Benchtest.Com - Workshop - DoAll D624-8 Surface Grinder

I usually don't ask a seller to do a extra nice job of packaging, but since this plate was 70 pounds, I did on this occasion. This is what I got for my asking. The cheap cardboard, coupled with the heavy handling that packages normally get, left this box a bit the worse for the wear. Fortunately, the shipping gods were on my side (for a change) and the plate arrived without a scratch. I still don't know how I got that lucky.

I got out my diamond powder roller, which is nothing more than a roller bearing that has been attached to an axle and bolted to an aluminum handle. The procedure for charging then lap is to sprinkle some diamond powder on one area of the lap and use the roller to press it into the lap's surface. I tend to think that I get better results if I use a lot of pressure on the roller, but with the hardness of the diamonds, they will embed even if moderate pressure is used. Using a larger roller would speed up the process some, but my next step also helps embed more of the abrasives.

The next step was to empty the loose diamond powder from the lap on to the top of the surface plate I have been reconditioning. Using a coarse camel-hair artist's paint brush, I spread the powder out to form a square the size of the lap. The lap is then placed face-down on the surface plate and pushed and twisted into the powder. At this point the lap is lifted off the plate and the roller is used again to press more diamond powder into the lap. The process is repeated until the surface of the lap shows a uniform coat of the abrasive. The diamond powder pressed into the lap shows as a dull finish in contrast to the new shiny lap.

I wanted to try a test run on the old surface plate before I started lapping the critical top surface of the straight edge. This would give me an idea of how much granite will be removed in a few passes. Generally one thinks of lapping metal as a surface finishing technique that removes very little material. However when lapping granite, my experience tells me that even with a rather fine 3000 mesh powder, I can remove enough material to change the surface topology substantially.

I have mapped the surface of the surface plate I am reconditioning by drawing a grid of 2" squares that correspond to locations on the plate and using my shop made planekator copy I have record the relative height of each corner and center of each square. After looking at my map and finding the high points I needed to lower, I tried out my new lap by rubbing it over the high spots and feathering them into the areas that I needed to take less material from. After a few passes, I removed the lap, cleaned the plate and remeasured. After noting all of the relative heights on the map, I found that I had been successful in lowering the highest spots by a couple ten-thousandths. I had also achieved my other goal of breaking in the lap. The lap's surface was now quite dull where it had been in contact with the surface plate and less dull in the lower areas of the lap. The pattern on the lap of dull and less dull was pretty even and the area I had reworked on the surface plate was smooth and uniform. I was ready to work on the granite straight edge.

Since I didn't have much of a height difference between the highest and lowest areas on the top of the straight edge, I chose to not recharge the lap. I have read that by not having any loose diamond powder on the surface and only using the diamonds embedded in the lap that one can achieve a smoother surface with less variations in surface elevation. Since this was what I was trying for, I gave it a shot.



With a ball bearing roller to apply the 3000 mesh diamond powder, I rolled it in as evenly as I could. I then did a practice run on my carefully mapped old surface plate. I then reapplied more diamond powder and carefully lapped the top of the granite straight edge. After checking level in front and behind the straight edge, I checked the top surface of the straight edge. After a couple cycles, it was level and smooth.

After a few passes on the top surface of the straight edge, I set up my measuring gear so I could check the relative height of the top surface. This turned out to be as close to zero difference in height as I can measure. For the last check, I leveled my new Starrett surface plate with the straight edge centered on it. When I had the bubble centered on my 0.0002" per 10" box level while on both sides of the straight edge, I placed the level on the top surface of the straight edge. In this position, the level also centered the bubble. This was no surprise since the Supramess indicator that I used to measure the relative height of the top surface has much better resolution (20 millionths) than the level, but I was happy for the confirmation none-the-less.

So, one more mini-project is done and I now have a way to check level between the separated ways of the surface grinder. This should help me to make sure that both the saddle and table on the surface grinder stay level and square with the vertical column. As a part of the project, I have now acquired a lap that I will use only for 3000 mesh diamonds. The 3000 mesh diamonds leave a pretty nice finish on granite, but it would be nice to have one more lap that I can devote to 5000 mesh only for situations where I would prefer an even finer finish.


Left: Cast iron straight edges spotted on each other. Center: Closer view. Right: A nice pattern showing that the two cast iron straight edges print as well on each other as they do on the granite master straight edge. The one on the left (first pic), bottom (2nd), left (3rd) was pull-scraped and took about a third the time to do.