Lapping plates-3 Plate method

[hman]

Hmmmm ... Aren't LEDs monochromatic? I'd be tempted to try a high intensity red or green LED for use with an optical flat.

 

[Weldingrod1]

Reading says yes... use multiple and a diffuser to spread/even out the light. YMMV

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[savarin]

My old spherometers
The large one is in thous imperial and the smaller one is hundredths mm
These were made before I had a lathe so are assembled with epoxy.

 

[hman]

Neat! Are the corner balls located precisely enough that you can calculate curvature from the dial reading? I can well understand where such instruments would be of value in grinding telescope mirrors.

 

[savarin]

Its not so much where they join the plate but where the balls actually touch a flat surface,
The distance between each ball is measured then the calcs start to get the base reading.
Zero the dial gauge on a flat sheet of glass, then place on the mirror, the the dial reading gives the depth reading which is then worked on further using the three balls measurements to give the real sagitta which then can be used to get the focal length. Phew.
I havnt done it for years so am very rusty here.
But, zeroing it on a sheet of glass then reading the plates should be a pretty accurate reading of flatness.
If its a plus its a hollow, if its a minus its a bump

 

[rabler]

Hmmmm ... Aren't LEDs monochromatic? I'd be tempted to try a high intensity red or green LED for use with an optical flat.

Most are. To be clear for everyone, the white ones are not

 

[Janderso]

I bought a monochromatic light to go with the optical flat from the Ruskies.
https://www.ebay.com/itm/221081770653?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2648

The plates so far>>
I bought two 600 grit lapping products and one 1,000 grit. (silicon carbide and garnet)
Both are supposed to break down which is what I want.
I mix the powder in lighter fluid.
This is kind of a zen thing. I just put on my audiobook and follow the process.
A on B
B on A
B on C
C on B
A on C
C on A
Then begin again. I do one minute sliding 12:00 to 6:00, - 3:00 to 9:00 - 6:00 to 12:00 - 9:00 to 3:00 and one minute spinning the top plate in a circle.
The plate below is after 3 complete cycles.

 

[Weldingrod1]

Love the spherometers! I need to make one, or a repeat o meter...

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[Janderso]

After about four hours or so, I’ve found success. It’s a Zen experience.
I put a book on and carefully went through the timed sequence for lapping in the plates. The last two cycles, I switched to 1,000 grit.
I found a hardened little spacer, I surface ground one edge then lapped it on one of my new plates.
The little sucker wanted to ring to the plate.
I switched to diamond paste for the spacer.

 

[extropic]

Excellent progress !!!

You make it seem easy.