OKK HM600 with drawbar & tool retention issues



CHALLENGE

OKK HM600, 12,000 RPM, air/oil bearing lubrication, motorized

With major drawbar & tool retention issues.

This high speed horizontal milling spindle arrived in our facility partially disassembled. The drawbar assembly was stuck in the spindle shaft, and the drawbar nut had been removed in an attempt to extricate the drawbar from the spindle. Our customer could not provide all of the rear rotating parts of the spindle for balancing purposes as those parts had been installed on the replacement in their machine. We were able to remove the drawbar assembly from the spindle shaft and noted that the drawbar shaft was corroded & pitted with scoring from the action of the springs. We also noted that the rear threads had been galled when removal of the nut was attempted, and that the rear of the drawbar shaft was deformed or “mushroomed”. When we inspected the front end of the drawbar assembly, it was determined that the gripper fingers and the through-tool coolant plunger had significant wear. When all of these issues with the tool retention system were taken as a whole, the only safe course of action was to replace the entire drawbar assembly, front to back.  

SOLUTION

Fortunately, we observed no significant dimensional deviations in the tooling 50 taper, on the bearing journals or shoulders, or in the housing bores. When all components had been thoroughly cleaned, we reassembled the spindle with a new ceramic bearing quad set in front and a new straight bore roller bearing in the rear, setting the proper preload. We component balanced the shaft assembly on our Schenck machine, in part because we would not be able to trim balance the complete spindle (due to the missing rear rotating parts.) We installed the complete new drawbar assembly with grippers & buttoned up the spindle with all new o-rings & assembly screws. Our final QC process included a test of tool retention force and vibration & thermal analysis during our run-up.

DETAILED CAUSE OF FAILURE

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