Using the mortise gauge. If you start by pressing the spurs into the wood where the lines are to stop, you'll feel the gauge click into the marks at the end of the cut (above left). To center a mortise (above right), press the spurs into the wood from both sides, then split the difference between the marks. (The same trick works with a marking gauge.) Keep your thumb inboard of the fence for protection.
The mortise gauge is a deluxe item with a brass mechanism for moving and locking the second spur. It's hard to find one without brass inlays (see main text), but you can reverse the fence to work off its wood face.
Often it's necessary to disassemble the mortise gauge and dab a little grease on the threaded rod that adjusts the second spur, to help it move smoothly. If it still binds, look for a particle of crud or a lacquer drool that may be clogging the works. You may have to pick out the culprit at knifepoint.
When you disassemble the mortise gauge, watch out for a little metal disc that sits under the stock's locking thumbscrew. It keeps the thumbscrew from chewing up the movable brass strip, so if it gets lost, you must replace the disc.
Some mortise gauges have a third spur on the back side of the stock. The idea is that this spur can be used like a regular marking gauge—two tools for the price of one. The economy is false because regular marking just wears out the expensive mortise gauge's fence sooner than necessary. What's worse, the day will come when you plant that third spur bone-deep in your thumb. So pull it out and save it for replacing the spur in an old marking gauge.
right photo, page 45.) It makes two parallel lines, for laying out the shoulders of both a mortise and a tenon.
The techniques are the same for all three gauges, and it'll take you only a few minutes' practice to get it right. Don't try to make a deep line the first time. You can go back and forth to make it as deep as you want. Note that a deep line is more visible and helps prevent tcarout at the joint's edges. A shallow line is easier to erase later by planing or sanding.
When you buy a new or used gauge, avoid the model with two brass strips inlaid into the fencc. The brass is supposed to counteract wear, and it's cosmetically pleasing. But because brass and wood expand and contract at different rates, a plain wooden fence will stay flat longer than an inlaid version. And if the fence isn't flat, you'll get a crookcd line when you gauge across a board's thickness. (Sec lower left photo, opposite.) If you already own a gauge with brass inlays, turn the fencc around to work off its plain facc.
Next, make sure that the fit between the fence and stock isn't sloppy. You can usually relieve a too-tight fit, as long as it isn't frozen, by buffing some paraffin wax into the stock or by taking a single shaving off it with a plane. The latter may require removing the spur, which you can do by levering it out with pliers, or by grabbing it in a machinist's vise and tapping the stock away from it. But you can't do anything about looseness, so don't buy a gauge with this problem.
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