Convex Grinding jigs / setups (rotary platen, convex platen, etc.)

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catalytic

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I would like to set up a grinder to to convex grinds. I would like to machine a jig to help me do this, but would like input from you guys.

1. What do you think about the idea of having a platen that is curved to match a convex profile? I could knock out a bunch of these with different radii on CNC... Would this be inferior to a rotary platen (besides less adjustability)?

2. Does anyone have any info/pics on rotary platens? I have never seen one in action

3. Have any of you come up with a different sort of jig for convex grinding?
 
The one issue I see is that the belt will run straight between the plane ends and not follow the concave of the platen until it's forced against it. Even with pressure it's going to want to grind away the edge and spine faster than the centre of the bevels because the belt will want to come back straight immediately before and after the blade contact. A rotary platen with a flexible backer is different as the backing gives based on the pressure applied, and the distance between contact wheels is typically shorter than a standard platen. I may be overthinking the extent of the problem, but as I see it, it may be an issue. I guess the severity of this is going to be directly related to the severity of the concave as well. Very mild concave probably would not be much of an issue, as it would on a more extreme concave.
 
Are you talking about convex or concave? I have never seen a convex platen and have my doubts on the concept.
 
I've seen lots of grinding jigs in my tours around the state of GA visiting makers shops with the GA Guild but not for convexes. Its a touchy thing pressure wise to do a convex grind. We do all our convexing by hand on the rotary platen. I've seen folks grind convexes with thick felt on a flat platten. Damon Lusky has done this in my shop. Worked fine. I don't know what kind of felt he used. He has a forum here, you might ask him.
 
Rick Marchand said:
Are you talking about convex or concave? I have never seen a convex platen and have my doubts on the concept.
Click to expand...

Nope, I've done some customs for some kitchen knife folks. They're crazy. I have my doubts too, but food release is a religion so you must have faith. There are platens out there for convex grinds in production.
 
Nathan the Machinist said:
Nope, I've done some customs for some kitchen knife folks. They're crazy. I have my doubts too, but food release is a religion so you must have faith. There are platens out there for convex grinds in production.
Click to expand...

Nathan, can you explain this because I have no idea what it means.
 
Fiddleback said:
Nathan, can you explain this because I have no idea what it means.
Click to expand...

I guess that was pretty inarticulate...

Like Rick, I had my doubts on a concave platen for convex grinding. I asked around here and the consensus was it made no sense. You'd get the splice bump of a flat platen plus you'd have to push the belt into the hollow before you'd start grinding against it. So I had my doubts whether it would work.

However, some kitchen knife makers use them. They're not doing it for the cut or durability but for the food release. I take it that good food release is one of the more complex aspects of high end kitchen knives and some of these guys take their geometry very seriously. For example, some makers use both a shallow concave grind near the cutting edge followed by a shallow convex grind just above so the cross section of the grind would look like a shallow stretched out "S". The goal is to create geometry that breaks the suction that makes things like sliced potatoes stick to the blade and the folks that specialize in kitchen knives develop reputations for knives that cut a certain way, handle a certain way and have good food release etc and some prominent makers develop a following for it. One of the things that some of them have done is reduce the touchy feely aspect of slack belt grinding by grinding convex grinds against slightly hollow concave platens.

So, despite the fact that most of us aren't real sure that would work real well, they're doing it. In the name of food release.
 
You have done these? I want one. Kitchen knives suck to grind my way.
 
I've experimented with many ways of convex grinding over the years and have found the rotary platen to be the best choice for a convex contour. Depending on what type of convex geometry you want to achieve will determine what you use. A true convex grind aka "axe grind" is done on a slack belt but from what I've found it's creates to steep of a grind for knives not allowing good cutting geometry with thicker stock. Flat grinding first then slack belt will help. The rotary is better then slack belt because it allows better control of the convex contour. Convexing grinding is all about pressure applied while moving across the blade. I highly doubt production machinery will ever produce convex grinds. They are best done by hand. As far as a stable/stiff type of platen, I can't see this working because the belt needs to flex.
Scott
 
Thank you for the replies.

There's something about this that I don't understand: Why would a belt tend to eat the cutting edge with a concave steel platen more than with a rotary platen that you push on until it's concave? Would running the grinder in reverse help this problem?

