November 2005
- The Importance of Drawbar Retention Force-

Maintaining the correct retention force in your machining center spindle is often an overlooked maintenance item. It’s not uncommon for a group of identical machines to have vastly differing measurements. Often, a company will find they are struggling with chattering on parts, or even parts that are out of tolerance. Many items can contribute to these problems, but one of your first checks should be your drawbar.

Worn or broken belleville washers on the drawbar assembly can significantly reduce the amount of clamping pressure on the tool holder. Low clamping pressure can cause premature wear in the spindle taper area, which can affect the accuracy of the spindle, ultimately leading to poor surface finish and out-of-tolerance parts.

The best means for assessing the drawbar without having to remove it from the spindle is with the use of a dynamometer. Most steep taper types (ISO, CAT, BT, HSK) can be checked using a dynamometer and the process takes only a few minutes to perform.

This small maintenance practice can turn into a sizable cost savings when looking at the life of the spindle. Proper drawbar inspection and maintenance will increase the uptime of your spindle while reducing the cost of repairs when service is needed.