By Mike de Punte of CAPE FORGE
The sharpest possible edge would be only the thickness of one grain of steel and supported by steel no thicker than the edge. Unfortunately this won't work, it wouldn't have the strength to cut anything harder than butter. So what we're going to do here is look at all the compromises we must accept and find the best way to work with whatever tools we have.
One of the things that I discovered some time back is that there is no one best way to sharpen an edge. There are many and some work better than others. Very often critical things are overlooked, such as:
All of these points are important in order to decide the best way to proceed. In the following pages we'll look at these questions and then discuss some of the best ways to get that edge sharp and get the job done quickly.
The main focus of this information will be regarding carving knives used mainly for wood. Of course, the knives will cut a wide variety of stuff, but we'll focus on wood - both hard and soft.
READ ON AND KEEP SHARP
If I run my thumb across the edge and if the hair stands up on the back of my neck than I know the edge is truly sharp. The problem is this isn't the kind of thing that can be taught. You sort of learn it by experience. The next best thing is to use the edge for it's intended purpose - cutting wood.
Somewhere along the way you'll need to try a truly sharp knife to get an idea of the comparison between "WOW THAT'S SHARP!!!" and "THIS THING DON'T CUT SO GOOD". So we'll try and get every knife you use in the first category.
When your knife is really sharp :
To get an idea of sharp you might try a new single edge razor blade on a piece of wood. The blades are thin and flexible so they won't be easy to use and the edge will dull very quickly. Rubbing your thumb lightly across the edge will give you an idea of sharp. Try not to bleed.
The dictionary says sharp is "ACUTE ANGLE NOT ROUNDED OR BLUNT".
A cold chisel can be sharp and still won't cut wood for beans.
There are literally thousands of different steel alloys, but all we need to be concerned with are just a handful. Some will make excellent blades, others are just passable and still others are downright terrible. Let's talk about some of the good ones. All the good ones have one thing in common - HIGH CARBON. Carbon content is expressed in percentage of carbon in the alloy, 1% = 100 points. The lowest percentage of carbon we can have and still have steel that will harden is .30% or 30 points. Any less than 30 points and the steel will always be soft - it can't be hardened.
There are many other alloying agents used in making steel, such as: Chromium, Vanadium, Cobalt, Tungsten, Sulphur, etc. The list goes on and on. The only other one we will probably be concerned with is Chromium. Put enough of this in and you get STAINLESS STEEL. There is some concern that stainless doesn't hold a good edge. The key is some stainless will hold a good edge, most won't. Some carbon steel will hold a good edge, a lot won't. IT'S IMPORTANT TO KNOW THE SPECIFIC STEEL. And if possible how it was treated.
Let's talk about a few of the steels you might come across. (not in any particular order.)
|Table 1: Carbon & Chromium Contents of Steel
|Good all purpose tool steel
|Much the same as W-1 or W-2
|Stainless - popular for custom hunting knives
|Good choice for forging, very good edge
|Best choice for forging - scarce
|Premium stainless - expensive - scarce
|Saw steel - makes good knives
|Air hardening - might work well
|Die steel - stainless - tough - not best edge
|Low carbon - very tough spring steel
These are only a few of the steels available, but cover most that will be used in any blades you might come across. Any one of them if properly made will produce a good blade and can be better than any other not made well. Made well means, how the blade was formed and how it was heat treated, as well as the quality of the steel that was used in the first place.
Forging is one of the methods used in forming the blades, and I think it's probably the best way to make any blade because it's one of the only ways we have of being sure of getting a good fine grain structure in the finished blade. This is also the most time consuming and costly way, for this reason not many are made this way any longer.
How hard the steel is will play a big role in how well and how long the blade will hold an edge. If the steel is soft the edge will bend over. If it's too hard for the alloy used, the edge will chip and break. So how do you know how hard is hard enough? ASK WHEN YOU BUY IT. You'll probably get an answer giving the Rockwell#, such as "Rockwell 58 on the C scale". Or as it's usually written R58c. This scale runs from the 40's (unhardened steel) to 65-67 (the hardest most steel will ever get). Most knife blades are in the range of 50c-62c with the low end on the soft side. Why any blade should be this soft is beyond me, other than being the cheapest way to heat treat, and manufacture. Table knives are even lower but they're not really meant to cut only to spread butter so they don't need to be hardened and heat treated at all.
