Why do I get different angles with the Angle Calculator and the WM-200?

[Plekter]

I am a little confused....hope to get an explanation from the wise men here.

https://cockloftsoftware.weebly.com/online-calculator.html

When I use this calculator (link over here) for my T8 and a chefs knife,  I put in the
Wheelsize 251,5
Projection 147,5
Angle         15

and I get
USB to casing 176,42
USB to stone    85,84.

Actually no problems in that - I adjust as good as possible and grind the knife....
but when I then try to measure the angle with the WM-200 and let it fit along the stone and upper straight side of the knife - then I get a measure of 18 degrees.

Does it have something to do with that the knife is thicker at the spine than at the bevel so it tapers a little from "top" to bottom (sharp side)?

So should I just go for the calculator and forget about the WM-200...or...?

All answers will be appreciated 

Joergen

[Dutchman]

There are several reasons why the sharpening angle may deviate. See "More math for the Tormek grinder" on https://forum.tormek.com/index.php?topic=1849.
However, the deviation of 3° points to the use of the simplified formula:

"This leads to the conclusion that the grinding angles ... have to be corrected by 2.5" if the measuring point is not chosen on the center line point where the jig rests on the USB. "

I wonder what formulas are used.

[Ken S]

Plekter,

As you set the bevel angle of.a knife (or other tool), the Projection of the blade from the adjustable stop of the jig to the edge of the blade becomes one side of a triangle. The Distance between the support bar and the grinding wheel becomes the second side of the triangle.If you set up a second knife or tool to the same Projection and Distanc, your bevel angle will b identical.This basic trig is the basis for Dutchman's tables, which arre the basis of the kenjig and the other angle setting jigs.

The Anglemaster works well with large, flat planes like the back of a bench chisel. I occasionally use the recommended method when I have a chisel to sharpen. When sharpening several chisels, I use other methods. I have found the Anglemaster clumsy with small. Bevels and tapered knives like yours. For knives, I set the Distance first and then I set the projection to 139mm. This works for my kitchen knives. It is not a sacred number, but it works. You can vary either number if you wish by using Dutchman's tables. I like the speed and consistency of standardization.

For work with very tight tolerances, Wootz' applet does essentially the same thing within tighter limits.If you routinely work with dial calipers or the 1/64" gradations on a rule, you will wantWootz'' applet. If you find the 1/32" gradations suffice, as I normally do, a simple kenjig and Dutchman's tbles will do the jib.

Yo be fair to the Anglemaster, it needs very good light to work best. The handbook mentions this, and it is critical. Even with good light, it is still not my preferred tool for knives.

Ken

[wanderingwhittler]

Does it have something to do with that the knife is thicker at the spine than at the bevel so it tapers a little from "top" to bottom (sharp side)?

Hi Joergen,

Yes, I believe that's the problem. As Ken said, with something with a nice wide bevel, like a chisel, you can put the angle master directly against the bevel and should get an accurate reading. But, for something like a knife with a small bevel, if you follow the advice of resting the angle master up on the side of knife (the primary grind), there will be an error introduced because you are not measuring against the center line of the knife.

Three degrees sounds about right for that error. I recently worked the math with an Opinel No. 10 and found that I had to use a correction of 3.15 degrees for it.

Here are the calculations I used in case you or anyone else is interested in using them or wants to check my math:

Spine width: 2.14 mm
Distance from spine to edge: 19.45 mm

We are interested in the right triangle that is formed between the edge, the center line of the knife (running from edge to the middle of the spine) and the primary grind (the hypotenuse of the triangle), so we need to divide the spine width by two before calculating the primary grind angle:

primary grind angle = tan^-1((2.14 / 2) / 19.45) =  3.15 degrees

(Make sure your calculator is in degrees mode if it supports both degrees and radians).

That amount needs to be added to your desired bevel angle to get the angle to set with the angle master. In my case, for a 15 degree bevel, I would have had to use 18.15 on the angle master.

The method of setting the bevel angle using the projection distance and USB distance from stone or case does not have this problem (assuming the knife is centered in the jig). I use it when I care about grinding a specific angle so that I don't have to worry about applying a correction to the angle master setting.

Greg

[Ken S]

I use substitute targets to create wide flat surfaces. At first I used metal. Now I use two thicknesses of plastic gift cards. I place them in the square edge jig and set the projection to same 139mm Projection (or whatever your knife projection is). The gift cards give you a large flat surface which matches the ideal thickness of the knife jig.

