On Sun, 14 Mar 2010 00:55:03 +1100, "stu" no where just yet
wrote:


wrote in message
.. .
On Sat, 13 Mar 2010 20:55:17 +1100, "stu" no where just yet
wrote:

I've just posted a picture in http://www.metalworking.com/Dropbox called
"stusplate"(is it the done thing to put a direct link here?)
As you can see from the reflection of newpaper, the surface has a very
fine
finish like no surface plate I have ever seen. It comes in a felt lined
lockable box. Does anyone have any idea what this sort of plate is?

Could be an optical flat. Your jpg doesn't show the size.
Inspection grade optical flats are typically 5 or 6" dia and
polished flat to within a fraction of the wavelength of green
light.

Jim
So that would be pretty flat then lol
I have two, one is 170mm(7inches) x 35mm(1.5inches) thick. The other is
220mm(9inches) x 35mm(1.5inches)they are labeled "precison plate"
I thought about optical flats but I thought they were made from
glass?(although I'm not even sure I've ever seen an optical flat)

Most optical flats are glass or quartz. I have a 6"dia x 1/2"
glass flat. Although your flats are opaque, bearing in mind the
polish, the thickness and the careful packaging, I still think
they are likely to be flat to optical precision standard

Jim.