View Full Version : 3-jaw chuck with mounting holes in back

01-13-2005, 09:50 AM
i need a new chuck for my lathe (11" SB). the crazy spindle thread pretty much means i need to mount a chuck to one of the backplates i have. i'm thinking of just getting a 3-jaw chuck for now, as i'm not at the point where the extra 0.001 of precision a 4-jaw will give me is needed. so, do they sell chucks (6" or 8") with threaded holes in back for mounting to an existing backplate, or do i need to drill and tap to match the plates i have? the reason i ask is i plan on picking one up at Cabin Fever on Saturday. if i need to drill and tap, how difficult is this process since i don't have a rotary table to use on a mill. and yes, i'd like to buy one, but i don't see it happening this weekend.


andy b.

01-13-2005, 10:09 AM
What thread is the spindle?

Paul Alciatore
01-13-2005, 11:27 AM
reallt hard to answer. I'd bring a backplate(s) with me to check.

Paul A.

01-13-2005, 11:40 AM
You can certainly buy a chuck without a backplate, and the chuck will have drilled/tapped holes in it for mounting. Then you'd want to drill your existing backplate to match the mounting holes in the chuck.

The first thing you'd want to do is turn down the o.d. of the backplate to match the recess in the back of the chuck. Aim for a light push fit. Then you need to locate the hole positions in the chuck to drill the matching holes in the backplate. The traditional way I've heard about is to chalk the chuck's surface, put it on the backplate, tap, take it off, and in theory at least enough chalk will have transferred to the backplate so you can see where the holes should be drilled. I've never tried it, but I have my doubts.

I think I'd be inclined to make some pointed screws, screw them into the holes in the chuck so the points are just protruding, put on the backplate, and tap so the points mark the backplate. The only difficulty here may be getting the pointed screws into and out of the holes, since they may be blind holes. If that's the case, maybe you could cut small notches in the end the screw, one each side of the point, and manage to turn the screws in and out by spanning the notches with some needle-nose pliers or similar.

If the mounting holes in the chuck are through-holes, there is no problem, of course.

01-13-2005, 11:46 AM
the spindle thread was only used on the 11" model. i believe it is 1 5/8 x8, but i'd have to verify that when i get home.

andy b.

01-13-2005, 11:58 AM
Depending upon manufacturer, mounting methods will vary. Some mount with through holes in the chuck, the backplate being tapped, some with the chuck body tapped, the backplate being drilled to accomodate. SGW's advice on mounting is good.

South Bend had a couple of less common spindle threads, 1-7/8"-8 and 2-3/8"-6 that are not easy to find. If you have one of these, a threaded backplate may be difficult to locate. If it is the more common 1-1/2"-8 or 2-1/4"-8 mounted chucks should be readily available.

A four jaw chuck is not entirely about accuracy, but is much more versatile than a three jaw in holding odd shaped work and other functions. If the only chuck available is a three jaw, the use of the lathe is pretty much limited to reducing big round stuff to smaller round stuff, or round stuff with holes in it.

01-13-2005, 10:25 PM
<font face="Verdana, Arial" size="2">Originally posted by andy_b:
the spindle thread was only used on the 11" model. i believe it is 1 5/8 x8, but i'd have to verify that when i get home.

andy b.</font>

Yes, the SB 11" sported 1 5/8-8 spindle thread. It'll drive you crazy trying to find a back plate with that thread. I waited almost a year before I lucked out on this one (See Link). I needed it for my 1910? Vintage, 7" swing S. A. Potter bench Lathe.