Backplate thread?

All info relating to Viceroy machines - Lathes, sharpedge, drills etc...

Moderators: Martin, bradders, Steve, Mr Magoo, Andy B

Post Reply
YouraT
Posts: 34
Joined: Sun 22 Apr , 2012 19:38 pm

Backplate thread?

Post by YouraT » Mon 17 Sep , 2018 18:46 pm

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
CNC Guru
CNC Guru
Posts: 329
Joined: Mon 20 Nov , 2006 18:19 pm
Hardware/Software: 280 VS lathe, Denford Senior E-type mill, Senior Major Universal Mill
Location: East Midlands, UK

Re: Backplate thread?

Post by Andy B » Tue 18 Sep , 2018 16:46 pm

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

User avatar
dazz
CNC Expert
CNC Expert
Posts: 229
Joined: Sat 19 Aug , 2006 11:31 am
Hardware/Software: Viceroy TDS 1/1GB lathe, Imperial, stripped down and rebuilt.
Viceroy Taper/Tracer attachment, Shop made cross slide tracer attachment, VSD.
Location: New Zealand

Re: Backplate thread?

Post by dazz » Thu 20 Sep , 2018 11:07 am

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
Regards

Dazz

RobT
Posts: 3
Joined: Sat 23 Feb , 2019 16:20 pm
Hardware/Software: Viceroy TDS 1/1GB

Re: Backplate thread?

Post by RobT » Tue 12 Mar , 2019 18:29 pm

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!

User avatar
dazz
CNC Expert
CNC Expert
Posts: 229
Joined: Sat 19 Aug , 2006 11:31 am
Hardware/Software: Viceroy TDS 1/1GB lathe, Imperial, stripped down and rebuilt.
Viceroy Taper/Tracer attachment, Shop made cross slide tracer attachment, VSD.
Location: New Zealand

Re: Backplate thread?

Post by dazz » Tue 12 Mar , 2019 18:41 pm

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.
Regards

Dazz

rpwilson
CNC Expert
CNC Expert
Posts: 125
Joined: Tue 03 Jun , 2014 15:20 pm
Hardware/Software: None. I own a pre CNC Denford 280 Synchro lathe.

Re: Backplate thread?

Post by rpwilson » Wed 13 Mar , 2019 1:08 am

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.

Post Reply