# Lapping plates-3 Plate method



## Janderso (Jul 25, 2021)

Oxtool did a segment on this.
I thought I would have a go. I really enjoy working with cast iron! The one drawback is the graphite!
I'm going to cut the grooves with my 14" Delta band saw. 90 degree grid should be easy this way.

I have some lapping powder on the way. 600 and 1,200 grit.
The diamond paste will embed in the plate so I'm using garnet and silicon carbide


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## benmychree (Jul 25, 2021)

Milling machine would be lots better for grooves.


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## Janderso (Jul 25, 2021)

benmychree said:


> Milling machine would be lots better for grooves.


Well, I thought about it, the bandsaw took about 45 minutes total.
I have clean edges-sharp.
John, what are the benefits of using the mill?
I pondered a slitting saw on the mill too.


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## rabler (Jul 25, 2021)

benmychree said:


> Milling machine would be lots better for grooves.


I'd be curious why it would make that much difference?  Sure, a slitting saw on a mill would allow uniform depth and spacing, but how much difference would that make in using it to lap?


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## extropic (Jul 25, 2021)

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

Watching.


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## benmychree (Jul 25, 2021)

Janderso said:


> Well, I thought about it, the bandsaw took about 45 minutes total.
> I have clean edges-sharp.
> John, what are the benefits of using the mill?
> I pondered a slitting saw on the mill too.


Benefits? For one, being able to sit down while the machine does the work. Also, one could gang several slitting saws to hasten the work.


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## Ulma Doctor (Jul 26, 2021)

interesting project!!!
i'll be watching!!!


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## SLK001 (Jul 26, 2021)

benmychree said:


> Milling machine would be lots better for grooves.


On a horizontal mill, yes, but a vertical mill will require setting up the disks on a 90º surface.  Plus, there's the headroom required for the lower cuts.  As for sitting down and watching the cut, once you get the angle right on the bandsaw, it seems like it would be the easiest method.  After all, these aren't precision cuts.


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## C-Bag (Jul 26, 2021)

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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## Janderso (Jul 26, 2021)

benmychree said:


> Benefits? For one, being able to sit down while the machine does the work. Also, one could gang several slitting saws to hasten the work.


"Gang several slitting saw".
John, with all due respect, I own "a" slitting saw, and to set up a gang of them would take me longer to set up than to just cut the darn grooves.
In the end, we are allowing a place for the slurry to go during the lapping process. To prevent a "bow wave" as Tom Lipton described it.
I appreciate the ideas of a lifetime machinist but some things are beyond my scope. In fact, most things are beyond my scope.


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## Janderso (Jul 26, 2021)

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.


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.


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## C-Bag (Jul 26, 2021)

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.


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## Janderso (Jul 26, 2021)

extropic said:


> 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.


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## savarin (Jul 26, 2021)

How are you going to test for flatness?


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## C-Bag (Jul 26, 2021)

savarin said:


> How are you going to test for flatness?


Optical flat like Oxtool?


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## savarin (Jul 26, 2021)

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


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## Janderso (Jul 27, 2021)

savarin said:


> 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
					

FastSHIP* Optical flat parallel double side 30mm / 1.18” Flatness 0.07µm λ/10 in Business & Industrial, CNC, Metalworking & Manufacturing, Metalworking Inspection & Measurement, Inspection Gages | eBay



					www.ebay.com


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## Janderso (Jul 27, 2021)

savarin said:


> a home made 3 ball spherometer


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


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## C-Bag (Jul 27, 2021)

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.
> ...


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.


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## savarin (Jul 27, 2021)

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.


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## hman (Jul 27, 2021)

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


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## Weldingrod1 (Jul 27, 2021)

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

Sent from my SM-G892A using Tapatalk


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## savarin (Jul 27, 2021)

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.


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## hman (Jul 28, 2021)

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.


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## savarin (Jul 28, 2021)

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


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## rabler (Jul 28, 2021)

hman said:


> 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


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## Janderso (Aug 5, 2021)

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.


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## Weldingrod1 (Aug 5, 2021)

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

Sent from my SM-G892A using Tapatalk


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## Janderso (Aug 6, 2021)

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.


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## extropic (Aug 6, 2021)

Excellent progress !!!

You make it seem easy.


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## Janderso (Aug 7, 2021)

extropic said:


> Excellent progress !!!
> 
> You make it seem easy.


Easy, Ha.
It is actually. It’s the dedication of time to perform the tedious steps of the lapping process.
I shou have started with 280 or 400. Probably could have saved some time.
I have an optical flat ordered just for fun.


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