Col,
I know that by lapping 3 near flat surfaces against one another you can get all 3 'flat' - that is A to B, B to C and C to A. I have tried to find info on the process and but have come up with a blank.
My thoughts are to use a grinding medium (I have some 240 grit Silicon carbide powder) mixed with either water or oil? then :
1. Spread a small amount of grinding mixture onto plate A, place B on top and move in a figure 8 pattern a few time.
2. Next Plate B on bottom and C on top
3. Then C on bottom with A on top
Ground areas should show up as dull.
My questions are:
1. On the next round do I rotate the bottom plate so the cutting action is in another direction? If so how much 45 degrees, 90 degrees?
2. How do I see progress as the shine will have gone on the first round or do I just keep going until all three plates are dull over the complete surface?
Once all is done I would then like to test each plate against the others.
Your basic approach is sound. If there is a college with an astronomy program within easy contact range, go to them and ask to look at their lens grinding set-up. It's the same technology. You will find that 240 grid abrasive is way too rough for anything but gross finishing. The people who grind lenses will have better polishing compounds that you can usually con them out of for an appropriate bribe (whiskey was the standard bribe back when I was a physics lab machinist 35+ years ago). As you are going for flat, your task is actually easier. A set of ground steel guides with a (UHMW or acetal) glides could be useful during the rougher phases.
There's a book that refuses to leap into my hands (meaning that it is probably in one of the 75 or so boxes yet to be unpacked) written circa WWI with a title like unto "Precision Means and Methods" -- I can't remember if it is a Franklin D. Jones or Frank Colvin book (neither Google nor Wikipedia helped). It has a section on lapping various things (including surface plates) in it. Various reprinters (such as Lindsay Publications) resurrect it from time to time, so it's not a lost cause to find a copy. It is a truly wonderful book.
E-mail Starrett. They will be willing to give you more information than you ever wanted to know. It's not as if they will not understand your situation. They are all about expanding skills. I know that they bent over backwards helping Guy Lautard when he did a write up on surface plates. It's not as if you are going to set yourself up as a competitor to them (is there granite in Australia?) and they know it. However, if you lap out a surface plate, you WILL come away with a better understanding of the VALUE they bring to the marketplace, right?
What kind of indicating medium do I use that will show up on granite (like using bearing blue when testing light coloured metal surfaces on surface plate)?
I thought of using white oil based artist’s paint as I have heard bearing blue called Prussian Blue supposedly because it is Prussian Blue oil paint. I tried got a tube of Prussian Blue oil paint from an artists supplier and it seems to work. It is also cheaper than bearing blue from machinery/bearing suppliers.
The oil base to paints will fill quite a large gap. True toolmaker's prussian blue (aka bearing blue) is so thin that a chunk the size of a small pea will coat (and transfer and transfer and transfer) a 4 ft X 8 ft (1.2 m X 2.4 m) plate thoroughly. Violin makers use ground chalk (such as for snap lines) to check fits of various components. They wipe it on one side of the joint to be fit and see what transfers to the other side of the joint. It can be vacuumed up and cleaned off of wood with a sharp scraper -- you won't have to go that far as you can wipe it off your granite with a solvent rag.
Auto body polishing compounds work will as abrasives for this type of operation and are much finer than most of the lapping compounds you can buy today.
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