Knife tip grinding - To Pivot or Not To Pivot

[wootz]

Hypothetical assumption voiced on this forum was that sharpening towards the tip should use a combination of lifting and forward pivoting to maintain the contact with the stone at the same line.
It seemed logical because the belly upward curvature positions the knife tip closer to the support as compared to the heel, and shorter distance to the support increases the grinding angle and narrows the bevel; pivoting forward was assumed to compensate for that.

Practical trials showed this assumption was wrong, moreover, thicker long knives with little belly curvature require pivoting the tip towards yourself/support, i.e. opposite to the hypothetical assumption.

I used 3 knives:
Thin & long knife - 19cm Global chef knife 1.8mm thick at the heel and tapered to 0.6mm at the tip.
Thick & short knife - 9cm Linder knife 4mm thick at the heel and tapered to 1mm at the tip.
Thick & long knife - 18cm custom camping knife 4mm thick at the heel and tapered to 1mm at the tip.




Knives were mounted in the jig as shown by Jan in
http://forum.tormek.com/index.php?topic=2562.15

This is a clever approach to avoid jig position effect on the bevel described in the Tormek Handbook p.53.

All knives were sharpened to included angle of 30 degrees by pulling across the stone WITHOUT pivoting, only lifting the handle of the knife as taper tells to maintain contact with the stone.
To help myself maintain the same blade position as the knife is pulled across the stone, I used Ken's idea of a laser to mark the initial line of contact, "LOC" in Elden's (kb0rvo) terms, as they discussed in http://forum.tormek.com/index.php?topic=2562.15

The same steps for all 3 knives:
- mark the bevel;
- set the 15 degree angle at the heel by manually rotating the wheel and checking the marking;
- sharpen from heel to the tip WITHOUT pivoting.







RESULTS

Thin & long chef knife - uniform bevel along the edge of 1.3mm.
Thick & short knife - uniform bevel along the edge of 2mm.
Thick & long camping knife - widening bevel, from 1.5mm at the heel to 1.8mm at the tip.







For these knives no pivoting was needed, even for the last knife the difference was practically negligible.

The only explanation I can think of is contrary effect of taper and upward curvature:
Because of the taper contact with the stone extends and bevel widens.
However, the belly upward curvature brings the tip towards the support, increasing the grinding angle and thus compensating for the taper.

Note that curvature of the thin & long chef, and thick & long camping knife used in the trial is comparable.
And note the bevel widening towards the tip of the thick long knife, though slightly (by 0.3mm).

Following the same logic, a thick and long knife with little upward curvature, sharpened without pivoting, will have even wider bevel to the tip. Sashimi knives and pig stickers are good examples of such knives.

And true, look at the bevel widening to the tip in the next picture, also sharpened without pivoting; now there is almost 1mm difference between the heel and the tip.


To produce a uniform bevel along the edge of this sashimi knife, I had to pivot the tip over 1 cm towards the support/myself.

CONCLUSION

Regular kitchen knives, as well as thicker but short knives do not require pivoting; lifting on the handle as taper tells is all you have to do.

Thick, long & narrow knives that are tapered towards the tip > 2mm, and have little upward curvature, require combination of lifting on the handle, and pivoting the tip towards yourself.
As a rough guide, you move the tip towards yourself about the same distance as the knife lifts over the edge of the stone, sort of translating the vertical movement into horizontal shift.

Knives with false edge require extra backward pivoting to increase grinding angle towards the tip (otherwise you'll grind it off).
With double edged knives and daggers, where you have to make two edges meet at the middle of the tip, as your grinding nears the tip, pivot the blade tip towards yourself , and stop grinding as the water flow reaches the very tip.

[Elden]

Hypothetical assumption voiced on this forum was that sharpening towards the tip should use a combination of lifting and forward pivoting to maintain the contact with the stone at the same line.
It seemed logical because the belly upward curvature positions the knife tip closer to the support as compared to the heel, and shorter distance to the support increases the grinding angle and narrows the bevel; pivoting forward was assumed to compensate for that.

