Technique Required to Make Accurate Backplate

I think that I might have found the cause of my trouble.

I made a dummy spindle nose and tested it in the register of the original chuck. I then took my wobbly back plate, turned it around, trued it up and cut a new, accurate register, all without removing the plate from the lathe. I then turned it around, screwed it on the correct way around and trued the working face. I used a DTI to check that it ran true and registered a slight flicker only.

However, each time that I removed and replaced it, it ran out of true again when measuring against the face of the plate.

In frustration, I checked the face of the spindle, just behind the register, with the DTI. The DTI ran true except for one spot where there was a significant difference. I examined the face of spindle with a magnifier and found that there is a rough blemish that is causing the DTI to move. I am assuming that this is the prime cause of the problems.

The decision I need to make is what to do about it: live with the problem or try to true up this face.

James.

Edited By James Alford on 08/09/2016 22:06:36

Edited By James Alford on 08/09/2016 22:06:56

Edited By James Alford on 08/09/2016 22:07:15

Is the blemish raised? If so it needs to be flush or lower than the rest of the register face. I would consider stoning a small area to remove a raised blemish an acceptable way of repairing it. If it is lower than the rest of the register it should cause no problems.

Martin

The thread should of course be as true to the register as possible, but all it does is secure the face plate to the spindle, the register is what dictates the true running, so if the face plate bore is a good fit to the register and the thread is free right up to its finial position then the plate will clamp in exactly the same position on the register every time and so once machined will always run true to it.

A small dead smooth file used carefully on the stationary spindle should remove that blemish.

Thank you for the replies. I had a more careful look tonight and confirmed that the spindle seems to be running true except for the dark spot at the bottom in the picture below, which stands proud by 2 thou. I shall have a go at flattening it today, using the ideas suggested or, perhaps, a hard rubber point in the Dremel.

Don't turn up the face. The best option is probably a slip stone but a fine diamond lap or a bit of emery cloth and finger or stick should do as it does't matter if the damaged area finishes up a little lower than the rest. Or Hopper's suggestion. With the stone, lap or file it's possible to feel that it's flat onto the work. If it sticks up for instance it will rock a little. The Dremel wont be so controllable.

Test the result lightly with a finger. It's possible to feel micron step changes that way.

 Just one more thing noticing a comment. Male threads in female threads that have tapered flanks tend to centralise themselves as they tighten so have that off centre and it can interfere with a back plate running true. Same problem if the thread is out of square.

John

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Edited By Ajohnw on 10/09/2016 09:45:19

Edited By Ajohnw on 10/09/2016 09:45:36

Edited By Ajohnw on 10/09/2016 09:50:32

Well, I have tried again, but still have a run-out.

This time, I managed to hold the blank for the back-plate in the three jaw chuck, face it off, drill, bore and tap the thread and then cut the register all in one setting on the Flexispeed. I then span it on to the spindle the correct way around and faced it off.

Using a DTI, I then checked the turned face for run-out. Immediately after facing it, I had zero run-out on the face at the edge (it is about 70mm in diameter. However, when I removed it and replaced it, I measured approximately 2.5 thou run-out at the periphery of the disc.

I do know why I cannot get a zero run-out: perhaps I am expecting too much. However, I shall persevere to see how truly a 1/8" milling cutter runs when the holder is finished.

James.

Success at last!

I have now finished the back plate and mounted the collet chuck. Measured on the shank of a 1/8" slot drill, I have a run-out of 0.0005". Most pleasingly, the run-out is the same each time I remove and replace the chuck, the run-out is the same.

Thank you all for the help.

James.