|Microbevels front and back.|
|Use a jig.|
|Copyright (c) 2002-15, Brent Beach|
OK, you don't have a belt sander, but do have a regular grinding wheel and want to grind the primary bevel then continue on with using the jig and the 3M abrasives.
Let's assume that you hollow grind your blade leaving a very narrow lip on both sides of the hollow grind. It is important to distinguish between the nominal angle of this grind and the actual angle. The nominal angle is the angle of the blade when resting on the two lips around the hollow. The actual angle is the angle of the hollow just in front of the lip near the edge.
Most people who hollow grind use the two lips to hone the iron after grinding. That is, they hold the blade at the nominal angle during honing. This allows them to hone without a honing guide. They can repeat this honing until the front lip gets so wide that honing takes too long. At this point it is necessary to repeat the grinding of the hollow.
The belt sander page includes a brief section on grinding wheels. There it shows that the actual angle at the edge and the nominal angle differ by about 4 degrees on a 6" grinding wheel.
If you use a nominal angle of 25 degrees, then the resulting bevel will not be as stiff as a blade with a flat bevel at 25 degrees -- the hollow has reduced the thickness and hence the stiffness of the bevel.
A slightly different honing strategy gets around this problem. Start with a 30 degree hollow grind, then hone at 30 degrees with all three abrasives. The resulting actual angle is a little more than 25 degrees. The thickness of the bevel back from the edge is actually thicker than for a flat 25 degree grind -- resulting in a stiffer bevel. Note however that you have to hone the entire bevel with each abrasive. I believe this produces an inferior edge.
An even more arcane solution is possible. Hollow grind at 30 degrees then using the jig with the blade at 25 degrees and the 15 micron abrasive, remove some of the back lip. This will allow you to then hone the front lip (the lip near the edge) at 29 degrees with the 15 micron abrasive, following up with the 5 micron and 0.5 micron abrasives as in flat bevel case.
Most people guess at the angle -- roughly set the tool rest, put a blade with the correct bevel on the tool rest against the wheel, fiddle until the blade appears to be resting on the wheel mid way across the bevel. Some people have a small plastic 'diamond shaped' jig with four standard angles at the point ends. Again, you rest the jig on the tool rest and fiddle the setting until the edge appears to be resting on the wheel at the correct position.
This method is surprisingly prone to error. A small misjudgment in where the wheel meets the blade or jig can produce a big error in the nominal angle. You may well grind the blade and find the two shoulders are of very different width. Play around some more until the two shoulders are the same with after grinding.
You can make a small angle setting jig as follows. Use a piece of wood the same thickness as the tool you want to grind and about 1/2" wide. Bevel one end at the desired angle. Hollow the bevel using a round file or a chisel leaving shoulders at both ends of the bevel. Use this jig when setting the tool rest - make sure both shoulders of the jig touch the wheel.
You can use the sharpening jig to check the grinder angle. Put the hollow ground blade in the jig and set the blade extension for the desired nominal angle. Run the blade lightly (start with a pull!) on the 5 micron abrasive. Both shoulders of the hollow grind should be equally honed. Try to get this angle pretty accurate -- it will make a difference.
Accuracy in the grinding step is important because the difference between the primary angle and the first microbevel angle is important. If the difference is too low, the first microbevel will be too wide and you will have to remove more metal to get the same depth of metal removed.
If you use a dry grinder, be very careful that you do not grind all the way to the edge. If you grind to the edge, no matter how careful you are, you will produce a surface at the edge that is difficult to hone and may have had its temper drawn. The page on grinding discusses the importance of not actually grinding the edge.
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