All knives were sharpened to included angle of 30 degrees by pulling across the stone WITHOUT pivoting, only lifting the handle of the knife as taper tells to maintain contact with the stone.
To help myself maintain the same blade position as the knife is pulled across the stone, I used Ken's idea of a laser to mark the initial line of contact, "LOC" in Elden's (kb0rvo) terms, as they discussed in http://forum.tormek.com/index.php?topic=2562.15

The only explanation I can think of is contrary effect of taper and upward curvature:
Because of the taper contact with the stone extends and bevel widens.
However, the belly upward curvature brings the tip towards the support, increasing the grinding angle and thus compensating for the taper.

Regular kitchen knives, as well as thicker but short knives do not require pivoting; lifting on the handle as taper tells is all you have to do.

   Let me start with saying, excellent work and stating of your findings, Vadim.

   After thinking about this for sometime, I want to express my opinion. I state it as opinion because I have not verified it with measurements. In order for the bevel "angle" to remain the same, contact between the grinding wheel and the knife edge will have to be maintained at the LOC (line of contact). Any deviation from the LOC will result in a change of bevel "angle". If one is to keep the "width" of one bevel face grind consistent as Vadim is striving to do, contact with the LOC will not be the proper approach for the entire blade, as he points out. To maintain the sameness of "width", one will have to vary sharpening procedures according to the knife.

[Herman Trivilino]

The bevel angle can vary even if the same LOC is maintained. However, if you keep the knife edge the same distance from the universal support (US) then you are assured that maintaining the same LOC will maintain the same bevel angle. This is what Steve has taught us. The Tormek knife jig doesn't always keep the knife edge the same distance from the US. This is the case when the knife edge curves at the tip. Wootz has shown us this in some detail and explained that the compensation mechanisms don't always work the way you might think they would. I get around all these difficulties by not using the knife jig. I instead use a platform. Easier to set up and the same bevel angle is maintained regardless of blade tip curvature.

And even if you vary the LOC it is possible, although seemingly very difficult, to keep the same bevel angle.

Note that when using a platform you keep the LOC the same and you also keep the distance from the US to the knife edge the same. At all times. Regardless of how the blade is shaped.

[stevebot]

Wootz, a nice piece of work.  Thanks.

[wootz]

Thank you for your replies, guys.

Yes, this trial was all about handling Tormek standard setup with their knife jig.
In adapting my bench stone skills to recently acquired Tormek, I had to clear up the jig behavior for myself.

My past experience, and Steve's decades of sharpening, and everyone else's who sharpens freehand tells to keep the edge at the LOC.
However, when I clamped a Muela pig sticker into the jig, and sharpened at the LOC, the bevel I got was less than satisfactory; frankly, I ruined the knife tip.
It's in the first picture.
The pig sticker is 4.5mm thick at the heel.

I just had to resort to experimenting to sort this out, to save the pig sticker and my reputation.
The above trial helped me to understand that with thick & long knives taper comes into play, partly due to lever effect, and partly due to knife jig design discussed in http://forum.tormek.com/index.php?topic=2577.0

For these knives, pivoting the tip away from the LOC appeared to be the right technique when you use knife jig in Tormek standard setup.
It's the second picture, and I am happy with the result, and the Tormek.


Sharpened at LOC


Sharpened pivoting away from LOC

[Herman Trivilino]

In that latter photo, Wootz, you can really see that the bevel doesn't have the same mirror finish as the rest of the knife surface. I wonder if the Japanese Waterstone (SJ) would produce something better.

As I've said before, when I grind with the standard SG grindstone carefully prepared fine and then polish on the honing wheel I see a surface with my 40X scope that has a mirror finish on the flat spots between the scratches. Without the scratches there would be a mirror finish, it seems, and I wonder if the SJ would remove these scratches.

