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[in the axis of the jig and the knife]

Grind a single-bevel blade and a double-bevel and observe difference between the resulting edge angle and the calculated.
Then grind 2 double-bevel blades of differing thickness behind the edge and observe difference between the resulting edge angle and the calculated.
Then take the thicker blade and grind it at 20 degrees, and at 12 degrees, and observe how the deviation from the calculated angle increases.

TormekCalc calculations are made in the axis of the jig and the knife, so results are valid for all symmetrical double bevel knifes with defined angle at the top of the edge. Error given by material thickness is suppressed there completely.

For single bevel knives is a correction necessary to get "right" axis of the jig. For this you can change diameter of USB or jig diameter value and "virtually" move the jig axis to the right place. Then you get exact results for single bevel knifes with defined angle at the top of the edge. Error given by material thickness is suppressed by the jig axis movement.

What is still present is error given by "roundness" of the wheel – see picture Edges.jpg – which caused that angle increases along the hollow. As you can see, the error is bigger with thicker material.

The essential thing is that you cannot eliminate* this error when grinding on rounded wheel. The only thing you can do is shift desired grinding angle closer to the edge heel. Then you get the desired angle at the edge heel but angle at the top of the edge will be smaller accordingly. If you shift the desired angle to the middle of grinded edge then you get smaller angle at the top of the edge and higher at the edge heel.

In view of the above, it is necessary to ask: Which point on the edge is the right one for the defined angle? Top? Middle? Heel? Something between?
It's no problem to do it in TormekCalc – just change the jig projection length and you will get desired angle at the point where you want. Small corrections are sometimes needed while grinding because of change of the jig projection length, or measurement errors. It's really not dark magic or rocket science.

What Ton does not mention, but I've found out, is that grinding with the wheel rotation goes differently to grinding into the wheel and requires additional mathematics. Therefore, you will see that the angle calculated for the Frontal Vertical Base by _TormekCalc.xlsx deviates from real edge angle even more.

Sorry, without detailed explanation this doesn't make a sense.

I don't have any problem with grinded angles while using my FVB (regardless of direction of rotation). It should be noted here that all constants in TormekCalc have been set for my T-8 and FVB and may differ in other devices.

I understand that accepting that may be frustrating to you both, but we have to adjust maths to the real world, the vice versa doesn't work.

It's not about frustration, it's about precise inputs and corresponding outputs.
I made a robust testing on TormekCalc and I didn't find any problem with calculations. Here I write about some limitations and explain why it is so. TormekCalc is free and everyone can test it and check its outputs. I have no evidence that there is something wrong except your claim, which is not specifically substantiated, so I try to make it right.

I have indicated that I have doubts about the accuracy of your measurements, which affect the grinded angle and the subsequent interpretation of the results.
There is why:
1. Jig projection length (video): Minimal supposed measuring/reading error ±0,25 mm
2. USB height (video): Minimal supposed measuring/reading error ±0,1 mm
3. Wheel diameter (video): Minimal supposed measuring/reading error ±0,1 mm

All these minimal errors together (in the worst scenario) will change the desired angle by ca ±0,25° at the exact jig projection length. Errors caused by FVB constants and by hand grinding aren't included.

4. CATRA Hobbigoni protractor (video)
Scale division is poor, supposed reading error can be ±0,5°, big reflection pattern can be other source of errors. Declared accuracy by Catra "measures the sharpened angles to an accuracy of ±2°".

Therefore I am skeptical of the reported results which seems to me inconsistent and I cannot simulate or verify them.


*) Actually it's possible with flat grinding on diamond wheels or much more difficult way by changes of jig projection length while grinding.

O.K., this is what's going on when grinding on any wheel - top of the edge has defined/counted angle (e.g. 20 °). Different angle is at the edge heel and it vary in dependency on thickness of the blade. This error is given by "roundness" of the wheel and can be eliminated by flat grinding on diamond wheels side only.

If you change grinding angle / heigth of the USB / jig protrusion length you will get better result at the edge heel but this will change the angle at the top of the edge accordingly.

All these errors exist but they are countable with pure math without any approximation and can be taken into account while grinding.

I still have doubt about your test procedure and result interpretation for some reasons. The main one is accuracy of used equipment. How you can see on Catra goniometer difference between angle 12 and 12,2°? The next big problem is height of the grinded edge. If you grind sharp angle or thick material you get very high grinded edge which makes wide reflection on laser goniometer. How do you evaluate such large reflection pattern with high accuracy? What is the accuracy of measurement of the jig protrusion length? What is the accuracy of measurement of the USB height? All these values (and many others) affect results and repeatability.

TormekCalc.xlsx doesn't use Jan's tweaks at all. All calculations are made in the axis of the knife, results are valid for all double bevel knifes (symmetrical) at the top of the edge. For single bevel knives is possible to make a correction of dimension JC by changing diameter of USB.

[+]

I did trial sharpening by Jan's formula / JVH _TormekCalc.xlsx with a knife 2.5 mm thick at the spine - the results:



If you have a laser protractor, you can see to yourself that neither Jan's formula nor _TormekCalc.xlsx by JVH grind to the exact angle, and the thicker the blade, the greater is deviation from the target.

While our updated Grinding Angle Setter and the FVB applet grind exact angle, as I've shown in the testing results.

Hello,

With all respect to you work I have doubt about test procedure and result interpretation.

