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Chuck back plate material

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GingerLathe24/02/2020 10:59:24
5 forum posts

I have an old 1940's Murad lathe and I need to make a new backplate for a replacement chuck. The spindle is 1 3/8" 10TPI. Unable to find a replacement chuck to fit that spindle, I need to make a new backplate. I've decided to buy a Pratt Burnerd standard accuracy 125mm chuck which is made out of nodular iron.

Can I make the backplate out of steel or does it need to be made out of cast iron?

Thanks for your advice in advance.

KWIL24/02/2020 11:02:17
3165 forum posts
62 photos

Your choice, I have both types in use. Whichever material you have to hand or find easier to work with.

GingerLathe24/02/2020 11:04:49
5 forum posts
Posted by KWIL on 24/02/2020 11:02:17:

Your choice, I have both types in use. Whichever material you have to hand or find easier to work with.

So there isn't a disadvantage with using a steel backplate with a cast iron chuck? My lathe is very low speed.

Thoughts?

ega24/02/2020 11:17:32
1511 forum posts
123 photos

Won't the CI backplate be kinder to the spindle?

Jeff Dayman24/02/2020 11:42:05
1762 forum posts
45 photos

One advantage of cast iron for a backplate is that it will damp vibration better than a steel backplate. As ega mentoned, a CI backplate is less likely to ding a steel spindle or start fretting it than a steel backplate. However in practice you may never have any issues at all with a steel one, with careful handling during install and some grease between plate and spindle.

SillyOldDuffer24/02/2020 12:06:48
5369 forum posts
1090 photos

Nodular cast-iron is three times stronger than ordinary cast-iron, which is good when centrifugal forces are applied by a high-speed lathe. Chucks are sometimes marked with a max speed rating.

Steel is stronger than cast-iron of any type, so a backplate made of it is safer. In theory a cast-iron backplate would damp vibration better, but I doubt the difference is detectable, especially at lowish rpm. So I agree with Jeff and wouldn't go out of my way to get a cast-iron backplate blank if I already had some suitable steel to hand.

Interesting machine - nothing to do with the question, but can you share some photos?

Dave

larry phelan 124/02/2020 12:53:17
606 forum posts
11 photos

I have made two backplates from MS for use on my Craftsman lathe [No kidding ] and they work fine.

I have never worked cast iron, never came across any, and the steel was to hand.

I just make sure to clean the thread on both the spindle and backplate before fitting, little bits of grit or swarf do no good.

So, steel should be OK.

GingerLathe24/02/2020 13:43:27
5 forum posts
Posted by SillyOldDuffer on 24/02/2020 12:06:48:

...

Interesting machine - nothing to do with the question, but can you share some photos?

Dave

Hey Dave,

Link to some pictures.

One as the lathe is now (little cleaning and a replacement tool holder) and some others when I picked it up. At some point the power switch must have broke and was replaced. It also has the badge of the shop in London which originally sold it (Buck & Ryan) which I managed to google some pictures of.

The best place for historical information I've found for any lathe is lathes.co.uk - Link to this lathe on that site: **LINK**

Thanks for the interest!

GingerLathe24/02/2020 13:43:54
5 forum posts

Thanks everyone for your help. Much appreciated!yes

old mart24/02/2020 14:51:21
1251 forum posts
116 photos

If I had a choice, I would always make backplates out of nodular (SG) cast iron. Why, it is easy to machine and doesn't make the mess of dust that ordinary cast iron does. The common blanks available, however, are the messy type, and having a magnet close to the tool tip can catch some of the dust. Steel is perfectly acceptable if your tooling works well with it. Having to make the register and thread without a gauge can be overcome quite easily. If the blank backplate has been faced off flat on the side which the new chuck will mount, and then turned round and clamped to a faceplate, the register can be bored to about 0.001" larger than the spindle register. The thread if Whitworth form will be 0.064" deep, so the starting bore will be 1.247" The thread if UNC form will be 0.065" deep, so the starting bore will be 1.245"

Just touching the threading tool in the bore and then cutting to a finish depth of 0.064", or 0.065" will give you the nominal size. Adding an extra 0.002" on a thread this coarse will not do any harm whatsoever. To check the fit on your spindle, just unscrew the faceplate with the backplate still clamped on and try it out. Should it prove to be too tight, just remount the faceplate, and with such a coarse thread, you can visually check the alignment of the tool in the thread in case it requires a slight adjustment.

Martin Connelly24/02/2020 17:31:44
avatar
1026 forum posts
122 photos

I suspect that the reason cast iron blanks are readily available but steel blanks are not is that a cast iron backplate is the cheapest way to supply material for machining to size. I have made a steel backplate from a piece of plate but didn't machine the back apart from the register area. Just seemed unnecessary for the chuck it was for. A cast blank requires a lot less machining to reduce the weight.

Martin C

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