Fitting a lathe backplate to a chuck

[Alcap]

I bought a Phase 2 10” rotary table last year and it came with a 8” 4 jaw Bison plain back chuck in like new condition . My lathe is a Clausing 5900 lathe with L-00 spindle . I bought a 8” semi-finished backplate ( by the way will need truing to the taper before trying to mate to the chuck ). My question is what chuck’s flat surfaces should touch the backplate and which would be for clearance ? I’m linking a site that shows a section view . The smaller diam. has the mounting bolts so is that what is supposed to make contact or the outer edge ? https://www.smalltools.com/bison-8-4-jaw-independent-manual-chuck-plain-back-7-850-0800/ Or try to make the step as close as possible ?

 

[Ed ke6bnl]

I made one for my loo 5C hardinge speed chuck. But bolt holes where on the outer portion and I made the center of the Chuck.. O05 smaller to tap the Chuck to the least runout and mine is center and it is right on. I am no machinist but where the bolts go is the mating surface. Machinest can correct me. I stand corrected, no need for slack for a 4 jaw was referring to 3 jaw and in my case a 5C holder

 

[machPete99]

I vote for the smaller diameter ("D") and turn the backplate OD to fit losely within the outer recess.

 

[mmcmdl]

Nice chuck , same one I have on the 5917 . Mine is registered and trued to the larger diameter and c bore .

 

[Alcap]

mmcmdl , so should I make the backplate register on diam. “ C” but keep it a few thousands from bottoming where the bolts go through ?

 

[benmychree]

Chucks should fit with no clearance on the diameter fit unless they are the AdjustTru type. Normally the fit should be on dimension "D" I fail to see why one would want to make the fit loosely on a 4 jaw chuck, there is perhaps a case to be made with a 3 jaw chuck, absolutely not on a 4 jaw.

 

[Alcap]

The question is for where the flat surfaces touch , sorry for the confusion

 

[benmychree]

Both the diameter D and the front of the backplate and the bolt up surface of the chuck should make contact without clearance. you would turn the backplate to diameter D and face it off to true up the semi finished face.

 

[Jimsehr]

I think it’s ok to have a bit of slop with a 4 jaw chuck but with a 3 jaw I think a close fit is best. If you have an undersized dia you can fix it by smacking the edge with a hammer about every 45 degrees around the edge of turned dia which will swell the dia enough to return to slip fit size.
Jim

 

[Alcap]

Here’s the backside of the chuck , the arrow will be the diameter I’ll use for concentric ,the outer flat and where the mounting bolts go through noted by black marks will seat it . Before machining I want to check the bearing clearance seem to have slight play when a indicator was on the chuck and moved some .It seems to run smoothly but want to check anyway. The backplate will need the taper trued ,its only touching on the inside with about .012 feeler gage gape .