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Lapping Plates

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  • Lapping Plates

    I haven't seen a lot of comments on this BB about lapping. Maybe 'cause it's so tedious! My only experience was the first project in the first machining course I took. (a drill gage) Anyone got any interesting lapping stories (tips, how to's, do's/don'ts, etc.) to share?

    One question that's always puzzled me: It would seem that the lapping plate surface itself would quickly wear to the point where the flatness is lost. ( the bottom surface of John S.'s shop ) Is that true? or Why not?
    Lynn (Huntsville, AL)

  • #2
    Let's hear about it, you veterans! I've been thinking about getting a cheap granite plate for this purpose since I fear to use my Starret plate for anything but worship!



    • #3
      I don't think you would want to use a granite plate to lap on, the lapping compound, or lapping grit sticks to the softer material and thus laps the harder of the two. I may be wrong, but that is why they use cast lapping plates, or brass.



      • #4
        When I mount a scope on a rifle or handgun I get the bottom portion of the rings in place, shim or machine if needed to get them as close to parallel as I can. I them use a 1" dia. steel rod and 3A Clover lapping compound and lap the bottom half of the rings untill at least 90% clean. This way when scope is placed and the top portion of the rings is tightened down you will not bend the scope due to rings that are not parallel. Not only will it not bend but you will have more surface area of the rings in contact with the scope and it will better resist movement during recoil.

        Paul G.
        Paul G.


        • #5
          OK, if I'm understanding correctly then, the abrasives latch onto the softer material (ie. the plate) and remain stationary relative to it; and then the relative motion is between those fixed abrasive particles and the harder object being moved around over the plate.

          Of course that suggests that trying to lap a cast iron workpiece on a cast iron lapping plate would cut both surfaces equally.
          Lynn (Huntsville, AL)


          • #6
            I have a lapping plate 24 x 36, actually an old metal surface plate Blanchard Ground to within .00005 corner to corner about three years back.

            My technique is different than others on this board, I protect my plate. I use 320, 400, 600, and even 1000 wet or dry paper on the plate, and use paint thinner as the solvent for the lapping with the paper. Learnt this technique in my dear old apprenticeship. For the real fine stuff, I actually have some shimming stock I bought - plastic- from MSC, I put the lapping compound on a piece of paper, and the shimming stock, .010 on the plate, then the compound on the paper. Have used a lint free rag on occassion. but this affects flatness. I have been able, through these methods, been able to hold about .0002 flatness all said and done.

            I have in the past also made my own 6 x 6 SS or D1 lapping plates for direct lapping, blanchard ground (I have one of these, and it is a true gem), hardened the plates, and gone direct.

            I have seven lapping plates in the shop for deburring, flatness, and finish considerations.

            The trick is getting the tool steel to do this with, call Bergeron Tool Steel and ask for some "short cut" ends.

            CCBW, MAH


            • #7
              Like spope, I use good wet/dry sandpaper, (taped down) and a lubricant on my cheap granite surface plates. Works great for me. I reserve my good plate for the important stuff.
              Location: North Central Texas


              • #8
                The lapping plates I have worked with are cast iron and crosshatched.I also wondered why the lapping plate didn't wear. All of the articles that I have read refering to lapping suggest a nonferrous lap be made for the purpose which suggests that the softer material holds the lapping compound as noted here on the board.

                I have also lapped aluminum small engine heads on a thick piece of lucite with a piece of 320 grit wet/dry soaked in parts cleaning solvent layed down on the lucite. The solvent kept the paper from sliding around on the lucite and lubricated the paper so that it didn't load up. If you have a lot of lapping to do you can take a parts cleaning brush and swab the paper a couple of times as you are working. Of course the lucite was on a solid flat surface but this was not high precicion work. The old Tecumseh engines had some issues with blowing head gaskets and this seemed to cure it.



                • #9

                  I buy cheap plates from - I use PSA backed abrasive on it for flat lapping small items. A grooved Cast Iron plate that has been lapped flat with a bronze ring or disc (you need both to keep it flat, and it should be checked before lapping) works well with Diamond, SiC, and Aluminum Oxide abrasives. I also use glass plate for lapping with ultrafine Diamond powder (100,000 mesh).


                  • #10

                    I use a CRS steel plate about 9" X 12" and use various grits down to 1500 grit wet/dry paper. So, I predominantly use emery or silicon carbide cloth/paper. I have also used 220 grit silicon carbide "powder" to flatten rough surfaces before moving to the finer wet/dry grits.

                    I've made several depth gauges from 1018 CRS based on Guy Lautard's design in "Machinists' Bedside Reader (#1) and lapped them to almost a mirror polish. I start with 220 and procede to 320, 400, 600 and end up with 1500 grit. It takes some time, but the result is worth it.




                    • #11

                      Can you explain the use of the "ring and plug" to maintain flatness of the lapping plate? Thanks.

                      John Alsina


                      • #12

                        You cut a thick plate into a circle and then remove the "doughnut hole". If you start with a 6" diameter, the inner disc would be under 4" diameter.

                        When you check your lapping plate against the granite plate and it is low in the center you would use the ring lap to flatten it. Conversely, if it is high in the center you use the disc lap.

                        This is an old trick from grinding telescope mirrors - among other things...


                        • #13
                          Another thing you find in grinding/lapping glass is that to start with the grit rolls between the glass surfaces and pits the surface instead of scratching it. Removing material in chunks. Grinding.
                          Later when you switch to the pitch lap or metal discs then the grit can embed and start planing or scratching the glass. This would be polishing.

                          Since metals are also cryataline do they behave similarly when alike surfaces are used with grit. Like cast iron on cast iron. Would the grit roll and pit the surface?


                          • #14
                            I have done a lot of lapping and I ahve the arms to prove it I use plate glass with wet dry at first moving the part straight across in a cross hatch pattern until flat(cutting over the whole surface) then switch to lapping compound in a figure eight pattern I usually stop when the parts being lapped will wring together.I use the whole surface of the plate to keep it flat.When the glass is frosted over with pitts wash it clean and use it to hold diamond dust.
                            I just need one more tool,just one!