Knife tip grinding - To Pivot or Not To Pivot

[cbwx34]

Some blades can be sharpen from heel to the tip with no need to pivot while other blades with no need to lift the handle.

Based on my limited experience, the compromise between lifting and pivoting is the suitable approach.

Because it is crucial how the blade is clamped in the jig, some two years ago, I have prepared simple "Knife Tip Settig Template" for kenjig projection length of 139 mm. https://forum.tormek.com/index.php?topic=2654.0

This clamping guarantees that you will get the same bevel angle at the heel and at the tip, more or less with no need to lift the handle. The template is not a universal tool, it works fine only for limited set of blade shape's.

Wootz has shown several examples for which this approach does not provide consistent bevel width.   

Jan

P.S.: The template design was extracted by back engineering the radius of the adjustable stop of the knife jig.

Thanks for posting this.  Very helpful.

I think your template also illustrated why I feel the jig radius doesn't 'feel' quite right, and why Wootz found it didn't work for the knives he was sharpening, (and as you stated only for certain blade shapes).  If you look at the attached picture, you see the clamped knife doesn't fit the radius unless the knife is moved, which equals the the projection length being shortened if I'm 'reading' it right.  (So perhaps Wootz could have got it to worked if he had altered the projection length?)

(One thing I've learned in dealing with other guided sharpeners with a pivot... it's a little more complicated than it first appears). 

Your template is a good reference though... definitely helps define the process of what is going on. 

[Jan]

CBWX, thank you for your kind words, highly appreciated. 

Jan

[Kavik]

Thanks for the verification Jan

Soooo... Based on that example from cbwx:
In theory, it seems the only thing that could make for a truly universal setup/technique would be to have collars with different profiles to match different blade types?
I mean, you're obviously not going to have one to match every knife... But say you had even just 2 collars, so that you have the stock pivot angle, a more extreme pivot angle like the one needed for cbwx's example, and then a very gradual curve for something like that pig sticker scenario

Would that allow for a consistent method, using line of contact as a guide, while maintaining proper bevel angle?
(i almost feel like there should be two threads here. One that can discuss technique for maintaining bevel angle... And one to discuss how to keep a knife looking pretty, ie: maintaining bevel width at any cost  )

[cbwx34]

Thanks for the verification Jan

Soooo... Based on that example from cbwx:
In theory, it seems the only thing that could make for a truly universal setup/technique would be to have collars with different profiles to match different blade types?
I mean, you're obviously not going to have one to match every knife... But say you had even just 2 collars, so that you have the stock pivot angle, a more extreme pivot angle like the one needed for cbwx's example, and then a very gradual curve for something like that pig sticker scenario

Would that allow for a consistent method, using line of contact as a guide, while maintaining proper bevel angle?
(i almost feel like there should be two threads here. One that can discuss technique for maintaining bevel angle... And one to discuss how to keep a knife looking pretty, ie: maintaining bevel width at any cost  )

I think, if I read my picture right... you could use just one collar, and adjust the projection length to get everything to fit.  (Of course that eliminates the Kenjig... since you'd have to adjust the USB distance....).  Guess it comes down to what your needs are, for example the Kenjig speeds up the process for volume sharpening.  If you start introducing multiple collars, it might defeat the purpose, to some extent anyway.  (Plus, I'm not sure a lot of people have the means to make collars?)

I don't think "maintaining bevel angle" vs. "bevel width" as separate issues... they are definitely tied together.  And, to me, it doesn't have to be one or the other... as Jan has stated, a combination of "lift" and "pivot" may give a compromise to both issues.

[Kavik]

Again, I'm just spitballing here based on my very limited experience with the current jig, and observing some of the comments here.
But it just seems to me that with a fixed radius on the stock collar, if trying to maintain line of contact while pivoting, won't you always be compromising  between the belly or the tip either extending past or falling below the LOC?
It seems to me that that's where the lifting becomes necessary to counteract that... At which point it becomes guesswork in how much to lift.

Whereas, again  in theory, if the radius of the jig were an exact match to the profile of the knife, maintaining LOC should keep the bevel angle exact with NO lift required. Similar to laying it on a tool rest and pivoting to follow the curve of the edge.

