Opening the spindle bore on a Mill
Model Engineers' Workshop|March 2020
Brian Wood had to enlarge the bore on his Dore-Westbury Mk II miller
Brian Wood

Summary

I recently replaced the 3 Morse taper ER 25 collet chuck on my mill but without really thinking about the implications of the drawbar diameter it was going to need.

On taking delivery I discovered it needed a drawbar with an M12 thread, the spindle bore was drilled through at 3/8 inches diameter.

This short article records the steps I took to open the bore out and the method of attack used on this particular machine.

Introduction

The change of chuck became necessary when the first two threads on my previous ER 25 collet chuck started to pick up on the nut, making it difficult to engage the nut without risking a crossed thread. Other threads on the chuck were also beginning to crumble. The chuck wasn’t actually that old either, maybe three years or so, but It uses a drawbar thread of 3/8 inch Whitworth which of course suited the Dore Westbury mill very well indeed.

The chuck is hardened throughout which effectively ruled out the possibility of lightly chasing the 1.5 mm thread for the closing nut. In the end I made a working repair to it by turning off the first two threads and followed that with a little fine-grade valve lapping paste to bed the remaining threads of the chuck and nut together again. Photograph 1 shows the result.

It is still a useful chuck but is now ‘under notice of eviction’ in case of further thread deterioration.

However, it is such a useful piece of tooling I decided to replace it anyway. The replacement came this time from Arc Euro Trade, the previous one was from RDG. Both are well-respected companies and there was no bias made in my choice of replacement other than to avoid another fully hardened component if I could.

On receipt, I realised I couldn’t fit the required 12mm drawbar down the mill to secure the chuck, oops! Photograph 2 shows a section of M12 thread held off in the mouth of the drive tube on top of the quill.

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