Cheap suppliers of aluminum?

MillenniumFalsehood · Mon Jul 20, 2009 12:32 am

Okay, I've got a rather large project which needs a lot of aluminum. The stock I'm needing has to be at least a couple inches thick and quite wide, say 36"x60" pieces. Yes, I'll be milling them down, but I need to know if there's a source for the stuff which is discounted or something. I'm working with a shoestring budget, so the cheaper the better.

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Mon Jul 20, 2009 2:44 am

wow,.. uuhhm,...

I bought a sheet about that size, only it was about 1/16" thick...
From a surplus scrap yard (metal recycler) that sheet was $50 bucks..

So I would guess about $1600 bucks for a 2" think piece...

$546 bucks for scrap aluminum @ 1,29 per pound..
But that size isn't often considered scrap...

kenlilly106 · Post by **kenlilly106** » Mon Jul 20, 2009 11:20 am

A scrap/surplus dealer would be your best bet, I'd figure out how many sheets you'll need and negotiate with the dealer if you buy everything in bulk.

Based on the most recent price I was quoted for 3/4" thick 6061 grade aluminum, a new 30"x60" slab 2" thick would run $1300+ and weigh 353#.

Ken

MillenniumFalsehood · Post by **MillenniumFalsehood** » Mon Jul 20, 2009 3:41 pm

Okay, thats actually better than the estimate I had in my head, believe it or not.

Still, I'll definitely either need to rethink my build or use something other than aluminum. Are there cheaper metals out there which would still be relatively light-weight?

kenlilly106 · Post by **kenlilly106** » Mon Jul 20, 2009 7:38 pm

No warranties expressed, implied or given in regards to my pricing estimate.

Magnesium is about 2/3 the weight of aluminum, a quick search shows it available in thicknesses up to 14" in tooling plate grade, it can be a problem to machine though due to the combustibility of the fines.

The cheapest metals would be steels or cast irons, weight would be the biggest drawback. The lightest metals are magnesium, aluminum or titanium, the last one can be hard to machine.

Any reason you want to use metal?

MillenniumFalsehood · Post by **MillenniumFalsehood** » Sun Jul 26, 2009 3:53 pm

Toughness, for one thing. The parts I'm making are the components for the leg skeleton of an AT-AT, and they need strength.

I would worry about fatigue in plastics, though I know aluminum has a relatively high fatigue rate as well.

Would it be better to to create the parts from sheet metal?

kenlilly106 · Post by **kenlilly106** » Sun Jul 26, 2009 8:33 pm

Are you talking about bending sheet metal into the appropriate shapes and then welding them together?

It would work, it'll just take a lot of work.

Have you considered using tubing for the leg structure and skinning it as required? I know they make extruded rectangular aluminum tubing.

I used to do metal fabrication for a living, any questions just shoot me a PM.

Ken

MillenniumFalsehood · Post by **MillenniumFalsehood** » Tue Jul 28, 2009 2:27 am

I considered tubing for a second. But the problem is that I'd have to weld on new sections if I needed to revise it and I don't own a welder. With a sheet metal part I can just drill a couple of holes and rivet on a shelf for new parts.

Also, I have some experience folding sheet metal to create a part. This was from a 'Mech I had under construction(before I realized that using steel would make the leg impossible to move for the servos I have):

The parts before I welded them.

After welding.

I sprayed them with paint to protect the metal as soon as I ran the parts against a wire wheel.

With an AT-AT leg, the shallow angles that are involved will allow the motors to bend them easier than the Mad Cat's legs, so I think steel will work in this case. Perhaps even just cutting a piece for the inside face of the leg section and welding it to a couple of L-beams, with strips welded to the ends to stabilize and strengthen them.

Chacal · Post by **Chacal** » Tue Jul 28, 2009 9:33 am

How heavy will the rest of the AT-AT be? That's the important question, which will define what material to use for the legs.