For reference, I'm actually looking to make an axe grind -- I want to try making my own competition axe. From what I understand, other people have done this type of grind with a KMG rotary platen (reference: http://blueandwhitecrew.org/resources/tips/sharpeningaxes.php#sharpeningwithabeltgrinder )
 
interest piqued! ...I have "convexed" all of my knives to a degree since I started making them... at first due to the lack of proper equipment keeping me from a true flat grind, then because I started liking the way mine looked and performed. then reading about it in magazines and eventually the internet made me think I was onto something. tell me there is an easier way.. please!
 
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Nathan the Machinist said:
I guess that was pretty inarticulate...

Like Rick, I had my doubts on a concave platen for convex grinding. I asked around here and the consensus was it made no sense. You'd get the splice bump of a flat platen plus you'd have to push the belt into the hollow before you'd start grinding against it. So I had my doubts whether it would work.

However, some kitchen knife makers use them. They're not doing it for the cut or durability but for the food release. I take it that good food release is one of the more complex aspects of high end kitchen knives and some of these guys take their geometry very seriously. For example, some makers use both a shallow concave grind near the cutting edge followed by a shallow convex grind just above so the cross section of the grind would look like a shallow stretched out "S". The goal is to create geometry that breaks the suction that makes things like sliced potatoes stick to the blade and the folks that specialize in kitchen knives develop reputations for knives that cut a certain way, handle a certain way and have good food release etc and some prominent makers develop a following for it. One of the things that some of them have done is reduce the touchy feely aspect of slack belt grinding by grinding convex grinds against slightly hollow concave platens.

So, despite the fact that most of us aren't real sure that would work real well, they're doing it. In the name of food release.
Click to expand...

Nathan, have you ever seen these platens or pics or video of them being used? Could you elaborate on what makers are using these things. I make kitchen knives and have not heard of this and would like to wrap my head around it and investigate more.

Thanks!
 
What about a jig to hold the knife that rotates a bit to get a convex grind on a flat platen? Harder to make sure as you need precision moving parts and tight tolerances but it would seem to me to be an effective means of mass production.
 
Interesting concept for "Food Release"

Grind a slight hollow just above the cutting edge.
Follow with convex grind on Rotary Platen.

I'm gonna give it a try soon and see how it works.

Robert
 
JMJones said:
Nathan, have you ever seen these platens or pics or video of them being used? Could you elaborate on what makers are using these things. I make kitchen knives and have not heard of this and would like to wrap my head around it and investigate more.

Thanks!
Click to expand...

I read about it over here: http://www.kitchenknifeforums.com/forumdisplay.php/51-Shop-Talk

That would be the place to ask around. I know a maker, I'll send him an email and see if he wants to chime in.
 
I use Nathan's large radius platen (36") and then a slight convex by grinding just above a standard flat glass platen.
Works for me, and the chefs love it.
I noticed years ago that the girls all absolutely loved the Cutco knives- for some reason the way that the hollow kicks flakes of potato etc. off the blade is really desirable, plus they love the contoured handle. I'm always liek...Can I make you a real knife that does all that, please?
 
Nathan the Machinist said:
I read about it over here: http://www.kitchenknifeforums.com/forumdisplay.php/51-Shop-Talk

That would be the place to ask around. I know a maker, I'll send him an email and see if he wants to chime in.
Click to expand...

I routinely use such a platen or platens for convex grinds on kitchen knives; I have a 2.5 degree, 3 degree and a five degree platen, much like how a rotary platen has different spacing between its wheels. I was taught to use this by a much more expert professional knifemaker who uses it with every kitchen knife he makes. I understand that it seems like the belt would not conform to the hollow, but it does quite well. I am able to get a very controlled and consistent convex grind whereas using a rotary platen requires more experience than I have to get a predictable result. So if a rotary works for you, then by all means use it, but a fixed platen works well too and may be easier to get used to.

Nathan made a couple for me and as usual they are done beautifully and function great. They are certainly cheaper than a rotary platen
 
pitonboy said:
I routinely use such a platen or platens for convex grinds on kitchen knives; I have a 2.5 degree, 3 degree and a five degree platen, much like how a rotary platen has different spacing between its wheels. I was taught to use this by a much more expert professional knifemaker who uses it with every kitchen knife he makes. I understand that it seems like the belt would not conform to the hollow, but it does quite well. I am able to get a very controlled and consistent convex grind whereas using a rotary platen requires more experience than I have to get a predictable result. So if a rotary works for you, then by all means use it, but a fixed platen works well too and may be easier to get used to.

Nathan made a couple for me and as usual they are done beautifully and function great. They are certainly cheaper than a rotary platen
Click to expand...

Thanks for the reply, do you have any pics or better yet a video of it in use?

Thanks
John
 
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