Friday, October 27, 2017

Mortise Gauges

I have several mortise gauges. The marking, or mortise, gauge is an incredibly useful tool; it would be possible to get by without one, but it would make one's woodworking life a lot more difficult.

If you don't know what a marking gauge is, it's used to scribe a line, or in the case of a mortise gauge a pair of lines, accurately relative to the edge of a piece of wood. What you do with it is set the block on the shaft at the distance you want the line, and then run the block along the edge of the wood so that the pin (or pins) in the shaft scores a line.

The top one is the first one I ever bought, and the simplest. It's made of beech, manufactured to quite decent tolerances, but it's not that easy to use as a mortise gauge because the secondary mortise pin is free-sliding, which means that you have to hold several things in place at once before tightening it up. It can often take several tries to get everything properly spaced, relative to each other. However, as a simple marking gauge (just using the single-pin side) it's great. I've modified it very slightly by re-shaping the pins from a conical section to spear-pointed triangles, so they cut the fibres of the wood rather than crushing them — this results in a much finer, more accurate line, but being so fine it can be difficult to see. especially in timber with a pronounced grain like oak.

The middle one I was given as a birthday present some years ago. It's quite old, and very well made from rosewood with brass inlays. It has a screw adjustment in the shaft for setting the mortise width, so it's a lot easier to be accurate when setting the relative positions of the block and the two mortise pins. Its only issue is that, being so old, it's been used a lot, and the pins have been sharpened and resharpened so much that they're mere nubbins. They could be replaced, but I'm not really confident enough of my skills to try it — it's not a straightforward disassembly job, and I fear that I might destroy it in the attempt.

The bottom one is one that I just bought by mail-order. It wasn't expensive, about twenty bucks or so, but it should see me out. It's made of Malaysian ebony with good, chunky brass anti-wear inserts on the face of the block. Like my rosewood one, it has a screw adjustment for the mortise pins

It has (or had) one significant issue though: the hole in the block had been machined too large, so the shaft rode quite sloppily in it. That would mean that you couldn't guarantee that everything was square when tightened up.

I fixed that by adding a pair of copper shims, one on either side of the shaft, to tighten everything up. You can see their ends folded around the face of the block (there's a similar flange at the back) to hold them in place. I cut a shallow recess in the face so that the shim rests below the surface of the block; that just ensures that it doesn't accidentally ride against your work-piece and send your line off its proper path.

I'll probably re-shape the pins the same way I've done on my beech gauge, but I'll give it some use first to see if that's really warranted.