[How-To] Threading a chuck backplate

I've read many times of 1.5x8tpi chucks fitting other lathes but not South Bends. A good exercise would be to make a copy of the spindle thread to use as a go-gage. Then do as others said, mount the backplate to your 4 jaw facing out, indicate off the internal register (non-threaded portion of the backplate) and then pick up the thread. Do a clean up pass first, check with your gage and then keep going until the gage threads in as far as the register. If the internal register is too small, carefully open it up until it isn't.

It's fiddly work that'll take longer setting up that it will to do, but it's perfectly doable. Making the go-gage will make it a lot more straightforward.
 
mattthemuppet2 said:
I've read many times of 1.5x8tpi chucks fitting other lathes but not South Bends. A good exercise would be to make a copy of the spindle thread to use as a go-gage. Then do as others said, mount the backplate to your 4 jaw facing out, indicate off the internal register (non-threaded portion of the backplate) and then pick up the thread. Do a clean up pass first, check with your gage and then keep going until the gage threads in as far as the register. If the internal register is too small, carefully open it up until it isn't.

It's fiddly work that'll take longer setting up that it will to do, but it's perfectly doable. Making the go-gage will make it a lot more straightforward.
Click to expand...

I agree with Mat’s approach. I’ve just finished 5 backplates with vee threads and the gauge was used not just for checking size but also as a mount for drilling hole patterns. The threaded portion should be a free fit, it’s the unthreaded portion of the spindle that has to be accurate.
Mal


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I agree with making a GO-GAGE for checking your threads. Otherwise, you'll have to unscrew the whole assembly from your lathe, turn it around and test the fit, then remount it every time you want to check the fit. However, it HAS to match your spindle threads fairly closely. Jim F stated that he has no way to measure the Pitch Diameter of the threads. If that is the case, then buy yourself some thread measuring wires and practice measuring thread PDs on some larger threads (it's awkward to do, but it can be easily mastered - and is a good skill to have).

Also, the horizontal region just behind your threads is NOT the register of the spindle. The register is the VERTICAL FLAT at the rear of the spindle. The horizontal region inside your chuck plate just has to be a larger diameter than that on your spindle (0.005 to 0.010" clearance is common here).
 
I think we are on the right track here. If you don't have a chuck to hold the unfinished back plate (chicken before the egg), you can thread it onto the spindle backwards, use that to bore out the ID (don't touch the thread!) and clean the back face. Then flip it over and try threading it on, bet it will work just fine.

@mmcmdl and @SLK001 got it right, the ID is a non precision surface that just needs to be a tight clearance over the register or vertical flat (new term to me). The accuracy comes from the back of the chuck pressing against the shoulder on the spindle and the threads aligning their pitch diameters.

Once you can get it mounted, then you can finish all the surfaces.
 
macardoso said:
@mmcmdl and @SLK001 got it right, the ID is a non precision surface that just needs to be a tight clearance over the register or vertical flat (new term to me). The accuracy comes from the back of the chuck pressing against the shoulder on the spindle and the threads aligning their pitch diameters.
Click to expand...

There seems to be some misunderstanding about the functions of the various surfaces of a threaded spindle: the thread simply tightens the chuck against the annular surface (aka vertical flat). The annular surface provides precise axial positioning but neither the annular surface nor the thread provide accurate concentricity - this come from the precisely machined cylindrical surface on the spindle. The better the fit that the backplate has to this plain part of the spindle the closer the chuck will be to the axis every time you screw the chuck off and on again. You won’t get a turned thread in cast iron accurate enough for the sort of repeat concentricity that a chuck needs. Make the thread a free running fit and the cylindrical surface as accurate as possible. You need clearance for a film of oil, that’s all, so about 10 micron or half a thou’ on diameter.
Mal


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Gonna have to wait till Monday. Fastenal will have me a piece of 1 1/2- 8 all thread.
2A thread, so I will have a gauge.
 
macardoso said:
I think we are on the right track here. If you don't have a chuck to hold the unfinished back plate (chicken before the egg), you can thread it onto the spindle backwards, use that to bore out the ID (don't touch the thread!) and clean the back face. Then flip it over and try threading it on, bet it will work just fine.

@mmcmdl and @SLK001 got it right, the ID is a non precision surface that just needs to be a tight clearance over the register or vertical flat (new term to me). The accuracy comes from the back of the chuck pressing against the shoulder on the spindle and the threads aligning their pitch diameters.

Once you can get it mounted, then you can finish all the surfaces.
Click to expand...
I only need the threads to fit, the chuck side has a slight taper to align it.
 
Any progress ? :) They killed me at work last night so I took tonight and tomorrow night off , no pay .
 
I have a piece of 1 1/2-8 rod coming from Fastenal Mon.
I stopped by a local job shop today, he did not have a 1 1/2-8 tap.
 
Plug gage ?
 
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