173rd Airborne Knife
From SarahWiki
This project came about because I happened to draw Gina's father in her family's annual Secret Santa roster this year. He's not necessarily an easy person to buy presents for, but in general I tend to prefer to give people things that I've made. I wanted to honour his military career -- he served in the 173rd Airborne in Vietnam -- and make something interesting. Since I've recently been getting increasingly interested in knife making, particularly making art knives, I decided to try my hand at making a knife for Fred.
This was actually my second knife project, though it was my first 'real' knife, in the sense that it was the first that was intended to hold and keep a cutting edge. I think the reason that knife making appeals to me is that it is possible to be artistic in design and execution, and combine my interest in sculpture with engineering something actually useful.
Contents |
The knife
How I made it
The design
I started with researching the 173rd Airborne, particularly it's logo and also I did a bit of research to see what standard issue combat knives would have been used during the Vietnam era. My first designs were based roughly on the military issue design, but ultimately though I decided to retain the basic shape of a fighting knife I wanted to make something prettier, more curvy. Then I had the idea to base the shape of the blade on the outer blade-like feathers on the 173rd logo. after some tweaking, I got a shape I liked, then I designed a handle and guard to match it based on measurements taken from my own hand, and tweaked and tweaked and tweaked until it visually balanced right. This all started in my head, then as pencil sketches, and ultimately as a computer 3D model.
The making
The blade started as a flat bar of D2 tool steel. This isn't a stainless steel, it's actually a very high grade tool steel that's used to make cutters for machine tools. I machined it on my little Sieg KX1 CNC milling machine -- it was profiled with a 1/4" rounded edge tungsten aluminum nitride coated solid carbide end mill under computer control. D2 is very hard, even annealed, so I needed to use something nontrivial to cut it. Then some final shaping and polishing was done with various grinding tools in a Dremel, followed by a lot of hard work with abrasives by hand. I heat treated it redneck-style, out in the back yard with a Bernz-o-matic propane torch, then tempering in the oven. The finish has a layer of oxide that formed during heat treating, which I kind of liked the look of so I left it in place. Since I was rather seat of the pants about the heat treatment, I'm not sure how hard the blade actually ended up, so I'd not recommend using it as a pry bar!
The guard and the pommel are phosphor aluminum (lead-free) bronze, held in place by brass machine screws. Everything was cut and machined on the milling machine, then hand detail shaped and polished.
The wood scales are made from an off cut of very hard, dense rosewood that I picked up from a hardwood supplier in Berkeley about a year ago. They look simple, but the back face of both of them have doughnut-shaped spigots that mate with cross drilled holes in the tang, securely locating them and giving a good mating surface for the epoxy that holds them in place. They were surfaced and machined on the milling machine. the engraving was again done on the Sieg, using a 1/32nd inch round nose end mill and Vectric Aspire generating the tool path. The wood treatment was a bit special -- I started by cutting a rough piece from the larger original, then I put it in the oven for a couple of hours at 170F to dry, then I put it in the vacuum chamber overnight. This will have removed most of the water from the wood. Next, it went back under vacuum submerged in Minwax wood hardener, which is basically a light penetrating varnish that under vacuum will penetrate all the way through the wood, plugging up internal voids with polyurethane plastic and rendering the wood far less likely to expand and contract with moisture. Once that was dry (another spell in the oven followed by a couple of hours under vacuum), I machined it, engraved it, shaped it and polished it with 2000 grit paper followed by a buffing wheel. This then got impregnated with Minwax polyurethane oil finish (basically a wipe on varnish/Tung oil hybrid, as best I can tell) by submerging the parts and sucking out the air several times. This then got wiped off, dried in the oven, vacuumed again, and finially two more coats were applied with buffing in between and at the end.
Videos
I shot some video while the machine was cutting the wood scales -- sorry I didn't get any of the blade. I shall have to remember to do so next time.
- Machining the back of the scales The front face was already surfaced flat, and is clamped face-down to the stack of HDPE plastic below. This means I can cut through the piece and into the plastic without worrying about any tear-out. Having said that, after wood has been treated with Minwax wood hardener under vacuum it tends to machine really easily.
- Cutting out the scales This is the same setup and tool as above, but this time we're profiling around the edges. Notice that I'm cutting both scales in one go to save time and complexity. You'll see the tool going up and down as it goes around each scale -- this is so that tabs are left around the edges so the piece doesn't move at the end of cutting, which often can cause a piece to be damaged or tool breakage. The result looks a bit like the parts in an Airfix kit -- a Dremel comes in very handy in dealing with cleanup afterwards.
- Engraving the 173rd logo I had to cut a special fixture to make this possible, because of the spigots on the back face of the scale. However, it did mean that the scale was located really firmly and just needed clamping down fairly gently, which reduced the chances of damaging it. I tend to make custom fixtures a lot when I'm machining, to the extent that I often spend much more time on it than I do on the actual machining, and this was a pretty good example. Here, the 173rd airborne logo is being engraved with a really small ball nose endmill. The Sieg is easily capable of the accuracy necessary here. Note the relatively high RPM and fairly slow feed rate necessary to get a good surface finish -- usual rules about surface feet per minute and chip load go out of the window when you're doing something like this.
You might also like to see the consequences of the project. Here is a video, shot by Gina, when Fred received the knife. The action is at about the 3 minute mark. :-)
