03-10-2020, 03:48 PM

As some of you know, I've been using grepper's KallyRest on my Kalamazoo sander. It has an adjustable angle guide, which is a table/rest that you lay the body of the knife on.

In previous sharpening adventures, I've used flat stones and plates and used wooden wedges as an angle guide. With these, you put the wedge on the stone and then lay the knife body on top of the angle wedge.

Many blades are roughly triangular in cross section, even before you get to the edge bevel. If you measure the included angle of the BODY of the blade, you normally will find that knives vary from around 3 degrees inclusive to fairly large angles. I just measured a Cold Steel Recon Tanto and it showed over 12 degrees inclusive.

So, if you want to know the true edge angle, and you're using an angle guide like I'm describing, you have to take into account the angle "offset" of the body. In particular, one half of the included angle is the offset. For a 3 degree inclusive blade, the offset is 1.5 degrees.

With a wedge where you set the body on top of the wedge, the edge is being lifted up by the body. So you have to add the offset to the reference angle. For example, if you have a wedge of wood cut at 15 degrees and you put a 4 degree inclusive knife on top of it, you have to add 1/2 of 4 degrees, or 2 degrees. 15 + 2 = 17 degrees. 17 degrees is the true edge angle that will be ground in.

Or said the other way, if you want a 15 degree edge angle and you have a 4 degree inclusive blade body, you have to account for the 2 degree offset: 15 - 2 = 13. You need a 13 degree WEDGE in order to produce a 15 degree edge angle on a 4 degree inclusive blade body.

Here's where it gets interesting to me. I've been doing these calculations for years and was pretty comfortable with them. When I got the KallyRest and later an angle cube, I planned to do these calculations to put some really low edge angles on a few blades. I'm planning on doing a few at 12 degrees per side and 10 degrees per side. I started to do the subtraction in the previous paragraph and something didn't seem right to me. I thought about it and drew some pictures. It turns out with the KallyRest (or any similar "angle table") the math is backwards!

You see, with the KR, the angle is not a wedge you are sitting on top of. Instead it's a SPACE between the table and the abrasive. So, when you set the blade body on the rest and it has (for example) a 4 degree included angle, you are getting CLOSER to the abrasive. With an angle wedge the body lifts further away from the abrasive! The tool rest is backwards from this. You get closer to the abrasive, therefore the edge angle is DECREASED by the body of the blade. Wow. I know, I know, it's just basic geometry but for me it was a wow moment.

Which means that if I set the KR to 15 degrees and I put my 4 degree (inclusive) blade body on it, I'm going to be grinding in a 13 degree edge angle. Or said the other way, if I want a 15 degree edge angle on my 4 degree blade body, I have to add the offset. 15 (edge) + 2 (offset) = 17. I need to set the KR to 17 degrees in order to get a 15 degree edge angle on this 4 degree blade body.

One of the blades I want to take to somewhere between 9 and 11 degrees (edge angle) is a full flat ground Delica. I've measured it's body angle at 3.7 degrees inclusive. So to take it to 11 degrees (edge). I need to set the KR to: 11 + (3.7/2) = 12.85 degrees. Neat.

I might make a video about this. I'm not sure if the words I've written are clear enough to get the meaning across. Let me know if this makes sense and/or if anyone would like to see a video about this subject.

Thanks for reading.

Brian.

In previous sharpening adventures, I've used flat stones and plates and used wooden wedges as an angle guide. With these, you put the wedge on the stone and then lay the knife body on top of the angle wedge.

Many blades are roughly triangular in cross section, even before you get to the edge bevel. If you measure the included angle of the BODY of the blade, you normally will find that knives vary from around 3 degrees inclusive to fairly large angles. I just measured a Cold Steel Recon Tanto and it showed over 12 degrees inclusive.

So, if you want to know the true edge angle, and you're using an angle guide like I'm describing, you have to take into account the angle "offset" of the body. In particular, one half of the included angle is the offset. For a 3 degree inclusive blade, the offset is 1.5 degrees.

With a wedge where you set the body on top of the wedge, the edge is being lifted up by the body. So you have to add the offset to the reference angle. For example, if you have a wedge of wood cut at 15 degrees and you put a 4 degree inclusive knife on top of it, you have to add 1/2 of 4 degrees, or 2 degrees. 15 + 2 = 17 degrees. 17 degrees is the true edge angle that will be ground in.

Or said the other way, if you want a 15 degree edge angle and you have a 4 degree inclusive blade body, you have to account for the 2 degree offset: 15 - 2 = 13. You need a 13 degree WEDGE in order to produce a 15 degree edge angle on a 4 degree inclusive blade body.

Here's where it gets interesting to me. I've been doing these calculations for years and was pretty comfortable with them. When I got the KallyRest and later an angle cube, I planned to do these calculations to put some really low edge angles on a few blades. I'm planning on doing a few at 12 degrees per side and 10 degrees per side. I started to do the subtraction in the previous paragraph and something didn't seem right to me. I thought about it and drew some pictures. It turns out with the KallyRest (or any similar "angle table") the math is backwards!

You see, with the KR, the angle is not a wedge you are sitting on top of. Instead it's a SPACE between the table and the abrasive. So, when you set the blade body on the rest and it has (for example) a 4 degree included angle, you are getting CLOSER to the abrasive. With an angle wedge the body lifts further away from the abrasive! The tool rest is backwards from this. You get closer to the abrasive, therefore the edge angle is DECREASED by the body of the blade. Wow. I know, I know, it's just basic geometry but for me it was a wow moment.

Which means that if I set the KR to 15 degrees and I put my 4 degree (inclusive) blade body on it, I'm going to be grinding in a 13 degree edge angle. Or said the other way, if I want a 15 degree edge angle on my 4 degree blade body, I have to add the offset. 15 (edge) + 2 (offset) = 17. I need to set the KR to 17 degrees in order to get a 15 degree edge angle on this 4 degree blade body.

One of the blades I want to take to somewhere between 9 and 11 degrees (edge angle) is a full flat ground Delica. I've measured it's body angle at 3.7 degrees inclusive. So to take it to 11 degrees (edge). I need to set the KR to: 11 + (3.7/2) = 12.85 degrees. Neat.

I might make a video about this. I'm not sure if the words I've written are clear enough to get the meaning across. Let me know if this makes sense and/or if anyone would like to see a video about this subject.

Thanks for reading.

Brian.