Mats Bengtsson More Advanced Wood Working and Do It Yourself

Using Leigh D4R to do mortises in small frames

Mats Bengtsson mib over the years

Using Leigh D4R to route mortises in small frames

The Leigh D4R is a fantastic jig for doing through dovetails, and through finger joints. But D4R is also a very good jig for doing mortises on small frames (using the M2 mortise and tenon template).

A small frame in pine 45 mm wide and 16 mm thick with a mortise in the bottom flat sideIf you are building frames and join them with tenon and mortises, you now and again need a mortise in the flat side of the frame (pointed to on the left side of the pine wood frame in the picture). The mortise shown is here on the flat side of the frame, and parallel to the bottom of the frame, square to the long edge. Placing a mortise in that way is not so easy as a mortise which runs along the long edge and is centered. Doing a mortise can be done with a number of tools.I find the Leigh D4R jig a very good solution for this specific type of mortise, the job becomes a breeze.

Choice of router bit size is among the first things to think of

First thing to do is to select the proper router bit size. This kind of mortise is not centered (not routed from both sides, assuring the mortise gets in the center). The best way to achieve a good result with this kind of mortise is thus to let the router bit become the width of the mortise, and only do one pass with the router per depth. With the router bit selected, the guide bit selection is easy.

The distance to the jig fingers decide the mortise width

Leigh D4R jig with a pine wood frame insterted and a mortise routed, indicating how to go about to adjust the mortise routingWith both router bit and guide bit selected, the distance needed from the Leigh guide finger (①), to the wanted routed mortise is pure mathematics (guide bit diameter minus router bit diameter, then divided by two).

I use that knowledge to calculate the distance I want the finger to be from the side of the frame, and then place the frame with the marking in place. Then I adjust the width of the fingers to stop the router at desired mortise width. However, many times measurement is not neccessary. I often want minimum 3 mm left towards the edges from the mortise. So if above formula indicates 3 mm, I can place the fingers just at the edge of the test bit.

As can be seen in the picture, I utilise the square side of the fingers, and the metal bracket they are fastened on as the edges the routing bit travels between. I do that to avoid using the plastic distances Leigh otherwise suggests using.

The Leigh jig is good at easily achieving repetitive results

To get the correct distance from the bottom, the Leigh mortise jig uses a stop bit (②). This can be a scrap piece of wood, but a straight scrap piece. The test bit (④) is inserted into the Leigh, following the guides (⑤), until it is stopped by the stop bit. Doing that, it is repeatedly placed at the same distance every time. Also, by feeling at the edge (⑥) where the test bit and stop bit meet at the left side, it is easy to control so the test bit is repeatedly inserted straight, thus placing the mortise at the same place every time.

The routing edge and stop bit decide the mortise distance from bottom

Now comes the time to adjust the routing edge to the proper distance from the stop bit. The guide bit on the router will travel along the Leigh mortise jig routing edge. Thus tthe distance of the mortise from the edge of the test bit is determined by where the routing edge (③) is. The adjustment of the mortise placement is done by the normal Leigh procedure, move the scale (⑦) forth or back. Do not interpret the scale, just use the markings as a guide for how far it is moved. Move the scale until the distance between the routing edge and the stop bit is the desired. Control the distance with a vernier caliper from the routing edge, and when satisfied, rout the first mortise on the test bit.

The tenon is used to tune the mortise distance from bottom

Next step is to route the tenon, and when that is done, to test the tenon in the mortise. When the tenon fits, it is time to fine tune the depth of the mortise (the tenon is always in the middle of the piece, due to routing it equally from both sides, and is thus not adjusted). The adjustment needed is determined by feeling the two pieces when they are joined:

  • If the piece with the tenon feels perfectly aligned with the bottom of the test bit, we are done.
  • If the piece with the tenon feels almost perfectly aligned, but the bottom of the test bit is slightly proud of the tenon piece, then we are also done (the difference is adjusted after assembly with a hand plane or sand paper).
  • If the bottom of the test bit is slightly lower than the piece with the tenon, then we need to adjust the mortise depth.
    • This is done by noting the current setting on the Leigh scale, and moving the scale slightly inwards, so the routing edge gets further from the bottom (remember to move the scale at both sides, they should have an identical setting). The error in the intial setup is not likely to be more than one millimeter.
    • Make the change, reroute the mortise and try again. Now, the fit of the tenon will be lose, but the only thing we want to test is if the bottom of the test bit is not perfectly aligned, or slightly proud to the piece with the tenon, and that can be done also with a lose fit, as long as you press the tenon piece from the bottom side of the test piece..

Finally route all mortises needed without further adjustments needed

With all settings ok, the Leigh shines:

  • Remove the test bit
  • Insert the bit to make a mortise in, pressing it towards the stop bit and the guides
  • Feel the edge between the bit to make the mortise in and the stop bit so they are perfectly aligned
  • Fasten the bit to make mortise in
  • Route
  • Remove and repeat until all mortises are done.


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