How to fixture thin-ish wall Al tube that is not all that round?

Hi guys,

I'm machining a few 100 pieces of 3" OD x 1/8" wall 6061 tube. Pieces are ~8" long and I just need to do some simple end work, +/-.005".

The tube came in at average 3.02" OD, but it is generally ovaled between 3.01" to 3.03".

The challenge is how to hold onto it without distorting it. I'm doing it in the mill, where I can control the clamping pressure better.

I made an exquisite fixture, which gave a loving 95% contact conformal grip ... for 3.00" tube. OK, the tube is oversize, so redefine the conformal surface to 3.02". Oops, the tube is too big in one orientation and jams in the fixture. When rotated 90* so it goes in the exquisite fixture, I end up with 2-point contact and ~.01" distortion which caused the part to be out of spec when taken out of the fixture and the material springs back. However, the amount of distortion is very unpredictable due to the 2-point contact.

So I tweaked the conformal fixture surfaces a bit to relieve the center and outside areas, and now I have 4-point contact which is at least predictable, such that I was able to write the distortion into the program and am getting finished parts within a couple thousandths of where they are supposed to be.

There's got to be a better way though. Some way of holding onto asymmetric thin-wall pipe without distorting it, but accurate enough to locate within a few thousandths so the machined features are reasonably coaxial with the tube.

Ideas?

Thanks, and regards.

Mike

how consistent is the egg?

If it is regular in oversize similar in out of round, measure them and mark them on the high spot or low spot and put them in the fixture the same way every time.

If they very more than your tolerance, you are in trouble.

gustafson said:

how consistent is the egg?

If it is regular in oversize similar in out of round, measure them and mark them on the high spot or low spot and put them in the fixture the same way every time.

If they very more than your tolerance, you are in trouble.

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They vary more than the tolerance unfortunately, so I have to find a way to grip them with minimal distortion. The variance in and of itself doesn't fail the part, as long as the machined features are +/-.005".

Regards.

Mike

I would make a cathead to hold them.

Couple other considerations:

1. I'm doing several hundred, and the cycle time is ~50 seconds. Whatever the better solution is has to be fast.
2. It is surprisingly easy to distort 3" x 1/8" 6061 tube. I can "egg" it .005" with my hand.

Regards.

Mike

Can you make a plug (or series of plugs)?

To combat the oval, you could make it similar to a dimond pin used for locating.

Just thinking out loud.


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Thanks for the idea. I wouldn't know what shape to make the plug. The tube ID varies between exactly round to egg-shaped by + and - .01". If I coerce the tube to be any one shape, some of the parts will come out off-spec.

Regards.

Mike

I cannot see the sense in this exercise at all.

- The material arrives oval by double the tolerance you say you think you are hitting. How would you even know, whom do you have to convince, and how can one?

- The error varies. Redundant information. You already ID'ed the alloy.

- You can move it by how you grip it in the Mark One Human Hand. This is a stable part? Just how?

- You don't want to "distort it", but have no firm reference to even judge what would constitute "distortion? I mean - some might even consider it righteous to process in a manner so as to reduce the amount of "ovalness" by half or even 3/4. Butttttt:

If I coerce the tube to be any one shape, some of the parts will come out off-spec.

Click to expand...

Get real about the feature's chance of meeting a realistic spec. Measured by whom and "Just HOW?"

ELSE get real about starting with materiel that supports hitting the spec if it even NEEDS to BE hit.

Clever clamping helps just HOW if the material is still sidegodlin once released?

Garbage In // Garbage Out. Perhaps all you realistically NEED?
Average in // Average out.
Excellent in // Excellent out. You want it? It costs more.

Pick any ONE.

Pay or earn accordingly.

You do not get the choice of mix and match.

That would be like trying to "measure" Aluminium. How's THAT been working for yah again?

What about a quick turning operation?

FredC said:

View attachment 243830

What about a quick turning operation?

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If you are allowed to furnish and bill-for a mandrel with each part?

HELL yes!

Just leave the Mike Foxtrot installed, add its cost to the bill, and hold whatever tolerance you need!

Otherwise, we now have a ten or twenty thou +/ one thou variance in wall thickness added atop the original problems?

I don't think it is legal, let alone fair, to then stamp the product "Made in China", and blame some other poor sod, is it?

Or is that perchance where the oval-shiney-wood CAME from?

Inflatable bladder with textured surface inside tube?

I would try vise jaws with 4 spots on 45 degree quadrants, 2 per jaw. Maybe line those sposts with self-adhesive rubber, neoprene is pretty sticky. It sounds like you need to talk to your customer about this, +/-.005" doesn't sound loose enough.

4miller1 said:

Inflatable bladder with textured surface inside tube?

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Drum sander hub?

