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02/22/2011 |
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.
It in the picture at position 1 the fence setting was eyeballed and the pin hole 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 pin holes 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 tweek, 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.
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Be consistent about choosing a reference face for all of your joints, and you'll be fine. And even if you make your mortises and tenons with a router or other power tool, the same principle applies.
All I am trying to do is show how it's done. I leave the application or not application of the technique to when it's appropriate on a particular job. It's one more technique that is easy to learn and useful to know.
Yes, I own a dial indicator, dial calipers, etc... But it's simple techniques like this that make woodworking enjoyable.
BTW, folks... This same technique works for finding the center of a piece of stock with a combo square.
Precision centering of M&T joints, in fact many joints, is a modern creation of the machine manufacturing age. Unless you are planning on a Henry Ford assembly line, it's overkill.
Relax and enjoy your work and don't worry about centering.
You should always use reference marks to keep parts correctly aligned but knowing how to trivially center something which takes only a couple of seconds more than a first guess is a good technique to know. Why find yourself in the situation where something that should be centered isn't? A Through mortise that's not centered on a chair leg will look odd. If there is no need to find the center don't do it but if finding center would improve the look or ease of construction of a project, not finding center because you don't know how is a shame - and that is what I am trying to address.
While it may not be strictly necessary to center your mortise, I think this is a good practice when working with relatively thin stock. Given that the mortise walls should be of equal thickness for maximum strength, I find this little technique helpful for at least ensuring that my mortise is not so far offset that one of the mortise walls is unnecessarily thin. A 1/4" mortise in 3/4" softwood stock, for example, really should be as centered as possible. And through-mortises should be centered for merely aesthetic reasons--an offset mortise in a stile is just going to look bad.
This technique is also helpful for resawing. I always use this method with my regular marking gauge if I am going to resaw stock, whether on the bandsaw or by hand.