|Forge tinkering; small knife
||[Feb. 25th, 2006|11:40 pm]
Well, I had a busy Friday.
I made a small knife from the cable damascus experiments; came out OK, though I'm not terribly impressed by the "pattern" the cable makes, and made some improoooovements to the new forge.
I really like the new forge. With a Michael Porter style venturi burner ( http://www.amazon.com/gp/product/1879535203 ) it gets nice and toasty, a good forging temperature, a cheery orange with a gentle mutter of the burner. I do a fair amount of forge welding and damascus, however, and while you *can* weld at low temperature, you have to get every detail right, and it's not as certain. To overcome this limitation, I built a new burner--a forced-air type. Blown. The canonical brute-force approach to heat.
The new burner is very simple. Unlike the venturi burners, which you have to get every detail right on for them to work properly (alignment of injector, size of venturi slots, choke plates, nozzle diameter, size of forge opening, etc.), the design is simplicity itself: blow air in a tube, mix in propane, shoot it out into the forge, where it burns.
In particular, in case anyone wants to duplicate my efforts, the new burner consists of a 1/2 hp blower, some ducting, a 1.25" brass ball valve, a 10" long 1.25" pipe, and the needle valve and connector assembly from an old weed burner.
To assemble, I first forged one end of the 10" x 1.25" pipe to about a 3/4" nozzle. I'm thinking this may have been too much, and I'm going to buy another pipe and forge it down to only 1" and see if it still works. The idea behind making a nozzle like this is to increase the rate of flow of the fuel/air mix to above the flame front propagation velocity -- keeping the flame in the forge, where it belongs, and not in the pipe. With the one I made, at higher volume the flame didn't completely combust before hitting the wall of the forge, resulting in a cool spot. The 3/4" nozzle worked fine so long as I didn't crank up the air/propane mix -- it was still quite functional (did a couple of folds on a piece of damascus kicking around the shop and forged out another blank for a pizza wheel), but for when I really want to make things hot, the larger diameter should get me a complete burn before the flow impinges on the forge wall.
Anyway, once I had my pipe forged to a nozzle, I drilled and tapped a hole for the propane line. The propane assembly was from an old Harbor Freight weed burner, and is just a tank connector, a hose, and a needle-valve with a bit of piping on the end. The old burner assembly broke off years ago, but the needle valve and propane hookup made making this burner dead simple. I put the hole about 2" from the end of the pipe opposite the nozzle. If you're doing this from scratch, I'd use a 1/4" pipe attached to a needle valve (you could use a regulator, but for this application it's the volume of propane that counts, not the pressure behind it), and an off-the-shelf propane hose and fitting.
To the nozzle and propane assembly I attached the ball valve, to which I affixed a length of 1.25" flexible plastic pipe, which in turn I connected to the blower. It's really that simple.
Optimal burning with propane/air is roughly 4 parts air to 1 part propane -- though honestly you can tell when it's working right. To light I opened up the propane a hair, and lit using a torch igniter. Once the propane was lit, I crack open the air, then up the propane, and the air some more, and repeat until I have the amount fuel/air I want. In contrast to the gentle murmur of the venturi burner, this sounds like a rocket, and instead of a nice orange glow, this gets the inside of the forge lemon yellow and white. Very loud and very hot.
I should also note that you can clearly see when you've got a reducing or oxidizing atmosphere -- both from the color and shape of the flame jet, and from whether the gas is burning complete or if some (or a lot) is still combusting outside the forge. You'll also hear the sound change as it moved through reducing to oxidizing and back.
With the venturi assembly, I can heat up a piece of 1" x 1" x 6" and have it at forging temperature in about 5 minutes from a cold start. In the same time, using the forced air burner, I'm welding, probably on the 2nd heat.
The new forge is impressive in another way, as well: even running it with the blown burner, for a couple of hours, the outside of the forge was just barely at the boiling point of water. I like this forge.
Having done some experiments on pieces of cable, I decided to try to make a couple of knives. Probably because I hadn't made the new burner yet, I had a lot of cold-shuts and bad welds in the test pieces, but after grinding off the bad bits I still had a couple of small knife blades worth of material. I picked the smaller of the two pieces for the first knife, ground in bevels and profiled it to a gentle sweeping curve, polishing it down to 15 micron, heat treated the blade (the cable I use, super-improved plough steel, is about the same as 1085-1095, simple water-quenching low temper steel), and cleaned it up.
After trying several other etches, I finally just etched the heck out of it using ferric chloride, which got me a decent 3-d effect. Since it's all the same steel, I guess I shouldn't be surprised at the lack of contrast. I'll have to fiddle with some different etches and patinas for future work.
The handle is a nice piece of antler, which I polished up the highlights on, rounded the butt-end on, milled a hole for the tang, and then epoxy'd the blade in place, using a piece of leather to cover up the ugliness of the raw antler end. It came out pretty nice.
I'll take some pics of the new knife, the new burner, and the forge and post them up tomorrow. I have another damascus blade etching now as well, hopefully I can get that finished tomorrow as well.