How Much Play in 4-jaw Chuck?

MtnBiker

MtnBiker

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A new to me 12" Pratt Burnerd independent 4-jaw just arrived. Looks to be in good cosmetic condition. Noticing some play in the jaws - maybe 1/16" side to side. What is normal? Does the design generally self-correct for slop when tightened?

I'm good with vague answers to my vague question. This is just a sanity check to see if it is worth the trouble of more testing (about 100 lbs of trouble).
 
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I would check for bell mouthing of the jaw faces if there is enough wear to wiggle the jaws side to side
Bell mouthing prevents the full jaw face from gripping the workpiece
Mount a round workpiece and check for light showing thru at the very outer portion of the jaw
Ideally there should be full contact out to the jaw tips
 
@MtnBiker My Pratt Bernard has almost no play side to side, but what would worry me is law tilt. it the jaw tilts by hand, someone has put the wrong jas into the chuck, they are made for that model, but not fitted to the chuck.

PB chucks are all individually fitted to their chuck. You can buy replacements for a chuck, but you have to fit them yourself. One set of jaws is about the price of an excellent used Bison chuck.
 
Dabbler said:
@MtnBiker My Pratt Bernard has almost no play side to side, but what would worry me is law tilt. it the jaw tilts by hand, someone has put the wrong jas into the chuck, they are made for that model, but not fitted to the chuck.

PB chucks are all individually fitted to their chuck. You can buy replacements for a chuck, but you have to fit them yourself. One set of jaws is about the price of an excellent used Bison chuck.
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As luck would have it, assessed the jaw with the most play and assumed they were all that way. Measured jaw lift is (inch): .0045, .003, .0025, .001

Looks like jaws were not installed in the right slots (I'll check once mounted). Guessing the lift across all the jaws would be in the .0025 range if I reversed the first and last jaws above.

Horrible? Good enough? Not trying to hold tenths on finished parts but don't want my 4-jaw to be less accurate than a decent 3-jaw scroll chuck.
 
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Got a reply from the Pratt Burnerd tech as follows:

"We discontinued these chucks in 1987. .003” of lift is usually where I draw the line and scrap them. The jaws are hardened so if you have any lift it’s in the chuck body which is not a replacement part."

So the "great condition" chuck is right at end of life/scrap. This doesn't make me happy.
 
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PB is a great name for great chucks. If not abused, they last forever. Yours was abused and not maintained.

Mine are both 1980 vintage, and are as tight and accurate as the day they shipped from the factory. But the guy that used them kept them lubed and didn't over tighten them.

Failing to lube your chuck and over tightening are the fastest ways to ruin a good chuck.
 
Dabbler said:
PB is a great name for great chucks. If not abused, they last forever. Yours was abused and not maintained.

Mine are both 1980 vintage, and are as tight and accurate as the day they shipped from the factory. But the guy that used them kept them lubed and didn't over tighten them.

Failing to lube your chuck and over tightening are the fastest ways to ruin a good chuck.
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The seller is willing to refund. Hopefully this is educational for folks in the market for a used chuck.

Update: chuck re-crated and shipped back. An expensive lesson given shipping expenses (seller covered 1/2 of return shipping).
 
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