What happened to the convex edge?

S

shane justice

Joined
May 12, 2003
Messages
1,606
I read, more often than not, the advice given to new makers is to flat grind.

Most of the pictures showcasing custom knives show that is the trend. It also seems that the big companies are making knives that are flat ground.

Many years ago, when I was just tinkering at making knives, I can remember reading about the the beauty and extreme sharpness of the "Moran" edge. So I always beleived this was the "Best" edge geometry for a knife. I dreamed of the day when I could put a convex edge on one of my knives.

The Japanese favor the "appleseed" shaped edge too...and the guys in the HI Cantina absolutely covet a khukuri with a convex edge...

SO what's the deal?

Does the flat ground blade perform better after all? Is it stronger cross sectionally? Does it flex better? Does it cut better?

I always thought the flat ground knife was best in the kitchen but now I see the guys in the cutting competitions all seem to use a flat ground blade.

Wondering if I should change my grinder set up to increase the perfomance of my knives?

Anybody have any thoughts?
Shane
 
I always thought the general concensus is that beginners benefit from making hollow-ground blades first. Flat grinds can be tricky, but for the record, most of my blades are flat-ground with a cannell edge. This just comes from sharpening on a slack belt - more out of using what I have available than anything else. I'm working on building a slow-speed belted sharpener right now. Then, I'll have some true performance in terms of a predictable edge geometry.
 
Jeff - email me what you have for the slow speed sharpener....I am way interested ;)


I grind Flat because I think its a good performing blade. Convex is fine but I dont think it would be much different than flat but others may know better.
 
The convex blade is alive and well in my basement! Mine are pretty convex to begin with, then I draw file forever to thin it out a bit, then I put a regular edge on it so you can sharpen it like a normal knife, so it's basically a flat edge on a convex grind.
 
I think, like most things in knifemaking, the flat grind is just a good compromise between varying factors. A flat grind is thin and slices well but still has lots of meat and strength behind the edge. Ease of sharpening is nothing to overlook as well and convex edges can be a bear to sharpen unless you are using the aforementioned slack belt method. Most of my knives that I call flat ground are a bit convex but have a conventional secondary edge bevel that can be easily resharpened on stones.
 
I think Guy has it right, flat grinds with a secondary edge are a little easier to sharpen, though a convex edge is not hard to sharpen if you do it free hand on a stone and know how.

I do a convex edge on all of mine, no mater the grind, but from my testing a full convex grind has the greatest streangth and cutting ability. It alows a fine edge with enough meat behind the edge to suport that fine edge during hard use, and it has nothing to hang up on. The blade flows through the material being cut.
 
I think part of the problem with a convex is the perception of sharpness. I almost always use flat grind ending with a convex edge.

From a customers standpoint though, the convex edge just doesn't feel as sharp. It cuts better. It holds the edge better IMHO, it just doesn't have that scary sharp feel.
 
I find it hard to get crips lines of anykind grinding on a slack belt. A lot (not all so don't bust my chops :p ) people seem to look down on a knife that does not have at least one crisp clean grind line. Just my .02 cents.
 
Mark Nelson said:
I find it hard to get crips lines of anykind grinding on a slack belt. A lot (not all so don't bust my chops :p ) people seem to look down on a knife that does not have at least one crisp clean grind line. Just my .02 cents.
Click to expand...

Good point, Mark. There are a lot of knife buyers out there that want something that looks cool especially if they are going to drop a large sum of money. Cutting ability/efficiency is often not the main goal of a buyer. That being said, my feeling as a user is that the convex grind rules from a purely utilitarian standpoint. I agree with what Peter said about cutting efficiancy and edge holding - other variables being equal. Edge rolling or chipping can still be an issue if the edge is too steep for a particular steel to handle, convex or otherwise.
 
I still convex grind all of my blades. I start by grinding them rough on the flat platen of the KMG and then at a certain point I switch to the rotary platen.

I think the key is to control the edge geometry....in other words, thin the edge out with the flat platen, then "roll over" the flats with the rotary.

A problem I had early on was leaving too much meat behind the edge on a convex grind.....the blades would shave, but not cut well.

Frink's rotary platen allows you to select the tension of the "slack" and with this you can really control the thickness of the edge without worrying what the belt is doing all the way up to the spine.
 
