Knife Jig Setting Template

[meltzer1]

On 10/27/2015  JAN presented a setting template for clamping the knife jig along the blade so that as the blade is lifted (airplane roll, not yaw) the change in angle to the wheel tends to zero out the change in angle from contacting the wheel further back toward the support.  Is there a development or presentation of the math for this.  I would be most interested to see how this done.  Thanks.

[cbwx34]

On 10/27/2015  JAN presented a setting template for clamping the knife jig along the blade so that as the blade is lifted (airplane roll, not yaw) the change in angle to the wheel tends to zero out the change in angle from contacting the wheel further back toward the support.  Is there a development or presentation of the math for this.  I would be most interested to see how this done.  Thanks.

Do you mean this?

https://forum.tormek.com/index.php?topic=2654.0

If so, I'm pretty sure that template is for setting the knife so that you pivot it, not lift it... so you maintain the same location on the wheel.  If I recall, it's based on the pivot point of the side of the Stop Collar.

[meltzer1]

Thank you.  I misunderstood the use of the template.  The derivation of the template is much easier to see, once it is realized that it is for pivot rather than roll. 

When one considers roll as opposed to pivot, the math seems much more complicated.  One is trying to have a certain angle at the edge.  This angle is measured in the plane perpendicular to the edge.  For a curved edge, it is the plane perpendicular to the tangent to the edge.  Even without taking into account movement back on the wheel with a curved edge, effectively a shortening of the blade projection, things seem rather difficult to calculate (at least for me).  Do you know if this has been worked out?

There does seem one advantage to sharpening with roll alone as opposed to some variable combination of roll and pivot, at least for knives with shallow curves.  That is repeatability.  For example, when one watches Wootz (Vadim) sharpen, which is always a pleasure, his body is positioned above the blade, his hands are positioned on and control both sides of the blade, and he has a very precise motion like a practiced golf swing.  He is, of course, not only taking several passes with one stone or grit, but also using multiple grits at precise settings.  Unless the angle is repeatable in the curved portions, this will not make much sense.  For most purposes, it will not be so important if the edge varies from, say, 12 dps to 15 dps along the blade, more that the angle at a given place along the blade is constant from one pass or step to the next.

Once one moves from hard wheels to paper or felt, the compressibility, or even runout of the wheel,  may make up for small differences, but this will be at the risk of rounding the edge.

[cbwx34]

Thank you.  I misunderstood the use of the template.  The derivation of the template is much easier to see, once it is realized that it is for pivot rather than roll. 

When one considers roll as opposed to pivot, the math seems much more complicated.  One is trying to have a certain angle at the edge.  This angle is measured in the plane perpendicular to the edge.  For a curved edge, it is the plane perpendicular to the tangent to the edge.  Even without taking into account movement back on the wheel with a curved edge, effectively a shortening of the blade projection, things seem rather difficult to calculate (at least for me).  Do you know if this has been worked out?

There does seem one advantage to sharpening with roll alone as opposed to some variable combination of roll and pivot, at least for knives with shallow curves.  That is repeatability.  For example, when one watches Wootz (Vadim) sharpen, which is always a pleasure, his body is positioned above the blade, his hands are positioned on and control both sides of the blade, and he has a very precise motion like a practiced golf swing.  He is, of course, not only taking several passes with one stone or grit, but also using multiple grits at precise settings.  Unless the angle is repeatable in the curved portions, this will not make much sense.  For most purposes, it will not be so important if the edge varies from, say, 12 dps to 15 dps along the blade, more that the angle at a given place along the blade is constant from one pass or step to the next.

Once one moves from hard wheels to paper or felt, the compressibility, or even runout of the wheel,  may make up for small differences, but this will be at the risk of rounding the edge.

That's why I like pivoting vs. lifting.

Seriously though, you're right in your analysis, but I don't think anyone's ever done the math on that.  The angle is set by how far from the belly to tip area you clamp the blade, and there are even more variables than you lsted, for example how the blade thickness changes in that area.  Fortunately, with a bit of practice, not that hard to get the desired result.

If you keep the knife clamped in the same place along the blade, the angle difference should be negligible between setups.  "Rounding edges" is harder than some would lead you to believe.

I often post consistency is just as, if not more important than accuracy... so note where you clamp the blade, etc. and it will work out fine.