What's new
What's new

SB 10K Machined back plate for 4-jaw 6" chuck - How I did it with some GLOAT!


Oct 8, 2019
Lathe: South Bend 10K with VFD and 12 speed gearing.

I just finished turning a new back plate for my “new old stock!” 4-jaw 6” Skinner chuck.
I used a back plate blank #1919 from “Little Machine Shop”.

Blank was pre-threaded. Threads “look” sort of rough but cleaned and oiled spun onto spindle VERY NICELY.
As expected, the gray casting appears to be on the lower side of quality but not the worst by what I hear.
Looked similar to photo’s others have shared.
No soft spots, no hard spots and a very thin hard shell.
Machined very comfortable with HSS at ~100fpm.
I could vary the RPM with the VFD and hear when the cutting sounded/looked "happier".
The variable RPM was nice (although not necessary) for facing from 6.25" diameter down to 1.5".

Part looked fine when finished.

My steps to do the job:
1. First, I mounted plate on the spindle and did a very light facing ~0.005" off the “chuck side” of the plate.
2. Removed the back plate.
3. Placed a 1 ½ ID x 0.200” spacer ring onto the spindle.
4. Replaced the backing plate on the spindle, chuck side facing the spindle. Normal mount side OUT.
5. Faced and removed about 0.300” from mounting side.
6. Re-bored the thread hole 1.510” x 0.200”to allow spindle mating on spindle side of back plate against spindle bearing surface.
7. Removed, flipped and installed the plate back on the spindle in “normal orientation” chuck side OUT.
8. Turned OD down to 6.00.
9. Re-faced chuck side mating surface (inner 3.5”)
10. Faced plate down to 3.625” and register depth of 0.200 (Mount bolts pass through 3.5” diameter)
11. Did final sizing of register diameter to 3.49~ until slip fit.
12. Re-faced large 6.00” diameter 0.003” to clean up inward to the registration diameter.
13. Test mounted of chuck.
14. Re-moved backing plate and place ON chuck.
15. Transfer punched the 5/16 mounting holes to the back plate.
16. First, I processed ONE hole, drilled and tapped, then remounted back plate to chuck and did the other 3 holes.
17. Did final mounting of backing plate to the chuck.
18. Mounted chuck on lathe spindle.
19. Run out tests: ALL faces less than 0.001”!
20. :D Did a happy dance around the lathe! (Thanked the lord for my GOOD LUCK!)

NOTE: At various process changes as appropriate thoroughly cleaned surfaces and light oiled.
Used cutting oil to tap holes. Worked easy and seemed to make very nice threads. Material seemed similar but maybe softer than aluminum. Made sure to have 0.500” bolt thread engagement hole.

Not perfect instructions I’m sure, but hopefully someone who has not processed a backing plate back will find this helpful. I learned how to do it by watching many various videos and reading lots of documents etc.
Just trying to give back to all of that information “I” learned from!

I know that a 4-jaw independent chuck does not have to be perfect in run-out but…. it does make sense that the rotation of this larger chuck for the light 10K does run potentially smoother.

Added pics:
Last edited:
How much clearance did you give yourself between the plate back counterbore and the spindle unthreaded area?

You said that the counterbore was 1.510", what was the spindle diameter?
1.5 spindle diameter.
Clearance of 1.51 was enough and could be more with no issue.
Depth of counter bore was 0.25+, enough to allow full threading and seating of the back of the plate to the spindle. Check with gauges to verify.