Warped aluminum plate

[blueskull]

I know this is not a mechanical forum, but seems like there are quite a few machinists here, so I will just post it here.
I've just ordered a few 7075T6 plates from McMaster, and so far the thick plates turned out fine.
However, one thin plate, a 0.09" plate, came pretty badly warped to a point that it won't sit flat without external force.

So my questions:
1. How should I elastically plastically flatten it (I need to machine it, then it will be the lid of a machined enclosure, I don't want it to warp in front of my customer)?
2. On the plate, it says "ALCLAD 7075-T6 AMS 4049 KAIS", so it's an aluminum clad, right? Then what could be the core it clads onto?
3. I take the "KAIS" as Kaiser, if that's true, should I blacklist Kaiser (sounds stupid though considering its prevalence in US of A)?

Thanks for your input.

[mzzj]

https://en.wikipedia.org/wiki/Alclad

High strenght 7075 aluminium core with corrosion resistant (pure) aluminium cladding.

[calexanian]

Unless you are buying precision flat stock some curl can be expected in most sheet stock. Unless it is specified for flatness you are more or less on your own.

[thm_w]

Yeah that is normal, no flatness is specified, eg: https://www.mcmaster.com/8885k851

Some examples of precision flatness plate: https://www.alustock.ca/MIC6-Aluminum-Cast-Plate https://www.midweststeelsupply.com/store/castaluminumplateatp5
They only go down to 0.25".

To make it flat would take a huge press or slip roller.
Or if you are machining it and don't care about flatness (because something will hold it in place later), then you would use vacuum plate or double sided tape to temporarily hold it flat, so the machining depth is consistent.

[KL27x]

In practice, sheet metal is only flat over certain area for the thickness. E.g., that 0.09" thick stuff might be made pretty darn flat in a smaller square a few inches per side. But a larger sheet might be futile.

Personally, I find 1/8" to 3/16" thickness is the minimum to remain structurally flat over about 12" square.

Same for anything under the sun, really. A whole 4'x8' panel of 3/4" plywood is never grossly flat until you cut it into smaller pieces. Just floating in zero gravity, you would find it's quite warped.

[T3sl4co1l]

Or worse if it's been bounced through shipping...

Might I suggest a double-stick-tape mounting base?  Or maybe a vacuum base, but that's probably not all that good considering the forces required in machining, and also give or take how many holes get machined in it?

If you aren't using the whole thing at once, it's probably flatter in smaller sections -- cut it up first?  Ideally using a nice wide shear that doesn't bend it up more.

Can always slap it with a hammer as needed, although that takes some understanding too (if it's domed, don't hit it on the dome, hit it everywhere else around to stretch out the rim around the dome).  And kinda wrecks the finish.

Tim

[Geoff_S]

You probably won't have much success with that approach.  There will be residual stress in the outer surfaces, so if you machine one surface flat and flip it over, it will distort again when you machine the other surface.

[KL27x]

^+1. I have tried to mill a thin cast aluminum sander table flat. The thing would change shape after every pass, so the goalposts just kept moving. By the time i got halfway across, the previously flattened parts were no longer flat. Belt sanding warped it worse. Wet sanding by hand over plate glass gave the best result, in the end.

Even without flipping and trying to mill the other side, the sheet might very well warp when removed from the sacrificial block you have mounted it on. When you buy 0.09" thick sheet and you want to mill it flatter, you might be doing it wrong. the thing is made with a roller to be pretty darn consistent in thickness. But you will find out for yourself. The housing and any bends in the sheet should provide adequate structure to keep things flat. You might be starting with too thin a sheet for the surface area.

[jpanhalt]

You can use slip rollers to get it pretty flat.   Will barely affect thickness.

[T3sl4co1l]

So in total: you're starting with a non-flat plate, and want it to be flat in an unsupported, naturally sprung condition, after machining, of a grade with built-in stress?

Is the machining step going to thin and slice the material much, or is it more like a few holes, small slots, that sort of thing?

How flat is flat, anyway?  If it's a few thou, it's not going to do it as-is just because of gravity.  If it's by eye, you can probably do well enough bending it by hand.

Yeah, if you have a slip roller, a few back-and-forth passes, upside down and right side up, along both axes, should be able to do the job, a bit better and faster than "by hand".

Tim

[calexanian]

As in the example above for a sander table, the only real way to ensure flatness of a large surface like that is grinding, and in the case of a table blanchard grinding would be best.

[chickenHeadKnob]

If you want a flat plate the key words to search for are blanchard ground plate. I would get it in steel A36 not aluminum for a fixture plate. This treatment is quite affordable and results in near surface plate flatness and parallel. On youtube you can see various blanchard grinders in action for an idea of how they work:

[Stavos122]

Not sure how you can mechanically fix this but had a similar issue in the past.  I switched to Tool & Jig grade aluminum plate  - no issues with flatness.