Re-Conditioning Granite Surface Plate
The actual process of reconditioning my surface plates, although being the ultimate goal in my project, was nearly disappointing in it's simplicity. The preparation was arduous, the final inspection was greatly satisfying, but the actual process of reconditioning - Aahh.
The process used is "Dry Lapping" which requires us to embed diamond powder into our Cast Iron lap using a hardened steel roller and then run this lap over the granite surface until tolerances are achieved. The key here is "until tolerances are achieved" which is where experience, shall I say craftmanship, is required to become efficient in this effort. On my large 36"x24" plate I first used the Moody Method using an autocollimator to map the surface and created a plan, a guess at best, of what grit to use, where to concentrate my lapping, and how many passes to make. As you can see the plate was not too bad with a 577 mike dip in the middle.
The actual process of reconditioning my surface plates, although being the ultimate goal in my project, was nearly disappointing in it's simplicity. The preparation was arduous, the final inspection was greatly satisfying, but the actual process of reconditioning - Aahh.
The process used is "Dry Lapping" which requires us to embed diamond powder into our Cast Iron lap using a hardened steel roller and then run this lap over the granite surface until tolerances are achieved. The key here is "until tolerances are achieved" which is where experience, shall I say craftmanship, is required to become efficient in this effort. On my large 36"x24" plate I first used the Moody Method using an autocollimator to map the surface and created a plan, a guess at best, of what grit to use, where to concentrate my lapping, and how many passes to make. As you can see the plate was not too bad with a 577 mike dip in the middle.
With the above information we can begin the granite lapping process.
The lapping compound needed is "Synthetic Diamond Powder" which can be found on Ebay at reasonable prices. I used 600 Mesh (22-36 Micron) for rough lapping and finished off with 2000 Mech (4-8 Micron). You'll also need some Isopropyl Alcohol as a fluid medium, a hardened steel roller to embed the media into the cast iron plate, and a Fantail brush to help distribute the diamond powder evenly. The video below shows how to prepare the cast iron lap with diamond powder & lapping the granite surface plate.
The lapping compound needed is "Synthetic Diamond Powder" which can be found on Ebay at reasonable prices. I used 600 Mesh (22-36 Micron) for rough lapping and finished off with 2000 Mech (4-8 Micron). You'll also need some Isopropyl Alcohol as a fluid medium, a hardened steel roller to embed the media into the cast iron plate, and a Fantail brush to help distribute the diamond powder evenly. The video below shows how to prepare the cast iron lap with diamond powder & lapping the granite surface plate.
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Now the first time I did this I had no idea what the results would be and the only good way to obtain feedback was to re-perform the very time-consuming task of inspection by the Moody Method with the Autocollimator. Above you can see my first pass results of 214 mike was about 65% of the total starting error of 577 mike. I had to perform this process a few more times as the 2nd pass overshot the goal drastically and I had to basically start over. As you can see the 4th inspection was well into a Laboratory Grade AA plate. I've since reconditioned a few smaller (24 x 18") plates with great success.