#knife making

So in the timelapse video above, our teacher used an industrial water jet that fires a mixture of air, water, and garnet sand at extremely high pressure along a CNC mapped tool path. This was to cut the profiles of our knives and our tang attachment post holes off of a 2.7mm thick sheet of 1095 Carbon Steel.

When we get this steel, it's in an annealed state, which means it was most recently at a very high temperature that loosened its crystalline structure out and then it cooled from that heat slowly enough that the crystal structure settled into a stable and ductile configuration. It is SOFT, compared to a finished knife.

The fabrication of the sheet also left a very slight curve to the entire sheet, barely perceptible to the naked eye. That curve was worked out by hand with cold forging: hitting with a hammer on an anvil without heating it.

I don't have a picture of it, but the water jet cutting also leaves a slight slope to the cut faces of the knife blank. Whichever side was on the bottom stays wider because as the erosive stream cut through it, the top and middle layers were eroded for slightly longer. We square off all of these sides on belt grinders using a relatively aggressive belt (36 grit or 60 grit)

In this picture you can see little radial circles intermittently along the length of the knife blank. These are left by the hardening impact with the hammer during coldforging.

Each impact condenses the atoms in that specific area, making it a much tighter and harder crystal locally around the impact site. This variability of hardness can be a big problem for longterm knife life, because soft areas will dull faster and hard areas will be the point of break if too much leverage is applied to the knife.

SO! We heat treat it. In our space, there's an Evenheat Kiln. It's a deep but narrow electric kiln that we preheat to 1515 degrees Fahrenheit, with some rectangular scrap steel stock to function as spacers.

Then we open up the door and slide each knife blank in to a spot between two chunks of scrap, along their spines, "edge" facing up. The scraps help to evenly heat the sides with the top and bottom of each blank, and after 20 minutes the blanks are at the temperature of the kiln.

Then we very carefully carry the red hot blanks one at a time out to a big drum of vegetable oil to quench them, rapidly cooling to an extreme hardness. I didn't get a video of this, but you want to stir the knife through the oil like you were using it to cut something, not moving it with the flat like an oar. By slide it through the oil, it contacts colder oil quickly and evenly cools at the pace to quench successfully.

After quenching, the knife is covered in a patina of scale, also known as black rust. It's chemical structure is different from red rust and it's much more stable, but it also is generally only formed under the catalyst of extreme heat. We quickly grind off the surface to remove that scale, then put the knife into a conventional home oven to temper it.

Tempering is the process of raising the steel to a specific temperature of your choice between around 375 and 600, and letting it chill at that temperature for a couple of hours, then removing it from the oven and allowing it to gradually cool in the air. This normalizes the hardness of the knife much more completely than the heat treat did, and it gets to a hardness where you can reliably sharpen the knife, and it's regained some ductility so that if it has a hard impact by dropping, it's less likely to shatter. Also, the outermost layers of the alloy tend to achieve a pretty color depending on which temperature you go to. First it hits Straw, then Magenta, then Indigo at the higher temps. The higher temp you go to, the softer your knife will be.

You can see after my temper that especially the remaining bits of scale turned a very pretty blue, while the parts that I'd ground down to homogeneous steel didn't take much of a hue.

After tempering, we grind the surfaces back down to clean steel for visual contrast purposes. You can see the appearance and geometry of my knife at this stage in these three pictures.

The side with the little dark circles is the face I mostly struck during coldforging, and there is still some detectable trace of that, though the tempering process was broadly a success. These spots vanish as I grind material away to gain my primary bevel.

A knife must necessarily have a bevel between the spin (wide, flat, blunt) and the edge (narrow, V-shaped, sharp). You can make a single-bevel knife, which would look like a chisel's edge, and would have certain properties when making cuts. Those properties are different from the properties of a V-bevel or double-beveled knife. The advantages between them are a bit too nuanced for me to explain well, because I haven't felt them myself, but expert chefs and butchers would better outline the reasons.

You arrive at how to remove the right amount of material from both sides by painting your knife with marking fluid and then using calipers or marking gauges to scribe guide lines at specific measurements into the marking fluid. Then you grind away the areas with the fluid, using large belt grinders.

I unfortunately didn't take progress photos at each grit, as I ground the blade down. We started at 60 grit, then 80 grit, 120 grit, 150 grit, 180 grit, 220 grit, then four ranks of scotchbrite conditioning belts.

From 60 - 120, the belts look like that. It's a ceramic aggregate of various sizes, suspending in a red polymer. They spin at high speed and gouge deep trenches into the knife, leaving grind lines.

Each grit successively is able to remove the imperfections left by the last grit, but if you move on from 80 to 120 before you've ground off the last grindlines from 60, you won't be able to remove them with the 120, so you'll grind over the surface, but the depth will leave that mark there.

So we generally revisited belts back and forth until we had the geometry we needed. You can remove the majority of the steel you have to remove with a 36 grit or 60 grit, but you spend a similar amount of time on each grit, you're just removing less material but achieving a smoother surface as you increase grit.

The 150 - 220 grit belts are an aluminum oxide sand on cloth belts, so they engage with the hardened steel differently than the ceramic does, rounding off the prominent ridges left by the ceramic belts grinding.

Finally, the scotch brite conditioning belts we use are a woven nylon with Silicon Carbide coating the fibers, which polish to a really nice surface with minimal risk of undercutting through the steel you want to keep.

From 150 and up the friction aspect gets really intense, so it's important to spread your work across the surface, regularly cool the work piece with water, or grind it with an active cooling jet of pressurized water mist targeting the knife as you grind. You just have to have a basin to catch the water underneath you when you do this. I did get a couple of blisters on the tips of my fingers from pressing the piece against the higher-grit belts because they went from cool to hot like THAT, when an imperfection engaged the friction, and steel famously transmits heat pretty well, especially when it's approaching a mm thick or thinner.

After all is said and done, we took posts of brass rod and handle blanks of wood, did some sanding to refine the general shape of the handle, then used 5-minute two part epoxy to fuse the posts and handle parts around the tang. We masked off the area closest to the handle to catch any spill out as it cured.

So there's the knife, with white oak handle and brass posts, clamped and curing. After twenty to thirty minutes, I took off the clamps and the masking, used the highest grade conditioning belt to work off any excess epoxy near the spill out, then used more aggressive belts again and a scalloped edge belt that can naturally dome back and work into finger-grip valleys on the handle to finalize the profile of the tang and handle.

And here it is after all's said and done and I've put the approximately 30 degree secondary bevel along the edge. I haven't sharpened it to 1000 or 6000 grit yet because we were missing the stones when I wrapped up, but I'll have it sharpened soon. And I think I'll make it a nice sheath of wood to hold it.