loose diamond sharpening update
bill tindal, E.TN
>To review the advantages of diamond sharpening, diamond cuts even the most abrasion resistant tools steels very aggressively. As a result sharpening is quick and huge steps between grit size are practical thereby increasing speed and reducing inventory of sharpening stuff. Furthermore, diamond powder or paste is very cheap. A $25 vial of powder could last a life time. A disadvantage is that a cast iron substrate is ideal and they are not commercially available. (editorial, why do you plane blade makers (metal workers) not make and sell these things? Good money could be made at $60 retail)
We continue to explore how to best take advantage of diamond powder. Someday Steve is going to tell us how fine we need to go for practical work, but for now we are going to 1 micron powder.
Background: I sharpen when I can not get a good shaving any more and by then the edge is well rounded. One micron is too fine to remove the dead edge of these dull edges. Fifteen or 30 micron diamond powder is sufficiently aggressive to get a wire edge on a very dull tool. But, we have never solved the problem of grit carry over. The loose diamond particles stick to the tool and carry to the next smaller sharpening step where they contaminate the substrate with this coarser grit. I like a hollow grind. Less material is necessary to remove to get the wire edge and the hollow surface indexes confidently on the slick cast iron surface for jig-free sharpening.
definition for purposes of this discussion- "lap" = moving the tool across the flat abrasive surface by hand, jig-free. I always pull the tool but I don't know if that is best or not.
Latest recommendation:
Step 1 Grind a hollow grind on the tool edge. I use a 7"-100 grit soft friable wheel. I use a VFD drive on a 3- phase grinder for low speed control, but that is a luxury not a necessity. The following is important and don't overlook it. **Use the polished edge from the previous sharpening to measure the progress of grinding**. Grind nearly to the tool edge. If the edge was straight to begin with and you don't grind the last traces of this bright edge away, the tool edge will still be straight after hollow grinding. At this point you have established a hollow almost to the tool edge, but you have not removed the dead edge and established a wire edge.
Step 2 On a grade "fine" fixed diamond plate lap the tool until a wire edge is established. It should take but a few strokes. If you see a wide band from this grit at the edge and still no wire edge, go back to the grinder and grind back down to the edge, again without removing it. Because only a few strokes are necessary on this plate , it will last for ever. I use a dinky 6" plate (it is what I was experimenting with), but a 12 " would be nicer.
Step 3 Sprinkle 2-3 milligrams of 1 micron diamond power about the cast iron plate (or a 3-5mm worm of paste), add a few drops of WD-40 and lap until the scratches from the previous grit are gone. It should not be but about 10 strokes. Lap the back a few strokes to break any wire edge that survives, a stroke or two back on the bevel and you are done. One micron diamond will quickly remove the scratches of even the 100 grit wheel, but it is slow going establishing a wire edge on a very dull edge with 1 micron.
The intermediate step we added of the fine fixed diamond plate solves many problems. It takes great skill to grind to a wire edge on the grinder while keeping the edge straight. If the edge isn't straight it takes for ever to get it straight with the 1 micron grit. So, the bulk of the metal is removed by the fast wheel grinding, the fixed plate quickly establishes the wire edge and the 1 micron loose diamond quickly refines the edge. The intermediate step could be done with 3M mylar sand paper just as well. Using the reflective polished band (from previous 1 micron step) at the edge as a grinding guide is a very handy way to keep the edge straight as well as gauge progress in grinding.