[tikka308]

I purchased the below-picture 1/8" x 3" stock from onlinemetals.com and, after spending some time milling it to the precise size, realized that it's slighly bowed!! I don't have a feeling gauge, but I know the piece of paper that I can slip under it is 4thou.

Any suggestions on how to straighten this? I tried using my fixture plate + clamps to squeeze the bow out but didn't have any luck. I don't have a press brake either. Appreciate any thoughts. thanks!

[fretsman]

Do you have any room to machine the thickness?

[tikka308]

Fretsman - no, unfortunately i don't have any room in the thickness.

If I did, that would solve the problem, but i've got 4 pieces like this - and machining both sides of the four pieces on my TAIG would take quite a while!!

[fretsman]

I see, unfortunately I'm no help then, sorry-

[tikka308]

No need to apologize!

[MetalBlade]

I tell you what I would do. Put it on an extremely hard and flat surface and smack it with a flat 1 pound hammer. Work over the bow and start by using modest hits with the hammer until you see improvement.

[fretsman]

That's kind of why I was asking about the amount of material he may have but since there is none, I'm thinking that's out -

[cjdavis618]

Are there any holes that are being machined into the stock. I'm thinking if you have a way to "Secure" the plate and pull it true, you would be able to complete the part.

If you don't mind, what shape of part are you making? Since is .125, if we know the outside dimensions, we may be able to help more.

I have a 1/4" plate that is the same way. I use it for perishable material though so I didn't care. If it is that critical that it be flat, I would contact them and let them know there is a problem and see if they will make it right.

I live a couple hundred miles away from them and pretty soon, I will make a road trip there to stock up. Let me know how they treat you if you contact them.

[cam1]

Why does it need to be so flat? What is the part being used for?

regards

[Robin Hewitt]

You can't simply crush it flat it will spring, you have to bend it the other way, past it's elastic limit.

I'd put it down on a thick bar, shim under either side, narrow bar down the middle on the top then crush it with G clamps.

Look see if it was flatter, if not flatter repeat with thicker shims until it is.

Hitting it will probably make it bend in other ways and not fix anything.

[fretsman]

Actually, Robin, you can peen material that is cupped on the concave side with short strong blows to bring it back as it relieves stress. We've done this with heat treated steels when we have something like 4340 that's oil quenched and goes nuts after heat treating.

Aluminum is pretty soft though so if these have to be "that" flat and have no room to remove material, I'd like to hear from others on some tricks because personally these would be scrap as after all this time it would be easier to just machine up 4 more blanks.

What is your tolerance, tikka?

[Robin Hewitt]

You can bend something by peening if you have a wide support on the back and strike a narrow blow to expand the face you are hitting.

Wouldn't say it relieves stress though, just conterbalances it

Makes a mess of the surface in all.

[tikka308]

Thanks for the thoughts, everyone.

The four pieces I'm struggling with are to be the four sides of a box. The "bottom" of the box is 0.25" and has no problems, which is great. But the bow in the four pieces mean the corners of the box have gaps which are not acceptable.

I tried "peeining" it for about 20 minutes last night and can't say I had any luck. I think the only way to fix it (at least with the tools available to me) is like Robin Hewitt said - shim it & "crush" with clamps...

I need to keep the 0.125" thickness due to drawing requirements.

[fretsman]

Absolutely, Robin, and as for the stresses, it just always seemed like that what's going on in that situation (heat treat). They go in ground flat and come out looking like a cup, lol!

I need to keep the 0.125" thickness due to drawing requirements.

That's why I like to make parts first..... and then draw them up......I kid, of course -

[cam1]

Fretsman: nothing wrong with that approach, as it yields known results....that's where experience comes into play. Alot of crap is "designed" nowadays, and the designer has never stepped onto the shop floor.

regards

[fretsman]

you're not kidding, tru-er words were never spoken.......

[praetor]

On the show "How it's made" there was an episode about how safes are made and in one part of that subject (safes) a guy was using a hydraulic shaft to get the 1 inch thick very large plate to exact flatness using said hydraulic shaft...bite the bullet, by yourself a cheap hydraulic jack, and make a fixture/jig to hold your pieces and push it [hydraulically] to straight but first, view the episode to get an idea of how it was done. Dude, I would've ordered the pieces slightly larger then 1/8 (possibly 3/16 to a quarter inch in a very long one piece, enough to cut up to desired lengths after getting it to desired thickness, you do have a cnc equipped mill don't you? let the mill run to desired thickness, it won't hurt the machine, right?) if anything I stated doesn't help...bite the bullet and start all over...live and learn.

[scrapman]

The only way you can realistically expect a flat part from thin material is to set an oversize piece in a low-temp solder, machine on side flat(something that is more difficult than you can imagine given a thin workpiece to begin with), then recast and mill the opposite side flat. My question is what are you making and what kind of flatness tolerances do you have to meet? Perfection is an illusion that we only think we can achieve. If you lack the ability to accurately measure the deviation, I question just how accurate you really need to be.

[tikka308]

Scrapman - the other 3" x 1/8" 6061 Aluminum that I've purchased in my brief tenure as a hobbyist has not been bowed like this. Although I could re-mill it, I want to explore what my options are to salvage the existing pieces - and possibly even learn something new in the process. You're right - I don't own the appropriate tools to accurately measure this deviation - but I don't believe that says anything about my interest in making accurate work. It's the carpenter, not the tools. My flatness tolerances for this particular piece are "optical" - i.e. when you hold the piece against a flat-surface, naked human eye should not see a bow; I would estimate this to be about 1thou. Like I said, the other 3" x 1/8" stock has met this test. Clearly one lesson learned is to check your stock before investing time in it.

[scrapman]

Please! No offense was intended. I'm a toolmaker by trade and was attempting to qualify the degree of accuracy you needed, and if you didn't need a high degree of flatness, is it really worth the effort. The aluminum is worth less than a buck, while your time is certainly worth many, many, many times that. It's always the little things that hang us up, isn't.