Aluminum extrusions ????

[lkenney]

I recently tried to machine a bunch of hollow aluminum extrusions, these are roughly square and rectangle tubes about 2.5" by 2.4" by 24" long, the wing sticks out about an inch from one side about 3/4" from the closest edge with a wing projecting from one side.

We needed to drill and tap screw holes on both ends and spaced down the wing. They are table or machine legs of some kind. We also milled a couple of slots in one side and of course everything was mirror with "minor" differences on each leg set.

It looked like a pretty straight forward job, complex in keeping all the holes and features orientsed correctly as we switched from one side to the other. I made special soft jaws and reinforcment bars to keep things true as I could. In the machine we were true and my programming right on the dot but when we measured them at the other shop it was like I had done the work drunk. Nothing seemed to be where it should have been or where it was when I machined it.

I finally backed out of the job and lost everything that i had put into it. It has bothered me ever since. It seems to me that these extrusions move and are never straight or consitant. how do you guys hold and machine them?

Thank you
Lowell

[Donkey Hotey]

I'm not entirely sure what shape these extrusions ended up (the 'wing' bits left me scratching my head), but the fact is that tolerances are pretty loose on shaped extrusions (hollow as well as angle & channel).

Look it up: you'll see that flatness, twist and even thickness are actually allowed to be all over the place. They generally hold tolerance pretty well but you can occasionally get a lot that will leave you wondering what happened.

This all leads me to question how they dimensioned the drawings and how you held and machined the parts. I can see why you'd be scratching your head, since this really shouldn't be that tough. I suspect that what they wanted was not clearly communicated in the way the drawing was dimensioned and the tolerance of the tubing stacked up to cause problems.

[ImanCarrot]

I don't know about anyone elses experience of extruded ally, but I work to pretty tight tolerances and will absolutely not touch the stuff. I always go for cast ally. Extruded "boings" all over the place.

I've found that "annealing" the stuff helps, but not o the degree that I need.

Heat treating or ultrasonic destressing (prior to machining) might help?

[lkenney]

Gregg,

If you looked at the end of the extrusion, they sort of looked like a "P" with the lower leg (or wing) being moved forward of ther rear of the "P" about 1/2 and inch. The tube is extruded with an opening down one side the whole length. The tolerances were very tight in relation to the rotation of the work around each end. Someone must know how to do this as these legs were for a large medical equipment firm, but they sure whipped me.

Lowell

[RDesign]

I would expect the hollow portion of the "p" shape to be inconsistent in location. I made some split clamps out of 2.5" OD 1" ID extruded 6061 tubing and the ID was up to .080" from being concentric to the OD.

[kendo]

We bend the stuff into window frames with an old pines tubing bender
Heavy extrusions for windows that go in trains and buses
its a nightmare sometimes machining all the details before or especially after its bent