D1-3 Camlock Backplate Fix

[JohnC]

I acquired a D1-3 camlock backplate for my Boxford STS 10 - 20 lathe. On trying to fit it to the lathe it was apparent that the plate was not seating against the face of the spindle. A search of various sites revealed plenty of information and there were many with the same problem. I found that with the backplate not seating against the face of the spindle I could not reliably remount the plate. The run-out measured on the face of the plate - the 'swash' if you like - was different each time. I concluded that it was essential to have the back of the faceplate tight against the face of the spindle. Here you can see a spot of light from an LED behind the nose. And, yes, I did clean up the faces, despite this grubby looking picture!



I checked that there were no high spots on the backplate that would stop it seating properly.



All seemed OK. I also checked the fit of the taper on the plate - it was a good fit but too small. The blueing didn't show up in the photo I took. The pins were also in the correct distance to allow the cams to tighten correctly.


My problem was that the internal taper in the backplate was too narrow, holding the plate away from the face of the spindle. I considered mounting the plate backwards on a faceplate and boring the taper. However, as the clearance between the plate and the spindle face was in the order of 0.008 ins, a bit of trig showed that I would need to shave 0.001 ins from the inside of the taper. Too much and I would have to face the mating surface of the backplate and try again......

So I decided to lap out the taper. First, I needed a copy of the spindle nose - I wasn't going to do the lapping on the spindle itself. So I set the top (compound) slide to the exact angle of the taper using a finger gauge mounted on the top slide, and tapping the slide round very gently.





I then turned a block of ally to a replica of the spindle nose. I used ally because (a) I had a suitable piece and (b) I thought it would adjust to any slight discrepancy in the taper, rather than change the taper in the backplate.



As measuring was not really possible, it was a case of plenty of try for size.



I removed the chuck and applied valve grinding paste to the ally nose replica, and let the weight of the backplate do the work.



Just like grinding valves 'in the old days' I made a few oscillating strokes then lifted the backplate slightly, turned it through a quarter turn and repeated. I did not want to go too far, so again it was lots of try and see for fit. It didn't take too long before I could just get a 0.003 in feeler in the gap. The ally nose was showing signs of a step so I remounted the chuck and skimmed the ally - I had left the topslide set. I changed to fine paste and gently crept up on a perfect fit - no discernible gap between the mating face of the backplate and the face of the spindle but a tight fit on the taper.


I can now fit and remove the backplate with very repeatable runout on the face and edge. The runout on the face which will disappear when I skim it. I have marked it to ensure it goes on the same way each time.

So now just to turn the recess and mount the chuck -

I know some will say I should have sent it back. Well I did, and the replacement one was the same! So unless I am mistaken, and an unmachined backplate is supposed to be like this, the solution was either a 'named' one at a significantly higher price or a bit of elbow grease...... Yes, you do get what you pay for!