[bzo]

I've been buying surplus T6061 rectangular bar and plate lately for my CNC project, and it seems that more often than not, the rectangular bar stock, which all seems extruded, is warped with up to a few millimeters of deflection per foot. The plate stock usually seems to be in good shape though.

Is this typical? I'm guessing the warpage is caused by shear cutting or rough handling.

Is there any way to straighten the stuff to reasonable flatness or do I just need to buy some more material in better shape?

[HuFlungDung]

Hi Bzo,

Commercial straightness is quite a broad spec, usually about 1/8" in 5 feet is considered straight

I'd wonder about the handling your stock is getting, because most extrusions I've handled, seem quite good for straightness. Your alternative from custom selecting the stock yourself, is to acquire a small hydraulic press and straighten the stock yourself.

[Rekd]

Look into Mic6 or K100 alum tooling plate. It's not bound with stresses that cause the extruded stock to warp. Don't know if you can get it in bar stock. It's a bit softer than 6061-T651 but it's a lot more stable.

[edit]
You might be able to straighten the bar stock out with a hydrolic press, provided the pieces aren't too long. (i.e. you prolly won't be able to straighten out a 1"x3"x12 foot bar, but you could straighten out a 1"x3"x18" bar fairly well)
[/edit]

[bzo]

thanks for the thoughts guys. I tried straightening one of the pieces by clamping the piece between two pieces of thick straight stock. However, because the bends are so mild, the aluminum tends to undergo only elastic deformation and springs back to the original shape.

Will using greater pressure in a hydraulic press overcome the elasticity?

[HuFlungDung]

Yes, T6061 is quite springy and takes quite a bit of overtravel to make a permanent bend. There would be no use clamping it between two pieces of straight stock. Use short protective pieces under the press ram and above the supports to prevent gouging the finish.

From my experience, I would suggest that for 1/2" stock, you would want the supports about 8" apart, that is, 4" each side of center when straightening in a press. Although a longer piece may have one long continuous bend in it, you cannot straighten it properly by only supporting the ends and pressing in the middle. You'll need to work your way along it, with the supports always about 8" apart. The distance between supports would be roughly proportional to the thickness of the stock, ie., for 1/4" stock thickness, supports about 4" apart.

[BT1]

Interesting topic:

Just machining it can be a tedious task. You machine one face and it bends. Machine another face it bends.

Different grades are worse than others.

Ahh the joys!!!

BT1