How Does Monitoring Drawbar Clamping Force Prevent Machine Downtime?

By: Harold Goellner, OTT-JAKOB Drawbar Specialist

The drawbar in a machine tool must supply enough clamping force to hold the toolholder in the spindle when the cutting load on the tool is too large. A stack of spring elements are commonly used to provide the drawbar clamping force, which holds the toolholder in the clamped position. This allows it to be released when the spring stack is compressed (usually by a pneumatic actuator).

The spring force decreases as the clamping cycles increase. This causes the clamping quality to decline through a weaker tool drawbar clamping force. A decline in clamping quality produces micro-movements, vibrations, and other malfunctions in the machine that lead to poor machining results, rejects, and even machine breakdowns.

Drawbar Clamping Force Monitoring with a Telemetry System

If the drawbar is not being monitored then the drawbar, tool, and spindle taper can permanently wear, leading to machine downtime.
If the drawbar is not being monitored then the drawbar, tool, and
spindle taper can permanently wear, leading to machine downtime.

A modular telemetry system is designed to monitor the spring force in rotating components. Future designs will have vibration and temperature monitoring. It consists of an inductively coupled transponder system that transfers energy between the stator and rotor electronics with an integrated radio transmission.

A DMS (Distance Measuring System) sensor on the rotor evaluates and calculates the spring force, and sends the raw data to the stator. The measured drawbar clamping force are output to a 4-20 mA interface in a linear manner. Vibration measurements are also provided through an RS232 interface.

There are several advantages to monitoring your drawbar clamping force with a telemetry system, including:

  • Operation at High Speeds: The telemetry system can be used at high speeds of up to 32,000 rpm due to its zero-contact construction and small installation area. 
  • Continuous Monitoring: There is an approximate 2-mm air gap for continuous measurement during machining and when the machine is idle. 
  • Stable Data Transfer: The send/receive structure is independent of the angle and guarantees robust measurement logging and reliable radio transmission in any tool position. 
  • Monitoring in any Position: The axial displacement between the spindle stator and spindle shaft makes it possible to measure spring force during all spindle statuses (unclamped, tool clamped (clamped with tool), or clamped without tool position).


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