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 tends to open the spindle nose as the spindle rotates. The most common way to provide clamping force to the drawbar is through a stack of Belleville washers, which is a kind of disc spring. The drawbar passes through the ID and makes contact with the inside corner of the washer on a shoulder. 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. It is important to detect a reduced clamping force and to replace a worn stack of Bellville washers before spindle damage occurs. Drawbar dynamometers are inexpensive, easy to use and can quickly detect serious problems before damage occurs. The drawbar force should be checked every month or so and record.