I think you just covered the major drawback to a convex grind, Greg. There are way too many blades being made that are WAY too thick right behind the edge and have more of a grind like an axe and less like a knife. Sorta like someone went to a loose slack belt with barstock and as soon as they got an edge they stopped.

Done properly, it's great. Done lazy and it sucks.
 
I flat grind at a very low angle, <10° included angle until I get the bevel down the where there is .202"-.030" of steel left and then do a fairl shallow convex grind to 0 on that flat (For wide blades it may be down around 5°). It is not a full convex grind, so maybe not as strong, but gets very sharp and cuts very well. The edge retention is very good also.
 
I sure hope convexes aren't going out of style, they're definately my fav and the majority of my blades end up with it.
 
I grind my bigger blades with a convex grind and no secondary bevel. Like Will said, they're not hard to sharpen on stones, once you learn how. Most of my smaller and utility knives are flat ground with a convex edge and no secondary bevel. The only things that I flat grind and sharpen conventionally are some kitchen knives and some damascus blades. Convex grinds that are sufficiently thin behind the edge are great for all types of cutting for me.

Todd
 
You guys have shown what's been my own problem with full convex grinds: too much meat behind the edge. Maybe I should tighten up the belt on the rotary platen. Most of mine are flat ground with a convex edge. This is great for scary sharp edges but most customers are hopeless at sharpening them. I always resharpen for free, which gives me a chance to see how things are holding up and freshen up the finish for my customers. All that being said, you've got me thinking about full convex again... Oh boy!
 
The convex grind is my favorite. I will flat grind at customers request. All my knives even my factory knives, I sharpen with a convex edge. As stated above, I too sharpen on a slack belt. To maintain all my edges, stropping is all that's neccessary. My earlier knives were too thick behind the edge but with the KMG, they are tapered more and cut better.
Scott
 
A nice, thin convex grind is just about the ultimate in my opinion. It pushes away material from the knife as it slices, has better edge retention, and is stronger. A thin one doesn't have nearly as my trouble with sharpening either, you can just sharpen it normally.
 
80% or more of my knives are convex when they leave the shop. Of those, at least half are full convex.

I'm not sure why someone said they can be made scary sharp. On the contrary, I find it terribly easy to keep them "murderously sharp".

http://www.pendentive.biz/blades/magvid1.wmv
(sorry Jennifer Connelly fans)
http://www.pendentive.biz/blades/magvid2.wmv

You can see some mags in the trash and some on the table. Went through all of them, no problem.

Knife in the video:

attachment.php





Re: "Looks"

I think a full convex grind is prettier. True, it's not as nice as a full flat grind on a large straight bowie.....but if you have a curvy knife, why not give it a curvy grind?

Example:

attachment.php




As to the thick/thin question - grind to fit the application...easy enough.
 
I read a tip for sharpening the convex grind that should allow anyone to do it. You take a block of wood and cut a strip of computer mouse pad and glue it to it. Cut some slits in the back of the block and wrap emery paper around it and anchor it in the slits. Use a draw stroke and the mousepad gives a little giving you a similar situation to the slack belt. Haven't tried it yet, but it seems like it should work OK, and takes a little of the mystery out of it for the perspective buyer who is afraid to try and sharpen a convex edge.

I'm a recent convert to convex edges after listening to Jerry Fisk give a talk about edge geometry at a couple of hammer ins. He talks about materiel flow. And how a convex edge is the most effecient. Made a pretty convincing case.
I use a flat grind with a convex edge, and they have performed pretty well.
Ed
 
Not sure if I read correctly, but I initially thought there was some confusion betwwen a convex edge and a convex grind.

I believe the convex edge is very alive and very well. I feel, IMHO, that a full height flat grind, and convex edge is the way to go for MOST knives. The convex grind is used less often for the aforementioned reason - many who do it are finding it more difficult to do and customers are demanding some cleaner grind lines. (Tell that to Ed Fowler...)

As for thick vs thin convex edges - I think there is a role for both. A thick convex edge is still way better than a thick flat ground edge cannel. However, I think the thick convex edge should only be used on very high-impact blades. I consider it acceptable and reasonable to make a heavy broad bladed bowie with a thicker convex edge that is slack belt ground 1/4inch up from the edge, while a fighter should have a thinner bevel behind the edge and hunters - well, even thinner.

My 2 cents. Jason.
 
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