Lapping plates-3 Plate method

[Janderso]

If I remember right Tom used a circular saw to do his cuts. I don’t think he has a horizontal mill either.

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.

 

[C-Bag]

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.

 

[Janderso]

Interesting project. Please provide details of the CI you've got (dimensions?, grade?, source?).

Watching.

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.

 

[savarin]

How are you going to test for flatness?

 

[C-Bag]

How are you going to test for flatness?

Optical flat like Oxtool?

 

[savarin]

I use a home made 3 ball spherometer to test for curvature but they are not "ultra precision" accurate

 

[Janderso]

How are you going to test for flatness?

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.

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]

a home made 3 ball spherometer

I was checking out the one Tom Lipton made. Could I make one? I don't know.

 

[C-Bag]

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.

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

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.

 

[savarin]

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

http://www.gagesite.com/documents/Metrology%20Toolbox/How%20to%20Measure%20Flatness%20with%20Optical%20Flats.pdf

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.