Thread: Knifemaking photo-essay (2)

The previous post in this series was: https://www.nzhuntingandshooting.co....ay-1-a-104989/

Heat treating.

I don’t have any images showing the heat treating process, however, I’ll describe what I do and show the gear I use.

1) With the furnace at 840°C, insert the knife blanks edge uppermost in a steel rack. By using a rack the blanks heat as evenly as possible – uneven heating can cause the blanks to warp.

2) A pair of scissor-handled heat treating pliers I put together more than 20 years ago.

3) Imagine the tray is 4/5 full of used engine oil. The blank has been in the furnace at 840°C for at least 10 minutes. I dip the red-hot blank edge-first into the oil and move it up and down to cool it as quickly as possible. (DON’T move the hot blank from side to side in the oil – such movement can cause it to warp.)

4) When the blank has cooled sufficiently to be barely touchable, I wipe it down with a rag, and put it edge-up into a benchtop fan oven set at 150°C – the pink arrow indicates the blank. Note also the thermocouple probe sticking out from the top of the oven.

I usually harden a dozen or more blanks at a time. Once the hardened blanks are all in the 150°C oven, I bring the temperature up to 220°C, and temper them for an hour. Then I let the blanks cool to room temperature, put them in the freezer for two hours, bring them back to room temp., and temper for another hour at 220°C. With this heat treatment protocol, my hardness tester usually shows the blades range in hardness from 57 to 59 (Rockwell C scale).

1) This is a hardened, tempered blank I made six or seven years ago. After heat treatment the blanks have a hard, adherent skin of black iron oxide, which I sandblast off.

2) Surface grinding the blank with a 60 grit belt.

I now check the blank for straightness by looking along the top from the butt end. With practice you can see a bend of about 0.5 mm per 100 mm of length. I won’t go into detail except to say that I can generally either “hide” or correct a bend of less than 1 mm per 100 mm.

3) Surface grinding the blade flats with a 180 grit belt. I take off about 0.001” (0.025 mm) per pass until the 60 grit scratches are removed.

Tapering the tang.

The surface ground blank is about 3.7 mm thick. If, while tapering, I aim to remove a maximum of 1.0 mm per side on the tang, that would leave the butt end 1.7 mm thick. Given that the tang is about 110 mm long, the required taper is approximately 1 mm per 100mm of length. The magnetic vice is 300 mm long, so I would need to raise it 3 mm to get the desired taper.

4). The setup for tapering the tangs, with one end of the magnetic vice raised 1/8” (3.175 mm).

1) Grinding the taper with a 60 grit belt.

2) How NOT to taper the other side of the tang. In the setup shown here, I support the tapered end of the blank (pink arrow), then grind the second taper. However, after once doing it this way, I discovered the new taper wasn’t flat -- it had a bow in it.

3) How to grind the second taper. The magnetic vice is raised 1/4” (6.35 mm) as indicated by the pink arrow. Make sure the recently-tapered side is attached to the magnetic surface!

4) The tip of the blade is supported, and then the new taper is ground.

5) The tapered blank.

1) I’ve swapped out the surface grinder’s belt sanding attachment for a fine white grinding wheel. A machine vice, which is bolted to a thick steel plate, is placed on the magnetic vice. I’m aligning the vice jaws at a 90 degree angle to the face of the grinding wheel; once aligned, I switch on the magnetic vice to hold the machine vice in place.

2) Grinding a recess for the guard, taking multiple shallow cuts.

3) The pink arrow points to the newly ground recess.

4) Fine burrs are left on the blade flats, which I’m sanding off with 320 grit abrasive cloth.