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.

Over time, the contact surfaces of Belleville washers begin to wear and the clamping force gradually decreases. A typical set lasts about a million clamping cycles. That may sound like a lot, but a machine that runs two shifts, or 16 hours, per day, 200 days per year and makes two tool changes per minute reaches that number in less than 2.5 years. 

Eventually, fatigue, corrosion or overload causes one of the washers to fracture. The remaining washers then carry an increased load and, therefore, are more likely to break. Broken washers also reduce the clamping force, and the loss of clamping force reduces stiffness at the tool tip. 

Telltale signs of clamping force loss in ISO- or CAT-type spindles include changes in cutting performance (cuts that once were stable now exhibit chatter) and the characteristic fretting corrosion seen on toolholders and the spindle nose. Fretting is a wear process that occurs at the contact area between two materials under load that are subject to small relative motion, such as one caused by a cutting force. The surfaces with fretting corrosion often exhibit a reddish or copper-colored oxide layer and are pitted. A spindle nose damaged by fretting corrosion should be repaired immediately.

Figure 2. Belleville washers in parallel and series.

In HSK-type spindles, the loss of drawbar force can be more problematic. If the drawbar force becomes low enough to allow the separation of face contact under cutting load, the connection stiffness falls dramatically, and the toolholder may pull out of the spindle. 

It is important to detect a reduced clamping force and to replace a worn stack of Bellville washers before spindle damage occurs. A user can measure spindle drawbar force with a drawbar dynamometer. These devices can be adapted to have the outer profile of a variety of toolholders and are periodically clamped in the spindle as if they were toolholders. They are instrumented with strain gages between the toolholder profile and the retention mechanism. The measured strain is converted to drawbar force and displayed on the unit. 

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 recorded. Belleville washer stacks should be checked and replaced immediately if sudden changes in drawbar force are detected or when the force drops by about 10 percent. CTE

About the Author: Dr. Scott Smith is a professor and chair of the Department of Mechanical Engineering at the William States Lee College of Engineering, University of North Carolina at Charlotte, specializing in machine tool structural dynamics. Contact him via e-mail at kssmith@uncc.edu.