Ken

[cbwx34]

There are several reasons why the sharpening angle may deviate. See "More math for the Tormek grinder" on https://forum.tormek.com/index.php?topic=1849.
However, the deviation of 3° points to the use of the simplified formula:

"This leads to the conclusion that the grinding angles ... have to be corrected by 2.5" if the measuring point is not chosen on the center line point where the jig rests on the USB. "

I wonder what formulas are used.

The formulas used are your "new and improved" versions.   (For those that don't know, Dutchman is the "creator" of the math and its derivations... and the forum thread he linked to is worth a read).

Wanderingwhittler has probably given the best answer here... assuming your measurements are correct, the difference is most likely because of the blade taper from spine to edge... the Anglemaster doesn't account for this.  (So, your assumption that it was the taper was correct).

I use substitute targets to create wide flat surfaces. At first I used metal. Now I use two thicknesses of plastic gift cards. I place them in the square edge jig and set the projection to same 139mm Projection (or whatever your knife projection is). The gift cards give you a large flat surface which matches the ideal thickness of the knife jig.

Ken

Just to clarify... there is no need to do this if using the calculators... this is if you're trying to be accurate using the Anglemaster.

[Plekter]

Thank you for all the answers. I am less confused now :-)

So for chisels and other "straight" stuff with a bevel only on one side - there I can trust the Anglemaster WM-200.

But for all double-sided bevels on knifes etc - there the calculator seems to be a much better solution to use.

Joergen

[wanderingwhittler]

I use substitute targets to create wide flat surfaces. At first I used metal. Now I use two thicknesses of plastic gift cards. I place them in the square edge jig and set the projection to same 139mm Projection (or whatever your knife projection is). The gift cards give you a large flat surface which matches the ideal thickness of the knife jig.

Ken,

This is interesting. Are you saying that you (1) set the substitute target to have the same projection distance in the SE-77 as the knife has in the SVM-45, (2) set the USB height using the angle master against the substitute target in the SE-77, and then (3) switch to the SVM-45 and grind?

I must be missing something, because I would expect that method to not be accurate because the SE-77 holds the tool (or substitute target) much higher above the USB than the center line of the knife in the SVM-45 jig. Please help me understand.

Thanks,
Greg

[cbwx34]

I use substitute targets to create wide flat surfaces. At first I used metal. Now I use two thicknesses of plastic gift cards. I place them in the square edge jig and set the projection to same 139mm Projection (or whatever your knife projection is). The gift cards give you a large flat surface which matches the ideal thickness of the knife jig.

Ken,

This is interesting. Are you saying that you (1) set the substitute target to have the same projection distance in the SE-77 as the knife has in the SVM-45, (2) set the USB height using the angle master against the substitute target in the SE-77, and then (3) switch to the SVM-45 and grind?

I must be missing something, because I would expect that method to not be accurate because the SE-77 holds the tool (or substitute target) much higher above the USB than the center line of the knife in the SVM-45 jig. Please help me understand.

Thanks,
Greg

I'll "bump" this question up... Ken probably missed it, but I'm curious too... I think he puts the "substitute targets" in the SVM-45... not sure why he said "square edge jig".... 

[RickKrung]

I'll "bump" this question up... Ken probably missed it, but I'm curious too... I think he puts the "substitute targets" in the SVM-45... not sure why he said "square edge jig".... 

I'm bumping it as well.  I am interested (more correctly confused). 

Using a projection of 139mm in the square jig seems odd and awkward and doesn't make sense to me.  Projections for the SE jig are supposed to be between 50-70mm or so.  Unless he has a substitute target that is sized for the SE jig.  I don't really get why there would be any need as the substitute jigs are meant for getting the apex right down to the stone with a totally flat top side also all the way to the apex for more accurate positioning of the AM.  Why would that be needed for chisels and plane blades, which already have flat backs? 

Rick

[Jan]

Square edge jig SE-76/77 has different geometry than the knife jig SVM-45 and for this reason we cannot expect that for the same stone diameter, the same protrusion and the same stone – USB distance we get the same edge angles.

You can make a calculator for the square edge jig SE-76/77 but it is more tricky than for the knife jig.

Jan

[Ken S]

Oops...... It was quite a while ago that I made the metal substitute target. I did use the SVM-45 knife jig, not the square edge jig. For those who prefer to use the Anglemaster, the combination does work well. (Personally, I prefer the kenjig.)

I did use the square edge jig with gift cards for one experiment, long forgotten now. Two thicknesses of gift cards actually works well with the knife jig if the cards are cut down in size.

Ken

[wanderingwhittler]

Ah, that explains it. Thanks for the explanation Ken.

And, thanks for the bumps CB and Rick.