Many of you will have chisels of the Stanley/Marples/Irwin ilk. I’ve had a set of Marples for, idunno, 20 years or so, but was never particularly impressed with their edge holding when used in a guitar building context. My blue handled Irwins were added maybe 10 years ago and they were of similar capability, i.e. not really cutting it.
- Irwin-Marples.jpg (127.17 KiB) Viewed 43314 times
You can see from the pic of the Irwins that I had had a go at hardening the brace carving chisel, with no real beneficial effect. I took to using a variety of old chisels bought at junk sales which performer better, but still not as well as I wanted. I even made a chisel from an old file, which turned out to be the best chisel I owned, holding an edge way sharper and longer than anything else I had.
- File Chisel.jpg (162.46 KiB) Viewed 43314 times
I was just about getting to the stage of junking the Stanleys and Irwins when a correspondence with an old mate happened to mention a method of hardening some of the alloyed tool steels that somehow I had in mind couldn’t be heat hardened. I’d previously tried and failed with the Irwin brace carver and a couple of old Stanley plane blades, which reinforced that thinking. I thought “nothing to lose here”, so give it a go.
The process is to heat the last 50mm of the tool to ~750°C, hold at temperature for ~ 1 minute then plunge it into olive oil. That’s it! To get to 750°C I needed two butane torches flat out and matched the hot steel colour to the chart attached (cherry).
- Colour Chart.jpg (66.45 KiB) Viewed 43314 times
I was expecting some “fireworks” when the blade hit the oil, so tooled up with a stack of safety gear, but was disappointed. Just a bit of mild bubbling. If you try this, I’d suggest that you use all the safety gear, at least the first time around. When the chisel has cooled, clean off the scale (at least) and buff out the discolouration if it bothers you. A previously sharpened chisel will be rather brittle on the cutting edge due to it being so thin (and so cooling too fast) and it won't take an edge. Just grind it back a bit, but keep it cool, then sharpen it up.
Well, I was impressed! I used the brace carver to flush trim a figured gidgee heel cap, reckoning if it could do that well, it would be fine. (The gidgee was some of that “mad” figured stuff from Pete McCurly; hard as the hobs of hell).
- Starting heel trim.jpg (67.97 KiB) Viewed 43314 times
Not only did it flush trim it fine, afterwards it would still shave hairs off my arm without a strop or re-hone. Not even my “file” chisel will do that. So I’d call that a pass. Previously, that chisel would have curled up its toes after one stroke.
- Heel cap trimmed.jpg (85.29 KiB) Viewed 43314 times
Here's a detail of the brace carver after doing the heel cap.
- Irwin brace carver.jpg (119.67 KiB) Viewed 43314 times
I did the rest of the chisels with similar results, but left out the 38mm chisel because I only just had enough heat to get the 32mm one to temperature.
- Chisel Set.jpg (135.08 KiB) Viewed 43314 times
On the wider chisels, if you get the back of the blade into the oil first, you can get the back to dish slightly like some of the Japanese blades, which eases honing.
- Dished back.jpg (150.1 KiB) Viewed 43314 times