I'm in the office today, and don't have one in hand to look at at the moment, but my theory for keeping it simple, fast, and more flexible would be as follows:

• create 3 collars, all of equal thickness (yes, i increased from 2 to 3, so the length of the jig doesn't need to change to match the kenjig system).
  
  
  
  • collar 1) a shallow, gentle sweep
  • collar 2) match the stock collar profile
  • collar 3) a more intense curve
• redraw the kenjig alignment template with 3 arcs drawn and labeled to match the 3 collars
• setup the knife in the jig using the template the same as always (with whichever collar you have on, as they're all the same thickness
• compare the curve of the blade to the 3 lines and match it up to the one that's the closest match
• swap out for the collar that corresponds with that line
• start grinding

My idea for the collar would be something simple, turned on the lathe, then cut in half, using pins and magnets to reconnect around the shaft.
Should add all of 10 seconds to the setup time for any knife that doesn't match the collar you had on the jig from the last knife you did

[Jan]

I don't think "maintaining bevel angle" vs. "bevel width" as separate issues... they are definitely tied together.  And, to me, it doesn't have to be one or the other... as Jan has stated, a combination of "lift" and "pivot" may give a compromise to both issues.

In my understanding the handle lifting approach is more aggressive in changing the bevel angle and also allows to sharpen long thick blades. From this point of view, lifting is more universal than the pivoting approach, which is smarter, but in pure form applicable only for thinner blades with "Tormekian" belly.

The compromise to both issues give us some freedom to find suitable trade-off between bevel angle and bevel width consistency along the edge.

Jan

[cbwx34]

...

• create 3 collars, all of equal thickness (yes, i increased from 2 to 3, so the length of the jig doesn't need to change to match the kenjig system).
  
  
  
  • collar 1) a shallow, gentle sweep
  • collar 2) match the stock collar profile
  • collar 3) a more intense curve
• redraw the kenjig alignment template with 3 arcs drawn and labeled to match the 3 collars
• setup the knife in the jig using the template the same as always (with whichever collar you have on, as they're all the same thickness
• compare the curve of the blade to the 3 lines and match it up to the one that's the closest match
• swap out for the collar that corresponds with that line
• start grinding

My idea for the collar would be something simple, turned on the lathe, then cut in half, using pins and magnets to reconnect around the shaft.
Should add all of 10 seconds to the setup time for any knife that doesn't match the collar you had on the jig from the last knife you did

My .02.... I think the effort would be better spent making and using one of Herman Trivilino's jigs.

I don't think "maintaining bevel angle" vs. "bevel width" as separate issues... they are definitely tied together.  And, to me, it doesn't have to be one or the other... as Jan has stated, a combination of "lift" and "pivot" may give a compromise to both issues.

In my understanding the handle lifting approach is more aggressive in changing the bevel angle and also allows to sharpen long thick blades. From this point of view, lifting is more universal than the pivoting approach, which is smarter, but in pure form applicable only for thinner blades with "Tormekian" belly.

The compromise to both issues give us some freedom to find suitable trade-off between bevel angle and bevel width consistency along the edge.

Jan

I don't disagree that doing both may be a good compromise... but lifting the handle, for the most part, gives the same result.  I don't think it's more aggressive... because it goes back to placing it in the proper position relative to the pivot (just a change in pivot point).

Edit to add:  Jan, you might find this of interest... Geometry and Kinematics of Guided Rod sharpeners by Anthony Yan   While the Tormek isn't really a "Guided Rod" sharpener... it has a lot of info about sharpening in relation to a pivot point.

[Kavik]

My .02.... I think the effort would be better spent making and using one of Herman Trivilino's jigs.

Would love to see what that is, but... Photobucket 
I'll have to look through it later when I have time to read and follow all the links

Honestly, I was wondering why people wouldn't opt for a tool rest type system in the first place, for "volume sharpening"...or heck, even for personal sharpening.
I just assumed since people still use and discuss how to do it with the jigs, they must want to find a better way to make the original jigs work for them *shrugs*

And for what it's worth, I wouldn't expect making the collars would really be all that much effort

[cbwx34]

Would love to see what that is,...

https://youtu.be/AcWAkQmoU8c

[Jan]

I have horizontal platform for sharpening in both directions, but for precise sharpening I use the guided knife jig despite all the issues discussed here.

