5c collet - serious run out..

[pmurdock]

ok.. so I'm not the sharpest tool in the shed. I've got a piece that is 0.940" in diameter.. I just grabbed the closest collet to that size (a 61/64") and turn the piece. Beautiful.. nicely cut no run out.. I take it out to look at it and then place it back in the collet. Bam.. the piece has runout 0.008" TIR..

Is it just me or is a standard 5C collet at 1/64" spacing just not going to cut it for this particular diameter??? does that mean that I have no other choice but to use an emergency collet or buy a hardinge collet custom ground to .940"??

As I understand it the work piece should never be larger than the collet size and should never be smaller than 0.010" of an inch.

This would mean my 0.940" piece will not work in 61/64 (0.9531") which is ~ 0.013" smaller than the collet which means I risk damaging the collet and certainly can expect problems with runout..

am I crazy or is this how it works??

cheers!

[One of Many]

You are no crazier than anyone else that discovers this about 5C collets. They only remain parallel at their machined size. An E-collet would be the cheapest solution.

I'll usually just tap the part true with a plastic mallet while indicating it in when it is critical. Either that or I'll finish turn between centers.

For best results, machine the part complete before removing it. Secondary operations will always have these conditions to some degree using stock collets for off nominal grip diameters. Some of that depends on the quality too, but isn't all that reliable to just grip and go. If you are expecting TIR on minimum runout, due diligence is in your hands until the setup proves otherwise. The skill here is to be prepared for these anomolies......and improve your chances for success on the fly!

It's a hurdle, not a road block?

DC

[pmurdock]

Originally Posted by One of Many You are no crazier than anyone else that discovers this about 5C collets. They only remain parallel at their machined size. An E-collet would be the cheapest solution. I'll usually just tap the part true with a plastic mallet while indicating it in when it is critical. Either that or I'll finish turn between centers. ... It's a hurdle, not a road block? DC

So if I tap it in then it could easily revert back to the original position during cutting or do you find that that is usually not the case?

cheers,
Paul

[tobyaxis]

Originally Posted by pmurdock ok.. so I'm not the sharpest tool in the shed. I've got a piece that is 0.940" in diameter.. I just grabbed the closest collet to that size (a 61/64") and turn the piece. Beautiful.. nicely cut no run out.. I take it out to look at it and then place it back in the collet. Bam.. the piece has runout 0.008" TIR.. Is it just me or is a standard 5C collet at 1/64" spacing just not going to cut it for this particular diameter??? does that mean that I have no other choice but to use an emergency collet or buy a hardinge collet custom ground to .940"?? As I understand it the work piece should never be larger than the collet size and should never be smaller than 0.010" of an inch. This would mean my 0.940" piece will not work in 61/64 (0.9531") which is ~ 0.013" smaller than the collet which means I risk damaging the collet and certainly can expect problems with runout.. am I crazy or is this how it works?? cheers!

Usually I machine an E-Collet for Off Size Diameters. It adds an extra operation but your parts will have a matched perpendicularity and concentricity. The only question to ask is " do the tolerances on the print, or the application intended require it".

As an additional note for the future 5C collets as I have been told are only accurate within +-.002 of their marked diameter.

[pmurdock]

Originally Posted by pmurdock So if I tap it in then it could easily revert back to the original position during cutting or do you find that that is usually not the case? cheers, Paul

Just as an update on this.. I used a dead-blow nylon hammer to tap it into place after putting an indicator on it and was able to dial it right in with the hammer. I then ran some operations and used the indicator afterwards and it held my corrected position..

thanks for the tips! I'll certainly pick up a few Emergency collets to have on hand in the future for just such applications!

cheers!
Paul

[One of Many]

Originally Posted by pmurdock So if I tap it in then it could easily revert back to the original position during cutting or do you find that that is usually not the case? cheers, Paul

Only if you are a bit lucky. There are a few variables in how the part is grasped in one operation to the next. I often refer to this as nesting error. In a flat plate with 3 flat sides, it should repeat its placement reasonably well.

In taper collet work holding, controlling the part in 3 planes becomes an issue of roundness, surface contact and parallelism of the mating surfaces. Variations in those relationships work against you. Of those variables, the least reliable is part alignment repeatability over the parts length or face perpendiularity to its centerline. In other words, the part may have acceptable TIR at the collet nose, but 2 -4" out it gets worse. As you have found, you can tap the part back on center while checking its runout with an indicator, but the chance of getting it back to the original grip alignment is unlikely.

DC