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Dealing with ovality.

SmlG54

Aluminum
Joined
Apr 5, 2018
Nuts and bolts:

4140 6" OD, Wall thickness .2", 2.75" OAL
Issue: after part is taken out of chuck and destresses, parts warps and suffers from small ovality.
4 different od readings give +- .002 part has tolerance of +- .001

OD is critical and be read differently at the moments:
1. When OD is finished and wall thickness is 1" (before ID work)
2. When OD is finished and wall thickness is .2" (after ID work)
3. """" and taken out of chuck.

Which of these measurements do yall think is the one use?

Thank you
SML
 
Nuts and bolts:

4140 6" OD, Wall thickness .2", 2.75" OAL
Issue: after part is taken out of chuck and destresses, parts warps and suffers from small ovality.
4 different od readings give +- .002 part has tolerance of +- .001

OD is critical and be read differently at the moments:
1. When OD is finished and wall thickness is 1" (before ID work)
2. When OD is finished and wall thickness is .2" (after ID work)
3. """" and taken out of chuck.

Which of these measurements do yall think is the one use?

Thank you
SML

.
most parts warp its just a matter of how much. its one of those things that programmers often dont appreciate the severity.
 
OD is critical and be read differently at the moments:
1. When OD is finished and wall thickness is 1" (before ID work)
2. When OD is finished and wall thickness is .2" (after ID work)
3. """" and taken out of chuck.

Which of these measurements do yall think is the one use?

Thank you
SML



Seriously???

Too bad Mr Customer.. I know it wasn't correct when I shipped it, but it was right
at one point in time...

That'll go over like a lead balloon.

Sucks when it WAS right, and NOW its not right, but that's part of the game.....
Occasionally, if you know what its used for, and how its going together etc,
you can get away with it (sometimes even with your customer's blessings), but
sometimes you can't, and you've got to figure it out and do it right.
 
OD is critical and be read differently at the moments:
1. When OD is finished and wall thickness is 1" (before ID work)
2. When OD is finished and wall thickness is .2" (after ID work)
3. """" and taken out of chuck.

Which of these measurements do yall think is the one use?

Thank you
SML

#3, if your customer is going to verify the parts to print.

Is the material springing out of round due to internal stress, or clamping pressure, or both???

Figure out which of these is your culprit, and affect the needed change.
It could be simply stress relieving the material prior to, or during manufacturing.
Or it could be clamping pressure @ 120° (assuming 3 jaw chuck)
The solution is to use bored, soft jaws.
 
I am confused. The finished part is out of print from what I understand. You either call the customer to see if they are still usable or throw them in the scrap can and start over. That has to be a really, really, bad set-up if you are getting that much distortion with that thick of wall in 4140. How much clamping pressure are you using?
 
many parts warp even if you use pie jaws at low torque.
.
warpage is one of those things you have to measure and look for it. many parts warp more after a day or 2
 
I am using bored soft pie jaws at around 125 psi.

These parts are for our company. So far I was told that if it measures in the chuck correctly then it's fine. (many would stop there)
Trying to pick some brains and see how others approach it.
The part measures true and concentric after #1 but changes by +- .002 after id work.
and then changes a little more after unclamping.
 
Thanks, I like that.

So, rough od, rough id and then rechuck?
And let me ask this, the part will stay in the chuck the whole time and just chuck pressure reduced before final passes?

And 125psi, is that high/low and whats a safe minimum chucking pressure??
 
Thanks, I like that.

So, rough od, rough id and then rechuck?
And let me ask this, the part will stay in the chuck the whole time and just chuck pressure reduced before final passes?

And 125psi, is that high/low and whats a safe minimum chucking pressure??

.
obviously you need higher chucking pressure for heavy roughing cuts and do not need as much chucking pressure for lighter finish cuts
.
as to pressure needed depends on part shape. often you try various settings to see what works best
 
Thin wall tubing

Depending on quantity what I have done in the past is finished the ID in Pie jaws around 60-80 psi, and then finished the OD on an arbor with a plate and stud to hold the part.

If its really finicky
1st op- rough ID to fit an U/S Arbor
2nd op Rough and finish OD on arbor
3rd op- Back in to Pie jaws to finish bore

I had a Daily Average of 390 good parts to 10 "bad" that could be pressed back into round by hand. Tolerances were +/- .002 on OD and ID and round within +/-.001

I had my Pie jaws bored to the finish OD Diameter and used them for op 1 and op 3.

If this seems slow this also included a 4th operation that added slots at 90 degrees for lubrication.

Edit: Also is it toleranced for Roundness or just the dimension? If its just toleranced for the dimension without a Roundness callout I'd find your High and Low reading and average it to get your "True" Diameter.
 
If I'm reading this right.

Step1) Turning OD, and leaving wall thickness at approx 1"..
Step2) Pie jaws, and turn ID...

How are you holding it in step1?.. By the ID???

Is this tubing or solid??

And where the problem may be, Going from a 1" wall(with mill finish?) to a .2" wall..
I'd probably try taking out more of the ID before finishing the OD...

Another thing, if the OD is the critical feature.. Do it last, and hopefully by then
the material has finished moving around.


Bonus that the customer is yourself..
 
Used to call these things wedding rings. I’d second all the “balancing the operations more” advice, as taking a 0.400” section out of a 0.600” section for the ID is a lot for the last OP. The pie jaws have to be the correct size as well as you can make them.

Things like these go oval just cause they feel like it, so you hold the OD or ID hard and then hold the thickness hard. If you measure tenths better with OD mic then the OD is the next to last OP, if you like a tenth setting bore gage better then reverse the OPs.

Inspection always has a hard time with slim rings. Old school they’d usually Pi tape for OD & measure thickness with a ball mic a number of times around the circumference (this catches size and concentric really quick). Trying to work inside of .002” quicker/better would take a fixture to roll the ring over a mandrel while underneath a tenth indicator.

This is also one of those parts a CMM driver can take more time trying to inspect than it took to make it...

Good luck,
Matt
 
Im going to try and balance the cuts. For starters I will just do all finishing operations last, OD as the final finishing op.
If that doesnt work, I will reprogram the roughing passes in my down time.

Let yall know results. Thanks for all the contributions.
 
Another thing to consider, is that if the Jaws themselves are causing the Ovality-don;t Chuck on the part at all. Saw them long and finish the part, while holding onto a post.

R
 
Another thing to consider, is that if the Jaws themselves are causing the Ovality-don;t Chuck on the part at all. Saw them long and finish the part, while holding onto a post.

R

Hey Rob, I didn't follow what you meant. I get saw long but not holding onto a post.
And jaws are soft pie jaws matching part radius (dont know if that comes into play)


AND edit: The more I look at it, the more it looks like trolling to me. Is that whats going on or is this just some way over my head stuff I dont know the lingo for?
And if it is trolling, well done, and go f-yourself (but I say it like a new yorker so we all know how they say it )
 
Hey Rob, I didn't follow what you meant. I get saw long but not holding onto a post.
And jaws are soft pie jaws matching part radius (dont know if that comes into play)


AND edit: The more I look at it, the more it looks like trolling to me. Is that whats going on or is this just some way over my head stuff I dont know the lingo for?
And if it is trolling, well done, and go f-yourself (but I say it like a new yorker so we all know how they say it )

What I mean is, saw the blank long enough to hold only enough to Machine the part complete (except the part you're holding on to) a post, is in reference to the extra material that is only in existence for clamping on. After that you can figure out the best way to remove the post without having the Chuck on the part itself. I hope that makes sense.

If I were Trolling, it would be funny at least. I don't care enough to not get some laughs out of it. :D
 








 
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