Jan

[Kavik]

Cheers for the YouTube link 

Yeah, something like that seems so simple, practical, versatile, and easy to setup and adjust....really does make the idea of modifying the regular knife jigs seem kinda pointless
(as opposed to  some of the other platforms people here have built that, as awesome as they are, would be cumbersome for me to store and would be a more complex build)

Now that my brain has shifted gears from just focusing on improving the use of the SVM45/100/140, to considering just the overall better approach.... I'm left wondering why mess with the SVM jigs at all? What benefit is there over a platform style?  (if this is too far off topic let me a know, I can always create a new thread for it)

Seems to me there is one issue that can present itself with either setup, and that's grinding more than is needed if you get careless.
With the platform this could be caused by staying in one spot too long.
With the SVM this could be caused by staying in one spot too long OR by applying more pressure at a certain point, which I would think is fairly easy to do on a fulcrum held against a spinning wheel.
The difference though is that with a platform, removing too much steel is certainly unfortunate, but at least your angle remains constant.
With an overgrind on the SVM you will not only waste steel, but you'll also move the LOC below the original point, resulting in a more obtuse angle.
Now, I haven't done the math to see how far you can drop below the original LOC before the change in bevel angle is significant enough to become an issue, but i imagine it won't take much... And most of us don't have a laser line setup to keep track of that while we're grinding


Jan,
Your last post came in while I was typing this. Can you elaborate at all on why you feel the guided knife jig is more precise for you? As you can see from my thoughts above I'm not trying to be argumentative, just having a hard time wrapping my head around this.
Also, since yours was the comment that got me curious about different profiled collars for the SVM jigs, any thoughts on if you think that would improve some of the need for guesswork and lifting?

Edit: just saw your attachment after I submitted this post. That was the other platform jig I was thinking of when I said awesome looking, but more complex to build

[Jan]

Kavik, the explanation why I do not use my horizontal platform for precise sharpening is simple.

When grinding away from the edge you have to resist quite strong tendency pulling the blade in the direction of rotation. When grinding towards the edge there is some tendency to push away or lift the blade above the wheel.

Struggle with those issues results in less consistent bevel compared with bevel obtained using knife jig which rests on the USB and is secured by the thumb.

Jan

[Kavik]

Thanks, I'll keep the in mind when I eventually get around to trying the platform idea

No thoughts on the differently profiled collars?

[Herman Trivilino]

With the platform jig I grind towards the knife and find that it doesn't tend to lift if you do it correctly. It just takes some practice. On the other hand, more experienced sharpeners prefer other methods. Some don't use a jig at all. It's a matter of preference.

There's a discussion in the Tormek manual about platform jigs, and their philosophy is that it's a useful technique only when the edge angle is blunt such as on a scraper. Note that when using the Tormek Tool Rest jig, which is a platform jig, the edge angle must be blunt because the platform is too far from the Universal Support rod. The idea is that for shallow angles the operator has to apply more force to a platform than to the grindstone. I don't agree. The platform can be used as a guide where very little force is applied to the platform. After all, you can freehand with a Tormek, in which case there's no force at all applied to a platform and the edge angle can be rather shallow.

Using the scissors jig base to mount a platform lets you get the platform much closer to the grindstone so you can grind shallower angles.

[Kavik]

That was kinda my thought based on work I've done on other grinders with tool rests....grind into the wheel, with just enough pressure on the spine to keep it flat on the rest, and it shouldn't be an issue....that all comes down to practice and user control i would imagine.... .. But i didn't want to say that out loud before actually trying it myself on this one lol

Was going to work on it tonight, but instead I got stuck at work late, then started working on laying out the platform for my other grinder and the BGM-100 that just arrived, then got distracted from that with a new guitar I just picked up from a friend..... Cuz I needed one more new thing to get into right now lol