Even with a couple of tries before getting it perfect, centering a mortise on a stile or finding the center of the edge of a board is faster using a marking or mortise gauge than walking across the shop to find a specialized gizmo for the job.
Here is what you do.
A - Set the pins on your mortise gauge to whatever distance apart you want.
B - Set the fence at a rough guess which would center the pins on the wood.
C - Put the fence against the wood and press to put pin-pricks where the pins are set.
D - put the fence against the opposing side of the wood and repeat the pin pricks.
E - If the pinpricks from the gauge hit the same holes from either side, you are centered. If there is any offset, adjust the fence to split the difference between the pin marks. Repeat steps C and D until the pin marks match up.
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In the picture at position 1, the fence setting was eyeballed and the pinholes are a good bit off.
At position 2 I've split the difference between the hole spacing at 1, tried again, and this time I am pretty darn close.
The pinholes are pretty deep because I wanted to make sure they photograph. So on position 3, after I have eyeballed and split the difference at 2, I'm not pressing so deep. The marks are even closer. A small tweak, a light touch on the pins so I can see the maximum error (which is how I would normally do it for all tests), and you can see it's perfect. I'm done.
It takes a little practice on adjusting the gauge, but after a few times this will become second nature. The time to do this is under a minute.
To adjust a marking gauge with a single pin, do the same thing except, of course, there is only one pin mark. It's the same technique of testing and splitting the difference.
This new "Tools & Craft" section is provided courtesy of Joel Moskowitz, founder of Tools for Working Wood, the Brooklyn-based catalog retailer of everything from hand tools to Festool; check out their online shop here. Joel also founded Gramercy Tools, the award-winning boutique manufacturer of hand tools made the old-fashioned way: Built to work and built to last.