Why is the knife jig arched?

[Sharpco]

The jaws of the jig are not flat but arched. What are the benefits of this?

In other words, what will happen if I make it flat?

[wootz]

My understanding is it is on purpose to better clamp knives tapered to the tip.
As you may know, I had polished the jaws flat, and on tapered knives the grip is not as firm as with the unchanged knife jig.

[Sharpco]

My understanding is it is on purpose to better clamp knives tapered to the tip.
As you may know, I had polished the jaws flat, and on tapered knives the grip is not as firm as with the unchanged knife jig.

But to sharpen thick knives, your solution is absolutely necessary. So it seems I have no choice.

[Sharpco]

My understanding is it is on purpose to better clamp knives tapered to the tip.
As you may know, I had polished the jaws flat, and on tapered knives the grip is not as firm as with the unchanged knife jig.

The idea came up. How about shallowly milling the checked part?

[wootz]

For thick knives only the lower jaw should be milled. As you know, I've milled the lower jaw, and evened out & polished the upper jaw of my knife jigs, as detailed:
http://knifegrinders.com.au/05Equipment_jigs.htm


Milled or just polished, the jig clamps OK with a cloth tape on the knife spine.
Having changed your knife jig(s) remember to calculate the jig centreline offset using Rick's method:
https://forum.tormek.com/index.php?topic=3445.msg21498#msg21498

Knowing the jig centreline offset for each knife jig I have, I put as many layers of a cloth tape at the clamping site on the knife spine as needed to bring the knife spine thickness to the calculated for the given jig, and this places the edge strictly in the jig centre when you tighten the clamps.
This gives even symmetric bevels and better sharpness.

[Ken S]

The clamping force from the locking screw is concentrated at the midpoinr of the knife jig. Having the jig surface slightly arched insures that the edges will have full clamping pressure. Flattening the surface would eliminate that design feature.

I once filed a slight bump off of a square edge jig before I realized its similar purpose.

Ken

[Sharpco]

For thick knives only the lower jaw should be milled. As you know, I've milled the lower jaw, and evened out & polished the upper jaw of my knife jigs, as detailed:
http://knifegrinders.com.au/05Equipment_jigs.htm


Milled or just polished, the jig clamps OK with a cloth tape on the knife spine.
Having changed your knife jig(s) remember to calculate the jig centreline offset using Rick's method:
https://forum.tormek.com/index.php?topic=3445.msg21498#msg21498

Knowing the jig centreline offset for each knife jig I have, I put as many layers of a cloth tape at the clamping site on the knife spine as needed to bring the knife spine thickness to the calculated for the given jig, and this places the edge strictly in the jig centre when you tighten the clamps.
This gives even symmetric bevels and better sharpness.

I think your cloth tape solution is the very good idea.

[Sharpco]

The clamping force from the locking screw is concentrated at the midpoinr of the knife jig. Having the jig surface slightly arched insures that the edges will have full clamping pressure. Flattening the surface would eliminate that design feature.

I once filed a slight bump off of a square edge jig before I realized its similar purpose.

Ken

Your explanation is clear.

[cbwx34]

The jaws of the jig are not flat but arched. What are the benefits of this?

In other words, what will happen if I make it flat?

I was going to ask if this was causing you an issue... or just something you "noticed"... but I think I have my answer.

My understanding is it is on purpose to better clamp knives tapered to the tip.
As you may know, I had polished the jaws flat, and on tapered knives the grip is not as firm as with the unchanged knife jig.

But to sharpen thick knives, your solution is absolutely necessary. So it seems I have no choice.

Freehand, Herman's jig, or if it's the occasional knife, just adjust for each side (a PITA though), are some options...

The idea came up. How about shallowly milling the checked part?

I wouldn't, unless you have more than one jig... (I think you do, but just saying for those that don't).

Knowing the jig centreline offset for each knife jig I have, I put as many layers of a cloth tape at the clamping site on the knife spine as needed to bring the knife spine thickness to the calculated for the given jig, and this places the edge strictly in the jig centre when you tighten the clamps.
This gives even symmetric bevels and better sharpness.

Symmetry is mainly for aesthetics, and will affect how the knife will perform, (although in most cases it isn't noticeable)... but it does not lead to "better sharpness".  Chisel grinds, and the variety of asymmetric grinds on some Japanese style knives will demonstrate this. 

[wootz]

Asymmetric grind in a knife jig is not the same as on bench stones - I tried to explain this here:
https://www.australianbladeforums.com/vb4/knife-grinders/37166-bevel-symmetry-sharpness.html