Seeking flat table - Gunner?

Buy a length of 8" C-channel and have it Blanch ground.

Old Craftsman or Atlas lathe beds go for just about scrap price. I have two

42" but it would cost quite a bit to ship them. You can have one for $10.00. Steve

LBGARY fired this volley in news: snipped-for-privacy@enews5.newsguy.com:

There's a honeycomb sort of flat table made by furniture makers for just that purpose. The method to build them is simple and quick, and I've seen a writeup of it on the web... just can't recall where.

Part of the process is a "truing" method that guarantees perfect flatness over the surface. Even though it's made of wood, it's quite rigid, and adaptable to any size you might want to make it in.

Hmmmm... It MIGHT have been in an amateur holography site where I saw that.

My personal solution to the problem of a flat for testing laser projector filters and galvos was to shim a piece of plate glass on my concrete floor until I was assured it was flat within a few thou... I used shims where it needed a lot of take-up, and clear packing tape for the thin areas. (have to press down on the surface while checking with a good straightedge to make sure it's well-supported all over).

Then I poured about a 1/2" layer of portland-based leveling compound on it, and slightly roughed the surface up when it got cheezy (with a dinner fork ). After it cured hard but not dry, I added another

2" of pea-mix reinforced with #6 fence wire strands, and let the whole slab cure.

It wasn't surface-plate perfect, but perfectly flat enough for the job. Custom "flat" for tiny bucks.

LLoyd

dags torsion box

Thanks for the offer, but the Hardinge lathe bed I mentioned is a flat piece of steel, about 1" thick. Most other lathe beds don't provide a continuous flat surface to use a gauge on, but two parallel rails.

The wooden torsion box is plenty flat and rigid for building furniture on, but what I need has to be able to resist very localized bending forces, like cold bending a bicycle tube while the bottom bracket is clamped to the "true" surface, so I think it must be made of steel. Gary

I'll look into Blanchard grinding. Is C-channel pretty stable?

You might pay less for enough flatness and stiffness by having someone flycut a piece of wide-flange beam.

I have a Hardinge lathe bed that I'd sell if it's not too worn for your needs. I think it's about .007 low at the worst point. It weighs over 70#, so it'll ship by UPS or FedEx ground, but not cheaply. The email address on this message is good if you're interested.

I'll look into Blanchard grinding. Is C-channel pretty stable?

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I've used ground C-channel as machine bases with great success. I use the thicker stuff not the bent stuff.

Not as good as I or H beam.

I'll look into Blanchard grinding. Is C-channel pretty stable?

If it's that critical, weld a piece of 1/4 or 5/16 plate to cover the open side, and have it stress relieved before grinding.