A knife with a "bent" or "crooked" blade is used to carve concave hollows, such as the bowl of a spoon. Many people are unfamiliar with these, and it is not something that most stores carry, but making one is a pretty easy blacksmithing project.
A bent knife can be made in many different sizes and degrees of curvature. Some have a blade only one one side, some have blades on both sides. I mainly carve spoons, and this knive works well for carving out the bowls. I only sharpen one side, so as to have fewer sharp edges for accidents, but this means that you need to pay attention to whether you are making a "right handed" or "left handed" knife. The basic design could be easily modified for different sizes or amounts of curvature.
I like 3/8" spring steel round stock from coil springs. After you have straightened out a section. draw out the end for the tang of the knife. For this knife, the tang will experience a lot of torque, so don't make it too small.
Flatten off the next 1-2 inches. Don't make it really thin, just flatten it some.
Cut off the flattened area with the tang. Round off the cut side, flatten more (keep the sides straight and parallel) and true up the tang a bit if necessary.
Now form the cutting edge. You want the cutting edge to be on the outside of the curve of the knife, as close as possible to the wood being carved. So for this knife you do not want a double bevel, just a single bevel. If the side currently facing up is going to be the inside of the curve, then for a right-handed knife you want to bevel the bottom edge (with the tang on the right).
Once you have a good tapered bevel, make sure the blade (flattened area) is flat and straight. Then heat to red and stick into ashes to anneal. Grind/file and then sand. At this point I only sand to 100 grit. You should have one flat side and one side with a bevel to a cutting edge. Blade shown is right-handed, put the bevel on the other side for left handed.
Beveled side (bevel at bottom)
Back to the forge! You can put the curve in the blade freehand, or on the anvil horn, but if you have a swage block it's easier. I don't have a swage block, but I do have some swage hammers, and one has a curve about right for a spoon knife. Get a piece of round stock a little smaller than the swage, and it's easy. I like to bend the tang at about 90-degrees from the curve, as shown. Place the blade on the swage with the beveled side up, so the flat side will be on the outside of the curve. This puts the cutting edge as close to the wood being cut as possible.
Heat the blade to red, quench in oil, and temper. I like to draw the edge to about a yellow/straw, but the back of the blade somewhat darker. Also try to get a good temper on the bend between the tang and the blade, as this area will be under a lot of stress. I often will dip the cutting edge in water periodically while temping, so I can get a good temper on the back of the blade without overtempering the edge.
Return to Sanding, this time start with 100 grit and work down to a nice smooth surface (I go to 600 grit, but I think somewhat coarser or finer would probably work about as well. For sanding the inside of the curve, a strip of sandpaper wraped around a piece of wooden dowel works well.
Time to make a handle. (to be continued)
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