PDA

View Full Version : Aluminum sheet - standards for flatness?



Bill Hopcraft
03-24-2007, 02:10 PM
I'm making some jigs for a woodworking project. I needed something fairly stiff and dead flat, so I picked 6061-T6 aluminum sheet, 0.160 thick. I bought a piece 12" x 24" (about twice what I need) online and was pretty disappointed to find the material was far from flat. There's better than a 1/8" bow over the 24" length when you lay the sheet on a flat surface. I've never bought sheet goods before, only bar stock, so I need your advice before I complain to the supplier. Is a bow like this common, or was I sold substandard material?

Paul Alciatore
03-24-2007, 02:31 PM
Sheet metal is made in rolling mills. It may have a fairly uniform thickness, but I doubt that there is much of a spec for flatness. Even if the sheets are flat off the rolls, it will be subjected to various bending stresses on the way to your shop. As an example of handling methods, I recently saw a stack of 1/2" sheet rock on a forklift being loaded on a truck. It bowed over 2 feet. Sheet rock will straighten itself after such handling, aluminum will not. Heck, thin gauge sheet metal is even sold in rolls. I doubt that the supplier can guarantee a flat sheet.

If you need a flat surface, you may need to add stiffeners to the structure or go to a much thicker material. This is why machine tools have hundreds of pounds of cast iron in them. Even cheap saw tables are made with cast aluminum that is 3/4" thick or more.

Another possibility for a thin, flat surface is glass. Plate glass is quite flat and can be purchased in thicknesses of 3/8" of more for some uses. I just purchased a piece that is about 5/8" thick and about 2 square feet for shop use.

John R
03-24-2007, 02:34 PM
Plate that thin nearly always has to be stiffened . If you are cutting small parts from the sheet then the small bend in the part may not matter.
John R

ptjw7uk
03-24-2007, 03:51 PM
You could mount the aluminium onto a ply sheet? It all depends what type of jig you are making, jigs used for wood routing are usually on the thick side (like the user) and get some of their requred rigidity by being clamped the piece being routed. A lot of post form jigs are made from phenolic or high density polyethylene sheet. The type of material used is dictated by a lot of different, such as how often will it be used so that heavily used would require hard wearing and stable material wheras one offs could be made from quite thin material.
I have used birch ply with a bonded aluminium sheet of 3/16 thickness which is a reasonable compromise.
On the aluminium sheet not being flat, I think you are expecting a lot from something only 0.16" thick which will bend under its own weight.
Peter

topct
03-24-2007, 04:11 PM
Sounds like they cut across the plate to get the 12 in. instead of lengthwise.

If you can get them to do anything about it?, good luck.

Nothing wrong with the material for your use, but it would have looked better if cut up the right way.

darryl
03-24-2007, 04:28 PM
Maybe what you're looking for is tooling plate- though in that thinness I don't know how flat it would be. All my working life I've used the 'sight and select' method. If I need something to be flat, I have to verify that property myself. I bought some tooling plate in 6061, and that was nicely flat. That was 3/8 and 1/2 thickness though. Other times I've needed very flat and thin material, and have made it using fiberglass and resin, cast between sheets of glass. That's a bugger to work with, though, if you need holes, etc. Dulls bits in a hurry.

If you do use glass as a flat reference surface, you still have to make sure it's sitting flat. May need to shim, even if it's fairly thick glass.

Sigh- there's a deal on right now at Liquidation World on some glass tables. The glass is 1/2 thick, and there's gotta be about 10 sq ft of it in total, four base pieces and one top. So far I've resisted buying it, but dang, I'm going in there again today- that would make a nice, but fragile, low grade flat.

DR
03-24-2007, 06:43 PM
Next time order Mic6 (or is it MIKE6?).

It's flat and uniform thickness. It doesn't have exactly the same machining characteristics as 6061-T6, almost powdery with small chips in comparison, but still easy to machine. This is what I would consider to be tooling plate.

I'm not totally sure it comes in that thin a size though. I only know it in nominal inch thicknesses like 1/2, 3/4, etc.

