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Tony Ennis
01-08-2011, 10:25 PM
A pal gave me a 3-jaw that someone gave him. (Danger Will Robinson! Danger!)

To my surprise, it fit on my spindle though only about 3 threads engage. The backplate didn't meet the register on the spindle, so I made a spacer this afternoon.

When I chucked an aluminum rod, I read about .025" of runout at the chuck jaws and .045" at about 6". I marked the high spots, re-chucked the bar, and re-measured. I did this a few times. The 'high spots' move around. From this, I surmise the rod isn't bent and that the chuck is wonky.

I took the chuck apart, cleaned and oiled it. There was some crud in it but not too much. Afterwards, the chuck felt better mechanically but performed about the same. I noticed that one of the jaws is milled slightly differently - the 'threads' where it engages the scroll are very square while the other two jaws' threads have eased corners.

Is the chuck worn out? Abused? Is there anything to be done?

tdmidget
01-08-2011, 11:08 PM
" The backplate didn't meet the register on the spindle, so I made a spacer this afternoon."
It's the wrong thread.

J Tiers
01-08-2011, 11:23 PM
" The backplate didn't meet the register on the spindle, so I made a spacer this afternoon."
It's the wrong thread.

Maybe........ usually if wrong, only one, maybe two will engage.

If it is a close metric, it might nearly fit like that. An 8 tpi, which is quite common, is only about 7 thou per turn off from a 3mm. I don't know how far that would allow it to go, it may depend on details, as well as which of them is the metric one, the spindle or the chuck.

The other alternative is that the thread spec is different, but same nominal thread.... I have a Logan lathe. I got with it a Southbend faceplate that didn't fit... it went on a couple three threads, just like the OP says.

Turns out the thread fit spec is a bit different between them, and after I scraped it out by hand a little with a threading bit, the faceplate fits fine.

Black_Moons
01-08-2011, 11:49 PM
Eh, not all is lost. You could put it on some crap bearings, add a DC gearmotor with speed control and have an awsome pipe welder rotating setup.

Or to rotate round objects for polishing/sanding with another power tool.

tdmidget
01-08-2011, 11:55 PM
Black moons; LOL

wierdscience
01-09-2011, 12:03 PM
After you get the chuck to screw on all the way-

Pick a pinion,mark it with an X or an O,use that pinion to tighten the chuck down on a steel or aluminum ring held in the jaw teeth.Mount up a Dremel tool or die grinder in the tool post,start the lathe up and grind the jaw contact faces true.

Pop the ring out and clamp a rod in it using the previously marked pinion only(it's now the master pinion)check the run out.Switch to a different diameter rod,chuck it using the MP and check the run out.

If there is less than .001 run out at both settings on the scroll,congrats on a new usable chuck.

BTW,did you look for the MP mark on the chuck the mfg left there?Most have one next to one pinion and not the others and it is the only pinion that will chuck a part with accurate run out <.005" the mark could be an x or an O or just a single hash mark l.

moe1942
01-09-2011, 12:34 PM
Why not check the threads with a screw pitch gage..Could be the threads just need cleaning up to remove a burr or ding..

Rich Carlstedt
01-09-2011, 01:11 PM
[QUOTE=Tony Ennis.................... I noticed that one of the jaws is milled slightly differently - the 'threads' where it engages the scroll are very square while the other two jaws' threads have eased corners.
Is the chuck worn out? Abused? Is there anything to be done?[/QUOTE]

Forget the spindle fit, you have a major issue with the jaws!

The jaw threads need to be square and clean. Rounded edges is not the norm for a decent chuck.
Get new matching jaws, then take on the spindle fit. Without equal jaws , the chuck is garbage
Rich

JCHannum
01-09-2011, 01:47 PM
I see the description of the jaws as cause for concern as well. It is possible they have been abused, or that they are a mis-match. Reversable jaws, which are common in smaller chucks have a rounded or almost oval profile thread while non-reversable are straight and square. It is possible that two are reversable and the third not. In any case, it sounds like there are major problems with the chuck.

It is not uncommon to see chucks mounted on motors for polishing or on as a welding or vertical fixture, but as a lathe chuck, it sounds like a lost cause.

jugs
01-09-2011, 02:28 PM
A pal gave me a 3-jaw that someone gave him. (Danger Will Robinson! Danger!)

To my surprise, it fit on my spindle though only about 3 threads engage. The backplate didn't meet the register on the spindle, so I made a spacer this afternoon.

When I chucked an aluminum rod, I read about .025" of runout at the chuck jaws and .045" at about 6". I marked the high spots, re-chucked the bar, and re-measured. I did this a few times. The 'high spots' move around. From this, I surmise the rod isn't bent and that the chuck is wonky.

I took the chuck apart, cleaned and oiled it. There was some crud in it but not too much. Afterwards, the chuck felt better mechanically but performed about the same. I noticed that one of the jaws is milled slightly differently - the 'threads' where it engages the scroll are very square while the other two jaws' threads have eased corners.

Is the chuck worn out? Abused? Is there anything to be done?

Thats a bad accident waiting to happen to you, :eek:
would you drive your car @ 60mph with only 1 stud holding a wheel on ???

The backplate must meet the the register without spacers, the threads must be fully engaged, it sounds like the thread is either damaged or the wrong size.

If you run it as it is the threads will strip & you'll have a few lbs of iron flying out of the machine at YOU.

If you get out of hospital you'll probably need an expensive rebuild or even a new lathe. Is it realy worth it for a few $$ ??

john
:)

Tony Ennis
01-09-2011, 02:42 PM
Don't worry Jugs, I didn't run it under power.

I checked the threads. Unsurprisingly, they are indeed the wrong size. Once I took the backplate was off the chuck it was a lot more apparent. Thanks for the clue.

I just faced my spare backplate so I should have something suitable before long.

Tony Ennis
01-09-2011, 02:51 PM
The odd jaw doesn't look different than the other two, cosmetically, and the three jaws seem to fit together very well when then jaws are closed tight. I'm hoping it's simply that someone skipped a step when the jaws were being made and didn't ease the edges.