the toolholder in the spindle when the cutting load on the tool is large. Most drawbars allow for automatic operation, and they must retain the toolholder in the spindle if power to the machine is lost.

The most common way to provide clamping force to the drawbar is through a stack of Belleville washers. A Belleville washer is a kind of disc spring, and a typical one is illustrated in Figure 1. This stack of spring elements passively holds the toolholder in the clamped position and allows it to be released when the spring stack is compressed, usually by a pneumatic actuator.

All images Courtesy of S. Smith

Figure 1. A Belleville washer’s geometry, where t = thickness and H = height.

The drawbar passes through the ID and makes contact with the inside corner of the washer on a shoulder. The stack of washers is supported inside the spindle shaft with a clearance on the OD, making contact with the stack on the outer corner with a shoulder.

As the washers are axially compressed, they flatten and increase in diameter. The outer edge of the washer is in tension as it flattens, while the inner edge is in compression. The force-displacement characteristic is not linear, can be quite complicated and depends on the material, the ID, the OD and the washer thickness and height. The force can be adjusted by changing the number of washers and their orientation in the stack. If the stacked washers are in the same orientation (nested), their stiffness is added like springs in parallel, but if they are mounted face to face or back to back, their stiffness is added like springs in series (see Figure 2 below).

A typical machine tool spindle may have 150 or more stacked washers. The drawbar force varies by spindle manufacturer but is typically about 2,000 lbs. for a 40-taper spindle and about 4,200 lbs. for a 50-taper spindle. The drawbar force in an HSK spindle may be more than twice as much as an ISO taper of similar size.