Author Topic: A frank opinion about grind geometry  (Read 19765 times)

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Offline PetrifiedWood

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A frank opinion about grind geometry
« on: August 23, 2012, 01:33:00 PM »
After all the heated debate, I decided it was best to move this article here to my vendor forum, since it's primary purpose is to explain why you don't see many convex knives coming from me.

A frank opinion about grind geometry

I won't presume to present this as other than my opinion. However there will be some facts discussed here. I do this in order to explain my position on grind geometry and why you rarely see a convex grind from me. When I first set out to make knives I wanted to hit the ground running. I had a few goals in mind. First among those was to make some nice, high quality handmade knives and do so quickly. Before deciding to make my own, I was in the market for another handmade knife. (I had already bought two and been gifted one.)  I asked around a little about this or that knife and was really discouraged by the wait times I was quoted. The fastest quote I got was 3 months. I figured in three months I could buy my own equipment and make my own knife. For about what it would cost me to buy 4 or 5 custom knives, I bought the materials and equipment to make them myself, from initial grinding all the way to heat treating and finishing. I also bought some books on the subject and read them, supplemented with plenty of online reading of various knife forums, tutorials, etc., and a good bit of help and encouragement from some of the experienced knife makers here on the internet, for which I am deeply grateful.

One of those books (that is highly recommended for new knife makers) is Wayne Goddard's "$50 Knife Shop". In the book, he discusses the various knife grinds and how they are formed. He also mentions the ease with which each are produced. Unless you've read the book or ground a few knives yourself you might not be aware of this. But the convex grind is the easiest grind to manufacture. My own experience agrees with Goddard's assertion. I can grind convex freehand with no jigs, platen, angle guides, table, etc. and have a nice looking blade as a result. The convex grind hides imperfections and can mask poor grinding technique as well. It doesn't require the precision and attention to detail that more conventional grinds require. Scandi grinds sort of fall in the middle ground because while they do require more work to come out precise and even, they don't require a second set-up like a flat or hollow grind with a secondary bevel does. Which brings us to flat and hollow grinds? These are ground at two different angles. One for the main bevels and one for the edge bevel. They are even more time consuming and involved than scandi grinds and require even more care to execute correctly.

All that said, one might ask, "Why are you telling me this?". As a knife maker it might be in my best interest to keep quiet and take advantage of the current popularity of the convex grind so that I can minimize the time and effort spent making knives. But as a knife user I just can't let it go.

When a convex knife leaves the maker and arrives at your door, it should be nice and sharp and ready to take on the world. But one day, it will need to be sharpened. This is where the convex grind really drops the ball. If you don't have a belt grinder, you are going to have to learn some new skills, use some unorthodox tools, and jump through some narrow hoops to get it sharp again. Knives have been sharpened on flat stones for centuries to a shaving sharp edge. There were no special spine tilting techniques required. You couldn't use sandpaper because it didn't exist. And mousepads as an invention are younger than most people alive today. So why would you put yourself through all of this extra effort when there are grinds that are easier to sharpen using readily available tools that have an exponentially longer useful life than sandpaper? Why put yourself out just to "make it work" with a particular grind when there are simpler grinds to maintain in the field?

Let's look at some of the other grinds. Both the hollow and flat grinds will have a secondary bevel. The secondary bevel's purpose is to make the edge tougher by increasing the inclusive angle. If a flat or hollow grind knife were "zero ground" (i.e. with no secondary bevel, like a scandi) it would be an extremely thin and fragile edge like that found on a razor. So these knives are left with a little thickness at the edge, and then given a second, less acute angle. So let's bust a few myths while we're discussing this secondary bevel.

1) "Scandi grinds are more delicate than other grinds."

This is a myth. I purchased a set of plans from SWC, the current maker of the Ray Mears knife. That is the quintessential modern scandi grind knife. The specified edge angle in the plans is 30 degrees inclusive. Folks, there are MILLIONS of 30 degree secondary bevels on flat and hollow ground knives all around the world. If a 30 degree edge isn't fragile on a flat or hollow grind, then it isn't going to be fragile on a scandi made from the same metal. In fact, the scandi grind knife will be stronger than a flat, hollow or convex grind if all three knives are made from the same profile dimensions. The reason for this is the scandi knife has the least amount of material removed of the various grinds.

