10-30-2018, 05:58 PM
This is a very common issue with almost all powered sharpening. It's simply over grinding in that spot, and there are a couple ways it happens. Mr. Sharpco mentions higher tension in the center of the belt, and that's pretty accurate for belt grinders that use dome faced wheels like Kallys. The belt tends to take the shape of the wheels. Belts with light (J weight) backing like Trizacts present the sharpener with pretty "slack" and rounded sides of the belt compared to the tauter center of the belt.
I think this "overstretching" of the center of the belt happens anytime a domed tracking wheel is used in conjunction with thin, flexible backing. It even happens on very expensive 2x72 knife making grinders using flat wheels on top and bottom of a flat platen. The center of a 2" wide belt doesn't touch the center of the 2" wide platen until you push something against the belt firmly enough.
In other words, if you're just "tickling" the belt, with the blade parallel to the platen, only the center of the belt touches the blade. If you're using a 1" wide belt, the hollow will be 1/2" from the heel. On a 2" wide belt, the hollow will be 1" in front of the heel.
Always starting your pass at the heel is the most common cause, because as you place the knife against the belt the knife isn't moving Sideways. There is built in hesitation if the belt touches the blade before the blade is moving sideways. Not much, but positively some. You might imagine what would happen if you had a hesitation in the middle of a pass. Instant divot in the middle of the blade.
There are two ways I try to overcome this. First, I don't initiate contact at the heel. I start well in front of the heel and go to the point, then without losing contact, reverse from the tip to the starting point. You don't have to worry about getting all the way to the heel until the rest of the edge is done. You will always have better control at the heel if you end your pass at the heel. It's so much easier to control your contact area and pressure that way, pressure being the most important thing of all.
I think this "overstretching" of the center of the belt happens anytime a domed tracking wheel is used in conjunction with thin, flexible backing. It even happens on very expensive 2x72 knife making grinders using flat wheels on top and bottom of a flat platen. The center of a 2" wide belt doesn't touch the center of the 2" wide platen until you push something against the belt firmly enough.
In other words, if you're just "tickling" the belt, with the blade parallel to the platen, only the center of the belt touches the blade. If you're using a 1" wide belt, the hollow will be 1/2" from the heel. On a 2" wide belt, the hollow will be 1" in front of the heel.
Always starting your pass at the heel is the most common cause, because as you place the knife against the belt the knife isn't moving Sideways. There is built in hesitation if the belt touches the blade before the blade is moving sideways. Not much, but positively some. You might imagine what would happen if you had a hesitation in the middle of a pass. Instant divot in the middle of the blade.
There are two ways I try to overcome this. First, I don't initiate contact at the heel. I start well in front of the heel and go to the point, then without losing contact, reverse from the tip to the starting point. You don't have to worry about getting all the way to the heel until the rest of the edge is done. You will always have better control at the heel if you end your pass at the heel. It's so much easier to control your contact area and pressure that way, pressure being the most important thing of all.