Lapping plates-3 Plate method

C-Bag said:
If I remember right Tom used a circular saw to do his cuts. I don’t think he has a horizontal mill either.
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Tom showed how a guy/gal could use a circular saw if he had to but he used his band saw the same way I did it.
 
More proof my memory is not right. :) Its another one of those things that you don't think is that big of deal because it doesn't have to be perfect but is more of a pain to do in real life.
 
extropic said:
Interesting project. Please provide details of the CI you've got (dimensions?, grade?, source?).

Watching.
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I purchased Durabar-80-55-06 Cast Iron.
5 5/8" wide, a little over 1 inch depth.
I must say, this is a real pleasure to work with. I think the end surface finish will be very good.

I assume cast iron is used for these plates because of the inherent stability and hardness??
You can see the heat affect zone clearly after I turned the face. I'm going to leave the outside mill scale as is. It's about as round as an egg, but who cares.
 
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How are you going to test for flatness?
 
I use a home made 3 ball spherometer to test for curvature but they are not "ultra precision" accurate
 
savarin said:
How are you going to test for flatness?
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Good question. I don't have an optical flat. I do have a surface plate and contrast material. I have a surface gauge with a tenths dial test indicator

What about this on ebay?
I don't know what all the symbols mean.
www.ebay.com

FastSHIP* Optical flat parallel double side 30mm / 1.18” Flatness 0.07µm λ/10 | eBay

Find many great new & used options and get the best deals for FastSHIP* Optical flat parallel double side 30mm / 1.18” Flatness 0.07µm λ/10 at the best online prices at eBay! Free shipping for many products!
www.ebay.com
 
Janderso said:
Good question. I don't have an optical flat. I do have a surface plate and contrast material. I have a surface gauge with a tenths dial test indicator

What about this on ebay?
I don't know what all the symbols mean.
www.ebay.com

FastSHIP* Optical flat parallel double side 30mm / 1.18” Flatness 0.07µm λ/10 | eBay

Find many great new & used options and get the best deals for FastSHIP* Optical flat parallel double side 30mm / 1.18” Flatness 0.07µm λ/10 at the best online prices at eBay! Free shipping for many products!
www.ebay.com
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yeah that whole optical flat thing is way beyond my pay grade. Thing with that one you posted is it's basically just a piece of glass without the special light. A while back while that whole vid was more fresh in my mind on CL some dept here a CalPoly wanted to get rid of a set of flats and the special light. I was soooo tempted, just like what you are doing with the 3 plate. But I know if I fall down that rabbit hole I would get lost to the obsession of perfection.
 
That seems to be a good quality flat.
The special light only has to be monochromatic so a sodium lamp should work.
have a look at this it may help
I've only ever tested concave surfaces for telescope mirrors so cant be of much help here.
My 3 ball spherometer is very simple, an aluminium triangle with a steel ball on each point with a dial gauge in the centre.
There are a set of formulae to work out the sagitta of the curve but for a flat a same reading all over the plate should indicate a reasonable level of flatness.
I leave it to other more in the know as to how accurate this method is.
I will get a couple of photos and try to find the formulae tomorow.
 
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