The drawbar is a critical component of a milling spindle that is responsible for holding the tool holder tightly in the machine spindle’s taper. It is composed of a shaft, a spring mechanism (coil or Belleville washers), and a collet. No matter how well your spindle is running, if the tool isn’t properly held, you will never realize full performance. At MZI, we follow a meticulous process to ensure the drawbar is functioning at or above OEM specifications.
Drawbar Terminology:
Pull Force (AKA Clamping force or Drawbar Force): The axial force, measured in kN, exerted by the machine’s drawbar mechanism to secure the tool holder firmly into the spindle taper. In other words, how much force is pulling the tool up into the spindle.
Retention Knob (AKA pull stud): The critical, high tensile strength steel component that threads into the back of the tool holder where the drawbar interfaces with the tool holder.
Collet (AKA Gripper Fingers or Gripper Set): The segmented component that threads into the end of the drawbar and is responsible for clamping and pulling on the retention knob.
Belleville Washers / Coil Spring: Belleville Washers are conical disc shaped springs that are stacked in a specific configuration to generate the clamping force required. Coil springs achieve the same purpose, but using a high force wound spring.
We always start by testing the pull force on any incoming spindle. On disassembly, the drawbar length is measured, photographed, and all components are carefully inspected. Corrosion and broken Belleville washers are the most common cause of drawbar failure, but other issues like scoring and gripper set damage can also present. Whether your drawbar requires a simple cleaning and re-lubrication, or full chrome, grind and replacement of all components, when your spindle leaves MZI, your drawbar will be performing at or above factory specifications.