The job of the grinding and honing stages is to shape the bevel so it is a flat plane that meets the also flat plane of the back at your chosen angle. The goal of the polishing stage is to remove the scratches that are left from the grinding and honing stages without adding more.
During all three stages both feel and sight are very important. Clean cutting stones like Arkansas stones will form a burr that can be seen as a black/dark line and felt with your fingers. If your stone forms a surry, that surry can interfere with burr formation and you may not be able to feel a burr when polishing on harder JNAT stones and some WaterStones. In those cases you must rely on sight alone to judge if the cutter is sharp. BTW, shine does not indicate a good long lasting usable edge.
Maximum light reflection requires a flat surface to reflect light, if the surface is rounded the light rays are scattered. Here come the weeds.
The cutting particles of Arkansas stones do not detach in any great amount and those that do are irregular. Another factor is the cutting surface of the stone is also random. A cutter polished on a Hard Black or Translucent Arkansas oil stone will have random shallow scratches with rounded edges. It will not appear as shiny as a bevel polished on a WaterStone but if looked at with a very high power scope the saw teeth on the cutting edge are smaller, smoother and will last as a working edge longer.
The cutting particles of synthetic WaterStones are more or less cubic that break free of the stone's substrate but do not break down. Because there are always new abrasive particles WaterStones cut very fast but, and there is always a but, leave a more or less ordered scratch pattern of relatively deep and straight sided scratches. They are great for shine but not so good at cleaning up the saw teeth on the cutting edge. To the unaided eye the bevel looks great but it is not as smooth cutting or will it last as long as a cutting edge formed by a natural stone.
JNATs abrade by forming a surry of small flattish abrasive particles that will break down, mostly in thickness, as the tool is being abraded/polished. JNATs benefit from the use of a nagura in two ways, the nagura used correctly will help keep the stone flat and it will help form the surry. The scratch pattern from a JNAT is much like that of the Arkansas stone, random, shallow, with rounded sloping sides. A bevel polished with a medium hard JNAT may appear cloudy, which is highly prized. When looked at with 10X or more magnification the surface will have a matte appearance with no deep or ordered scratches and the cutting edge will be much smoother that the edge of a cutter polished with a WaterStone.
I hate to say it, but more to come,
ken
Great post, well written and explained. My only quip is the comment about Quote if the surface is rounded the light rays are scattered Unquote.
ReplyDeleteTrue but it seems to imply you need a square edge to see reflections. As long as we have not achieved "Zero" (Nirvana :-) thickness, we still see light.
Bob, sipping his bedtime drink. Need more ice :-)
Bob,
ReplyDeleteThanks, that was reference to the scratches. One of the reasons WaterStones give such shiny bevels and backs is because the scratches are sharp sided. Where with natural stones, less shine because of more rounded scratches that scatter the light.
You are correct, if you have not honed or polished to the edge, the edge will look back at you (reflect light).
Yep, I'm getting ready for nachos, beer, and maybe a little TV. I still have a couple of months left of no adult supervision, living is easy but I kinda miss being fussed at.
Take care and wear your mask,
ken
It seems a frequent newbie mistake is to round the very edge of a plane iron.
ReplyDeleteThen with no relief angle, the plane doesn't cut (except at the edge of the board.)
The iron is skidding on the wood.
Then people try to augment the depth setting but to no avail.
Sylvain
Sylvain,
ReplyDeleteTrue, one of the problems with multiple bevels is ending up with a convex bevel without a relief angle.
ken