And no, it's not unusual to have that much out-of-flatness on aluminum sheet.

darryl
03-24-2007, 07:32 PM
I read this again- you're making woodworking jigs-

I've had pretty good luck making jigs out of mdf core and laminated with formica or arborite. One jig I use is made with 1/8 inch core and gp laminate, which is the thicker of the two commonly available, at 1/16 inch. That makes a pretty solid jig, and it's only 1/4 inch thick total. I have some which has a 3/8 core of mdf, and it's very strong. In that you could implant t-nuts for fastening, and I imagine you could make t-slots, depending on how you would clamp into those slots.

I recently epoxied two pieces of laminate together as a test of the adhesive integrity. Well, that sure beats contact cement for strength! It cuts well on the table saw without any apparent delamination at the edges. One of my next tests will be to laminate a fiberglass layer between plastic laminate. I expect that to be very stiff, and the thickness will be around 3/16 total. There wouldn't be room for a t-slot in that, and barely enough room for a small t-nut to be implanted, but you could probably get away with an 8-32. With that small size, the flange could be flush with the bottom, yet leave room for the glass cloth and the top laminate above it. You'd be cutting some off the length of the t-nut to make it flush with the top of the lamination stack, but you'd still have about 1/8 inch length of threads in it.

Just some ideas of what could be done and be workable. In my case, I wanted the laminate package to be fairly thin so I didn't use up too much of the capacity of the blade height on the table saw.

Another material that works well instead of the mdf core is the thin veneer plywood. We had some 1/8 thick stuff at work which has about 4 or 5 layers, can't recall now, but once laminated on both sides it became very stiff. I used contact cement on that, but doing it again I would use epoxy. The only problem is that you have to make it flat and keep it flat as the epoxy sets, otherwise you end up with a permanently curved sheet. Same goes for contact cement- you can usually get one side laminated flat, as long as your working surface is flat, but for the second side you may have to fasten the sheet down first to take any curl out before laying on the second laminate.

I'm tending to like plastic laminate construction for jigs because the surface is quite durable, doesn't normally raise burrs (which aluminum can do), and it just feels better to use.

Thanks for posting this topic, it's reminded me to do the fiberglass lam test. I'll post some results once I have some.

Your Old Dog
03-24-2007, 07:45 PM
I use Baltic Birch plywood for all my jigs. Very nice product but just comes in weird sizes of 5'x5'. Not much I can do with aluminum in the wood shop that I can't do with Baltic Birch.

CCWKen
03-24-2007, 08:24 PM
Most all sheet products are cut from rolls. The rolls can be as large as 8' in diameter. They're run through a straightener then sheared. Your piece makes up less than 7% of a full sheet so no telling what it's been through or where. You can't expect a small piece (that's most likely a drop) to be a precision cut piece unless it was so cut.

Don't expect 6061 sheet to hold a flat surface. Or any sheet for that matter. It's not until you get into plate thicknesses ( > 1/4") that you can expect to keep jig rated stability. Also, expecting "dead flat" from ANY mill product is asking too much.

If you were closer, I have a piece of 3/8" 306SS plate I've been tripping over. But even that wouldn't be "dead flat" unless you had it ground.

Rich Carlstedt
03-25-2007, 12:29 AM
For flatness in Aluminum, only CTP (Cast Tooling Plate) will give you the flatness you want. This material is also called Mic6 by some.
CTP is very rigid and ground flat at the mill. It is then covered with a plastic protective sheet. It is hard and brittle due to the very large grain structure.
CTP is not in any way 6061. 6061 will move any time you cut it or machine it.
CTP has no formula !!! you will not find one, nor is any tensile strength guaranteed. the reason is the Aluminum mills use junk /recyled/ ??? for their own alloys, to produce the large crystal, and don't care what the formula is because the goal is "nonwarp and flat" We bought sheets 5x10 feet directly from the mills, in 20 to 50 sheet batches. I heard that 1/4 " was available, but have never seen any. All ours were .5, .75, 1.00,1.25, 1.5 , and 2.00.
You may see less than .010 from flatness in these in 4 feet .
It is gummy when machined, so coolant/fluids are needed.
Rich