A fairly common archaeological find from viking age
Scandinavia is the krumkniv or curved knife. It came in many shapes and sizes, but most were fairly compact with a highly curved edge, hence the name. I have not been able to figure out what archaeologist have deduced as far as the purpose of the knife, but it is clearly made as a utility knife of sorts and not intended for combat. The shape would be reminiscent of skinners, herb knives and leatherworking/ fabric knives of today and I would think it is likely that the krumkniv served a similar purpose.

In this instructable, I aim to show you how to make a krumkniv from an old leaf spring with a limited amount of tools.

Supplies

·Hammer

·Anvil or anvil like object

·Heat source (As long as you’re able to heat your steel to glowing hot it’ll work and doesn’t have to be fancy. For this project I dug a hole in the ground, filled it with charcoal and supplied air with a shop vac. For the heat treatment I used a small coffee can forge and the kitchen oven)

·Quenching oil (I use canola)

·Angle grinder (with cutting discs and flap discs)

·Vice

·Files (a good bastard file is invaluable)

·Sandpaper

·Tongs

·Paper and pencil

·PPE (I would recommend using at least long pants that dont like to burn and good shoes, along with eye and ear protection and a respirator).

·Leaf spring

(Note that these are just guidelines and what I had available. Work with what you have whether that is less or more than what is on this list).

Step 1: Design

Step 2: Cut

Step 3: Fire in the forge

Steps 4-7: Forge

Steps 8-10: Refine shape

Step 11: Hand sand bevels

Step 12: Heat treat

Step 13: Sharpen

Step 14: Personal touch

Step 1: Design

Draw your design. It is much easier to know how to approach
your project if you have even a rough sketch of your design. Draw the outline of the cuts you need to do on the leaf spring. Since leaf spring is generally fairly thick, and you draw out the steel later on, it should be cut slightly smaller than the design. Also account for having to draw the steel from straight cuts into curved shapes.

Step 2: Cut

Cut out the design with the angle grinder (Or any other means you have for cutting steel).

Step 3: Prepare for Forging and Fire Up the Forge

As you can see, my forge is just a hole in the ground and my anvil is a sledgehammer head lodged in a large log. Work with what you've got!

Step 4: Forge the Bevel

Heat up the steel and start shaping! I started by drawing out the edge into a more rounded shape. Next, start forming the bevel. Do this by holding the part of the blade where you wish to form the bevel on the anvil at a 20 to 30 degree angle and strike with your hammer on a similar angle relative to the blade. When you forge, move the work piece around, not your hammer. Strike the same spot on the anvil every time. Make sure there is solid steel underneath where you're striking the work piece. Careful not to catapult hot steel off the anvil!

Step 5: Handle

To make the handle. First start by drawing out and tapering the steel designated to become a handle from the blade end towards the back.

To create a twist in the handle, heat the handle to red hot, attach in a vice and with a pair of tongs, well… twist.

Step 6: The Curve

Making the curve of the knife involves additional work on both the handle and the blade. Strike the blade as indicated by the black arrows. When striking, the metal will curve upwards to the sides of the hammer.

The red arrows on the one photo shows where the metal will go when struck where the black arrow indicates. The red arrow also show where you can strike the bevel to form additional curvature in the blade. When you strike from the top like this, you will inevitably deform and thicken the edge again somewhat. When you then go back to correct the bevel as indicated in step 4, you can make the blade curve upwards and in on itself when you strike the extreme parts of the blade.

This step may involve a bit of trial and error. Essentially, when you strike a part of your knife, the surrounding metal is going to want to move in the opposite direction of the piece of metal you are trying to shape. Use this to your advantage.

The shaping of the more aggressive back curve of the handle would probably have been easier if I had an anvil with a horn. "The modern blacksmith" by Alexander Weygers has a good explanation for how to make decorative curves. Holding the blade over the anvil and striking from the side did not quite work for me though, so I bent the back of the handle over the edge of my anvil and held the point of the knife to the log where my sledge hammer anvil sits and gently tapped around the heated handle end until I achieved the desired curve.

Step 7: Straighten

The final forging step was to straighten the knife. I didn’t get a photo of the straightening step for this knife, but here is a kiridashi I did in the same way. Put the heated knife on a large flat surface (I used an old rifle target) and gently tap with the largest surface area hammer you have until it is straight. Reheat and repeat as required.

Step 8: Refine Overall Shape

Time to refine the shape with some abrasives. Outline the parts you wish to remove, and have at it with an angle grinder with flap discs. I like the flap discs as there is very little chance of kickbacks and a 40 grit disc will remove metal at an incredible speed!

Step 9: Refine Edge

Do the final shaping of the edge with a file (or a belt grinder if you have one), the goal is to get an even edge with no high or low points.

Step 10: Refine Bevels

Time to refine the bevel with abrasives. First outline your bevels, then remove the bulk of the material with your angle grinder/flap disc combination...

…And refine your bevel with a file.
Draw filing is a good technique to learn. I know a lot of people like to use a jig for filing bevels. I made a jig for shaping a guitar neck once, as I was not confident I could make the correct shape by hand. I haven’t used it yet and suspect I never will. By Paul Sellers advice I started sharpening my tools freehand, and though it seems like a big plunge, doing it this way is probably easier than you imagine (not saying that it is necessarily easy, just easier). It is certainly faster. I started out making blades freehand filing and so far, I am very happy with the bevels that have turned out.

Leave the edge about 0.5mm thick so you don’t warp it during the heat treat.

Step 11: Hand Sand

And next is hand sanding of the bevel. I only went up to 240 grit on this blade, but you can take it however far you like. Sand in one direction until you get an even scratch pattern, then change the grit and sand across the scratch pattern on the bevel until all the scratches move in the new direction, then you move up in grit size again and sand across the scratch pattern until it is even again. Repeat until you get to your final grit (You probably don’t want to take it past 400 grit before the heat treatment though).

Step 12: Heat Treat

Time to heat treat. I have a small diy coffee can forge that is perfect for the purpose of heat treating small blades, certainly easier than digging up the backyard. You can find a lot of instructables on making a small forge. Warm canola oil works well for quenching (the canola oil is pre heated to increase viscosity and thus it actually works as a faster quenchant than cold canola oil).

First you want to normalise your blade to refine grain structure and relieve stress from the forging process. Heat the blade until it no longer attracts a magnet and allow it to cool down until black ( the whole heat treating process apart from tempering is advantageous to do in low light). I repeat the normalising process two more times.

To harden the blade, heat it until it is cherry red and quench it into the warm canola oil. Test whether the hardening process was successful with a file. The file should skate over the steel instead of biting into it.

5160, which is the most common steel used in car springs has an optimum tempering temperature of between 190 and 205 degrees celcius (375-400 f), where the balance between hardness and toughness is optimal. Unless you know your oven very well, it can be helpful to use a digital thermometer.

I gave the blade two tempering cycles of two hours each and quenched in water after each cycle.

After the heat treat, sand the bevel once again, as the polymerised canola oil will form a black layer on the whole knife. Start on the grit you ended on in step 11, and go as high as you like in grit size following the same procedure as previously.

Step 13: Sharpen

Time to give the blade a proper edge and sharpen it up. I use a course diamond plate for putting the final edge on the blade. It is by no means the most efficient way of doing things, but it is cheap. Thin diamond plates are only about five dollars each on ebay.