How to solve warping when milling aluminum

conceptdevelopers · Jul 29, 2018

I do the solid modeling and programming. We cut a lot of 2024, 6061 and 7050. I'm having a disagreement with co-workers on how to stop parts from bowing or twisting. I think the solution is done with multiple solutions.
1.) If we can, normalize it before starting
2.) use SFM and depths of cut recommended by cutter manufacture
3.) If we have access stock take skim cuts on the cold rolled material and straighten that before it's cut.

Machinist in the shop tell me to take light cuts and cutting slow will result in straighter parts. I said it is heat that is one of the culprits and the cutting it slow doesn't help but hurt.

I'm looking for a solution not a need to be right

What say you?

guest · Jul 29, 2018

Normalizing is not a process used on aluminum, you would anneal which will kill the temper.

The cutter manufacturer's recommendations are under ideal conditions, and usually pretty aggressive. They don't sell tools by promising lower performance than what you are using now.

There are two things going on when you are machining. You are relieving internal stresses by removing material, and you are inducing stresses by machining.

If you are having warping problems, you can reduce the induced stress by taking less aggressive cuts. When there is a lot of material coming off, you should rough the part, then unclamp it and let it spring. Reclamp it in a relaxed state and finish. Leave enough finish allowance for a full cleanup after the part springs.

crazygoat · Jul 29, 2018

Flip, flip, flip.

guest · Jul 29, 2018

^^^Yep. Balancing the material removal helps on flat parts, and lets it spring between flips.

Hard to say without knowing the part. If it's a critical part, you can do a low temp stress relieve between roughing and finishing ops. Gotta be careful about the temps. Keep it under 400 deg F.

EmGo · Jul 29, 2018

guest said:
If it's a critical part, you can do a low temp stress relieve between roughing and finishing ops. Gotta be careful about the temps. Keep it under 400 deg F.

Or cryo, which might be safer ? Even a dry ice soak, if that's too extreme ?

metlcutr55 · Jul 29, 2018

are we talking 6" x 3/8" x 12, or 4" x 4" x 4"? Id pound the shit out of one roughing only. let it sit a day. measure the distortion. adjust the roughing to leave finishing stock per your normal successful procedures. finish. if its the thin flat stuff, you may need to flip it once or twice. and rough and semi finish. what are you talking about for finish? .005 tolerance or .0005. will make a big difference.

crazygoat · Jul 30, 2018

conceptdevelopers said:
Machinist in the shop tell me to take light cuts and cutting slow will result in straighter parts. I said it is heat that is one of the culprits and the cutting it slow doesn't help but hurt.

What say you?

I say your machinist is right. Walk out in the shop one day and do a part both ways. Touch it with your hand afterwards.

conceptdevelopers · Jul 30, 2018

Actually I started with a 6" x 3" x 4 ' and had to make an "I" beam out of it. I flipped it to whittle it down to 4" x 2.1" x 4' then cut the 2.1" sides followed by the wide sides of the I beam. Top of the "I" .256" thick, bottom.156" center rib .090" thick

DMF_TomB · Jul 30, 2018

many parts are stress relieved or annealed, rough and semifinished machined then sent out for heat treatment then finish machined
.
if it cannot be stress relieved you need to take rechuck and take semi finish cut and rechuck again and finish cut depends of tolerance.
.
often with larger parts when machined alot they can warp easily over 0.100", biggest mistake is not leaving enough allowance for warpage and not expecting more warpage with each cut taken. i often see .001" warpage after only taking .020 off
.
and some parts when straightened with a press often warp back after a day or 2.
.
as for machining fast or slow you want to avoid large temperature changes. a part can warp just getting hot unevenly and warp differently as it cools off. if cutter is dull that can induce stress or put a work hardened skin of the surface.

crazygoat · Jul 30, 2018

conceptdevelopers said:
Actually I started with a 6" x 3" x 4 ' and had to make an "I" beam out of it. I flipped it to whittle it down to 4" x 2.1" x 4' then cut the 2.1" sides followed by the wide sides of the I beam. Top of the "I" .256" thick, bottom.156" center rib .090" thick

4 feet long? It doesn't matter what method you use, with that kind of whittling it is going to bow,twist and a lot of other things

rcoope · Jul 30, 2018

Custom extrusion! Custom extrusion! If this is a production part a custom extrusion would be worth a look. Otherwise this seems a hard part to make and one would think there must be different way to do it that would be easier. Time to do a Dan Gelbart and break out the round tubing and sand blasted epoxy shear joints.

Can you describe the application any further without abrogating any NDAs?

conceptdevelopers · Jul 30, 2018

rcoope said:
Custom extrusion! Custom extrusion! If this is a production part a custom extrusion would be worth a look. Otherwise this seems a hard part to make and one would think there must be different way to do it that would be easier. Time to do a Dan Gelbart and break out the round tubing and sand blasted epoxy shear joints.

Can you describe the application any further without abrogating any NDAs?

It was customer supplied material. Boeing had an extrusion, BAC...., as the first option but did approve machine it out of a block. Our customer knew it would be much harder out of a block but the extrusion lead time was to great for their needs. It was an AOG and they where willing to pay AOG hourly rates for however long it took us to whittle it

(AOG for those of you not fimiliar means Aircraft On the Ground)

DMF_TomB · Jul 30, 2018

crazygoat said:
4 feet long? It doesn't matter what method you use, with that kind of whittling it is going to bow,twist and a lot of other things

.
.
i have seen 6061 T6 piece 6 foot long was easily warped over 0.100" especially if it starts as 2" thick and ends up 0.5" thick in spots
.
and have seen stuff straightened in a press to .002 bend back some after a few days. dont think you can hold .001 flatness and have it stay stable flat

metlcutr55 · Jul 30, 2018

without web(s) thickness, final flatness & straightness req info, impossible to answer. mill this flat, parallel, straight? depends on if you are standing 5 feet back from the machine and eyeballing it, or sitting it on a plate and checking it with indicators. are you looking for results of 1/8" of an inch or .010 overall? big dif.

rcoope · Jul 31, 2018

Well in post #12 the OP did point out it was an Airplane on the Ground situation. And perhaps it required 7075 though one would imagine even that it could be cold set (as they called it in aluminum bicycle welding) back straight a bit.

CalG · Jul 31, 2018

Talk to the grinder hands on this one! ;-)

thermite · Jul 31, 2018

rcoope said:
Well in post #12 the OP did point out it was an Airplane on the Ground situation. And perhaps it required 7075 though one would imagine even that it could be cold set (as they called it in aluminum bicycle welding) back straight a bit.

"Airplane" changes so many things.. grounded or no. Best to say-so, first post. It gets more appropriate expertise focused onto the issue a great deal faster.

Whatever it takes to meet ALL specs and reduce risk of it falling out of the sky. Folks making lawn furniture or go-carts should just keep the beer cold and let Aerospace mavins sort it.

I got space in the fridge...

BSCustoms · Jul 31, 2018

rough, rough, finish, finish.

How to solve warping when milling aluminum

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