an opposit approach, and grinding philosophy
Bill Tindall, E. TN
>To make this discussion clear let me define bench grinders, LV sharpening system, belt sander, hand crank grinder, etc. where the abrasive particles move past that to be sharpened at relatively high speed as "power grinding". When the object is moved by hand at slow speed across the abrasive i will call it "hand grinding". And, we will not further define what we are doing-lapping, sharpening, etc. and just lump it all into metal removal.
The most important thing I have learned in sharpening is that all of the worn edge must be removed to establish a new shap edge, be it planes or scrapers. Therefore, the objective of the first stage of sharpening, "Step 1" is removing enough metal to remove the old worn edge. Formation of a burr indicates that this objective has been achieved. The rest of the sharpening process(I will call these steps "Step 2") consists of removing the scratches of the previous grit size to establish a finer scratch size. This process continues through successively finer grits until "it's good enough". Compared to Step 1, very little metal is removed in all of Step 2.
These steps can be easily followed with a 10X lens and it is informative to do so.
Fundamental Principle** If in Step 2 the reduction in grit size between steps is not huge, these steps require very little abrasive action(time). If you have read our article on sharping with abrasive sheets (Articles Section) we show that only a few hand grinding strokes are necessary to remove previous grit scratches and the abrasive lasts a long time. The secret is to not make big jumps in grit reduction.
It seems to me that power grinding has its greatest benefit in Step 1 where a relatively large amound of metal must be removed. The turning community enthusiastically employs this practice. If the scratches in step 1 are coarse and the goal is very small scratches at the final edge, then two approaches are possible. Big reductions in grit size can be made with the need for a lot of grinding at each step ( and resultant great wear on the abrasive).Either power grinding or tedious hand grinding would be required. If relatively smaller steps in grit size reduction are used in Step 2, very little time is required at each step. Hence, the desired result is easily and cheaply achieved with hand grinding. The "cost" is the necessary inventory and inconvenience of more abrasive devises (stones, paper, lapping wheels, whatever) to cover all the required steps.
Diamond cuts much faster than any other abrasive we have tried. Hence, the steps between grits in Step 2 can be bigger, maybe much bigger, that we recommend in the abrasive sheet sharpening article. For aluminium oxide and silicon carbide a size reduction of about 30% per step is recommended. For diamond a factor of two is recommended and our preliminary work sugggests a factor of 4 is possible for tool steel.
I don't know if the following will work, because I have not tried all the steps and materials in a controlled fashion. But I think it has a good chance of working based on what has been done. I mention this information to encourage others to experiment with what I think could be an effective approach that doesn't cost much to impliment.
Step 1- power grind with something that will quickly establish a burr.
Step 2- Using cheap diamond charged on cheap 3 x 12" acrylic plates, or more expensive cast iron plates (more on this in a later post), take about 5-10 hand strokes using 60 or 30 micron diamond. 30 vs 60 will depend on how coarse the scratches are from Step 1. Repeat with a grit 4 times smaller. Continue until it is good enough. I have been stopping at 6 micron but most will want to go to 3. My planning objectives are such that I have no useful information as to whether even finer grits can be beneficial.
Most of us already have a power grinder to achieve Step 1 objective. If one sharpenes more often than I tend to do, Step 1 may not even be necessary. Less than $100 will purchase the acrylic and a near lifetime supply of diamond paste or powder necessary to achieve Step 2. (12 x 12 acrylic plate is about $12 and diamond paste is about $15 for 5 grams paste or $24 for 25 carats of powder).
I do have a fair amount of experience using the above approach for lathe tool sharpening and have found it quick, effective and cheap(I stop at 15 micron for fine truning work). I am about to do a lot less latheing and more planning and chiseling so may have more to say later.
bottom line to this long winded answer, I think it makes more sense to power grind on the coarser grits and hand grind for the smaller.
