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Myford chuck not gripping

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Ro11/07/2020 16:29:31
31 forum posts
35 photos
Posted by JasonB on 11/07/2020 16:15:27:

I'd have thought you would have ended up with better being the jaws were trued in place.

yeah, to be fair, I was hoping for a bit better. At some diameters I’m getting a couple of tenths, and other diameters a bit more, with the worst being about 3 thou.

I’m blaming it on the scroll, which wasn’t in the best of conditions (and also maybe my file work when trying to repair it)

half a thou sounds fantastic. Have you got any recommendations if I decide to invest in a new chuck at some point?

JasonB11/07/2020 16:56:43
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Scroll could be worn/deformed and it is only true at one position per full rotation, probably previous owners giving to too much wellie.

Mine is a Bison 3274 picked up at a good price but about the same as a Pratt

SillyOldDuffer11/07/2020 18:09:22
5932 forum posts
1282 photos

Well done Ro, 3 thou isn't too bad for a 3-jaw. Run-out is mostly a pain when work has to taken in and out of the chuck because each time the job takes up a slightly different position and the turnings aren't concentric. Cutting stays concentric when the job is only set in the chuck once, in which case a bit of 3-jaw run-out probably won't matter.

The main advantage of 3-jaws is speed and convenience gripping round objects, as a breed they're not hot on accuracy. If low run out is essential, switch to an independent 4-jaw because these can be set up very accurately with a DTI. Their disadvantage is the extra time taken to set them up, though some use nothing else and say - with practice - they can be as quick as a 3 jaw. They're also required for holding odd shapes. Collets are good for speed and accuracy, their disadvantage is cost!

Just as a matter of interest, how exactly do you chaps measure run-out? My Chinese certificate calls for a 20mm diameter test bar and the run-out to be measured 50mm out from the jaws. This, I think, is a rather tougher test than finding run-out as close to the jaws as the DTI can get. Rude things said about Chinese certificates, but my chuck was about 0.01mm better than the 0.05mm @ 50mm claimed for it. Not quite so good now unless I fuss with it on the register. Wear and tear on an inexpensive item I expect.

Dave

Hopper12/07/2020 02:42:18
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4654 forum posts
101 photos
Posted by Ro on 11/07/2020 15:26:30:...

...I made up a quick top hat and trued the jaws with a carbide boring bar and the top hat held in the back of the jaws (then reversed the top hat to the front and did the same for the very rear of them).

The jaws now grip fully along the length and don’t slip at all!

I was slightly worried about the work on the scroll affecting the runout, but after testing at multiple diameters I seem to be getting a maximum of about 3 thou.

 

Possibly because when you held the top hat ring in the rear of the jaws and tightened the jaws inwards, the front tips of the jaws were pushed inwards, the opposite of the way they are pushed when in use holding a piece of round bar etc.

That is why the clover leaf plate I posted above works better. It pushes the front tips of the jaws outwards while they are being machined, replicating the position they are in during normal operation.

If there is a bit of slack between the jaws and the slots in the chuck body they run in, it can make quite a difference.

I got the chuck on my 1937 lathe to run within one thou using this method.

Edited By Hopper on 12/07/2020 02:43:19

Ro12/07/2020 07:40:44
31 forum posts
35 photos
Posted by Hopper on 12/07/2020 02:42

Possibly because when you held the top hat ring in the rear of the jaws and tightened the jaws inwards, the front tips of the jaws were pushed inwards, the opposite of the way they are pushed when in use holding a piece of round bar etc.

That is why the clover leaf plate I posted above works better. It pushes the front tips of the jaws outwards while they are being machined, replicating the position they are in during normal operation.

If there is a bit of slack between the jaws and the slots in the chuck body they run in, it can make quite a difference.

I got the chuck on my 1937 lathe to run within one thou using this method.

Yep, I understand what you’re saying, and can see how the clover leaf could be a better solution.

However, in my scrap box I had the material to make up a top hat, and didn’t have the material for a clover leaf.

When I find myself with a scrap piece of plate large enough and some spare time I’ll give it a go.

Hopper12/07/2020 08:20:43
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4654 forum posts
101 photos

You can actually substitute 3 pieces of flat bar maybe 1 inch by 1/8 . Cut the hacksaw slot in each after drilling the 1/4" hole to give a bit of spring to allow for slight dimensional variation between jaws. Bit more fiddly to get all 3 in place but ok if chuck is laid flat on the bench. Much easier to make than the full plate though.

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