Re: [CAD_CAM_EDM_DRO] Re: RepRap

Peter Reilley wrote:

>In "Foundations of Mechanical Accuracy" they describe
>how to make a surface plate with no reference surface.
>It involves grinding three square or round plates together
>in a certain order. If done properly the three plates
>will grind each other flatter and flatter. There is no
>theoretical limit to this process. You can go to optical
>flatness and further. All done without precision machinery
>or measurement tools.
>
>

I bought a decent surface plate on sale from KBC, and went to pick it
up at their St. Charles, MO location, as the shipping on these things will
run more than the plate itself. (They were a bit dubious about forklifting
a 350 Lb surface plate into a Toyota station wagon, but it did fine!)

It doesn't really make sense to make one of these, unless you have a
masochistic streak. I hand-scraped several precision tools on it, and
kept my straightedge true while doing my 6' lathe bed. Then, to
make some other tools, like squares for aligning a mill's axes, I needed
a precision hand-scraped square. A few came across eBay at high prices,
and I didn't manage to snag any of them. I finally got a rough casting
for one on eBay. It was listed as "precision hand scraped". Well, I took
that with a big grain of salt, and it turned out it had been "broken up",
meaning the hard skin of the casting had been hacked off with a chisel.
That was fine, I wanted to scrape it in myself, so I could KNOW it was
true. I bought a cheap 8 x 8 x 8 ground angle plate on sale and cut it
in half, making two 8 x 8 x 4" angle pieces with one rib each. I hand
scraped in one surface of each of these 3 pieces on the surface plate.
I labeled them A, B and C. I then put two at a time on the surface plate
in the order A-B, then B-C, then C-A and spotted and scraped the 2nd
face of the 2nd piece of the pair. When done, I had 3 plates that all
verified flat against the surface plate, and by definition had to all be
a true 90 degree angle. This process works the same as the automatic
generation of a flat, if you already have a flat surface to place both
squares on.

Jon