04-18-2016, 06:40 PM
We have a TroyBilt chipper/shredder that my wife uses every year to make a lot of mulch for our landscaping. Some years she'll make 30 to 40 big garbage cans full. That much wood going through it causes the shredder blades and chipper knife to wear pretty quickly.
The chipper knife is no longer available as a replacement part, so I make new ones every 4 or 5 years from some A2 bar stock that I get from McMaster Carr. I made two of them while I still worked in a lab where I had access to all sorts of high temperature furnaces. But now that I'm retired my furnace connection is gone.
I bought a piece of steel during the Winter because I knew the knife was shot and I needed to make a new one. It's pretty easy to fabricate a new knife. It's a standard 1/4" x 1-1/4" bar with a 45° beveled edge and three counter sunk holes. I actually cut the beveled edge with a hacksaw, then rough ground it smooth but not quite to a sharp edge. I drilled and countersunk the holes easily enough on the drill press, using the old knife as a template. OK, but now how am I going to harden it?
I tried just heating it with a Mapp Gas torch. Nadda. Couldn't get it to turn much color at all. That was a bit of a surprise. OK, I need a forge. I remembered I had a little SS boiler I had taken out of a dead coffee maker. After finding it I cut a hole in the side near the bottom into which I inserted a piece of 1" iron pipe. I drilled some holes near the top for some steel rods to support the knife and then wrapped the hole thing with some high temperature blanket insulation. A hair dryer provide the combustion air to super heat the charcoal fuel.
And here it is in action:
Holy cow the thing worked great, so well that I had to pull the hair dryer back away from the pipe inlet to slow it down. The photos are after maybe 15 minutes and just before I decided it had been heated to what I guessed was the right color for hardening. I pulled the knife out and let it cool in still air until I could handle it with gloves. My file danced off it so it definitely hardened. It's in the oven now annealing.
That was fun.
John
The chipper knife is no longer available as a replacement part, so I make new ones every 4 or 5 years from some A2 bar stock that I get from McMaster Carr. I made two of them while I still worked in a lab where I had access to all sorts of high temperature furnaces. But now that I'm retired my furnace connection is gone.
I bought a piece of steel during the Winter because I knew the knife was shot and I needed to make a new one. It's pretty easy to fabricate a new knife. It's a standard 1/4" x 1-1/4" bar with a 45° beveled edge and three counter sunk holes. I actually cut the beveled edge with a hacksaw, then rough ground it smooth but not quite to a sharp edge. I drilled and countersunk the holes easily enough on the drill press, using the old knife as a template. OK, but now how am I going to harden it?
I tried just heating it with a Mapp Gas torch. Nadda. Couldn't get it to turn much color at all. That was a bit of a surprise. OK, I need a forge. I remembered I had a little SS boiler I had taken out of a dead coffee maker. After finding it I cut a hole in the side near the bottom into which I inserted a piece of 1" iron pipe. I drilled some holes near the top for some steel rods to support the knife and then wrapped the hole thing with some high temperature blanket insulation. A hair dryer provide the combustion air to super heat the charcoal fuel.
And here it is in action:
Holy cow the thing worked great, so well that I had to pull the hair dryer back away from the pipe inlet to slow it down. The photos are after maybe 15 minutes and just before I decided it had been heated to what I guessed was the right color for hardening. I pulled the knife out and let it cool in still air until I could handle it with gloves. My file danced off it so it definitely hardened. It's in the oven now annealing.
That was fun.
John
