Wednesday, June 18, 2014

Custom camp knife

With all these tiny electronics projects going on, I had to take a break to do something with steel, power tools, and brute force. For many years I’ve used a Benchmade Nimravus as my wilderness knife, but a few things frustrated me about it: handle too large for my hands, uncomfortable thumb ramp, tip too steep, and semi serrated. I decided to make one from scratch to be exactly the way I wanted.

I started with sketches. The design borrows a lot from various knife designs that I like (including the Nimravus). I drew it 1:1 scale so I could hold my hand to the paper to design the handle to fit me. I went through a lot of iterations until it looked right. The final design has a 4” handle and a 4” blade, very wide flat grind, a fairly blunt tip, and slight concavity in the lower half of the blade. I was able to be pretty aggressive with finger notches in the handle since I knew it would fit my hand and didn’t need to be generic.

I wanted some parts to be recycled/repurposed materials, but I couldn’t find anything with dimensions that would work for my blade, so I bought 154CM steel. It will make a better knife in the end than a saw blade or file anyway. I wanted it to be indestructible since I will use it for heavy duty work like splitting logs, so I bought 0.125” thickness. I decided the handle scales at least should be recycled, and I found an old Pelican case to cut material from. I also used the stainless steel pivot pins as my handle pins.

When my steel arrived, I had to cut it to length since I bought it long enough for two. After wearing out two hacksaw blades trying to cut it and resorting to scoring and snapping, I figured the steel must have arrived already hardened. I was too excited to take the time to make a forge from concrete, so I just set up the outlet of my shop vac to blow into the bottom of my charcoal grill to do heat treat. It worked well enough to get the steel to critical temperature, then I let it air cool to soften.

I traced my design onto the blank and started roughing out the outline. I was very glad to have my angle grinder with a cutoff wheel for this, as it was still tough to cut with the hacksaw after softening. I either didn’t soften it enough, or 1/8th is just really thick, or both. After roughing, I spent a lot of time at the bench grinder setting the outline. After it matched my sketch, I spent a lot of time at this stage refining the handle profile since I could actually hold it. I also made slight tweaks to the blade edge shape.

I built a filing jig using a design I saw in a few places online. It holds a flat file at an exact angle through the whole process so you can make a precise grind. I didn’t bother to take measurements for the angle, I just played with it until it looked right. After the first filing session, I could tell it was going to take way too long to remove all the material by hand. To speed it up I went back to the bench grinder and very carefully did a rough grind to get a big portion of the material off quickly.

Even after that, the filing took forever. After the first couple hours I was getting worn out and had to stop. Later that night I went out and did a 20-minute filing session about once an hour for 3 or 4 cycles, and then the next day I finished the filing in one 2-hour session. Everything I read online says it should have taken less time, so I probably didn’t do the first heat treatment correctly. Towards the end I scribed some center lines along the edge so I could make corrections when I started grinding unevenly. Once the leading edge was down to 0.025” I decided the grind was done.

After that I set the edge using the same filing jig set to a steeper angle (eyeballed it again). I was really nervous about going too far and ruining the edge profile, so I left more than I probably should have before the final heat treat. It wasn’t really an issue though, since it was such a small amount of material to remove after hardening. The last step before heat treat was to drill holes for the handle pins. At this step too I was reminded that I didn’t soften it enough when I broke a drill bit. I think the fact that I was able to drill at all means it was softened to some degree, though.'

For the hardening heat treatment I set up the grill forge again, and it was easier to work this time since I only needed to get half of the piece to critical temperature. I quenched the blade in a soda can full of used motor oil. The motor oil left the surface darkly stained, even after thorough cleaning. I was disappointed since I had decided to do the surface finishing after heat treat, but I loved the look of the dark coating. I sanded it slightly and then decided to just leave it as-is. You can still see file marks but it feels completely smooth, plus it’s probably better if I don’t need to worry about scratching it up when I use it. After heat treat I did the tempering step in the oven at 400°.

I used a hand saw to rough cut the handle scales to shape, then drilled the holes in them. The holes were drilled slightly undersized in both the steel and the scales, so everything held together pretty well even before glue. I applied epoxy to both sides and then clamped the whole thing together. After the epoxy had partially set, I cut off the excess on the pins and ground them flush, then put the whole thing in the vise to apply more even pressure and left it for a few hours.

I was really wishing I had a belt sander to do handle shaping; I ended up doing all the rough shaping with a rasp and the sanding drum on my Dremel. I did the finer detail for finger grooves with a half-round file, and then finished everything with sandpaper.

I had to go back to the filing jig to set the final edge, which wasn’t too bad. After that, I used a combination of diamond whetstone and carbide sharpener to get a rough edge before moving on to ceramic sticks. It took an edge nicely, we’ll see how well it holds up with use. Next up is making a kydex sheath, and I’m thinking about trying some dye on the handle since the color isn’t exactly what I wanted.

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