Correct use of the Angle Master

[Richard A]

I have a question regarding using a knife jig and the Angle Master on a knife that has 3 angles - 1. The most parallel section that starts from the spine and goes half way down the blade. 2. A mid area that starts a new angle that meets with the bevel. 3. the bevel forming the cutting edge. If I put the Angle Master on the top area closest to spine and set a 20 degree angle, that becomes a 23 - 25 degree angle when measured off the mid area (depending on the knife). So my confusion has been over what area to use and what angle am I really sharpening at. My thought was to use the most parallel surface to the center line which would be towards the top.

On a typical chef's knife that has a long tapering blade to a narrow bevel, is it best to measure the angle from the mid point of the blade??

[Herman Trivilino]

Hi Richard.  You are correct in that the angle must be measured relative to the center line of the blade.  What you are overlooking, though, is that for the AngleMaster to measure accurately it must remain in contact with the grindstone.  Thus you cannot use the AngleMaster on the portion of that knife where the blade's surface is parallel to the center line because that portion of the blade is too far from the surface of the grindstone.  And as you point out neither can you use it on the portion of the knife blade where it touches the grindstone because the surface of that portion of the blade is not parallel to the center line.

So ... what to do?  Well, I stopped buying kitchen knives with this geometry so I wouldn't have to deal with this issue.   

But seriously, the solution is to do the math.  Let's say that the portion of the blade that makes contact with the AngleMaster is tilted at 2^o relative to the center line.  If I want the bevel angle to be 15^o, I'll have to adjust the AngleMaster to 17^o.

I think I did my math correctly, but I'd appreciate it if someone would check it for me.  I could be wrong.   

[Richard A]

Thanks Herman.... Just to be clear, the Angle Master in this scenario is always in contact with the grindstone, whether measuring from the mid section or the upper portion of the blade (closest to the spine). The third angle I referred to, the actual existing bevel, becomes irrelevant since it is the angle we are either recreating or modifying.

I like your idea of doing the math on what the tilt of the blade is relative to the center line, but how do we best and accurately determine what that tilt is??

[Herman Trivilino]

Thanks Herman.... Just to be clear, the Angle Master in this scenario is always in contact with the grindstone, whether measuring from the mid section or the upper portion of the blade (closest to the spine).

It must be in contact in two places, as shown in the figures on the 2^nd page:

http://tormek.com/media/448610/hb-10-en-v103-wm-200.pdf

I like your idea of doing the math on what the tilt of the blade is relative to the center line, but how do we best and accurately determine what that tilt is??

I suppose you could use a pair of straight edges, but it might be hard to hold two straight edges and the knife using only two hands.  I just tried it on my pocket knife using scissors as the two straight edges, but the scissors aren't long enough.  And I'm not going out to the shed to get my hedge clippers. 

The way I've done it is to mount a wood chisel in the square edge jig as if I was about to sharpen it.  Adjust the angle so that the cutting edge makes contact with the grindstone and measure the bevel angle using the AngleMaster in the same way as shown in those figures.  Then lay my knife blade on top of the chisel, again with the cutting edge in contact with the grindstone, and again use the AngleMaster in the same way to measure the angle of the upper surface of the knife blade.  The difference between those two angles is the angle between the two faces of the knife blade.  Divide that by two and I've got the angle that the face makes with the center line.

Here's an example.  In that figure we see a plane iron mounted in the jig.  The bevel angle is 25^o, as shown on the AngleMaster.  Now suppose I lay a knife blade on top of the plane iron.  Imagine it's in the position you'd use if you were about to sharpen it.  Now I put the AngleMaster in place and note that when it's lined up with the upper surface of the knife blade it reads 29^o.  That's a difference of 4^o, meaning that the two faces of the knife blade make a 4^o angle with each other.  Thus each face makes an angle of 2^o with the center line of the blade.  Now I remove the plane iron and mount the knife in a jig so I can actually sharpen it.  If I want the bevel angle to be 15^o, I'll set the AngleMaster at 17^o.

If this doesn't make sense to anyone let me know and I'll try to make up a drawing to illustrate. 

All this is really not necessary as the angle is so small that it doesn't introduce much of an error, and the edge angle on a knife is not that critical.  But when I have an expensive knife and I want to remove the smallest amount of metal possible, I will measure the existing edge angle using the notches on the AngleMaster, and then use this method to match it precisely.

I don't recall a previous discussion of this issue, but I do recall encountering it soon after I got my Tormek.  Like I said, I try to buy kitchen knives that have parallel sides for this very reason.  They perform the same, and I see professionals using them when I watch television.

Here's my most recent purchase.  Haven't needed to sharpen it yet, and I've had it for well over a year:

https://www.swissarmy.com/us/app/product/Cutlery/Category/Chef-s/Swiss-Classic-Fibrox-Handle/8-Straight-Edge-Chef-39-s-Knife/6.8063.20US1

[Richard A]

Nice Herman - That makes logical sense to me and seems like a good way to really drill down on the angle accuracy for the type of knife originally described. I agree with you that in most cases, the variance of the angle will not be that big of an issue. However, my concern was like yours....if I have an expensive knife that I want to be very precise and not remove more steel than needed.