Legs warping

[ChadInColo]

I have found a problem in our shop that we haven't been able to get around. We have some parts, 7050 aluminum, with 2 legs, 8 inches long, 3/4 inch thick, 2 inches tall, 1.5 inch separation. We are using a tooling tab in the middle, bench it out after machining. The cutting is taking place in a 4 axis horizontal, with liquid cooling/lubrication. The problem is, when the tab comes out, the legs bend in. This has been consistant, and is causing lots of rework.

My suggestion was to remove the tooling tab from the middle, but that would be replaced by 2 tabs, one on each leg, which would have the same restraining effect I believe. Anybody have any ideas? Are we dealing with residual stress in the material? Imparting some stress? Let me know if you need any other info and I would sure appreciate some help.

 

[metman]

Chad,
Here are some thoughts:
1. Although I have no direct experience with distortion of 7,000 series alloys during/after machining, I am told that it is common for these grades and I suspect that it is from both residual and machined-in stresses.

2. You say it is consistent. Can you allow then for the measured distortion by machining out-of-shape so that when the tooling tab is removed it actually distorts to the desired shape? If you are using a 3D computer model from which you create your toolpath, it would mean remodeling the original or creating a new CAM model.

3. Another possible solution might be to start with material approximately 1" x 2-1/4", machine off 1/16" from all exterior surfaces, then start over and machine a second layer of 1/16" from all exterior surfaces. Now you should have very little residual mill surface stress before you cut between the two legs. If you initally take off 1/8" from a surface rather than in increments, I would expect it to distort and make it troublesome to true-it-up. I chose 1/4" additional thickness out the air. You could call your Aluminum supplier and talk to their metallurgist or to their suppliers' metallurgist to get a more definitive number for material to remove to minimize residual stresses. If it still distorts, then it must be machined-in stresses also which requires shallower depth of cut and slower feed rates and/or better cutting tool geometry that has a good shearing action.

Before resorting to item 3., you should wait for others with more hands-on experience to respond.

Leonard

Jesus is THE life,
Leonard

 

[MechEng13]

Chad,

One other thing to note, if the parts are made from Castings, Aluminum is known to move quite significantly once the "skin" has been broken. If you can get them cast to "near net shape" you can avoid some of this distortion. Don't know if that is your problem or not.

HTH,


Alan M. Etzkorn
Product Engineer
Nixon Tool Co.

http://www.nixontool.com

 

[ChadInColo]

Thanks for the tips. I will have the QA guys look and see if the distortion is consistant enough to machine it to warp to straight. We machine a lot of 7075 aluminum here, don't seem to have much trouble with it warping, I don't know if it is the 7050 or just the geometries. I will look more into how heavy a cut they are making, see if that might be it.

This is a hog-out, not a casting, but thanks for the input.