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| Making a granite surface plate |
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| Colh:
Hi Guys A few years ago I picked up 3 pieces of black granite about a 300mm square and 30mm thick for next to nothing - probably sink cut outs from granite bench tops as they are polished on one side. I would like to lap these together to produce 3 surface plates. Here in Aus you pay an arm and a leg for a granite surface plate because they are imported and the freight is over the top due to the weight. 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. 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. Would appreciate the comments from those who are familiar with the lapping process and any other who may like to comment on my thoughts. Thanks Col |
| Artie:
Hi Col, are they not flat already? I couldnt detect any deviations on mine... then again I guess its determined by just how flat you need it to be.... mines in here.... http://www.homemodelenginemachinist.com/index.php?topic=9168.0 Cheers Rob T |
| Lew_Merrick_PE:
Col, --- Quote from: Colh on November 15, 2010, 03:33:21 AM ---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. --- End quote --- 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? --- Quote ---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. --- End quote --- 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. ??? |
| Jasonb:
As said the commercial kitchen worktops are as flat as we are likely to need, I measures a bit a few months ago as someone was asking on a woodworking forum about using them to flatten plane soles, over a 12" length I had less than 0.0005" movement on a DTI. Jason |
| AdeV:
--- Quote from: Lew_Merrick_PE on November 15, 2010, 12:11:05 PM --- 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. --- End quote --- Lew, I find Amazon to be invaluable in this cases. Searching for that title (as keywords) with both authors brought a couple of potential hits: "American Machinist Grinding Book: Modern Machines and Appliances, Methods and Results" - Hardcover (1912) by Fred H. Colvin and Frank A. Stanley (I presume you meant Fred Colvin - can't find anything written by a Frank Colvin) or "Modern Toolmaking Methods - a Treatise on Precision Dividing and Locating Methods, Lapping, Making Forming Tools, Accurate Threading, Bench Lathe Practice, Tools for Precision Measurements, and General Toolmaking Practice" by Franklin D JONES (Hardcover - 1915) 2nd hand copies of both available in the UK, I've not tried the States. |
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