Milling a plate to be flat

severial ways to tackle this. one I would put in my shaper. two clamp it in a 4 jaw chuck. face it off in the lathe or clamp the chuck to the mill table and mill it flat.
 
I recently made a fixture plate out of 6061 and was looking at the same problem. Since it was extruded material I expected stresses that would cause warping when I fly-cut the bottom and top surfaces. In addition, the piece, as received, was warped. I thought about drilling and countersinking the 2 mounting holes on each side of the plate, then screwing the plate down and fly-cutting it: but the plate was stiff enough to cause concern about repeatability of the hold-down forces to ensure a planar surface every time I mounted it to the table. It looked to me like the best approach was to sequentially flatten both sides, but the problem was how to do this and avoid spring-back when the plate was un-clamped.

To solve this, I made two low-profile clamps out of square steel stock. Both were a little less in height than the plate I was making, and both were drilled and counter-sunk for hex head mounting bolts, to attach them to my mill table. One of them then was flipped 90 degrees and drilled/tapped for two 10-24 set screws. I bolted the "plain" clamp to one side of the table, placed the workpiece I wanted to flatten on the table and pushed it into firm contact with the clamp. The other clamp was loosely installed on the opposite side of the table, with the set screws protruding about .1 inch, pushed into firm contact with the plate-to-be and bolted in place. I tightened the set screws to hold the plate firmly in place and fly-cut the plate. Flipped it over and did the other side.

The clamping arrangement did not impose any additional forces that would warp the plate -- they all were in line with the top and bottom surfaces of the plate. The internal stresses of the plate were the only forces distorting the plate. I figured that the plate WOULD warp after being machined, but the effect would be small enough to address with the clamping forces used to bolt the plate down. This turned out to be the case -- when installed the plate was flat enough for my purposes, anyway. For more exacting requirements it might be possible to improve things even more by repeating the sequence of milling the front and back.
 
Wow! All of the replies. I used some square unistrut washers to make "fake" low profile edge clamps. The held it ok.
Milled off about .020" to get a cut across the entire face. But Near the end, I discovered that in some areas, every time I passed the cutter over it, it took a hare more metal off. But not every where. So it must have been warping more as I made each pass. And this is all with one locked setting for the depth / vertical. So yes, I have ordered flat stock that has specs for flatness. The project is for my 3D printer - the stock bed is made from what must be just ordinary 1/4" thick aluminum. It's dished at least .020" I have 5 of these that I maintain, between the work owned ones, and mine. Every one has a different warp to the build plate. It causes problems with the first layerd of prints - they don't stay stuck, it matters where you place small parts on the bed, etc. The first 2 [for work] I made new plates out of 3/8" specified plate from McMaster, but it was EXPENSIVE. I just got my own printers, and figured I'd try the mill on the original plate, since it seemed like it could work, and I'm not making $ with them... I also recently got feedback from another 3D printer forum, someone found a source [direct from a metal supply house] for similar speced metal for a fraction of the price, so I decided to give it a try. Those should show up next week.

I still learned stuff trying this, also, trammed the mill, it wasn't off much, but I had most of the 8x8" plate flat with no line between passes that I could feel a step. So that's a good thing too.
 
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