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Lapping A Granite Surface Plate

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  • Lapping A Granite Surface Plate

    Do you need multiple grits of lapping compound? Is it diamond lapping compound that is used? How many lapping plates are used and how big do the lapping plates need to be?

  • #2
    A few years ago I took my surface plate to Starrett to have it certified. When I dropped it off the guy asked me "Do you want a quick tour"? Hell, yes! The plates getting recerted were measured with an autocollimator and then turned over to a guy who lapped it according to the map of measurements. The lap he used was a block of granite maybe 24" x 6" x 4" with a handle closer to one end. He moved it around in a circular pattern and checked his work with selection of Aluminum I-Beams with a mounted dial indicator. Some had pads on the end with a dial indicator in the middle sticking down, some had the indicator on the end. Once he felt it was flat again it went back to the guy with the autocollimator for another mapping. Don't know what type of compound was used.

    Steve.

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    • #3
      Yes it's diamond paste. No do not even think about trying this on your own. Our calibration service does this to way less than .00005, more like .00001 and you need the proper tools which cost a fortune. Remember it is done in place for a reason. If you move or change anything on a large plate you will likely affect it's flatness.

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      • #4
        Yeah what td midget said. Wllingness to lap a surface plate does not equate to success after the plate has been lapped.

        There's some fine technique and elaborate instrumentation required to ensure success and acheive cerfifiable accur4acy. At the minimum you would need the basic lapping materials plus a Rhom Planikator (?) system to quantify progress as you procede. You will also need the Federal Specification GGG-P-463c because it has in it the info you will need to properly support the plate for inspections and the acceptance criteria for the various grades and sizes. Unless there is calibration facility near you to document your finisheded surface plate you're better off in these days of cheap immports to demote the suspect surface plate to a paker;'s kitchen and purchase a replaement.

        I'm not saying a rank amateur couldn't do a servicable job of lapping and calibrating a granite surface plate. I mere merely suggesting that it's not too practical given the low cost of new replace ment import plate Vs the time to acquitre and the cost of the calibraion equipment and devevelopment of the skills. For exam0ple Iit took me over three years to dinf and urchase a Federal electronic differential leveling system resolving 1/2 arc second for l3ss than $1500.
        Last edited by Forrest Addy; 05-25-2009, 10:30 AM.

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        • #5
          Out of curiosity Neil, how were you proposing to measure the flatness of the surface plate?
          "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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          • #6
            Originally posted by tdmidget
            Yes it's diamond paste. No do not even think about trying this on your own. Our calibration service does this to way less than .00005, more like .00001 and you need the proper tools which cost a fortune. Remember it is done in place for a reason. If you move or change anything on a large plate you will likely affect it's flatness.
            For sure I'm going to try it "on my own". I don't consider less than $1,000 expensive to buy a used Rahn Repeat-O-Meter and a used Hilger and Watts Autocolliminator to be a "fortune".

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            • #7
              Originally posted by Forrest Addy
              You will also need the Federal Specification GGG-P-463c because it has in it the info you will need to properly support the plate for inspections and the acceptance criteria for the various grades and sizes. Unless there is calibration facility near you to document your finisheded surface plate you're better off in these days of cheap immports to demote the suspect surface plate to a paker;'s kitchen and purchase a replaement.
              Anyone can download the Federal spec you mention. I have the link at work if anyone needs it.

              When is your next scraping class going to be held?

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              • #8
                A Repeat-O-Meter just tells you localized flatness, and an autocollimator will tell you global flatness. It's going to be a bitch to extrapolate a flatness map to hand lap it...
                In Moore's book, they show the giant surface plates scraped together with the three-plate method, and the plates are checked for flatness by suspending a straight edge above the surface place, and exhaustively measuring each point with a gauge head.

                I've read that granite surface plates are lapped on a large rotary lap.
                "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                • #9
                  Originally posted by lazlo
                  A Repeat-O-Meter just tells you localized flatness, and an autocollimator will tell you global flatness. It's going to be a bitch to extrapolate a flatness map to hand lap it...
                  In Moore's book, they show the giant surface plates scraped together with the three-plate method, and the plates are checked for flatness by suspending a straight edge above the surface place, and exhaustively measuring each point with a gauge head.

