Convex

N

Natlek

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Jun 9, 2015
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What if we combine this to guys to grind convex bevels ? What do you think ? Has anyone tried it ?

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I don't think I'd do it that way. I did a couple convex recently and found that I twist the blade around quite a bit to keep the belt running straight over the edge, approaching the tip.
 
Richard338 said:
I don't think I'd do it that way. I did a couple convex recently and found that I twist the blade around quite a bit to keep the belt running straight over the edge, approaching the tip.
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Richard , you grind free hand as far as I know and edge up ? When you approach to belly and tip you pull/twist little tang towards you , right ? Rubber belt on rotary platen would twist when you put pressure on left or right side of belt .How much will twist depend from how tight is belt and between which two wheels you grind . When you approach belly and tip IF you pull tang down /instead of towards yourself/ pressure would stay in middle of belt and belt would not twist .But that is little tricky to do , at least to me .
On jig we grind edge down , blade is guided straight and pressure on belt would be even on belt? I don t know ,maybe combination of flat grind to some degree and then convex would do job .And with testing with rubber belt tension and distance between wheels maybe it would work .Maybe it will work for full convex grind, I don't know how it would look if the convex grind is lower then spine ... I will make jig this spring , light weight version from Micarta and steel where I must use steel . My rotary platen is double frame so it will be very easy to install jig and move it around between wheels .....
 
I put tape over areas that I want to avoid. I look carefully at what the effect is and guide the knife as necessary. I think it is different for different shaped blades. Last time I did a Santoku and a BBQ slicer.
 
eric brinkerhoff said:
If I was able to do do such nice hollow grinds as you do, Natlek, I would never even consider doing convex...
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Thanks my friend . But most credit goes to my jig and contact wheel I use . Especially the wheel , I think .Because here in Macedonia I can't get anything knife equipment related I have to manage the way I know and can .And this time I hit it right in the center :)
I have timing belt on my 17 inch wheel with as far as I remember169 teeth .And thanks to them, belt remove steel fast and steel stay cool. I can feel the teeth as I grind .That wheel remove steel like circular saw do .A small area of the belt touches the steel at one point and all pressure from me on jig goes there .That s why I grind so fast on that wheel .I can not do that on solid rubber wheel , I try .
Look this one , I am afraid to use micrometer to measure thickness behind edge ,it will break 100 % .I call it singing knife . Unusable, but I wanted to see how far I could go without overheating the steel.... :)
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I'd love to have one of those rotary platens, since I like my blades having a smooth convex geometry toward the edge. As it stands, I grind my primary bevels flat against my glass platen. As I approach where the edge will be, I nibble away against the platen until there is almost and edge. I made a little leather cover for the platen that I install, and then blend all the nibbles together until it's a smooth, convex plane. Generally, the edge is almost sharp at that point, so I move on to stones and finish it up by hand. I'm still learning quite a lot with each knife I make, so take my procedure with a grain of salt

One thing I'll be saving up for is a contact wheel set up because hollow grinding combined with convex edge geometry seems to me to be hard to beat, (aka; S grind)
 
Look for F3 felt pieces 1/4" thick and put them on a metal platen. It has some give to it to give a very nice convex grind! The felt has some memory, so it will form to the convex a bit to help keep you consistent, too. F5 felt is too soft. I also tried S2-32 felt and it is harder/firmer and doesn't have the give I look for to get a convex grind. I flat grind to 120 on a glass platen and stop before I get to the edge and then use the felt platen at 50 or 80 and work back up. The felt lets you use very fine belts w/o a belt split jump, so you can get a really nice belt finish!

Brian Housewert uses an old scotchbrite belt section covered by leather to do his convex grinds and it helps keep the plunge line crisper than the felt since the corners of the felt will wear in. The leather doesn't smoosh and give as much to mess up the plunge grinds at the corner, so I will be trying that next, or the softer felt and leather to see how that does. I had a KMG rotary platen before and it was nice, but the pressure was tricky. The belt liked to cup a bit and you could end up with 2" grooves if you weren't careful!

I saw another guy used a setup where he put a smaller contact wheel in the tool rest slot and a smaller contact wheel in the regular tool arm slot of a grinder and did convex grinding that way, basically grinding in the slack between 2 contact wheels. I wanted to try this, but never got around to it.

I go edge up and reverse the belt when I get close to the final edge, seems to give more control and better feel to thin the area behind the edge out better and I slow the belt way down.
 
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