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7075-T7351 Aluminum, Gummy? How to machine?

I can't remember how many probes there were, this was back in the early 90's. I do remember the inspector being very careful to calibrate the meter with the calibration coupons before testing parts.

It might have had only 2 probes, as a lot of parts they had to check were contoured with no flat surface for 4 probes to contact simultaneously.

If only two probes total, contact resistance will likely dominate, rendering the measurement useless. This would be especially true of aluminum, which forms oxides readily.

If the article has a standard, known shape (machined part or standard coupon), one can use two groups of two probes. Nor is it absolutely necessary that the four probes be in a line, although that's simplest.

I would guess that Inspection measured the resistivity of a known-good sample, and tested for the same answer (within some small tolerance), thereafter.

I was a contractor at McD, and the company i worked for were making parts for the Wind Tunnel shop at McD. One day I get called down and shown a part they had made, the inspector said it was supposed to be made from 7075, but the electrical conductivity showed it to be 6061 (there was only 7075 and 6061 at the shop so it was an either/or situation).

The difference in bulk resistivity is fairly large for those two alloys:

6061-T6 4.066e-8 ohm-meter.
7075-T6 5.210e-8 ohm-meter.
7075-T73x 4.310e-8 ohm-meter.

The smallest ratio, 4.310/4.066, is 1.06, or 6% difference.

Which way exceeds the density difference, and is easier to measure in the shop.

Turned out a part got scrapped out on night shift, they'd substituted the material that should have been 7075 with a piece of 6061. and being night shift did tell anybody what they had done. After that debacle every time new parts came in I'd get called down to the inspection lab so they could do an incoming inspection with me there. They were always hoping to find another part that had the wrong material. Luckily it never happened again.

Yeah. The night-shift folk thought that nobody could tell, and when they found out different, stopped attempting that dodge.
 
I'm not sure if he's gonna come back, but I have a question about what he was describing as gumminess.

Then again I'm probably beating a dead horse lol

What does it mean that it was gummy about a thou? That there was a burr after facing or endmilling? Or is this describing that "fuzz" you can sometimes get when machining a surface. Like recutting chips with an endmill or chips welding and depositing with a facemill?

I wouldn't either of those describe a material as gummy. I could be machining some 50rc steel and have some burrs. I wouldn't call that gummy.


Am I missing something?
 
I love machining 7075, it's much nicer than 6061. probably my favorite material, good thing because I do a lot of it! It face mills great for me, sounds like you need to talk to a proper tooling dude. [email protected] will hook you up with the right stuff and speeds and feeds as well. Why did you not buy your material with certs? Do so from now on and you will have no question about what you have.

Whenever I buy AL from McMaster, they automatically send me certs.

BTW, 7075 machines very nice for me too.

Bill
 
If only two probes total, contact resistance will likely dominate, rendering the measurement useless. This would be especially true of aluminum, which forms oxides readily.

If the article has a standard, known shape (machined part or standard coupon), one can use two groups of two probes. Nor is it absolutely necessary that the four probes be in a line, although that's simplest.

I would guess that Inspection measured the resistivity of a known-good sample, and tested for the same answer (within some small tolerance), thereafter.



The difference in bulk resistivity is fairly large for those two alloys:

6061-T6 4.066e-8 ohm-meter.
7075-T6 5.210e-8 ohm-meter.
7075-T73x 4.310e-8 ohm-meter.

The smallest ratio, 4.310/4.066, is 1.06, or 6% difference.

Which way exceeds the density difference, and is easier to measure in the shop.



Yeah. The night-shift folk thought that nobody could tell, and when they found out different, stopped attempting that dodge.

Aluminum is normally tested for conductivity (not resistance) using a device with a single probe (it uses eddy current). You could be pretty confident the material is correct with both hardness and conductivity measurements.
 
Do me a favor and keep your uneducated and insulting opinions to yourself. Your advice is not helpful.
He's correct though. I machine a lot of 2024 and 7075. I have never run into a piece that was gummy. You can drive 20 and 70 series aluminium very hard and it still makes a nice chip.
 








 
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