MIC-6 Warping

[cbauer]

I recently had some L-shaped brackets machined from MIC-6 with the hope that the parts would stay flat. Because they are L-shaped, there is a significant amount of material removal required to mill these, and I specified cast aluminum tool plate so that the resulting stress relief would be minimal. Dimensions are approx. .120" thick, 6" x 5" overall outside dimensiosn, and the L is approx. 1" wide. The parts supplied by my quick-turn machine shop are very warped, though: up to .060" in the thickness dimension (or about 50% of the thickness!) I'm wondering if my expectations for MIC-6 were wrong, or something else caused this. I can imagine several scenarios:

1. The parts were damaged in handling (for example, peeled off of a sticky part holder on the mill, or in packing/shipping).
2. The vendor did not use the material I specified (is there a way to tell? I did not purchase a material cert., but did get a blanket CoC.)
3. I was wrong about the properties of cast aluminum and need to adjust my expectations.

The vendor's doc. suggests that they started with 1/4" thick MIC-6 aluminum. Should I have expected this amount of warp from parts with this sort of aspect ratio? Any experienced feedback welcome. TIA.

CB

[PinkertonD]

Should I have expected this amount of warp from parts with this sort of aspect ratio?

Did you define an acceptable degree of flatness in the drawing?

Have you spoken to the machine shop management? They may be as unhappy as you if they were peeled off sticky mounting pads and bent in the process.

I find that calm, pleasant conversation will always elicit a much more reasonable outcome and going in with fault-in-hand and guns blazing.

I am also a little confused with the term "warped," when followed by the statement "up to .060" in the thickness dimension." Are you stating that the thickness tapers by 0.060" If so, is there a tolerance range defined for the thickness?

You need to state the case much more accurately if you want accurate help here. Having said that, I would first be talking to the machine shop management as stated above.

[cbauer]

Thank you for the response. I had these fabricated at a rapid-turn shop without a fabrication drawing (just 3D model--and perhaps against my better judgement), so no flatness spec. was provided to the shop.

The thickness of the parts appears to be fairly constant (within the shop's standard dimensional tolerance of +/-.005"). The warp I see is simply curl of the parts out of plane. On a granite surface plate I can push on one corner and see the opposite corner lift .060".

Your advice regarding a conversation with the shop management is good. I'm really less interested in assigning blame than in understanding the cause. I am in a prototyping stage and don't want to propagate a poor design, nor institute expensive or complicated fixes that aren't actually necessary. My hope was that someone here might have enough experience with cast aluminum tool plate to say, either, "no way, the material should not have curled that much--something else caused that" or "yes, that's normal, even for MIC-6." The truth is that I would have expected 6061 aluminum to do something like this, and I THOUGHT MIC-6 would be an adequate means of addressing it.

CB

[RWOLFEJR]

I hopped on Alcoa's site to look at the material properties. My guess is that the relatively large amount of stock removal... as compared to the original cast thickness... was were the problem started. It appears that 1/4" thick may be as small as they will cast? That's as small as they go in discussions of expectations of flatness and thickness tolerances on this stock.

So figuring maybe this stock is as close as you can get for your part requirements, I suppose you might have to find a way to make it work.

Could possibly be that the shop got the part too hot during machining. The relief of stresses at machining should be less in your cast stock than a rolled product but some relief accompanied by heat could add to the warpage. I've heard tell of folks machining thin wall parts while clamped to some material that will act as a heat sink to help draw the heat produced by machining and further reduce warpage. Don't know anything about that though because I've never encountered the need to look further into that.

Could also be a that they took all the extra stock off of one side. That could be the recipe for more twang when released from the machining holding fixture. That isn't to say that this is what happened or that they did indeed remove all the stock from one side only... or that by machining off half the extra stock from each side will necessarily help. Just saying that I'd expect better results in machining if you could purchase the stock cast for minimal clean-up cut and/or dust both sides of the flat legs.

Your solution might be a simple as taking a couple / few light cuts rather than all in one pass? If that still doesn't get it... maybe try a few parts machined on both sides and in a few light cuts each side? I'd also look into purchasing the material closer to size. That's 50% of your material going in the chip hopper.

Good Luck,
Bob

[Christoph]

I would be interested to know which way the plate warped towards the side it was machined on or away from it?

.c

I hopped on Alcoa's site to look at the material properties. My guess is that the relatively large amount of stock removal... as compared to the original cast thickness... was were the problem started. It appears that 1/4" thick may be as small as they will cast? That's as small as they go in discussions of expectations of flatness and thickness tolerances on this stock.

So figuring maybe this stock is as close as you can get for your part requirements, I suppose you might have to find a way to make it work.

Could possibly be that the shop got the part too hot during machining. The relief of stresses at machining should be less in your cast stock than a rolled product but some relief accompanied by heat could add to the warpage. I've heard tell of folks machining thin wall parts while clamped to some material that will act as a heat sink to help draw the heat produced by machining and further reduce warpage. Don't know anything about that though because I've never encountered the need to look further into that.

Could also be a that they took all the extra stock off of one side. That could be the recipe for more twang when released from the machining holding fixture. That isn't to say that this is what happened or that they did indeed remove all the stock from one side only... or that by machining off half the extra stock from each side will necessarily help. Just saying that I'd expect better results in machining if you could purchase the stock cast for minimal clean-up cut and/or dust both sides of the flat legs.

Your solution might be a simple as taking a couple / few light cuts rather than all in one pass? If that still doesn't get it... maybe try a few parts machined on both sides and in a few light cuts each side? I'd also look into purchasing the material closer to size. That's 50% of your material going in the chip hopper.

Good Luck,
Bob