# Machining a backplate



## Paden_v (Jul 13, 2017)

I have a Grizzly G9972Z Lathe I picked up last year for 400 dollars no tailstock is the only issue with it. Well I'm getting into machining more things all just practice and learning and what not. For my end goals I know I'll need a new and bigger chuck, got a 6 inch 4 jaw from someone but I need the backplate or faceplate or whatever you want to call it, I got one with the proper thread pitch but obviously need to do some final machining. The recess in the back needs a bit taken out so the shoulder fits on my spindle and the registration boss needs to have a little taken out. Mostly my question is how snug should all of this be. I have pretty good ideas but I like to have as much information as possible and from reliable sources (you guys) also any suggestions or the process you use would be appreciated I know there's more than one way to skin a cat and love to learn the different processes.


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## Technical Ted (Jul 13, 2017)

There may be specs available on-line so I would check that out. For the couple of chucks I've mounted, for the fit between the backing plate and the chuck, I shoot to make them the same size with a maximum of -.001 smaller where the plate inserts into the chuck. I like a nice tight fit, drawing the plate into the chuck with the bolts. A 4-jaw chuck is not as critical as a 3-jaw since you will be adjusting the jaws for concentricity anyways... for a 3-jaw I would want the fit nice and tight. 

YMMV,
Ted


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## ddickey (Jul 13, 2017)

I read .0004" is what you want to shoot for. You also have to make sure the height of the register is ~ .020" shorter than the back of the chuck recess.
The backplate needs to be as flat as possible as it will obviously affect chuck accuracy. I think Shars has some nice instructions. Also, make sure you cut a small amount of relief on the outside of your register for a good fit.
This is just what I understand from my own studying. 
I have very little practical experience but will be attempting another soon.


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## Paul in OKC (Jul 13, 2017)

.002 max should work fine. I have found anything less than .001 is hard to assemble. You need a half thou for any kind of slip fit. Unless you are making rocket parts..........


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## RJSakowski (Jul 13, 2017)

For Grizzly lathes, you can order a back plate as a replacement part.  You will still have to machine the chuck - back plate interface but the back plate - spindle interface is ready to go.


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## Paden_v (Jul 16, 2017)

RJSakowski said:


> For Grizzly lathes, you can order a back plate as a replacement part.  You will still have to machine the chuck - back plate interface but the back plate - spindle interface is ready to go.


Grizzly didn't have any backplates for my particular lathe but I got mine from littlemachineshop and got everything machined down with about a .002ish tolerance just need to get my transfer screws in to drill the holes through the backplate and I'm in business.


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## RJSakowski (Jul 17, 2017)

Paden_v said:


> Grizzly didn't have any backplates for my particular lathe but I got mine from littlemachineshop and got everything machined down with about a .002ish tolerance just need to get my transfer screws in to drill the holes through the backplate and I'm in business.


Did you go to the replacement parts list for your lathe?  The 1-3/4"-8 tpi thread on the 602 spindle is not a common spindle thread.   I had done quite a bit of searching for a plate for mounting a 5C collet chuck with no success.  I got the original lead from Fabrikator in one of his posts and it saved me a bunch of extra work as well as a few coins.

Grizzly didn't list a back plate in their catalog but it was in stock as a replacement part. I haven't checked parts lists for other Grizzly models.  I assumed that they would offer similar replacement parts.

It was actually cheaper than buying a blank plate and saved me boring and threading the spindle mount.

At any rate, you have found a plate so you're good.


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## benmychree (Jul 17, 2017)

I have machined backplates for a lot of lathes that I have owned over the years; although the following is not necessary for this discussion, if we're talking rough backplates, first of all, I make a thread plug gage to represent the spindle thread; generally, I make them from a solid piece of steel, one end turned down for a handle and chucking for threading, and knurled.  A large plug gage can be made by welding a handle on for chucking and handling in use. I like to have a thread mike to fit the thread exactly the same size as the spindle, but of course, thread wires will also work.  Next I'd rough out the casting all over, then finish bore, recess, and thread the ID and the face that will seat against the shoulder on the spindle.  I agree, the recess that fits the large diameter of the spindle should be about .002 or a little less,loose on the spindle; I have found over many years that the thread centers up the backplate and chuck, I have seen the recess incredibly loose, but the plate still centers up nicely. I then screw the finished thread on the spindle, making sure to thoroughly oil the threads (I have seen a galling with clean threads) I then finish face the chuck mounting surface and finish turn the OD where the chuck fits.  A metal to metal fit is OK, but up to .0005" interference is OK as well.  By this time with the cutting forces in play, frequently it is hard to unscrew the backplate, to do this, I change the chuck jaws to the set for holding large diameters and "chuck the backplate" and take a piece of wood that fits between one chuck jaw and the lathe bed and back the spindle, just as one would do to remove a chuck.  Of course, then come the transfer screws if the chuck does not have through holes for fastening from the front and tapping the backplate.


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