If I had to do it, I'd use aluminum profiles (square tubes, L beams etc. — which are easy to find and cheap down here) epoxy glued and riveted together (pop rivets to hold while the epoxy cures). If you need to add brackets to hold the surface panels, glue them on. With enough contact area, you get very good adhesion, even with hot melt glue.

kenlilly106 · Post by **kenlilly106** » Tue Jul 28, 2009 8:48 pm

Something to watch out for is if you start with formed sheet metal and then add stiffeners to it, you may increase the weight beyond what an aluminum tube would be.

Also, the more mass you add away from the pivot point makes it harder to move, so I'd minimize the amount of material at the ends of the legs.

For AT-AT legs, would rectangular tubing with the servos at the joints work? You could skin them with lightweight "greeblies."

Ken

MillenniumFalsehood · Tue Jul 28, 2009 11:16 pm

Chacal: I would say no more than 75-100 pounds ideally for the upper body weight. The legs need to contain most of the weight to keep the overall machine stable, so I want to shoot for that for the upper body. I'll probably just use an empty fiberglass shell for the main body, while the head will contain two paintball guns for the main cannons and two pellet guns for the weapons on the 'cheeks'.

Kenlilly: Yes, but with the legs not going over, say, 25 degrees (most likely less), will it still be too difficult if there is significant weight in the feet? I won't make the amount outrageous, but at the same time I want to keep the center of gravity as close to the ground as possible. In any case, I could use sheet metal, but put cutouts in strategic locations to save weight.

I planned on using vacuformed ABS or fiberglass as the covering.

It would probably help you guys who are commenting and offering help to know that this thing will be quite tall, probably on the order of 8-10 feet tall. The reason is that the motors I'm using are a certain thickness, and in order for the legs to be in scale the entire machine will necessarily be that tall.

kenlilly106 · Post by **kenlilly106** » Wed Jul 29, 2009 7:06 pm

How much weight in the feet?

What's the torque rating of the motors?

I'm guessing a 2-3' length for the lower leg?

Ken

MillenniumFalsehood · Post by **MillenniumFalsehood** » Fri Jul 31, 2009 2:46 pm

I'm not sure how much weight I am going to put in the feet. I was going to get some lead bars and trim the weight accordingly.

The motors have (I'm guessing) about 100 ft/lbs of torque, but I'm using a worm gear setup similar to the one seen in the ICS books which will slow the motor significantly but increase the torque accordingly. The entire leg including the feet will be about 5-6 feet tall.

I'm using geared-down wheelchair motors for the hip joints and DC power-drill motors for the other leg joints. I'm sure that putting the motors on worm gears will transfer the kinetic energy very efficiently and allow them to lift their respective joints much easier.

kenlilly106 · Post by **kenlilly106** » Sun Aug 02, 2009 2:45 pm

The motors sound strong enough, making the structure strong enough to withstand the moving loads is going to be the next problem.

Have you considered wood? Lightweight but strong enough for this application, it can withstand cycling loads and bending without permanent deformation like metals, easy to fab and modify, and cheap compared to metals.

Ken

MillenniumFalsehood · Post by **MillenniumFalsehood** » Sun Aug 02, 2009 8:16 pm

Perhaps. I thought it would be easier to mount the control boards and motors inside of it if the whole thing were hollow. It would sure be easier with metals because they don't require routing out of the places where the components reside(assuming of course I use a sheet metal box-type structure instead of solid aluminum). I don't own a router and using a dremel will take forever compared to how long it would take to cut the pieces using the bandsaw.

Another thing is the aesthetic appeal and bragging rights of an all-metal internal structure.

lauriegrove · Post by **lauriegrove** » Fri Sep 04, 2009 1:37 am

Perhaps. I thought it would be easier to mount the control boards and motors inside of it if the whole thing were hollow. It would sure be easier with metals because they don't require routing out of the places where the components reside(assuming of course I use a sheet metal box-type structure instead of solid aluminum). I don't own a router and using a dremel will take forever compared to how long it would take to cut the pieces using the bandsaw.

Another thing is the aesthetic appeal and bragging rights of an all-metal internal structure. Razz

Yes it would be much easier to control it .