                  I've read that granite surface plates are lapped on a large rotary lap.
                  The way I seen it done is to use a straight edge and a mirror mounted on carbide points with the autocolliminator to map out the plate and you follow the Federal spec

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                  • #10
                    Originally posted by Neil Jones
                    The way I seen it done is to use a straight edge and a mirror mounted on carbide points with the autocolliminator to map out the plate and you follow the Federal spec
                    An autocollimator is used to detect global flatness (for example, twist): it tells you how planar the two points are. The problem is that you need the flatness for all the points in between. You could conceivably move the autocollimator around thousands of times, and map out the entire surface, but it takes awhile to setup and register each time

                    I'm guessing you bought a big granite surface plate and it's not flat? You might be better off with a Talyvel -- an exquisitely sensitive electronic level (really a gauge head with a pendulum sensor). With a Talyvel and a lot of patience, you could record a 2D map of the flatness of the surface plate.



                    Mine has an analog display head, but the new ones have a digital interface with software that maps the 2D flatness of the surface (you can do this by hand with the analog head, of course):

                    Last edited by lazlo; 05-25-2009, 12:05 PM.
                    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                    • #11
                      What size and grade of surface plate are we talking about? I purchased a Grade B 12 x 18 surface plate from Enco for $25 with free shipping and you can buy a grade A plate (.0001") with two ledges for $36.95. Of course the larger size surface plates are more expensive (to buy and ship), but even so it makes it difficult to imagine how you could save much money especially if you are paying the shipping cost plus the redo of the plate. If you do it yourself wouldn't you need another surface plate to use as the lap?

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                      • #12
                        Originally posted by lazlo
                        An autocollimator is used to detect global flatness (for example, twist): it tells you how planar the two points are. The problem is that you need the flatness for all the points in between. You could conceivably move the autocollimator around thousands of times, and map out the entire surface, but it takes awhile to setup and register each time

                        I'm guessing you bought a big granite surface plate and it's not flat? You might be better off with a Talyvel -- an exquisitely sensitive electronic level (really a gauge head with a pendulum sensor). With a Talyvel and a lot of patience, you could record a 2D map of the flatness of the surface plate.
                        I have not purchased a big granite surface plate that is not flat but I'd like to.

                        Lapping a surface plate in is a skill I'd like to acquire. Same with scraping.

                        When I've seen surface plates inspected and lapped the only tools that were used is what I've already mentioned. I've never seen anyone use a Talyvel. Seems like it might make the job a lot easier.

                        Speaking of scraping are you familiar with this book and video and if so do you think they represent good value for someone who would like to learn how to scrape?

                        http://www.machinerepair.com/orders.html
                        Last edited by Neil Jones; 05-25-2009, 12:18 PM.

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                        • #13
                          Originally posted by sidneyt
                          What size and grade of surface plate are we talking about? I purchased a Grade B 12 x 18 surface plate from Enco for $25 with free shipping and you can buy a grade A plate (.0001") with two ledges for $36.95. Of course the larger size surface plates are more expensive (to buy and ship), but even so it makes it difficult to imagine how you could save much money especially if you are paying the shipping cost plus the redo of the plate. If you do it yourself wouldn't you need another surface plate to use as the lap?
                          I've always liked the idea of spending thousands of dollars to try and save a few bucks. You end up with a skill that can reward you (mentally) many years later when you forget about how much it really cost you to acquire that skill.

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                          • #14
                            Originally posted by Neil Jones
                            Speaking of scraping are you familiar with this book and video and if so do you think they represent good value for someone who would like to learn how to scrape?

                            http://www.machinerepair.com/orders.html
                            I have the book and video, and they're both excellent. The video covers predominantly hand-scraping, and the book -cough- dovetails nicely with technical background, instructions on sharpening the blade, to to measure for aligment, etc. Michael was a little disorganized about prices and shipping costs when I called, so I bought both from a Dapra distributor.

                            By the way, you asked on the other thread about the "Scraping for Alignment" DVD -- it was also excellent. Here's my review:

                            Bridgeport Scraping/Rebuilding DVD Review
                            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                            • #15
                              Short of attending a class the video looks like it will provide a decent framework for learning. As I recall the class I took from Forrest the new material I picked up included work holding, tool usage, marking and analysis, and the need for discipline in tool usage including stroke length, stroke angle and pressure, posture, preparing the work for marking (India stone, remove all debris!).

                              In addition I learned about the pace of progress which is surprisingly fast for the small blocks we were working on. Finally I became conscious of simultaneity. The object we were working on was intended to come to size, perpendicularity, and surface quality all at the same time, ending with a final stroke. Knowing when that final stroke has happened is elusive to say the least, but it is the most important thing to know - when to quit.

                              Along with that is recognizing you are on or not on a path to successful completion. If you are not on a path to completion you are on a path to failure, so you need a mental map of where you are and where you are going, and what the lay of the land is, and that your progress is taking you toward success. The understanding of simultaneity is useful in this analysis.

                              Not included was actual scraping processes such as frosting and working with dovetails and round work such as bearing surfaces.

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