Or - given the material, one of those bladder gadgets for clearing clogged sewerlines?

I actually HAVE ID mandrels and OD collets as could do this. But not a lot of point if have to start the job with QC reject material, is there?

Who is cutting them to length, you or a supplier? My saw's air vise will do a number on tube. Might be what happened to your pcs.

jamscal said:

Who is cutting them to length, you or a supplier? My saw's air vise will do a number on tube. Might be what happened to your pcs.

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Asked and answered. The shop that sawed the tube for me has a modulator for their autosaw vise, and are well-aware of the potential for vise-squash.

Regards.

Mike

Thermite nailed the problem. The material is inconsistent and can be made to be in or out of tolerance simply by holding it. You may have to mark each piece with where and how you hold it for inspection purposes.

Tom

Finegrain said:

Asked and answered. The shop that sawed the tube for me has a modulator for their autosaw vise, and are well-aware of the potential for vise-squash.

Regards.

Mike

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Seems so, if they delivered it as consistently as the evidence supports.

Or do you already have incoming QA that showed it was bad-news even before sawing?

"Perfect" tube - nor even solids - do not exist.

"More better" than what you have surely DOES exist.

I can't remember the brand and it was domestic, but had to deal with 2" schedule 80 aluminum pipe with an egg shaped hole recently.The hole diameter varied radially from 1.930-1.950. For me it was a new phenomenon all previous aluminum pipe and tubing I used had round O.D. and I.D. with just .020 or so T.I.R. I just had to make sleeves with a .055 wall, lathe only 2" long, they sucked. I ended up rounding out the hole holding the O.D. in 2" deep pie jaws then turning the O.D. on an expanding mandrel. The egg shaped hole was how it came from the mill, I cut it up myself.

Dualkit said:

I can't remember the brand and it was domestic, but had to deal with 2" schedule 80 aluminum pipe with an egg shaped hole recently.The hole diameter varied radially from 1.930-1.950. For me it was a new phenomenon all previous aluminum pipe and tubing I used had round O.D. and I.D. with just .020 or so T.I.R. I just had to make sleeves with a .055 wall, lathe only 2" long, they sucked. I ended up rounding out the hole holding the O.D. in 2" deep pie jaws then turning the O.D. on an expanding mandrel. The egg shaped hole was how it came from the mill, I cut it up myself.

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It isn't "just" Aluminium. I was careful for years as to Copper line that didn't want to sweat-solder properly or fought even compression fittings. Just had to buy pricier goods, such as Australian made vs Chinese made when out in Hong Kong.

Also bit the bullet and laid in the sizes of "rounding" plugs I found handiest. Dual-use, as mine are also meant for flare forming.

Schedule-wotever "pipe", "tubing", and conduit goods - this out-of-round is not uncommon, and matters not a great deal for their intended use.

Precision tube is "out there", too, but it does command a higher price.

All depends on the application which one is the true "least cost", what with extra handling to compensate or correct the lower-cost stock, if good-enough correction is even practical at all.

No guarantee the OP has one that is amenable to "rescue" at any lower cost than starting-over.

Metals can be dear. Time, and need of special fixturing or tooling, OTOH, is always EXPENSIVE.

thermite said:

... Or do you already have incoming QA that showed it was bad-news even before sawing? ...

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Yes, that. It's egg-shaped before sawing.

But, regardless of when it got egg-shaped, it's egg-shaped, and it's sitting on my shop floor. I suppose I could make a fuss with the saw shop and/or the supplier. I'll do that with the next batch. I have neither the time nor the inclination to reverse course on this batch.

It turns out that the only diameter dimension that matters is an inner machined 2.878" bore, which has to be +/-.005" size and roundnness. The actual tube OD and ID are an order of magnitude less critical so the OD ranging even as much as 3.00" to 3.04" is within spec, for this customer.

This is how I'm doing it:

The size is actually uninteresting, since it's a new-ish Brother Speedio and easily interpolates within tenths.

Roundness is where all the trouble happens. Squeeze the tube, and it deforms of course. The material doesn't shrink though, so if you squeeze it .005" in one axis, it gets bigger in the other axis by ~.005".

4-point conformal contact vise jaws (actually a complete fixture, but it acts like a vise). The contact patches are ~1/2" wide, and ~90* apart. The shape of the contact patches is 3.02" and round. The rest of the jaw surfaces are relieved. In CAM, I made entities that emulate a deformed 3.02" circle, close to an ellipse 3.015" x 3.025". I believe (and the results tend to prove) that this is the deformed shape of the tube when fixtured. The fixture closing pressure is very low, just enough to keep things from shifting or chattering.

After a little practice, and a few iterations of the ellipse in CAM, I'm able to hold the bore round within .002".

Regards.

Mike