[wootz]

Herman, the latter edge is after Japanese SJ.
The SJ definitely gives better result than going from a finely graded standard SG straight to the honing wheel.
I understand what you mean, and yes it is possible, but for the Japanese stone to hone out all SG scratches, I should have spent twice the time.
And the Japanese stone is most expensive, and soft and has to be declogged and trued frequently, wearing off quicker than any other stone.
So I limit knife processing on the SJ to acceptable minimum.

All my experience tells me that jump from finely graded standard SG stone to the 4000 grit SJ is way too much.
I have a dedicated SG stone which I grade only finely, and yet I wouldn't rate it at 1000 grit as the Handbook reads - I'd rate it at 600-800 grit.
And when you move the blade from that to the 4000 grit SJ, it's hard to level out all that bad scratches with this superfine stone.
Yet the Japanese stone definitely delivers more than the honing wheel, and for high end knives I cannot skip it.

I wish Tormek offered a dedicated waterstone of true 1000-1200 grit; using it after 220 grit, and then going from it to the 4000 SJ would be just perfect.

[Herman Trivilino]

Ahhh. I see that now. There are some places where you've got all the scratches out. Like near the tip.

And getting that mirror finish may not be desirable for many or most applications. A knife cuts a lot of things better when it has a tooth. Those scratches make the knife behave like a saw, albeit on a microscopic scale.

Removing the burr is more important, and that is what the honing wheel does well.

[Jan]

Vadim, congratulations to your great contribution! 
I just returned from my vacation in the Mediterranean and I was pleasantly surprised by your new thread.

I need some time to digest your contribution, because until now I never grinded a knife pivoting the tip towards myself.

Referring to your image below, I would like to ask you, how do you use the laser LOC to sharpen the tip without pivoting the knife?

I hope, I will come with some comments and geometrical considerations soon.

Jan

[Elden]

   Nice looking sharpening job on the pig sticker dagger, Vadim. I agree that your method for that knife definitely looks nice. Have you measured the degrees of the bevel angle along the entire length of the blade after sharpening? Also, what is the width of the bevel face grind at the heel and the tip?
   The thickness of the blade at the tip of the knife is quite thick because of the spine of the knife coming so close to the point of the knife. This would cause the width of the face bevel to become quite wide as your first picture of the knife shows.
   Great job, Vadim!

[wootz]

Have you measured the degrees of the bevel angle along the entire length of the blade after sharpening?

Edge angle in the above 3 knives sharpened without pivoting is about the same +/- 2 degrees along the straight part and into the belly, then increasing by 6-13 degrees near the tip (included angle).
So, set at the heel to the included angle of 30 degrees, and sharpened without pivoting, it resulted in 34-42 degrees near the tip.

Bevel face width practically doesn't change from heel to the tip, while the bevel base thickness does increase at the tip.

[wootz]

Referring to your image below, I would like to ask you, how do you use the laser LOC to sharpen the tip without pivoting the knife?

Hi Jan, just by eyeballing along the LOC and further to the staight part of the blade, kept them in line.

[Jan]

Hypothetical assumption voiced on this forum was that sharpening towards the tip should use a combination of lifting and forward pivoting to maintain the contact with the stone at the same line.
It seemed logical because the belly upward curvature positions the knife tip closer to the support as compared to the heel, and shorter distance to the support increases the grinding angle and narrows the bevel; pivoting forward was assumed to compensate for that.

Practical trials showed this assumption was wrong, moreover, thicker long knives with little belly curvature require pivoting the tip towards yourself/support, i.e. opposite to the hypothetical assumption.
.
.
.
CONCLUSION

Regular kitchen knives, as well as thicker but short knives do not require pivoting; lifting on the handle as taper tells is all you have to do.
.
.
.

You can of course sharpen the tip by lifting the handle without pivoting the knife, but you will grind a smaller bevel angle, than at the straight part of the blade. This can be compensated by grinding the tip near to the support, which on the contrary will increase the bevel angle.

I made some geometrical estimations for the above described procedure. I assumed that the edge angle is 30 degrees, i.e. the bevel angle is 15 degrees and the stone diameter is 250 mm = 10 ".

