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View Full Version : Metal Lathes New chuck for my SB 9A but with problems



Josh Bowman
10-17-2012, 8:01 PM
I just got in a Shars 6" 2 piece jaw, 3 jaw chuck with back plate, for my SB9A. I mounted it up and chucked up an arbor and found it to be over .015" tro. I may have isolated some of the problem to the back plate. It's got a .007" warp on the face and about the same on the edge. I've contacted Shars, but am waiting for a reply. I was wondering if I should turn the back plate true, or just let them send me another if they will. I appreciate any ideas here. The best I can tell, my spindle is <.0005" tro. It's hard to check it with the threads, but with the stock faceplate, that's what I got.

Adam Neat
10-17-2012, 8:44 PM
I would double check to make sure there isnt a burr or ding on the face (where it mounts) throwing it off. If all thats in good shape I personally would just turn it true.

Did you get soft jaws with your chuck? You may be aware of this but you can chuck up a short piece of material and turn the jaws. Then when you chuck up a part it will be dead on

Scott T Smith
10-17-2012, 9:01 PM
Ditto Adam's recommendations. Truing up a backplate is common practice for older metal lathes.

George Carlson
10-18-2012, 11:40 AM
If this is a new lathe to you, be sure to check the spindle. Like Adam said, check for obvious problems with the spindle nose, then check the shoulder and the smooth round area just behind the threads for runout and wobble. I've seen it on SB lathes before. People jam a chuck on the spindle. To remove the chuck, they put a piece of wood in the chuck and reverse the lathe. This is OK on the big machines, but it can bend the spindle on the little machines. If the spindle is OK, then mount the backplate and turn it true.

Josh Bowman
10-18-2012, 7:02 PM
Ok, here's where I'm at. Shars got back to me and wants me to send the chuck and back plate back at their expense to replace it. I need some info though to see it this is worthwhile. I disassembled the chuck and checked and cleaned, really didn't find much except oil (not grease) on the scroll. I reassembled it and put it back on the face plate. At my spindle with nothing on it I get maybe .00025" tro. With the faceplate and at the largest point (where it attaches to the chuck) I get .007" tro. If I chuck a 1/2" mandle or 1" aluminum bar or 3" aluminum bar, I get .025" tro at 5 inches from the backplate (where I measured .007" tro). This makes my brain hurt, but does that make sense to rise like that, so ONLY the back plate is the problem? It seems to make since and I want to try to make a deal with Shars to let me turn the backplate and save us all a lot of hassle.

Adam Neat
10-18-2012, 8:31 PM
Honesty you may be fussing too much over the run out. Many 3 jaw chucks and especially those in the "Shars" price range will never run dead true just chucking up. With your removable jaws you can turn the jaws of course.

With the 3 jaws and the scroll you may even experience different amounts of run out at different diameters, also your stock may or may not be dead true.

Best way to zero runout and what most folks do when they worry about that stuff is put on the 4 jaw chuck and indicate the part in.

When Im going to just turn down the diameter and part something off, a 3 jaw chuck is fine

Josh Bowman
10-18-2012, 9:13 PM
Ok, I hiked up my britches and trued up the backplates surface where the chuck seats, I reduced the size of the center boss slightly and after some tapping with a hammer the TRO is about .003 from a 1/2" mandrel out to a 3.5" round stock.
Darn wished I had greased the chuck before I remounted it, guess I'll get more practice tweaking it again. Frankly I didn't trust that the jaws were right, since I found one loose and was afraid the factory may have ground it like that, but they seem fine.
Thanks all for the help.

George Carlson
10-19-2012, 7:12 PM
I would not be suprised if a new chuck had a piece of swarf in the scroll. You might want to pull the chuck off, disassemble it, and clean the scroll and master jaws real well. 3-jaw chucks will have runout that varies with the chucking diameter. The reason for the 2-pice jaws is to allow mounting soft jaws. The soft jaws can machined to hold specific parts and be dead on. In the home shop, soft jaw are not very common since we tend to make "one offs" not production runs.