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Lathe Chuck backplate

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Kevin Murphy25/01/2021 21:06:43
3 forum posts

Hi All

I have just joined so thanks for having me, first off I must come clean and say that I am not a model engineer. My interest is riding motorbikes, general messing around with motorbikes and motorbike restoration. I have a Boxford VSL lathe circa 1972/76 which is a sort of a Boxford AUD with a variable speed drive and a L00 taper chuck mount. I put the lathe to good use making bits for my bikes and tend to mostly use stainless of one sort or another.
I am currently making a backplate for a 4 jaw independent chuck, I have machined the receiver and after much measuring, worrying and remeasuring it has come out good. I cut the receiver as close as possible to the same depth as the recess in the chuck which is 2.5mm It is a second hand chuck which came with a back plate for a different lathe that I have removed. On measuring the cut for the receiver on the old back plate it is 3mm in depth. This would obviously give a much more positive fit of the inner part of the backplate to the part of the chuck were the locating bolts fit , but leave a 0.5mm gap between chuck and back plate at the outer edge. My question is what is the correct or best way for a backplate/chuck to fit. I do not feel I see any chucks with a gap between them and the backplate. Thanks for any help

Martin Connelly26/01/2021 09:00:36
1856 forum posts
197 photos

It is usual to have the backplate spigot height less than the chuck recess's depth so that there is no gap visible. Does not need to be a big difference, just enough to close the gap. If there is a radius on the inside of the recess in the chuck then chamfer the edge of the spigot to clear it, not a big chamfer, just enough.

Martin C

Chris Evans 626/01/2021 09:06:29
1952 forum posts

Plus 1 for Martin's comments. Welcome to the forum from another motorcycle tinkerer, my interest is mainly pre-war bikes but I do plenty of work on 50s/60s bikes for friends.

old mart26/01/2021 15:37:51
3317 forum posts
203 photos

As already mentioned, the spigot should be slightly less than the depth of the step, so the outside rear of the chuck contacts the backplate.

Edited By old mart on 26/01/2021 15:38:28

Pete Rimmer26/01/2021 20:57:08
1053 forum posts
69 photos

I have never worried about the register being any thicker than is required to register. Come to think of it, I rarely bother to make the register a snug fit. Perhaps on a 4-jaw but not on a 3-jaw. I like the facility to adjust the part a few thou if required.

My 8" TOS everyday chuck has no register at all.

Kevin Murphy27/01/2021 12:07:51
3 forum posts


Thanks for everyones advice, So obviously the old back plate was incorrect. I will get the job finished off now, I left everything set up in the lathe so will make sure I have a slight clearance between spigot and rear of the chuck recess. Then on to the drilling of the 4 holes for the location bolts, I think I am going to try the grub screws turned to a point method, but will see once I get to it.

Also does anyone know where I can get a Hexagon chuck key for a Pratt chuck, I would say the size is around 17/64

I have looked at quite a few sites but they only seem to have square chuck keys.

Thanks Kevin

not done it yet27/01/2021 13:02:10
6285 forum posts
20 photos

Also does anyone know where I can get a Hexagon chuck key for a Pratt chuck, I would say the size is around 17/64

Allen key?

Edited By not done it yet on 27/01/2021 13:02:58

Chris Evans 627/01/2021 16:27:55
1952 forum posts

Rotagrip in the West Midlands are agents for Pratt Burnerd chucks. If you have no luck PM me an email address and a sketch then I will make you one. Double check the size. 17/64" is very small, a 1/4" allen key would be loose but  7mm should not enter if your size guess is correct.

Edited By Chris Evans 6 on 27/01/2021 16:30:57

old mart27/01/2021 20:29:08
3317 forum posts
203 photos

I made a set of screws for a Pratt 6" four jaw independent, and felt it would be easier to broach a hex in them rather than a square. The key was made from a piece of allen key pressed into a handle with a cross shaft through it.

not done it yet has already mentioned Allen keys.

Oily Rag27/01/2021 22:38:53
464 forum posts
147 photos

Hi Kevin,

Welcome to the forum. Good to have another biker on here, but we are all united in our love of making swarf, scrap, and the occasional useful item!


Howard Lewis28/01/2021 16:23:40
5241 forum posts
13 photos

If you have access to milling facilities (even a vertical slide? ) you could make your own hexagonal key.

At a pinch, take a piece of hexagon bar , (1/4 inch or 7 mm if you can get it ), and gently file a little off each flat in turn until it is a snug fit in the socket screws in the chuck.

Having done that, drill and tap the other end of the bar, before cross drilling it for a bar to act as handle, retained by a grub or capscrew in the tapping..


Kevin Murphy28/01/2021 18:18:10
3 forum posts


Big thanks for your interest and all the helpful advice that has been given, such a good response.. Thanks Chris for your generous offer to make a chuck key if required.
I have now taken a very fine skim off the face of the back plate spigot and now have a good fit with no gap at the edge, so that’s that bit done. The back plate is now out of the lathe and so the drilling of the locating bolts to think about now. I Have had to order the bolts (5/16 UNC) as I need to use socket head bolts because the holes are going to be very close to the sides of L00 mounting boss on the back plate.
Re the chuck key, the size is as suggested between sizes at around 6.5mm. The hexagonal sockets in the chuck are a bit worn, probably from someone using a 1/4 or 6mm Alan key i the past. I will give rotagrip a call and if they do not have a key I think I will give grinding part of a 7mm Alan key to fit then make a T holder to hold the resized part of the Alan key..

It’s all good fun and as you say, and good experience. Plus as the choice of L00 backplates is limited the one I got was quite large so I have made lots of Swarf getting it to size. 😅🙈
I will post an update with finished job hopefully in a few days.

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