The Shop > Finishing

cheapo flat surface lapping

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sorveltaja:
1: Take a finest grit wet sanding paper sheet that you have.
2: Then find the flattest material sheet you have (sacrificial).
3: Place that sanding paper sheet upside down to the surface.
4: Apply some abrasive paste evenly (be it toothpaste or something more fancy).
5: Take your time to lap (depends on paste and material you lap).

I have tested this only on aluminum with toothpaste so far, but the result was beyond my standards. Nice and even satin surface.

P.S Soaking that sanding paper sheet into water, that has some soap in it, to break water's surface tension, helps to keep the sanding paper sheet in place.

bogstandard:
S,

I remember seeing some old photos of how surface plates were made, even great big ones. They were stacked on top of each other, face to face, with a lapping compound between, and then just lapped together. They wore each other totally flat.

I suppose they do it all by grinding nowadays.

The plate glass platens out of old photcopiers are perfect for what you are doing. But mine couldn't resist being tapped with a big lump of steel. It lasted for about 10 years before giving in.


Bogs

John Hill:
If I am not mistaken any two 'flatish' surfaces rubbed together in a random fashion will form a portion of a sphere though the radious of the sphere may be very large and the plates then appear 'flat'.  Rub three plates together in the same say and you get flat surfaces.

Baldrocker:
Hi sorveltaja
Not wanting to hijack your thread

John
What sort of out of "flatness" are we talking here, critical to our needs
or just a poofteenth or gnats wisker? A long term project relies on the answer.
BR

John Hill:
BR, the home telescope people know how to grind surfaces to a fraction of the wavelenght of light! Quite a bit less than 1/1000th of a millimetre.

Google 'making optical flat', I am sure there will be a description out there!

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