Middle concaving problem reconsidered

[Jan]

The knife concaving problem was discussed in many forum topics, e.g. in Wootz's "Middle overgrinding problem". http://forum.tormek.com/index.php?topic=3055.msg16442#msg16442

The general recommendation how to avoid a hollow spot on the blade curve comes from Stig: "Use less time and pressure in the middle of the blade".

Wootz provided detailed analysis and recommendations concerning timing and pressure when pulling the blade across the grindstone.

Since I read about the concaving issue and also concaved my knife, I was wondering what is the time deficit at the heel and at the tip of the blade compared to the time spent at the straight part of the blade. 

In the attached drawing, there is a graph showing how much time we spend grinding one centimeter segment of a 15 cm (6") long blade. I have assumed that we move the knife slowly, with uniform velocity of 1 cm/s over the stone from the heel to the tip.

The simplified graph shows that the largest deficit is at the heel. At a distance 5 cm (2") from the heel the blade already receives full grinding time (5 s per 1 cm segment). This is true for all distances up to 13 cm from the heel. The last 2 cm of the blade near the tip are deficit again, because I assumed that we stop the pass 2 cm before the stone edge.

The grinding time deficit near the heel and near the tip should be compensated by spending additional time and/or applying additional pressure here. The graph is in compliance with the quoted guidance and also with relevant Wootz's recommendations. 

Jan 

[Jan]

A similar situation is encountered when sharpening a plain iron using square edge jig SE-76. The graph in the attached drawing shows how much time we spend grinding 1 cm segment of the edge. For simplicity I have assumed that we move the plane iron evenly across the stone with a velocity of 1 cm/s and that the tool rests with 1 cm on the stone in the turning points.

To minimize the concaveness it is necessary to maximize the tool movement outside the stone. That is why the handbook recommends that in the turning points the tool rests only with circa 6 mm (1/4 ") on the stone. The fewer the better.

Without any correction we can expect concave edge similar to the red curve shown in the time – distance graph. The correction is simple – spend more time on the sides of the tool.

Jan

[SharpenADullWitt]

Jan, at least on Chef's knives, what I have done so far, is use the start position also as the end position, and go slower on the tip.   (faster through the center, and slow down near the handle, almost to a stop, then change back.
So far, it has been commercial knives (which I also have at home, due to low cost verses my amount of use), which don't have a bolster to get in the way (bring the knife to about the mid point of the stone).
I can't say I time myself at all.

[Jan]

SADW, I think the most important measure to prevent knife middle overgrinding is to be aware of this problem and check frequently the edge straightness.

You are correct, Chef's knives and Santoku knives which have large blade width can be sharpen in a way similar to a plane iron. The passes are long and in the turning points the blade rests with less than 1 cm on the stone. This very significantly minimizes the need for timing/pressure correction near the heel and near the tip. 

Jan

[Ken S]

Whether or not one actually uses a timer with different parts of the knife arc, I believe keeping Joan's grinding time concept in the back of one's mind will help solve the problem.

Ken