Matching mystery Jaws to chuck

Evening all,

I've been fortunate enough to pick up a restoration project ML7 which is now fully up and running. I'm now at the point of sorting out the tooling and workholding for it. I have a 4 jaw chuck that needs a major strip as it is heavily rusted, but have already stripped, cleaned and rebuilt two 3 jaw chucks. One is a Pratt Burnerd International, the other is a Crown of some description. Jumbled in various boxes I have assembled 4 jaw sets. However...

Some of the sets don't result in a centred grip (several mm off).

Some of the sets result in visible runout on the workpiece.

Is there an easy way to identify what Jaws go with these chucks (if any)? They are matched triplets in terms of stamped numbers and 1, 2,3 sequence.

I'm debating just rigging up a grinder, loading the Jaws and grinding them concentric, but I'm wary that if the Jaws aren't matched to the chuck I could end up with slop issues.

Any and all guidance hugely welcomed! Thank you.

Hi Rick,

Welcome to the forum. On my self-centering 3-jaws the chucks have a number stamped for each jaw slot and each jaw is stamped as you describe and of course, go into the slot with matching number. I assume you have already cheked this thread?

Thor

Hi Thor, thanks for the welcome.

Yes I'm ok with that side of things, this is whether I can identify if mystery sets are suitable for the chuck I have or not. They engage with the scrolls ok and fit the slots.

Chuck may not be numbered - depends on quality and maybe previous owners.

The most important thing is to fit those jaws in their numbered order.

I bought a chuck with mismatched jaws for little money at the Doncaster show. It was a brand new chuck which had been returned faulty but with a bit of proper grinding of inside and outside and steps of both jaw setts it is perfect within a thou. I originally ran out by at least 20 thou.

https://youtu.be/2VKR45MYVEw

https://youtu.be/6fy5AlDiV0o

David

Welcome to the forum.

I don't know of a way of identifying which unknown jaws go with a chuck other than trying them.

Mention of 'several mm off', reminded me of a mistake I sometimes make, which is loading the jaws in the wrong order, or not engaging the scroll as expected at the beginning. To avoid this, I identify slot 1 and jaw 1, and, without inserting the jaw, rotate the scroll until the leading thread just appears in the slot. Then the jaw is inserted, the scroll turned to engage, and do the same with slot2/jaw2 and slot3/jaw3.

Dave

3-jaw self centreing chucks normally come with matched sets of jaws. Internal and external. and have to be assembled in the correct order The Pratt Burnerd (not sure about the Crown chuck) should have numbers 1-3 on the slots and the jaws should also be marked. If they are are not the correct set of jaws and are not exactly on centre you will have to grind them. You will have to make a special spider to hold the jaws while they are ground. Do a google search for how to do this.

Paul sums it up very well.

The jaws that do not meet in the centre, when inserted in the correct order, may be because one of the jaw has a tooth missing from the back.

The technique here is to give the scroll an extra turn before engaging that particular jaw. In this way, the damaged jaw will will engage at the correct time for the others. Hopefully, all three will all then meet in the centre.

Howard

Thanks all. I've read up on grinding jaws but wanted to check if I was heading down the right path before removing material!

I do already do the "turn until start of scroll is visible then turn back" approach, but hadn't even crossed my mind that an entire tooth could be missing from a jaw - I'll check, thank you.