Backplate thread?

[YouraT]

Hi all.

Is there an accepted "close enough" approximation to use for cutting the backplate thread on an all-metric machine ?

Long-term a set of translation gears and a banjo are on the cards, but not just yet...

Thanks,

Youra.

[Andy B]

Hi Youra,

See my thread here.
As mentioned there, various sources suggested up to 2% error will give a usable engagement.
I would aim for 1% or even 0.5% - the headstock thread engagement is quite short (8-10 threads?).

I've never actually tried it - do you have a bit of scrap to do a test thread on?

Andy

[dazz]

Hi
With any error in thread pitch, you will end up with (nearly) all of the engagement happening on a short length of thread. As a result, there is a significant risk of plastic deformation of the lathe spindle thread. Any sort of deformation of the spindle thread is going to hurt accuracy.

I recommend you buy a back plate that fits. It would be cheaper/easier than making a new spindle.

Dazz

[RobT]

I've always thought that the thread has very little to do with accuracy - it's simply there to pull the chuck, backplate or whatever down hard on to the register face which is where everything gets aligned. Having said that, I agree that you wouldn't want to damage the spindle irreparably, so my solution would be to cut a 3.2mm thread (which is pretty close anyway), but undercut it so there's a touch more to go then run a 1.5 x 8 tap down it to make it perfect. The taps are available from RDG Tools at a sensible price, but are essentially bottoming taps so aren't of much use for cutting the whole thread (and they're only carbon steel so probably aren't strong enough anyway). They should be fine for just "cleaning up" however. The tap will cost you a tiny bit more than buying a ready made backplate, but once you've got it the next one comes for free!

[dazz]

Hi
The tap idea sounds like a good solution.
I think of the thread as a rotary Vee. It has a major influence on chuck runout.

[rpwilson]

I've always taken the contrary view: the register ensures concentricity, the face of the flange behind the register ensures alignment, all the thread does is to lock the backplate firmly against the flange. So, the recess in the backplate should be a close fit over the register, but the thread need not be that tight. Before I made my translation set, I cut my imperial threads by the method described, cut most of the thread depth using the nearest available pitch on the gearbox, then finish off with the tap.