Let's talk about the range of 55c-62c where 98% of the carving knives fall. Most people are used to working with blades around 55c-58c. These are a bit on the soft side but easy to sharpen and must be sharpened often. They don't hold the edge very long. The upper range of R60c-R62c is where the CAPE FORGE blades are. This is all our blades from carving knives to drawknives and chisels. This hardness will keep a very sharp edge for along time with only stropping ever needed - rarely if ever will you need to use a stone. In order to make a good edge that is tough at this hardness special heat treating is necessary and forging can be a big help. All of our blades are flame hardened one at a time and triple tempered. Flame hardening leaves the blades at around R65c - triple tempering is a time consuming process that brings the hardness down to the R60c-R62c range and leaves the blades very tough. None of these processes are any kind of secret, just time consuming, adding several hours to the production time. We think this give THE BEST QUALITY. The other element in our quality is hand forging each blade. This give a fine grain structure, which is one of the only ways a hard blade can stand up to hard use. If you snap a blade in two you can see the grain. Below is an exaggeration of grain structure, but pretty close.
This is probably the most important factor in making an edge sharp. Basically this means the shape of the steel that supports the edge. Too thick and you end up with a cold chisel - good for cutting steel but lousy for wood. Too thin and you get a straight razor - good for cutting hair but not so good for anything harder. The shape of the edge is determined by the type of grind used in making it in the first place and how the edge is applied. The shapes you are most likely to come up against are:
Hollow - Double bevel - Convex. This is what they look like:
All of these various grinds also depend on how thick the steel is and how wide the blade is. For most carving purposes the blade doesn't need to be any thicker than 3/31" and not much wider than 1/2". This will ensure that the blade has a good chance of having the proper edge geometry without any special grinds other than a flat grind. A flat grind is the basis of the convex shape and gives the best combination of strength and shallow angle at the edge.
After the blade is ground flat, the angle created in the initial sharpening is rounded over to make a smooth transition from the edge back. This will keep the blade from acting like a wedge when cutting and make the cutting action smoother.
SMOOTH IS GOOD
Don't be too concerned about angles. Most of the angles
have been decided when the blade was made;
HOW THICK - HOW WIDE - WHAT KIND OF GRIND.
Remember, if the steel is hard enough and properly heat treated and ground for the job at hand it will take and hold that fine edge.
So far so good, now let's get it sharp
At last count I came up with 6.5 trillion different kinds of abrasives - from paper to stone and all sorts of stuff in between. All of them work. Some will keep you busy for days on end and others will turn your tools to scrap in seconds. We need to match the method with the material.
The only time you need to use a hard abrasive such as any one of the stones on the market, is when you want to change the shape of the blade as in repairing nicks or repairing the geometry of a faulty edge. For this purpose I think water stones in a fine grit do the job as well as anything I've tried - they're aggressive and leave a fine finish. None of the stones, oil or water, natural or man made, seem clearly superior to any other as long as we compare equal grits.
Here's the catch - NOT ALL MANUFACTURES USE THE SAME STANDARDS.
Coarse grits get the job done quick, but it'll take forever to remove the scratches and get back to the close to mirror finish that you need to cut smoothly. Keep the final finish in mind. If it takes 3 seconds to get rid of nicks in the blade with a coarse stone and another 3 hours to remove the scratches - maybe it was too coarse.
Use a stone when you need to:
Otherwise do all the edge touchups with the strop.
As with stones there are many types of strops and ways to use them. Again most of them work, but let's stick to one that's probably the most popular; leather for the strop and some sort of cutting agent applied to it. For our blades we recommend using a hard leather of 9-12 oz. (1.8"-14" thick) glued to a piece of wood to keep it flat. If the strop is too soft or not supported as in the old fashioned flexible strop used by barbers, the strop will curve around the edge and probably make it duller rather than sharper. The loose barber strop was designed for use with straight razors that were hollow ground. With the blade held flat on the strop the hollow grind kept the edge at about the proper angle - again a method that works well enough in it's intended use but not good for flat ground blades. Now that we have a piece of leather firmly mounted (smooth side up) let's put some sort of cutting agent on it. For our blades which are hard, R60-R62c, we've found that silicon carbide in an oil base works very well with little effort.
Silicon carbide seems a bit more aggressive and faster cutting than aluminum oxide or most any of the other compounds in use today. There isn't a huge difference but it does seem to work better.
To use the strop: Put the blade on the strop and raise the back side till the edge just starts to bite in when you lightly pull the knife across the strop - now lower it just a little so the strop is working on the area just behind the edge. Use a fair amount of pressure for one or two 6 inch strokes on both sides of the blade. That's all there is to it . Now the blade should be tuned up and ready for more work.
Use the compound sparingly. Use just enough to put a light coating on the leather, this is enough for several stroppings.
REMEMBER - YOU DON'T WANT TO FEEL THE EDGE BITING IN WHEN YOU STROP
If you have any questions or problems (about tools not the weather or stuff like that) please feel free to give us a call at the number listed.
Thanks and happy carving!