Forged Bent Knife Blanks

Basic Instructions For Making Your Own Bent Knife

First, decide all the details for your blade - length; double or single edged; bevel angle (for carving mostly hard woods and rough carving, the edge will hold up better with greater bevel angle); bevel geometry (where the main and secondary bevels will be). Some of these things may be beyond your scope. Maybe you can make some knives that would be somewhat useable, and enjoy them as tools you made anyway. You’ll learn a lot with every blade you make.
If you are new to knifemaking, you might practice (going through all the steps below) with some mild steel. 1/8" thick by 3/4” wide would be good for the larger blanks, half that size for the smaller.

Grind the profile.  If you are making a double edge blade, then it should be fairly symmetrical. It’s difficult to grind the bevels for a double edge knife. You might make your first bent knives single edge.

This is important:  It is ok for the blade to get hot (don’t burn yourself!), even getting red won’t affect the temper because it hasn’t even been heat treated yet. The important thing is that you should not cool the blade in water. If it gets too hot to hold, use pliers or just set it aside to cool for a while. Don’t cool it with any liquid before it's been heat treated.

Grind the bevels.  The geometry of the bevels at the edges, especially the bevels on the bottom that contact the wood, has everything to do with how the blade will function. I believe the cross section shown below is the best geometry for bent knives. The two flat bevels on the bottom allow for good control following lines (as in Northwest Coast formline carving) and for making rounding cuts without chattering (as for spoons and bowls).
The fineness of the combined bevels (top to bottom) determines how well it will cut and how well it will hold an edge. The finer the bevels, the easier it will cut, but with poorer edge holding. It's a trade off. All my bent knives have the main edge bevels of 10 to 12 degrees (top to bottom) ground from the bottom convex side (leaving the top concave side relatively flat across, side to side). Although the top concave side only needs to be fairly flat across, it's important that the bevels on the bottom convex side are both quite flat across.

Cross section of double edge bent knife blade Concave side is up, convex side down. Main bevels are about 12 degrees. Microbevels are exaggerated so you can see them. They should only be about 4 degrees more than the main bevels. This brings total bevels to about 16 degrees at the edges.

Please Note:   Only the main bevels are ground before heat treating. The microbevels should be done after heat treating, when the burrs are drawn. This is to avoid decarburizing the fine edges.

Grinding the bevels can be done a lot of different ways. For my classes, students use files. At this point (before heat treating), the steel is relatively soft and can be filed by hand (with a good, sharp file). Although this entails more “elbow grease”, it is a good way to learn the basics and to go slow. Power tools are not only dangerous, they also make mistakes faster. But if that is where you’re at, the tool most professional knife makers use is a 2 inch wide belt machine (typically 2”x72”). Alternatively to having a special machine, you could mount a hand held belt sander upside down. The benefit of any belt sander is that it doesn’t change shape (it stays flat), as opposed to a grinding wheel that changes shape as it is used. Most of the bevel shaping can be done before the blade is bent, while it is still straight. The initial grind would be with 36 or 50 grit, followed up with finer and finer belts (120 grit, then 240, then 400 if you want the bevels to end up fairly scratch free). The goal is to finish with the bevels even and uniform and the edges about 1/64" thick (about a thick hairline) along their entire length. Sound hard? It's more difficult than it sounds, but practice does make for improvement.

Bending  is kinda critical, because you’ve already put a good deal of effort into the blade. If you have a small forge, you can use that at a low heat, and reducing flame. Otherwise a regular propane plumber’s torch will work. Don’t use oxy-propane (or acetylene) – that’s way too hot. Heat the blade evenly to a dull red. Avoid overheating the delicate edge(s). Gently bend to the shape you want. You can use a small pair of pliers with smooth jaws or gently push the hot blade against something rigid, like a preheated piece of metal held in a vice. Best to go slow and be careful to stay away from the edges. If you bend it too far, it’s usually difficult to bend it back. It’s easier to bend more with another gentle heat.