1)   When we omit to pivot the knife by some 45 degrees when grinding the tip, we will decrease the edge angle by more than 8 degrees.

2)   When we grind the tip at a distance by some 9 mm = 0.35 " shorter to the support than the straight part of the blade, we will increase the edge angle by some 8 degrees.

So, the edge angle decrease caused by replacing pivoting by handle lifting, can be fully compensated by shifting the spot where we grind the tip near to the support.

In my thinking, to maintain constant bevel angle along the belly and the tip, it is more appropriate to pivot the knife so, that the edge meets the stone at the line of contact.

However, I can easily imagine that this concept may not work, when we need to maintain constant "width" of the bevel face along the whole blade, especially for atypical knifes.

Thanks again Vadim, for shearing your well documented practical trials!
As Goethe said, theory is grey, but the golden tree of life is green.

Jan

[cbwx34]

Hope you don't mind an "old thread" revival... (remember, I asked)....    but there's a lot of good info in it.

First, a picture to refer to (you can click on it for a larger view)...



First thing I wanted to point out, that when pivoting the blade (1 & 2 in the picture),,, on smaller blades it won't necessarily allow the belly/tip area to reach the LOC... but will drop below it.  So, length of the blade matters.

Second, depending on the setup, when you pivot the blade, you can actually "ride up" on part of the clamp (picture 3).  Can be avoided, but something to watch for.

The main reason I brought this thread up though, is honestly, even after reading thru this and the other related thread referenced... I've never been totally convinced that the original intent of the knife jig was to pivot the blade... but to merely lift the handle to follow the belly to tip area.  In part, based on Jeff Farris' video saying, lift don't rotate, (in fact he makes a point that pivoting is incorrect) and in part, it just doesn't seem a natural movement on many knives.  I understand the purpose... pivoting makes an attempt to maintain the same LOC... so the angle will stay the same... but am not really convinced that the curve of the stop on the jig was designed for this.  (I would swear that it wasn't even mentioned in the old manual... but maybe it's just Jeff's voice stuck in my head).

Lifting on the other hand, will also allow you to keep the same angle, depending on how you set up the clamp, (as seen in Jan's picture in this thread, although the pivot point would change), and to me, seems to be a more natural way to follow the shape of the knife when using the standard jig.  I try to illustrate in pictures 4 & 5... that when you lift the handle, you also change the orientation of the bevel in relation to the wheel... so that the angle is no longer totally dependent on where it is on the wheel, but also includes the "side angle" of the bevel in relation to the wheel.  Think of the extreme... if I stood to the side of the Tormek and sharpened a knife on top of and parallel to the wheel (doesn't have to be on top, but easier to visualize)... the angle would totally depend on the angle of the knife.  So, when setting the knife in the jig, by adjusting the belly/tip area closer or farther from the pivot point of the jig, this angle can be altered so that, even by merely lifting the handle...  the same (or at least similar) angle can be maintained throughout.  So, if you clamp the blade where the handle is lifted higher to reach the tip, you increase the angle... and the opposite if you clamp farther away from the tip.

It seemed in reading thru this thread, that the main focus was keeping the angle the same based on the LOC... and not what I'm describing.  And, obviously, I'm not saying pivoting doesn't work, (and it of course is the answer for freehanding where the blade is held level, or for Herman Trivilino's setup, etc.)... but I'm of the mind that with the standard jig setup, lifting the handle in many cases, may be the better option.

So, what say y'all? 

[RichColvin]

Wootz,

I'm having trouble following your work.  I think it would be good to create some language to describe what you are saying relative to movement. 

Airplanes use the language of roll & yaw (http://visual.merriam-webster.com/transport-machinery/air-transport/movements-an-airplane.php ).  That would work if you used them in terms of movement of the SVM-xxx knife jig, but that's just one option. 

Your testing is very worthwhile & I want to understand it better so I can replicate you good learnings. I may not be the brightest crayon in the box, but I am smart enough to learn from my betters & elders.

Kind regards,
Rich