Now a lot of you convex devotees will balk at that statement, but you might be confusing edge strength with overall blade strength. Another thing that needs to be addressed is that "convex" itself is a misnomer. The correct description of the blade cross-section commonly referred to as "convex" is actually "ogive", but for clarity I will use the familiar jargon. It is often said that convex edge can be made tougher than a scandi. But in doing so, some compromises are made that go beyond the sharpening difficulties mentioned above. Imagine a convex edge that doesn't remove any more metal from the blade than a scandi grind would. In order to do that, the grind would have to be very "low" on the blade. Which means that in order to come to an edge, the edge geometry will more closely resemble an elliptical ogive than a tangential ogive. This means that instead of looking like a rocket nose cone, graceful, sleek and sharp, the edge is shaped more like the roof of a grain silo... thick and blunt. In order to get any kind of good slicing performance with a convex grind, it must have a tangential ogive and that means removing a lot more material from the same blade than a scandi grind. Less steel means less strength.


2) "Scandi is for soft woods and convex is for hard woods."

Another myth. Grind type has far, far less to do with performance in a given material than edge angle does. "Convex" is a catch-all term that can describe such a wide range of possible equivalent edge angles that it is impossible to say "convex" is suitable for any particular purpose. We can say with confidence that an edge angle of 20 degrees will slice better than an edge angle of 40 degrees, and that 40 degrees will be more durable than 20 degrees. But it takes some complicated math (real, actual rocket science, in fact) to describe a convex edge ogive in terms that can quantify how it will perform in a given medium relative to a flat edge angle. For our purposes this can be simplified by calling them "fat" or "thin" convex grinds. Now, a thin convex grind can be made to slice as good as an acute scandi or secondary edge bevel. And a fat convex grind can be made to hold up to impact and abuse, but so can a less acute scandi or secondary edge angle. There is nothing a convex edge can be made to do that can't be done by an equivalent flat edge angle, except be easier to produce at the cost of more complicated end-user maintenance.

Ok, so we left off discussing secondary edge bevels. If you have a scandi grind with a 30 degree inclusive bevel, and a flat grind with a 30 degree inclusive edge bevel, then the same exact angle meets the material being cut. The sharpness potential for both types of grinds is identical. The edge's resistance to wear is identical. The only difference you will notice is when slicing through different types of material. For example, a full flat grind on a 1/4" thick 1.5" wide blade with a very narrow edge that has a 30 degree edge bevel will experience a lot of drag (as will a convex grind) while cutting through a block of cheese. However a scandi grind on a blade of the same dimensions will have less of the blade in contact with the cheese because of the wedging effect, and therefore less drag, equating to easier slicing. On the other hand, materials that offer less drag than cheese (fruits, vegetables and flesh, for example) can see better slicing performance with a full height flat grind and a narrow edge, given the same edge angle as the aforementioned scandi grind. What this equates to in practice is that a thin piano wire is best for cutting cheese, but a scandi is better than a flat grind. For general food prep, a full flat grind with a narrow edge is best. (Centuries of chef knives can't be wrong.) A full flat grind or hollow grind is best for skinning too, though a scandi can perform excellently in that capacity. For working wood, the wide flat scandi bevel will outperform a flat or hollow grind with a secondary bevel. (Think about how chisels and gouges for wood working are ground. They are most similar to scandi grinds with the wide flat bevel and no secondary bevel. Nobody in their right mind would use a convex edge chisel for working wood, hard wood or otherwise. Nor would they "convex" the blade in their hand plane.)

So with all that information, there still remains the question, "What grind do I need for an all-purpose knife?" Well, like everything else it is always a compromise. If you want a knife that can be batoned or chopped with then you are going to sacrifice some sharpness by necessarily increasing the edge angle for toughness. If you want a good carver and slicer you are going to sacrifice some edge toughness by necessarily decreasing the edge angle. How does this equate to convex grinds? Well, a "fat" convex is for chopping and batoning, and a "thin" convex is for slicing and carving.

