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Casting a Lathe Chuck Backing Plate

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tom osselton:

Thanks Russel, John, Tom!  :beer:  :beer:  :beer:

I bored the through hole and turned the back registers and drilled for the spindle mounting bolts. That was made easier because when I'd built the lathe, I made a gauge for the spindle mount. It definitely came in handy this time around. The one face fits the spindle, the other fits the fixture to be attached, and bolt hole patterns for 3 and 4 bolt fastenings are located.

I tapped the backing plate then bolted it to the spindle. I put a final light truing cut on the face, then I turned the front register to fit the new 6" 4-jaw chuck. I will mount this with hex head cap screws, rather than Allen screws -- much easier to remove than those on the import mini-lathes, as there's room on my lathe to swing an open-end wrench behind the spindle bolts, and the backing plate bolts.

I need to  drill the backing plate for the chuck mount. I think I'll also drill holes for my 5" 3-jaw chuck, as they can share the same backing plate. The 3-jaw won't have a register, because I just bolt it up lightly, and then tap it into center using an indicator on a bit of shaft held in the chuck. Then tighten the mounting bolts. No runout due to mounting. Worked great when it was mounted on the faceplate, and I'm sticking with that method.

The 4-jaw does have a register, just for convenience in mounting. Obviously you can true up any gripped workpiece in an independent 4-jaw chuck using the chuck jaws.

Plug gauge back and front, and the backing plate ready to drill for the chuck(s):

Okay, end of the day, how to locate hole centers in backplate to fit the chuck? I don't have any transfer screws, and didn't want to make some -- and especially not for M10, since they'd probably never see use again, and besides, my lathe isn't functioning at this stage to make them.

You could measure out the locations, using a square (there are 4 bolts), if you had a center point to work from, but the center of the backing plate was bored out. I could possibly stuff a piece of wood in there, saw off flush and find a center there to work from.

Or you could put it on a mill and drill there -- probably the smart way to do it. I don't have rotary table, so I'd have to do it by coordinates, no DRO either so I'd have to read dials remembering backlash, etc. But also my mill is in a really cold space right now, and the setup would be like minor torture.

So, I decided on another way. I found a piece of stiff cardstock and laid it on top of the chuck. Then poked a pencil through it to locate a threaded hole. I put one of the M10 bolts through that hole and screwed it in a short way. Did the same for all the other mounting bolts.

Then I took a small hammer and tapped all around the chuck back face outline. This is the same way you make custom fiber gaskets. The light tapping cuts the paper to the shape of the surface it is lying on.

The finished paper template lying on the chuck:

Here is the template on the backing plate. It fits over the register. I taped it down and the used a transfer punch of the same size as the holes to line up and punch the hole locations. Though a transfer punch doesn't bear against paper like it does in a hole, it is useful as a visual guide to center the punch.

This isn't exemplary precision machine layout, but it is as likely to be as accurate as doing it with a square and measurements from a wooden center -- in my hands at least!  :zap: And the register is what really counts in mounting the chuck, as long as the bolts pass though the plate in line with the tapped holes.


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