I've made kitchen knives, cleavers, bread knives, friction folders etc. I thought it was about time I stepped it up and tried making a proper folding knife.
It was fiddly and a bit troublesome and there are things I'd have done differently, but I got there in the end. It folds, it locks. Here's how I did it...
Step 1: Materials
High carbon steel. There are numerous types from numerous sources. As this is a stock removal knife it needs to be the thickness you require your blade. About 3mm in my case. I got all my steel from a piece about 160 x 40
Suitable handle material. As it will be pivoting, the handle material needs to be very strong. I used micarta.
A suitable pivot bolt. I removed mine from an old cheapo leatherman knock off. I have previously used a chainring bolt from a bicycle and it worked fine.
3.2mm (or similar) rod to make the pins. I initially used old drill bits but got fancy and switched to brass.
Cardboard (I used old cereal box)
Glue for sticking paper to metal.
Step 2: Tools
Something to cut the steel. Angle grinder is good. Hacksaw is ok.
Heat treatment stuff (forge or blowtorch, oil, etc)
Sharpening stuff (files, grinders, stones etc)
Step 3: A Man With a Plan
The most important thing in this build is to think it through and plan it out. You can't just wing it or it will neither lock nor fold.
I looked at photos on the internet of the mechanisms from commercial folding knives. Then I printed one out and copied it onto plain paper, changing it slightly for my aesthetic. I cannot include the photos as it's not my work. Sorry. But they are easy enough to find.
Print 3 copies of your plan for the mechanism at the correct size of the final knife.
Carefully cut the paper out and check that it will interlock, fold and unfold correctly. Make any adjustments.
Glue copy to cardboard and cut out (it's better for accuracy to glue first and cut out after).
Pin to a board through the pivot points and pin the spring piece (the tail) and test that your model will work properly and that the spring will push correctly.
All good? Good.
Step 4: Metal
Remember the last step? Sticking paper to cardboard and cutting out carefully and accurately? Good.
Stick another copy to your steel and very carefully cut it out. I used an angle grinder then refined the fine pieces with files and sandpaper.
The tadpole shaped tailpiece functions as a flat spring, so it is essential for the spring to work that you make it smooth and even with no sharp cuts or severe angles.
Very carefully drill out your holes. I went 3.2mm for the pins and 5mm for the knife pivot, simply because that matched what I had.
File/grind the blade edge to a suitable angle. I went for 20 degrees.
NB Pretty much all of the folders I researched had a 2 piece spring tail piece. If you look at the drawing above you can see that there would be a slot cut into the tail and a thin spring piece would be held in place. As I would have to temper the spring as, uh, a spring it seemed to me an unnecessary step to have it in 2 pieces so I fused the 2. I did make another small spring piece just in case, but the fused tailpiece works great).
Step 5: Testing Testing
(Some of these photos are after heat treating, but it doesn't matter)
Make a piece of scrap wood into a working model to make sure everything is dandy.
Drill a 3.2 hole for the rocker first (as this gives the alignment)
Hold the knife in it's locked place against the rocker To fix the correct place for the 5mm pivot hole. Use the actual knife hole as a template to (carefully) drill through as it MUST be accurate.
Put the tailpiece in the correct position for the spring to push up against the rocker. Drill both pin holes.
Use the correct pivot and pins and make sure everything works correctly. It needs to lock solidly in place under spring pressure (Do NOT put too much pressure on your spring before tempering - next step), and fold securely with the tab on the knife meeting the slight belly on the rocker preventing the actual blade hitting anything when folded.
Step 6: Harden
Without heat treatment it isn't a knife. Well, it is, but it's a crap one.
There is a world of info online on heat treating knives. Some of it is even accurate. You can do this as easily or as complex as you wish depending on your skills, requirements and equipment. Here's an easy version:
Heat the blade (or just the sharp edge) to the state where it is no longer magnetic (nice and easy to test for). I did it in a gas forge, but a blowtorch can do the job at a push. Plunge it into oil (any sort). Gently check with a file on a non critical place. If correctly hardened the file should sort of skate across the knife. If it isn't hard do it again until it is.
Do the same for the tailpiece and the rocker.
Step 7: Temper Temper.
What you now have is a very brittle knife (bad), rocker (not so bad) and spring (very bad indeed).
The knife and the rocker need to be tempered together. This can be done in your domestic oven when your wife isn't looking. The spring MUST be done separately
Heat the oven to about 220c and heat the knife and rocker for about an hour. Let them slowly come back down to room temperature. Repeat twice. You can now sharpen the knife fully.
The spring needs to get to about 400c. If you have a self cleaning oven that should do the job. If you don't you have to try something else. If you are careful and a good judge of colour you can heat it indirectly from below with a blowtorch (resting the spring on a thicker piece of metal) and watch it until it turns a darkish blue. See videos on YouTube for this.
I have a high temperature thermocouple thermometer (about £17) which goes to crazy high temps.
I rigged this so it was resting on a thick piece of angle iron which I heated from below until I could get a steady 400-ish. Then I added the spring piece until heated through. I let it cool then repeated it twice. I nervously test the tadpole tail for springiness and it worked beautifully. By 'worked beautifully' I mean it was springy and didn't snap.
Step 8: Scales
When there 2 parts to a knife handle they are called 'scales'. That's that cleared up then.
You need to very carefully drill holes in your scales that exactly match the ones from your board test model. You can replicate the method you used for the board, trace the holes, use the board as a template for the drill or something else.
Put the pins and pivots in place and test everything works properly. Then mark the INSIDE of one scale for the shape you want. Cut/sand/file it to size, testing for size frequently. Once you are happy, use the pins and pivot to hold both pieces together. Now cut/sand/file the other scale to match.
Step 9: Sticky Business
Test the action once again. Once you are completely happy that all is well...
Mix up some epoxy (the stuff I use is from the pound shop and works well) and glue the tailpiece in place, making sure the pins are fully through.
Very carefully glue the rocker pin in place (but NOT the rocker), ensuring you do not have any waste glue on the inside of the scale. I did this by carefully putting the epoxy only in the hole, poking the pin through some cling film, put the pin FULLY in the hole then pulled off the cling film (this dragged any excess epoxy away).
Wait for these parts to fully cure.
Test that the parts all line up before the next stage.
Mix up some epoxy. Cover the tailpiece and tailpiece pins with epoxy, ensuring that the tail can move and spring and does NOT get glued down. Put some epoxy in the new rocker pin hole from the OUTSIDE of the scale (it is vital that the rocker can rock and does not get stuck).
Carefully press the scales together while holding the knife, rocker and spring in place. Screw the pivot together.
Step 10: Tidying Up
Cut/grind/sand off the protruding parts of the pins. Give the scales a final sanding/polish.