Stresses in bar and plate aluminum stock.

[LONDONDERRY]

I have this part that is 193mm X 134mm 6 mm thick, that is made of 7075-t6 aluminum plate. One of the major issues is the part becomes stressed during machining top and bottom surfaces and potato chips. So now we are having the same part fab'd again.
This time around I need on surfaced lapped, so I went to a lapping house and talked to one engineer that has 40 years in the business. He recommended that the material be made from bar stock which has less stress that plate stock. So the idea is a machine shop would cut off a slice of bar stock, like a slice of bread and machine the top and bottom surfaces which will have less stress
So after talking to the lapping house, I paid a visit to the machine shop and they said in most cases bar stock has more stresses than plate stock.
So the question is who's right and who's wrong. Or are they both right and wrong. I was all so thinking of having the part made from MIC 6 plate. Any suggestions?

Frank

 

[unclesyd]

I don't believe that you can go wrong with the MIC-6 tooling plate. MIC-6 was the mainstay of our in house spinnerette manufacture process. All jigs, work tables, and bases were made from this material with no problems with stability. All our processes worked in tenths (o.ooo1).

The only thing I've heard about machining was that double sided machining and abrasive machining were desired for thickness reduction. If I recall correctly we got most of this material through TCI.

 

[Albigger]

If you don't have to make it out of 7075-t6 then you will definitely have an easier time with MIC-6. It is a version of cast aluminum tooling plate versus wrought plate, so it will have much, much less internal stresses to begin with.

You haven't provided many details about your geometry, but if you still have to use plate or bar, you may want to rough-machine your part, let the part spring/move/stress relieve, then final machine and polish it.

 

[SincoTC]

I agree with the previous replies, MIC-6 is very stable and has very little internal stresses. However, being a cast material it will have considerably less strength than wrought material. Its short chipping properties means that it will drill and mill like a dream, but its friable nature means that threads are not that strong, particularly small instrument types where the small chips can jam and tear out the thread on tap withdrawal (roll forming of these threads is not possible due to the low ductility). Finally, some finishing operaions may be compromised due to surface porosity.

I have used a lot of MIC-6 and it's brilliant for tooling, jigs and fixtures etc, but you need to consider the above points because it's not a simple, straight replacement for the wrought materials.

I hope this helps.

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

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[TVP]

LONDONDERRY,

Keep in mind that residual stresses in bars, plates, etc. are three dimensional in nature, meaning that they may be complex. For example, in a bar there could be a compressive stress in the circumferential direction, compressive stress in the radial direction, but tensile stress in the longitudinal direction. The following are a couple of interesting links on residual stresses in Al plate:

http://www.lanl.gov/contour/

http://www.lanl.gov/residual/alumsm.pdf