Now it's time to address "convexing" as a verb. A lot of folks will "convex" their flat ground knives. What they are really doing is breaking the edge on the secondary bevel while largely leaving the main grind bevel (be it flat or hollow) intact. There is nothing wrong with this if it floats your boat. But you will still have the same compromise between fat and thin convex performance that you had before with large and small flat edge angles. It doesn't serve any practical purpose other than to remove more material behind the edge. You can't add material by "convexing" so it isn't going to make the overall blade stronger by any stretch of the imagination. It will change the edge geometry, but you have to be careful not to "convex" yourself an elliptical ogive which will give you poor cutting performance. Another thing to consider is that each time you sharpen a convex edge using a resilient mouse pad and sandpaper method, it is very easy if you aren't careful to remove more material from the edge than from the sides, gradually converting a tangential ogive to an elliptical ogive, and diminishing slicing and carving performance with every stroke across the mouse pad.

So what's the solution? We're still searching for that ultimate do-all grind geometry. By now it should be apparent that you are going to have to make some compromises, either in the edge geometry itself, or in the way you use (and sharpen) the knife. To some extent edge geometry can be altered by the end user. But that always is at the cost of shortening the knife's useful life by removing material.

So for the bush, what's the best all purpose grind? If field maintenance is of any concern at all, scandi is hands down the best grind. There is nothing easier than a knife that has its own angle guide built right in. You lay the bevel on a flat stone (which will last you decades longer than a sheet of sandpaper) and push. People pay up to and exceeding $100 for complicated sharpening jigs designed to maintain a consistent angle between the blade and the stone. Scandi knives don't need it. A flat or hollow grind can be sharpened on a stone with the same ease, but it requires more careful attention to holding a consistent angle since the edge bevel isn't as wide as a scandi and is harder to see with the naked eye.

Once you decide on a grind, the next step is deciding on an edge angle. I think a lot of the sentiment that scandi knives are delicate is derived from people using Mora knives. Mora makes an incredibly nice knife for the price. But their edge is close to 20 degrees inclusive or less. That's a fantastic carver and slicer but it is a little on the thin side of the spectrum. On the other hand, ESEE knives (of which I have a few) come with literature that advises using a 20 degree angle (40 degrees inclusive) to maintain their factory edge. This is on the opposite end of the spectrum, but I can tell you from personal experience that a 40 degree edge can be made shaving sharp. The 30 degree angle SWC puts on the Ray Mears knife is right in the middle... a compromise! But it's a good compromise that retains a lot of carving and slicing ability while retaining more durability than a thinner edge like a Mora has. Edge angle is another place that convex grinds come up short. What's the convex equivalent of a 20 degree flat edge angle? How about 40 degrees? We have been using "fat" and "thin" as descriptors but how do you quantify convex edge geometry in order to tell your knife maker what you're looking for. Do you describe it like you would a steak? "I'll have the medium-thin convex grind, please." It all seems very unscientific and difficult to quantify. If you ask for a 30 degree edge bevel you will get exactly what you are asking for and you can easily understand how that angle's performance will relate to other angles.

From a maker's perspective scandi is a nice compromise, being a little easier to produce than flat or hollow grinds, but still requiring more care, precision and attention to detail than a convex edge. It requires more time, skill and expense to make, but the results are a knife that is faster to sharpen and is simpler to maintain for the customer. From a customer's perspective you shouldn't be paying more or waiting longer for a convex knife than a scandi made using similar materials and construction techniques. Just to give you some perspective, a while back I was making a knife for a visiting friend. I made a minor mistake putting a scandi grind on the knife and was able to salvage the knife by converting it to a full height convex grind. It was a really easy fix, and the knife came out looking great. He was happy with the knife, and I saved a piece of tool steel from the scrap bin. Looking at it, you'd never know it wasn't planned as a convex grind from the moment it touched the belt.

To summarize, a 30 degree scandi grind is the best all-around grind for toughness, versatility, slicing, carving, batoning and ease of sharpening. Flat grinds have a performance edge where game processing and food prep are concerned. In my opinion the convex grind's best application is for saving botched scandi blades from the trash.

So that's my thoughts on the subject. I totally respect it if you disagree. After all, this is just one person's opinion and experience. Some of you might have different results with the grind types and angles than I do, and some of you might use your knives differently as well. In any case, whatever knives you choose to use, use them safely, and most of all have fun using them! :)