Grinding a Convex Grind

C

Carson Falk

Joined
Apr 28, 2017
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I have run into a problem on a customers knife. He is wanting a convex grind but I have never done one before. Does anyone have any suggestion on how to grind one? He also wants it with a zero degree bevel, has anyone had any experience doing this particular grind? Any suggestions would be greatly appreciated!!!
 
I do a fair bit of convex grinds for fillet knives, and this is what works for me. It's not the most conventional approach, but the results are good. I'll start by laying out a flat grind so that the edge will be able half the thickness of the stock. I'll flat grind the blade until the edge is where I want it, and the grind lines are nearly at the spine. I'll then switch to a slack belt and convex it. I've found doing it entirely on the slack belt takes forever, and is easy to make something too axe like. Then from the 60 grit ceramic belt I'll go straight to the buffer. I start off with a 12" wheel loaded with 240 grit formax greaseless compound. This makes quick work of blending everything together, smoothing it out, and getting rid of all the scratches. Then I switch to an 8" buffing wheel with 400 grit greaseless, and refine the finish. From there it's back to the grinder, this time with an 8" contact wheel and a 1000 grit cork belt loaded with jacksonlea green stainless compound. I run the blade vertically (in line with the belt rotation) and rock it back and forth as I go to cover the entire surface evenly. This will quickly refine the finish everywhere except right in the plunge. The plunge gets the 400 grit scratches removed by hand, with cratex and then 1500 grit wet dry on a leather backer. From there it's back to the buffer, this time with the hardest spiral sewn 12" wheel I can get, loaded with the same jacksonlea green. A quick buffing will result in a bright mirror finish. From there a hand rubbed finish takes about a minute, or (what I almost always do) is switch to a Canton flannel buff with a fine jewelers compound. This will give the best mirror I know of, and even under magnification no hairline scratches are visible
 
I use a similar method. Start with a flat grind until the edge thickness and grind height is where I want it. Then use my rotary platen to blend and convex the edge to zero. Work up through my grits to cork belts on the rotary platen. When it's finished I finish the edge with a felt belt and green compound.

The rotary platen helped shorten the time this takes as opposed to using a slack belt for me. I don't use it often enough to justify the cost of buying one but it didn't cost much to make my own and it was worth the effort.
 
I've often thought a rotary platten would be nice to have... I might order in a 2" wide serpentine belt, turn up the required pulleys, and put one together sometime. I doubt it would replace using a wheel for most of it for me, but it would definitely speed up the convexing. Another thing I'll sometimes do for narrow blades is use the little 1" wide slack belt area above the platten of my grinder to convex it. It takes a lot more force vs a full slack belt
 
The other thing the rotary is nice for is blending dissimilar materials that don't abrade at the same rate. Like a bolster and scale. If you do it on a slack, the bolster is always proud, but on the rotary they stay together as long as you don't press too hard.
260J20 is the belt part number I used.
 
That could work well for cleaning up the edge on a full tang as well. Very easy to have the tang stick up if you're not careful.
Do you have a manufacturer to go with that part number? Probably help my local industrial supplier find one
 
That's a generic belt part number that crosses most aftermarket suppliers. Gates, A-Metric, Continental etc. They go by groove width and number of grooves, and it works out to be roughly 1.84 wide (20 grooves, .092 width). I just got mine on Amazon.

The biggest pain was cutting 80 grooves (4 pulleys) to uniform depth making the pulleys :confused: Not hard, just tedious.
 
Ahh, like bearing part numbers then. I'll order one, thanks. The pulleys shouldn't be too bad to make. I've done things like that before, they are just a bit tedious
 
As folks have mentioned,startoutwitha flat grind. If you try to do all of the grinding on a blackbelt, you will end up with a bevel geometry that looks kind of like you took a 2 inch tall flat ground blade and squished it down to 1 1/5 inches tall. WAY too thick. If you examine a later Bill Moran knife, you have to look closely to actually tell that it is a full convex grind.
 
I do convex grinds on almost all my knives, but i do not use rotary platen or slack belt. I grind 2-3 flat grinds so i can control where the convex is and how much. Then i blend them on a 120 grit belt, moving slow with light pressure and a subtle rocking motion. As of late ive only been blending my top 2 bevels and leaving the bottom edge bevel at 120grit and finishing entirely on water stones. Then I'll either handsand the entire blade or just the top portion and leave the hazy stone finish on the bottom bevel.
-Trey
 
Also the bottom bevel I will grind to zero or stop and thin to zero on a 140x diamond plate. Depends on the knife and my mood.
-Trey
 
I also do a series of grinds to define the specific shape I want, then blend them together.
 
For me the rotary platen has turned out to be one of the best investments I've made for convex work.
The awesome thing about it is that by applying pressure either at the edge, at the spine, or evenly give you so much flexibility in the convex.
Then there are the varying spaces between wheels, and belt tensioning.
All of these combined offer an almost infinite control of the grind. It's an awesome tool. :)
 
comet_sharp said:
I do convex grinds on almost all my knives, but i do not use rotary platen or slack belt. I grind 2-3 flat grinds so i can control where the convex is and how much. Then i blend them on a 120 grit belt, moving slow with light pressure and a subtle rocking motion. As of late ive only been blending my top 2 bevels and leaving the bottom edge bevel at 120grit and finishing entirely on water stones. Then I'll either handsand the entire blade or just the top portion and leave the hazy stone finish on the bottom bevel.
-Trey
Click to expand...

This is the traditional method of getting an appleseed grind ( Japanese name for the convex grind of a katana) in doing togi on water stones. It starts as many tiny flats all along the hira-ji. After you form the desired amount of niku, you blend the flats into a continuous curve on a softer stone.
 
Stacy E. Apelt - Bladesmith said:
This is the traditional method of getting an appleseed grind ( Japanese name for the convex grind of a katana) in doing togi on water stones. It starts as many tiny flats all along the hira-ji. After you form the desired amount of niku, you blend the flats into a continuous curve on a softer stone.
Click to expand...

Funny you say that. I made a skinner with a teacher and started making kitchen knives myself. After experimenting this method is just what gave me the best performing grind with my skillset. I also make wa handled Japanese inspired knives so great to hear.
Thanks for the info.
-Trey
 
Good stuff. I was just getting ready to try out some full convex grind methods. I'm used to using a flat platen and slack belt in the past, but I may try the 3 flats, then to rocking, then to slack. In the past I've had some issues getting the plunges I wanted with a full slack, and took WAY too many passes.
 
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