I spend a fair bit of time in the wood shop making projects for the boat and the house. Often times that involves some cabinet work of one sort or another and although it may be acceptable to simply glue and screw cabinets together if they are to be painted it is not the best way if they are to be finished bright or the showing any sort of fixings is unacceptable. One of the best ways is dado or as we say in British English cut some housings. These are simply a groove across one board into which another board slots. Useful for all sorts of things but the trick is making sure that one board fits neatly into the other. Sometimes the component parts will be solid wood but most often they will be some type of panel material; MDF, plywood etc. To a novice one would think that if you bought three quarter inch plywood you would only have to cut a three quarter inch dado and all would be well but plywood can often be slightly thinner or even slightly thicker than it's stated width so a more accurate method needs to be found to ensure that no matter the thickness of the material the fit will be snug and attractive.
Whilst it is true that you could simply measure the thickness of the boards and cut to that you are still faced with the problem of accurately machining the dado. This simple jig which is shown below is but one way of overcoming these problems. Simple to make it will work with almost any router and can be knocked up from offcuts in about 30 to 45 minutes.
The actual length of the jig is dependent upon the actual width of the material that you wish to dado but for most purposes I have found that 24 inches will suffice for all but the largest cabinet jobs.
First up is to prepare two lengths of 2 by 1 inch material. These have to be dead straight and true, any inaccuracy here will be transferred to any dadoes that you cut. I used poplar which is a stable timber and trued them up on the jointer but planing them by hand works equally well but carefully check them with a T square for true.
These are then screwed to a couple of pieces of plywood the same length as the timber that you just cut. I found that prefinished birch is ideal but use what you have. Make sure that the plywood is at least half the width of your router's base plate.
When the wood has been screwed to the plywood it should look like this.
The next stage is to chuck a router cutter into your router which can thinner than the width of the dado you wish to cut but not wider. I have found that a half inch cutter is ideal as I mostly work on three quarter thick boards, just remember that the cutter that you choose can be smaller than the material than that you will be cutting but not larger. This will become apparent as you become familiar with the process. With the jig clamped firmly down to some scrap make several passes and cut through the jig. Do not try to cut through the thickness of the ply in one go, you will have a cleaner cut with less tear out if you take it steady. Also make sure that the base of the router is held firmly against the timber screwed on fence. The width of the jig is now very accurately machined from the fence to the edge of the plywood. Do this by cutting through the plywood along both fences so you end up with two identical but separate pieces.
To cut a dado line up the edge of the ply with the marks on the board where you want your shelf or whatever to go and clamp the jig in place. Set the cutting depth of the router so that it removes between a quarter and three eighths of an inch for three quarter thick material (LESS if the material you are cutting into is thinner) Then keeping the base plate of the router hard against the jig cut the dado.
Without moving the first part of the jig squeeze a sample section of the material that is to fit into the dado between the section of the jig that you just used and the new section and clamp the second section in place. Remove the sample material and the first half of the jig and without altering the depth of cut on the router make a second pass.
TA DA. A perfect dado every time all ready for glue up.
When routing against a jig always move the router from right to left to avoid it snatching and running away from you.
Mark the jig with a sharpie to show which cutter you used as the jig will only work with that particular cutter and router combination. Using a different cutter will lead to inaccurate dadoes.