A while back I started making a marking gauge.
I set it aside for a while – in part I’ve been working on my bench, but also because I wasn’t sure what I wanted to do for the cutter on this gauge. I’ve seen folks use a pointed piece of drill rod, but that doesn’t work well across the grain. I’ve seen instructions on how to sharpen this kind of cutter to make it more of a knife part, but it still looks too thick to slice through the wood fibers nicely. David Charlesworth write about this in one of his books, this picture is from a Popular Woodworking article:
I really like my Glen-Drake gauge, I like the way that it slices through the wood and leaves a crisp line. I’ve seen marking gauges that use small blades and even X-Acto knife blades. The downside of that approach is that the attachment is a bit more fiddly. I wanted this to be simple and fast, here is what I came up with.
I’m using some 5/32″ O1 drill rod — this is an oil-quench tool steel. I chose the diamater because I wanted to fit it into a 1/8″ hole and have it be a snug fit. I cut a few short pieces with a hacksaw and de-burred them ends. Note that I filed one end flat and a small dome on the other end. (I swear, this was in focus when I took it)
I heated the rounded end using a propane torch, the hottest part should be right at the tip of the inner flame cone. I’m holding it a little too far into the flame in the picture, but I’m juggling the part and the camera at the same time.
Heat the tip until the first 1/3 or so is a cherry red. I’m going to forge the end into a flat, fan shape. Forge might be too strong a word, this is pretty light work. I used light taps and re-heated it after a few taps. Between the small size of the part, and the mass of cold metal in the hammer and “anvil” it cools off really quickly.
To get the shape I wanted I need to hold the hammer at an angle, and hold the part at the edge of the anvil so that the hammer can hang off the edge. This is a posed shot of course, I couldn’t hold the hammer, vise grips with the red-hot part and the camera all at the same time.
What I’m working toward is to thin the metal at the tip, tapering back to a round cross section, while widening the metal at the same time. It’s harder to type than to actually do it, trust me. It took me maybe 3 or four cycled of heating and hammering to get to the shape I wanted.
Next I needed to harden the metal so it would hold an edge. There is a lot of science in heat treating, and a fair bit of black magic when it comes to making edged tools. I’ve forged a handful of knives and chisels in the past, forging edged tools is a bottomless pit of austenitic temperatures, molten salt baths and cryogenic cycles. For the purposes of making a pointy thing to leave a deep scratch in a piece of wood, simple is fine.
Heat the part to a medium cherry red. If you have a magnet handy the part should loose it’s magnetic attraction at this temperature. Then stick the part into a small container of oil and swish it around. Done. Technically it should be “tempered” to draw out the brittleness, but it’s not necessary for this.
Please use caution if you do this yourself, it’s all too easy to burn yourself or set something on fire if you’re not careful.
You can confirm that you’ve hardened this by using a file. A slightly dull file (I have several of those left over from retoothing my saw) should skate across the hardened area without cutting. Compare it to a piece of the same material prior to hardening it.
Then I worked the tip with a grinder, then an oilstone. The tip is wide and thin, less than 1/16″ at the edge.
I still need to sharpen it, and finish the rest of the marking gauge, but I think this will work out well.