The Shop > Finishing
cheapo flat surface lapping
bogstandard:
I don't know which John you are referring to BR, but with a surface plate, just sliding a height gauge across it will instantly take it out of what would be called 'flat'. It all depends on how flat you want something to be.
The flatter your requirements, the more expensive it gets. There is a great deal of difference in the cost of a flat surface plate for general purpose workshop use, or one used in say a metrology lab used for calibrating instruments, or the flatness required by the flat earth society.
To someone in the distant future, John Hill's flat optics would most probably be classed as bent like a banana. It all depends on how well you can actually measure it for flatness, and how to get it there.
Bogs (John)
John Hill:
John Bogs, I think the amateur optics people accept that what they make as a 'flat' is really a section of a sphere of many kilometeres in radius. I do not know if a 'perfect' flat is physically possible, maybe by rubbing an infinite number of 'flats' together for an infinite length of time.
So no doubt what you say is true, the best flat of today will indeed look pretty poor in the future!
bogstandard:
John,
We used to get our 4ft diameter 6" thick cast iron lapping plates flat by doing what you said. We used 3 free rotatings rings left running for hours at a time, eventually the wear curve was flatted out. But you had to keep a very close eye towards the end because it would soon 'go over the top' and curve the other way.
The smaller 2ft ones I would face off on the lathe, and the 9" ones I faced up on the surface grinder. When the 2ft ones were checked for flatness in the metrology lab, if they were within no more than 0.0002" deflection, that was good enough for what we were doing. It was easy getting within that measurement on the suface grinder with the smaller plates.
John
jim:
i use a kitchen granite worktop saver from asda as a "flat surface", i couldn't afford a proper surface table and got one from asda for about £12. its about 3/4" thick and flat to within a couple of thou, grown attached to it now :D :D
ozzie46:
I got mine from a place that makes granite counter tops. Asked for the cutout piece that is removed for the sink. Price, FREE. :D :D :D
Ron
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