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Danny7

Well Known Member
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.
 
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.
 
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.
 
212F

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
 
az_gila said:
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")...
Click to expand...

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?
 
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.
 
Temperature

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
 
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.
 
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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).
 
Richard,
<<Off to the Guard base where we calibrated the eddy current machine (which can be used for sorting alloys)>>

I have a very good airplane friend who runs a scrap yard. Sometimes the incoming stuff includes billet aluminum machine shop cutoffs, some with mill marks, some without. The nice bits often find their way to my shop. Material without mill markings obviously can't be used for anything but fixtures and tooling. Is it possible to positively, absolutely identify aluminum alloys and temper with eddy current? Darn shame to deep-six a 4" round three feet long <g>
 
Dan,

That's one of the applications for eddy current testing. However, it was a function we virtually never used, so we had little experience with it. The day I checked my HS, a reservist from a nearby unit happened to be there and she had just used that function to check a part on several C-130's, so we had some help.

Eddy current is a great system for checking for cracks in aluminum and that's what we used it for.

The various machines vary a lot in terms of simplicity, ease of use and cost. The machine they had when I got there was a great little unit, easy to use and gave clear results for crack checking, which was what we used it for almost all the time. The Air Force, in their infinite wisdom, replaced it with one that was virtually worthless and we held onto and used the old unit as long as possible. Then a 3rd unit came along and it was pretty good, although most of us still preferred the original one, but it wasn't long before the last one we had went belly up. So you need to know the machine before you buy.

I see on the internet, they go for somewhere around $5k and up. One of the guys in the shop wanted to do NDI work after he retired and got a good buy on Ebay. There isn't much of a market for used eddy current testers. You might find one as gov't surplus.
 
Thanks Richard. I'm unlikely to buy an ECM, but it sounds like it would be worth asking around my area.

Yo, Smokey, you out there? Happen to know if they have one over at Dannelly?

Danny, the practical answer is the 140F pain threshold. I shape aluminum bits with a 6x48 belt sander. If you work barehanded, and one hand is supporting the part a few inches from the end, and you don't push the cut too hard, you'll get plenty of warning <g>

High pressure against the sanding belt isn't all that great a habit. Try a coarse grit, 80-100 area; I suspect whatever is loaded on your brother's knife machine is way too fine for aluminum. The bad habit problem shows up when you work 4130. Push hard and it will glow red pretty quickly. Dunk it in a water bucket and you're created a part with a quenched edge. Very hard, maybe brittle, maybe has a few tiny cracks. Not real good for something like a flying wire tab.
 
Hot Aluminum

All of the shaping and what-not of the aluminum I've worked with has gotten hot quick and cooled quick. 212 degrees???, I can't be sure but I don't wear gloves for that reason. I don't want the aluminum to get so hot as I can't handle it. I work slower or on different areas on the same part.
 
DanH said:
Richard,
<<Off to the Guard base where we calibrated the eddy current machine (which can be used for sorting alloys)>>

I have a very good airplane friend who runs a scrap yard. Sometimes the incoming stuff includes billet aluminum machine shop cutoffs, some with mill marks, some without. The nice bits often find their way to my shop. Material without mill markings obviously can't be used for anything but fixtures and tooling. Is it possible to positively, absolutely identify aluminum alloys and temper with eddy current? Darn shame to deep-six a 4" round three feet long <g>
Click to expand...

Ehh, if you have a few possibilities, or want to sort a bunch of stuff that's known to be 7075 by temper (T4, T6, etc) then it'l work, but if you hand someone a stick of aluminum bar, and bunch of standards and an eddy current instrument, the best case is that they come back with a number of possibilities of what it is. Now, if you're getting scrap from an aerospace shop that ONLY cuts 7075-T6 and 2024-T4, you could probably sort it out. But don't sell ME any parts that you've sorted that way.
 
Your part is OK

I assume you're talking about 6061-T6 aluminum, since there are few instances where you would need to shape a skin, and most of the structural aluminum parts (bracket and such) in a Vans kit are made from 6061-T6. T6 is a temper, and T-6 temper is achieved by the following heat treatment process:

"Solution heat treat at 990 F for adequate time to allow for thorough heating and then water quench. Precipitation hardening is done at 320 F for 18 hours and air cool, followed by 350 F for 8 hours and air cooling". (http://www.suppliersonline.com/propertypages/6061.asp)

Having said that, annealing will remove a metal's temper, rendering it softer, more ductile, and lower strength. "Annealing should be done at 775 F for 2 to 3 hours followed by controlled cooling at 50 f per hour down to 500 F, then air cool". (http://www.suppliersonline.com/propertypages/6061.asp)

I suspect that the AC 43.13 reference is very conservative and allows a mechanic to gauge temperature with a very simple sensor (that being a bucket or splash of water)

So the bottom line is, you're OK as long as your parts stay below the anneal temperature of 775 deg F. Since it's hard to know how hot that leading edge against the belt is getting, try to avoid a sizzle when you dunk it in water. But I don't think there's any way you are getting to 775F unless you are purposely shoving your part into the belt so hard that the motor is taxed! Also, keep in mind that annealing takes TIME at extended temperatures, too!

Your parts are fine. :)
 
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DanH said:
Thanks Richard. I'm unlikely to buy an ECM, but it sounds like it would be worth asking around my area.

Yo, Smokey, you out there? Happen to know if they have one over at Dannelly?

Danny, the practical answer is the 140F pain threshold. I shape aluminum bits with a 6x48 belt sander. If you work barehanded, and one hand is supporting the part a few inches from the end, and you don't push the cut too hard, you'll get plenty of warning <g>

High pressure against the sanding belt isn't all that great a habit. Try a coarse grit, 80-100 area; I suspect whatever is loaded on your brother's knife machine is way too fine for aluminum. The bad habit problem shows up when you work 4130. Push hard and it will glow red pretty quickly. Dunk it in a water bucket and you're created a part with a quenched edge. Very hard, maybe brittle, maybe has a few tiny cracks. Not real good for something like a flying wire tab.
Click to expand...

i believe the belt i was using was 36 grit. i could be wrong though. It didn't load up with al much at all. i was handling the material the whole time with my bare hands, it would get warm but i could still hold it no problem then i'd cool it in water.
 
thanks Noah and everyone else.

I only check email about 2-3 times a week, so i couldn't get back before now.

danny
 
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