Having trouble grinding convex blades

Ivan Campos

Ivan Campos

Knifemaker / Craftsman / Service Provider
Joined
Apr 4, 1999
Messages
2,500
Hello

I am having a certain trouble when I try to grind convex blades. The slack belt tends to make the blade convex at the sides and keep it flat at the center.
Can anyone help me?
 
Ivan, are you using the slack in the belt between a platen and your contact wheel, these should be about 3 inches apart hope this helps
 
ivan, there are two things that will get the radius you are after. one is distance beteew the wheels. i taught myself to grind and convex was the first and still my favarite. the first real grinder was a coote, which is two wheel. so when you take out the palten i must have 20" from wheel to wheel. this gives a nice radius so even the spine of the blade is shaped giving a apple seed profile grind.the other factor is to use J weigth or the most flexable belts possilble. Hermes makes the J weight superflex which is my belt of choice for this operation. i also have a Hardcore grinder but the distance beteew the 2" and 5" is about 10" so i start on the hardcore and go to the coote at 100 grit. hope this helps
 
Put a piece of canvas backed graphite on your platen
and grind as if you were doing a flat grind. Sounds
crazy but it works. You will not get a drastic convex but a gradual curve from spine to edge. The graphite will only last for one or two knives and then it will have to be replaced.
Micky Wise
 
Ivan,

It might help if you think of the convex grind as an infinite number of flat grinds, stacked one right next to the other from edge to spine and the slack belt as being used to blend all of the microscopic coners together. First start by setting the cross section geometry of the blade taking several cuts on the contact wheel. then go back and gring off the high spots that are left between your cuts again using the contact wheel. If you like the cross sectional geometry, then put on a fresh sharp belt and go to the slack belt portion of the belt and blend all of the previous cuts together. After you make the first pass, on the blade in the slack belt area look at the blade and see if you have fairly equal contact across the blade. If so great, If not make another light pass down the high spot using the contact wheel. Keep at it until you have the look you are after.
Another thing that may help is to tighten your belt and/or use less pressure on the blade. If you are having to push hard to get the belt to cut it is probably time to change belts.


Bondo, From the Willow Bow.


P.S. Ed says to tell you to only grind where you what to take off metal.: :D
 
Thanks a lot, gentlemen!
Tightening the belt to its limit and using a brand new belt seems to work. I just made ocnvex a flat ground blade i had laying on my bench and I found two problems only: the belt seems to ahve its edges folded, so it can't reach the beginning of the grind and the edge seems to have become too thin if I keep grinding until I get the blade sharpened. does anyone knows how to overcome this?
 
Ivan,

Don't grind where you don't want to remove metal.:D grind the edge portion of the blade last. ie. don't start thinning the edge until you have the rest of the blade nearly to the shape that you want it.


Bondo from the Willow Bow
 
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