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JOEL Joel's Blog

How to Quickly Center A Mortise Or Find The Center Of A Board Without Gadgetry


How to Quickly  Center  A Mortise Or Find The Center Of A Board Without Gadgetry 4Even 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.

Join the conversation
02/22/2011 jg
after many years of doing this, I now find I hit it the first time. Practice practice practice!
02/22/2011 jeremy
That's how I usually go about it but I often have a problem with the distance between the pins changing on me as well. Do the pins on that gauge set separately from the fence somehow? Or do I need work on my technique?
My mortise gauge is screw adjustable and consequently the pin distance is independent of fence adjustment. I taught many many years ago that slide adjustable gauges were not worth the trouble because of problems like you are having.
02/22/2011 Sean
I think it depends upon the rest of your work habits in making a mortise and tenon. I make my mortises first, and then saw my tenons to fit. If you use the same face on each of the mortise boards as a your register, exact center doesn't matter much. Or, if using a wheel gauge, you can just set it up so as take equal amounts from each side to arrive at a centered mortise of the desired width. Just some other thoughts and methods.
02/22/2011 Steve
My response is analogous to Sean's: Who cares if the mortise is perfectly centered? You make one half of the joint, either mortise or tenon, and then you gauge the location of the other half from the first.

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.
First of all the point here was to show how to find the center for a single point or double point mark. Certainly it is not essential for every operation and many times it doesn't matter. Other times it will be. Being able to find the center of a saw handle you are making seems like a good idea. Laying out the slot for the saw the back is also a place where centering is important. In a chair centering a rail might look pretty good and even if the mortise is offset, centering the tenons makes it easier to keep organized

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.
02/22/2011 Barry
I LOVE this stuff...

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.
02/23/2011 Gary Roberts
There's really no need to center this way. Decide on the width of the tenon, set the gauge by eye, gauge the parts of the joint from the same face (witness marks help here) and go for it.

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.
When I first started working wood a few years ago, I figured out this little trick on my own. It's nice to see "my" method validated.

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.
02/25/2011 Charlie Pitts
A great discussion - I learned TWO useful things. 1 how to mark centered m/t. 2 I don't always need to bother if i'm careful to mark the faces. THANKS!
03/01/2011 Anonymous Comment-Leaving Person
I find this to be very useful advice that has several applications. You never know when you're going to need to know how to do something, and coming across it here adds a another arrow to our quiver of shop knowledge. Thanks for sharing it Joel--that's why we come here!
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