Mortise and tenon joints are among the most popular wood joints and have been in use in one way or another for hundreds, perhaps thousands of years. They can be made in a variety of ways, both by hand, by woodworking machine or a combination of both and give a strong joint that is well suited for many kinds of builds. In this video, I am making a self-centering mortising jig for the wood router.

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Although I don't use this jig much anymore, it is a jig that I used extensively in the past and I liked it because it adapted to many different joints and sizes and was easy to use and gave good results ... 

I have made a few versions of this jig in the past and the version I like the best was the one that used a clear plastic bast the replaced the base on the wood router. The first thing you will need is a suitable piece of plastic that you can pick up many places, mine 6 inches square, and I found that 1/4 thickness works fine for this. 

Self Centering Mortise Jig

You will need either use the base of your existing router as a template, or if you have some pointed screws that fit your router base, you could use them for marking the base where you will need to drill holes for the bolts to mount the new base to the router. 
There is a process for making these bases because you will need to drill countersink holes, either flat or angled, depending on our model of router. 

After the holes have been marked, you will need to first drill some tiny holes through the plastic at each bolt hole. The purpose of these holes is to act as a pilot hole for your Forstner bit, which is the next bit you will need for a flat bottom countersink hole. These countersink holes need to be deep enough that the screw heads are below the surface of the base plate, but not too deep that they compromise the strength of the clear plastic base.

Once you have the countersunk holes drilled, the last thing to drill is the "through holes" that the bolts will go through, then check to make sure your new base plate will fit your router, and adjust as needed if required. 

 With the new router base in place, you will need to find the center of the new clear. 

Floating Mortise jig

plastic base plate. The quickest way is to install the smallest router bit you have. In my case, I have some 1/16 inch bits which make a tiny hole. The smaller the hole the better because they are easier to find the center of the hole with. 

Next you will need to find your comfort with the router because this router bit works by setting it on the wood, twisting the router slightly until the 2 dowel plug you will be installing are firmly up against the wood you are mortising, then the router is plunged and moved back and forth to make the mortise cut. SO .. you need to find where these 2 dowel plugs will go so they are in a comfortable spot when you are holding your router. 

When you are clear where they will go, draw a diagonal line, through the center of the hole you made in the clear part of the router base and etch a line end to end on the base.  Once that line is there, use some sort of a marking block on each side of the bit to mark the cross line where you will need to drill holes for the screws that will be driven down from the top to hold the dowel blanks. The cross lines probably want to be about 3 inches from the center of the new base, but this distance is not important as long as both dowel plugs are the same distance from the center point.

Next, you will need a couple of short dowel pieces, 5/8 or 3/4 diameter works best and about 3/4 to 1 inch long.  Attach these plugs appropriately, probably with flat head screws that you have countersunk holes for.

The final job to make the plate complete is to drill out the center of the clear base to accommodate which router bits you will be using. When I use this jig I primarily use either 1/4" or 3/8"  up-spiral straight bits depending on the joint, but you might select another size depending on what you are doing. My final holes size for the center of the clear base is 3/4" to allow plenty of clearance. 

Mortise router Jig

That jig is now ready to use. The best way of using this jig if you are using pure carbide bits is to make sure that you "plunge" the holes in your mortise, all the way along the length of the mortise before you clean them out end to end. Carbide bit have no tolerance to bending or sideways pressure and they will snap before they bend. If you are using steel bits with carbide edges, these are much more suited to sideways movement.

To cut the slots, you can set up "stops" front and aft of your cuts for more accuracy if needed, or cut them by eye, whatever works best.

Floating Tenons

Some people call these "loose tenon" but I prefer to call the "floating" because it leads to fewer questions and explanations. 
I make my own from the best wood I can find which is straight grain wood. I plane the wood down to the appropriate size on my planer and often leave it a tiny bit thick so that I can sand them down to a perfect size as needed. To round over the ends, I simply use an appropriate round over bit on my router table and in no time I have a series of perfect floating tenon blanks that just need to be cut to length as needed.

Floating tenons are a quick, reliable and easy way of making joints and with a bit of adjusting, can adapt to many situations and demands.

Copyright Colin Knecht

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Mortising Jig