Sorry guys, backplate again.

[snoopdog]

I can keep myself busy for hours using the search function, believe me. But anyway, shown is my 4 jaw that I'm fixing to work on, and I need to get to it for an upcoming project. First, the rear register only allows for 3/4 of an inch threaded engagement, at 8tpi. Is that enough? This is the same as my smaller 3 jaw, but .200 less as the larger 3 jaw that I just finished. Second, there's a. 030 deep machined surface between the outside edge and the bolt circle, is that critical? , with the forward facing register being the actual resting surface, seemingly. Thanks in advance, Charlie.

 

[mattthemuppet2]

you can make the threaded part of the backplate as thick as you want too, there shouldn't be anything that restricts the thread engagement length.

If it were me, I would mount up the backplate on the lathe and turn down the face to a boss that fits the first recess (between the chuck bore and bolt circle). Then leave the rest of the backplate flat. No need to worry out the slight recess of the back of the chuck where the webbed part is. Then transfer punch the bolt holes, drill and tap and you're done.

 

[snoopdog]

you can make the threaded part of the backplate as thick as you want too, there shouldn't be anything that restricts the thread engagement length.

If it were me, I would mount up the backplate on the lathe and turn down the face to a boss that fits the first recess (between the chuck bore and bolt circle). Then leave the rest of the backplate flat. No need to worry out the slight recess of the back of the chuck where the webbed part is. Then transfer punch the bolt holes, drill and tap and you're done.

So, do you think 3/4in is enough? If I turn the back face down, I'm only going to gain 1-2 threads, is it worth the effort? Your other thoughts were on par with mine.

 

[mattthemuppet2]

Oh, I get you, the unthreaded part of the backplate hole is too deep. That's easy. Turn and face a ring the same thickness of the unthreaded part of your spindle, maybe a little thicker. Then thread the back plate on backwards (boss facing towards the tail stock) and turn down the face of the boss so the unthreaded part is slightly deeper than the same part on your spindle.

Then take the backplate off, turn it around and do all the other stuff.

 

[snoopdog]

Oh, I get you, the unthreaded part of the backplate hole is too deep. That's easy. Turn and face a ring the same thickness of the unthreaded part of your spindle, maybe a little thicker. Then thread the back plate on backwards (boss facing towards the tail stock) and turn down the face of the boss so the unthreaded part is slightly deeper than the same part on your spindle.

Then take the backplate off, turn it around and do all the other stuff.

Thanks again

 

[Dabbler]

I have to do the same on my 12X37 lathe. The screw on spindle has about .400 of unthreaded shaft to the shoulder. This part has to be a very tight fit, and is responsibel for your radial alignment (at least in my case). Axial alignment is provided by a larger surface closer to the spindle bearings. (which the end of your ruler is touching)

 

[DAT510]

If I'm understanding correctly, you are thinking about shortening the "un-threaded" section?

I've alway understood that portion was supposed to be machined to a close fit, as it helps insure your chuck registers/indexes the same each it is removed and replaced.

When I did mine, I bored the "un-threaded" portion to be a close fit to the spindle, and I made sure the rear face of the backing plate seated against the face of the spindle. I basically matched how the OEM backing plate was machined.

Hope this helped?

 

[mattthemuppet2]

The unthreaded section of the backplate doesn't have to be a tight fit. It's used to ease the threading on of the chuck, not any alignment. Alignment is handled by the threads and the face of the spindle.

Generally speaking you want the unthreaded section to be a little larger in ID than the OD of the thread crests, but not much larger than that. You can make it a very tight fit (I did that once) but it'll be a bear to thread on and off

 

[markba633csi]

I made mine about 1 thou clearance. And I also had to make a spacer ring, thread the backplate on backwards and machine the boss down some to get
more thread engagement.
I think the couple extra threads are worth it- takes time to do it right

 

[Shotgun]

Whatever you do, be careful not to remove TOO much of the back, such that you run out of threads before the chuck is firmly seated against the spindle face.

Had Some Luck with the Stuck Chuck

I don't think this back plate has ever been off of this 1942 Sebastian lathe. Removing it required EXEME measures.

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