Heat treating  is a two step process – hardening and then tempering.
Hardening:   The entire blade needs to be about 1450 F then quenched (quickly cooled) in oil. If you don’t get it hot enough, it won’t harden during the quench. Getting it too hot promotes grain growth, which is bad for edge holding. 1450 F is a glowing red, but it is difficult to describe the exact shade. Fortunately, steel also turns non-magnetic at this temperature. So you can test the temperature with a magnet. When you think the blade is near 1450, take it away from the heat and quickly hold a magnet to it. If the blade is above 1450 the magnet won't stick to it. What you want is to bring the entire blade to just past the temperature it turns non-magnetic, then quicky quench in oil. Clean thin motor oil works fine. Hold the entire blade and tang below the surface of the oil and slowly move it around to keep cool oil contacting the surface. Don't bring it to the surface until it has cooled or it can catch fire! A minute should be plenty to harden it, but it may still be hot to touch. You can let it air cool until you can handle it. The blade will be fragile now, most likely break if it is dropped.
Tempering:  The stresses from hardening are relieved so the blade isn’t so delicate. The higher the temperature, the tougher the blade will be, but also softer (poorer edge holding). It’s a trade off. The idea is to heat the blade just to the temperature where it will be tough enough not to break or chip, but also hard enough to hold a fine edge. I have found this temperature to be about 420 F, depending on the tool. The blade is slowly heated to about 420 F and held there for at least an hour. This brings the blade to the best hardness and toughness.
Gently wipe the oil from hardening off the blade and gently sand (about 320 grit) most of it so you can see shiny metal. Be careful, the blade can break quite easily. Put it in a cold oven (a convection oven is better, for the more uniform temperature) so that the blade isn’t touching anything. Bring the oven up to about 400 F. As the steel heats, the surface will oxidize, producing patinas or colors. The progression of colors as the steel gets hotter are: pale straw, straw, dark straw, brown, purple, blue, gunmetal. Pale straw happens at about 410 F, dark straw about 440 F. However, if the blade is held for a long time (an hour or so), it will turn those colors at 20 to 30 F lower temperature – also producing a blade that is significantly better than one tempered at the higher temperature for shorter time. What you want is the whole blade to be medium to dark straw. Monitor the color of the blade often. If the edge goes beyond brown (to purple), you’ve gone a bit too far (making the edge softer than it should be for best edge holding). Then you should repeat the entire heat treating again, being sure to reheat the blade very slowly up to the non-magnetic. Be more careful during the tempering to avoid overheating.

Mounting the handle  can be done before or after final sharpening, but is easier before the blade is sharpened. It’s also nice having a big handle on the blade for holding it during the final sharpening.
Roughen the shank very well, and taper the end for pushing into the handle. I suggest using either epoxy or cyanoacrylate adhesive.

This is important:   Now that the blade has been heat treated, it will loose its temper if it gets too hot from grinding/polishing. If you use power sharpening equipment, don’t heat the edge to show any tempering colors (especially beyond straw). It’s OK to cool with water, but best to just go slow. If you do overheat the edge, it will probably be too soft to hold a good edge. Then you’ll need to go through the entire heat treating again.

Sharpening  should be started by truing up the convex side of the blade (it may have cupped or gotten misshaped during the bending) and removing the scratches there to at least 600 grit. This is the really critical part of the blade, as it controls how it will work. The bevel there should be fairly flat. A little bit of roundness is better for carving hollows like spoons and bowls, to round out through the cuts, but it should be quite flat for at least 1/4" back from the edge.
Drawing the burr  with a microbevel of about 4 degrees is done on the concave side of the blade (the side of the blade NOT in contact with the wood). Tightly wrap a dowel with 220 grit sandpaper. (Rubber bands on the ends will help keep it in place.) Try to get a small unform burr to form along the entire edge. Go slow. Follow that with 400 grit, then 600 grit sandpaper. The final honing can be done with leather strops with rouge rubbed in. Use a flat board with leather glued on for the convex side. A round dowel with leather is needed for the concave side. Rub the rouge in often – it's what is doing the cutting, and it wears away quickly. Try to maintain the bevel angles you formed earlier.

Follow my guidelines  to care for your carving tools: protect from rusting and avoid contact with other hard surfaces (sand, metal, glass…). Strop often!

One of the best indicators of how much sharpening a person has done is how careful they are with their tools when not using them. A simple towel makes a temporary tool roll. Make sure it is dry!