I have never noticed the dependency of the grinded angle and the thickness of the blade behind the edge. Math used in TormekCalc is precise and verified firstly in CAD simulations and then in real life. I have to say that I get the expected results every time I grind. It doesn't mean that small corrections aren't needed, but they are always caused by dimension changes or measurement errors.

The most common is the jig projection length shortening if you grind too much (under the axis). Wheel diameter change should be considered too in longer grinding.
Measurement errors also affect grinded angle (e.g. ±0,16 mm of the jig projection length changes angle by ±0,1%; ±0,14 mm of the vertical USB heigth changes angle by ±0,1%).

Next thing is laser goniometer accuracy (Catra Hobbigoni has accuracy ±2 °) and its scale division is very tight (low resolution). Interpretation of laser reflecting is another source of errors.

Last at not least your results in this trial sharpening (12° gave + error , 20 ° gave - error) don't make a sense. Result seems to me inconsistent and I cannot simulate/verify it.

Please do not misunderstand this post, it is not meant to be offensive. Just I didn't notice such problem and there aren't detailed information about testing procedure (used equipments, measuring accuracy for all parts etc.). Therefore I have a doubts and I see possible systematic errors. Of course I can be completely wrong, but I haven't found any convincing evidence yet.

jvh

Hello. First Time poster.
IF i have used the black marker method to get the same angle for The knife on the sharpening Stone, how can i change the lenght of the legs of the USB so that i can Quickly change from Stone to leather wheel?

Best regards
Nik3

Hello Nik3,

you can use TormekCalc https://forum.tormek.com/index.php?topic=3919.0 for reverse calculation of bevel angle through "marker method" (see "Bevel angle" sheet).
Then use this angle for USB heigth calculation for other stones/wheels (see "USB height" sheet).

[visible]

Edit:  One thing I've run across, on the "Bevel Angle" sheet, the first entry for "Wheel Diameter..." has a strange drop down menu that shows up...

It looks like a compatibility issue. Do you use MS Excel or other app?

It is dynamic drop down menu which shows wheels from "USB height" sheet in all visible columns. If you hide a column, corresponding wheel is exluded from drop down menu. Excel doesn't provide a function  how to do it directly, therefore auxiliary formulas are used, unfortunately they can be incompatible with other apps.

Here is slightly modified version with static drop down menu, which has limited functionality and always offer all wheels...

I can confirm that your results are identical with my results with an accuracy of 0.001 mm.  Congratulations!

Thanks for verifying, I really appreciate it.

Thanks all! 

I forgot to mention one disadvantage - when grinding and raising the handle at the end, the jig axis and therefore grinding angle changes a little bit, due eccentricity of the bushing. It's not a problem for me in the most cases because eccentricity isn't too big and method with jig 's rotation left/right is usable without any influence to grinding angle.

I've wanted to use the SVM-00 Small Knife Holder with pocket knives which have small blades, but the ones I sharpen always have multiple blades on each side.  This made the angles drastically different for each side.

See above - with high eccentricity there is a risk of high angle changes when raising the handle of the knife.

For such knives I'm using correction by turning adjustable stop. Set one side with Anglemaster WM-200, mark start position of adjustable stop, turn the jig and set the same angle on the other side by rotation of the adjustable stop. Count the number of turns, write this number to blade (e.g +2,5) and mark finish position of of adjustable stop. Now, when I'm turning the jig I make angle correction by turning adjustable stop to start or finish position. It's not comfortable at all but it works.

Thanks for sharing it here.
I could imagine some ticks on the eccentric bushing surface which will help to mount a knife with specific blade thickness.

Not here only, see knife.cz 
It's not that easy, the clamps would always have to be in the same position (e.g. paralel), otherwise angle will differ.

Hello everyone,

for precise grinding I am using TormekCalc - Excel spreadsheet which I made for:

- Height calculation of universal support bar (USB) from defined point on the Tormek housing.
- Height calculation of universal support bar (USB) from the wheel surface.
- Reverse calculation of bevel angle through "marker method".
- Knife database with the ability to load stored data from previous grinding.

Calculation is made for all used wheels (unused wheels can be hidden by column hiding).
Calculation is made for all USB's (vertical, horizontal and frontal).

Help is in comments (sorry for my bad English), drawing with used symbols and dimensions is included.

Table is locked (without password) by default, only input cells are editable. Feel free to unlock it if you know what you are doing. Improvement ideas are welcome.

Credit goes to Ton Nillesen and his book "More math for the Tormek grinder" which inspired and helped me a lot.

Enjoy!


jvh


Note: File is compressed to zip format (forum restriction), you have to unzip it before using.

Hello everyone,

as many others I had a problem with knife centering in jig SVM-45 and SVM-140, because jig does not center the knife correctly and bevel on one side is different from other side.

Therefore I made a centering bushing which is mounted on the jig. Bushing is eccentric and by turning left or right is a knife easily centered. Outside diameter is 15,3 mm, inside 12+0,1, eccentricity ca 1,6 mm, length 25 mm, lock screw M3x3, 5 mm from the end.

Outside diameter is still small to use adjustable stop without any limitation, moreove there can be a gap between the bushing and  adjustable stop (theoreticaly up to 6 mm), which allows to make angle corrections by turning adjustable stop. For angle checking I'm using Anglemaster WM-200 on opposite sides of blade.

jvh