Why Monitor the Drawbar Shaft Position in a Spindle?

By: Harold Goellner, OTT-JAKOB Drawbar Specialist

It is important to monitor your drawbar shaft position when rotating your spindle. Rotating your spindle in ‘unclamp position’ or ‘clamped without tool position’ can cause:

  • The tool changer to remove the tool from the spindle too early.
  • The spindle to start moving before the tool is correctly clamped.
  • More frequent maintenance, loss of precision, and misclamping.
  • Potential damage to the workpiece and the machine. 
  • A safety risk to the operator.

Traditional ways of monitoring drawbar shaft position involve more manual work from the operator and inaccurate results. An electronic sensor system built into the unclamping unit provides more precise information as well as safe clamping and unclamping.

Distance Measurement System with integrated electronics.
Distance Measurement System with integrated electronics.

Electronic Distance Measurement System to Monitor Drawbar Shaft Position

A built-in electronic Distance Measurement System (DMS) records the exact position of the drawbar shaft, and in turn, the current position of the tool--guaranteeing safe clamping and unclamping for the machine and operator.

Included in the unclamp unit, the DMS combines analog sensor electronics with a limit switch. It also features a compact design that comprises an inductive sensor and a target ring on the drawbar shaft.

The unclamping unit contains a cylindrical inductive analog sensor. This sensor is based on the linear variable differential transformer principle (LVDT). It consists of two coils that are connected in series out of phase to detect the drawbar shaft position. The evaluation electronics installed in the unclamping unit determine the change of distance and calculate the outgoing signal. If the target ring moves axially, the coupling factors between the coils change and identify the three clamping statuses:

  1. Unclamped
  2. Tool Clamped
  3. Clamped without Tool

The signal sent to the machine control system can be analog (4-20 mA or 2-10 V) or digital (S1, S2, S3), and it can be sent during rotation or when the system is idle.

Advantages with DMS Drawbar Shaft Position Monitoring

There are several advantages that a DMS provides for manufacturers, including:

  • More accurate and precise data with an internal, electronic solution that eliminates mechanical errors from traditional solutions. 
  • An all-in-one compact design that is ready to use as soon as it is installed. 
  • A fully automated, failsafe solution that prevents the spindle from rotating when a tool clamped is out of tolerance or when there is no tool.


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