Saturday, April 28, 2012

A Japanese-style mortise gauge

To some of my readers, perhaps only the word "Japanese" makes sense in the title. No matter. All will be revealed in good time. A mortise gauge is a device used to mark out the sides of a mortise. A mortise, in turn, is a sort of trench excavated in wood. In this trench fits a similarly shaped peg, the tenon. M&T joints are as old as the hills. This summer's program includes a brand-new machine storage barn. I am determined to do it timber-framed. None of this nailing commercially milled lumber together. We will cut mortises and tenons and peg the whole thing together. It will last much longer than I will, unlike the ticky-tacky boxes they call "houses" these days.

Anyway, a mortise gauge is a layout tool. It will lay out the long sides of the mortise. If this is perplexing, stay tuned. We begin with the slider; it needs a rectangualr hole. I am using wood from a footstool that had fallen apart and been deconstructed.
What we have to do is cut a rectangular hole in a piece of wood. This is the way I did it. First, the dominant dimension in the scrap wood was 19mm, or 3/4" RGU. So we drill overlapping holes 19mm wide, and chisel out the rest so's it's rectangular. Behold the result above. A reasonable rectangle. Now we make the arms. These are nominally 19mm square cross-section, arbitrarily long. I could have made them much shorter than I did, but here is one of them:
That's one arm. The other looks just like it. In the middle there is an aluminum separator strip. This is to keep the arms parallel. The arms have to be planed so they are a tight, but not impossibly tight, fit into my rectangle. It is much easier to plane the arms than to enlarge the rectangle.
When both arms are in it looks like this:
Now, in the arms go the cutters. These things score the wood, and prevent tear-out when you actually make the mortise. I made them out of an old hacksaw blade (never throw good steel away). They were annealed, ground to shape, hardened and tempered. Then they were sharpened. Tedious but necessary. Now we had to make a slot in each arm to accept the cutter. I did this by drilling 1.5mm holes in a line and cutting out the intermediate stuff with one of my miniature mortise chisels (handmade, of course). When we got this done we had a respectable-looking Japanese-style mortise gauge.
There were some details, in which, of course, the Devil always resides. I epoxied copper rubbing strips to the inside of the rectangular hole, which, by the way, is itself a mortise. A through-mortise to be exact. The arms require wedges to hold them in place. But on the whole I am very pleased with my gauge. It cost nothing. Perhaps in the next episode I will cut a mortise for you to show you how it is used, and it has been used, and very, very useful it is. I have a commercial mortise gauge, but this one is much better, because it scores the wood instead of scratching it. This helps a lot; prevents tear-out when you actually cut the mortise. Next episode I may cut a mortise for you to show how it is done.

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