Response to Bob's diamond query
bill tindall
>1. Grit to begin with- For fast material removal 60 micron on acrylic cuts very quickly. In fact, 60 micron on acrylic may be better than 60 on cast iron. It seems to me to leave a coarser scratch suggesting more aggressive cutting (probably the diamond is not crushing as quickly on the softer acrylic, but this is speculation). 60 micron is the grit I would start with for flattening. It is too coarse for normal sharpening.
2. Grit life- It takes about 5 cents worth of diamond to recharge a 3 x 12 plate. You say it lasts but for 15 min of grinding? Dang! that's 20 cents an hour for abrasive. Are you Scotch? :-)
If it takes more than a few minutes to accomplish a task with the grit at hand, the grit at hand is too small. I haven't done any flatening since we learned more about grit optimization so the following is a good guess. I suspect one can go from 60 micron to 15 or maybe even finer as the next step.
3. Substrate Selection I think that the hardest plastic will be the best nonmetal substrate and that would be acrylic(Plexiglas) (Note, hardenss is not to be confused with toughness.) However, if I already had Lexan, or something else that was flat, I would see how it worked. I don't think that I get more than 15 minutes of grinding per charge on cast iron, but I have never really timed it, so this is a guess. For sharpening it should not take more than a few seconds at each grit. I realize that flattening is another story.
Of late I have been doing a lot of turning and less plane and chisel work. I put a tiny dab of diamond on my acrylic or cast iron (I'm trying them both) lathe tool hones every time I use them. This may be overkill, but at a penny a dab it doesn't seem extravagant. I hone with diamond about 10 times and then wheel grind to reestablish the hollow grind. It takes but a few passes with 15 micron diamond to remove 80 grit wheel scratches at the tool edge.