how hot can you get AL while grinding?

Danny7 · #1 05-16-2008, 03:48 PM

I am building an rv7 emp, and had to contour the 710 and 714 reinforcement parts.

my brother has a knife grinder (fancy belt machine) and it removes metal really well. I ground the angles and radii on the ends of the parts, and the aluminum got fairly hot each time i ground, which i would then dunk in a water bath. about 2-3 seconds of swishing it in the water brings it back to 60 degrees or so.

so, was what i did a bad thing? how hot does the AL angle have to get to ruin it? does it have a heat temper?

i would grind about 1-2 seconds with lots of pressure, to remove lots of material and keep the belts in good condition. more pressure = new broken abrasive faces, rather than dulled edges.

N941WR · #2 05-16-2008, 03:55 PM

Danny,

I could not find a good picture of a "knife grinder" on the net and have no idea what one is. That said, if it is anything like a wheel grinder, you shouldn?t be using it to shape aluminum. Wheel grinders can load up very quickly when shaping aluminum and have a tendency to explode, which can seriously injure the operator.

Please make sure your "knife grinder" is approved for aluminum.

PS. Welcome to the forum.

tmbg · #3 05-16-2008, 04:42 PM

I believe the knife grinder he's talking about is a vertical belt sander with a very narrow (1" or so) belt. Oughta do alright for shaping aluminum.

What I WOULDNT do is quench the aluminum. That could change the temper undesirably.

Norman CYYJ · #4 05-16-2008, 06:30 PM

How hot do you want to make it?

az_gila · #5 05-16-2008, 06:32 PM

The max. temp the FAA recommends is in AC43.13 (where else?) and IIRC is 212F - below this, none of the usual alloys will change their temper (or get "quenched")...

Check my number for accuracy though...

gil A

RVNineA · #6 05-17-2008, 01:24 PM

Quote:

Originally Posted by az_gila

The max. temp the FAA recommends is in AC43.13 (where else?) and IIRC is 212F - below this, none of the usual alloys will change their temper (or get "quenched")...

212 deg F ? I wonder how hot the aluminum skin gets when a soldering iron is used on it to help remove strips of the blue plastic from lines of rivet holes?

ergie63 · #7 05-17-2008, 01:35 PM

Even though your iron is hotter than 212, the total quantity of heat it prodcuces is small and does it stay in one place long enough to get the aluminum anywhere near 212.

Its like a candle. The flame is hot enough to burn you, but you can snuff it out with your fingers without getting burned becasue there was not enough time for that heat to get into your fingers.

John Clark · #8 05-17-2008, 02:21 PM

This brings up a related issue. An avionics tech friend of mine was doing a whole panel update for a customer with a Cherokee '6.' The owner insisted that they powder coat the new panel. My friend refused, because the panel was a part of the structure and the heat of the powder coat cure would effect the temper of the part. Turns out that the powder coating cure heat was 390 degrees F. Not good on a structural, aluminum part.

John Clark
RV8 N18U "Sunshine"
KSBA

RScott · #9 05-17-2008, 06:40 PM

Time and temp are both factors, as I found out. Here's the tale:

Working on my HS in the winter. I went out to the shop and fired up a propane heater, 35,000 BTU & it was one of those units that is a vertical rectangle in shape, sitting about 30" tall. Had it sitting a foot or so beyond my work table, which was about 36" high. Grabbed my HS, set it on the table and it extended out maybe 18" beyond the table, right over the heater. Discovered it after about 30 seconds, and pulled the HS off the table ASAP.

Looking at the skin, there was kind of a lump maybe 8" by 15". It flattened out as the skin cooled and after cooling there was no visible evidence. . Checked 43-13, found the 212 number az_gila mentioned. I put a scrap of AL over the heater, let it cook for what I guessed was about the same length of time and checked the temp with a meat thermometer and it got over 400 degrees. (Yes, I know it was a crude way to measure the empt, but it was what I had.). Called Van's, checked the internet, couldn't find out much info. Van's said they had not heard of this particular screwup before and couldn't give me any advice.

But I was blessed with having retired from the Air Guard where I had worked in the NDI shop. I heated a scrap piece for what I knew was a longer time than the HS had been exposed so I had a test sample. Off to the Guard base where we calibrated the eddy current machine (which can be used for sorting alloys) on an area of the HS that had not been subjected to heat, then compared it with test piece. There had been a clear change. Then checked the heated area of the HS and found only a slight change in a small area, maybe 3" in diameter. In fact, this new reading we got was within the range identified for T-3, just different from unheated areas.

Later, I learned that tempering requires heating, then holding at that temp for some specified time, much longer than my HS was heated, then I believe there is a spec on how fast it cools.

I would not suggest that anyone heat or quench their AL, but recognize that when using a Scotch Brite wheel you will get some local heating beyond the 212 spec. FWIW, the Air Force says if a piece of metal is too hot to hold in your hand, it is at about 140 or hotter.

gtmule · #10 05-17-2008, 10:21 PM

Probabally more dangerous than somehow tempering your material is annealing it. If you holf ally at elevated temps for a while, you might turn your T6 into T7, but that'd be pretty hot (between 200 and 400F, IIRC), for a good while (hours and hours). More realistic is annealing the material which will happen very quickly, but at hight temps (>600F, IIRC).