Boggs Side Chair Build #28: Rung Mortising Part 2, Rear rungs in the rear legs

Drilling the rear rung mortises into the rear legs is similar in many ways to drilling the front rung mortises into the front legs. The big difference is that, for the rear rung mortises, in addition to accounting for splay, I also have to account for the rotation of the rear legs by the slats. To achieve this I begin by placing both legs in the drilling jig with a single slat in one of the legs. Sighting down from a vertical framing square I can see if the slat is vertical. If not, I raise one end of the jig to rotate the legs until the slat is vertical. Next I repeat the same process with the slat in the opposite leg. Typically a vertical slat in one leg does not match a vertical slat in the opposite leg so the final adjustment before mortising is to split the difference resulting in the slats in both legs off vertical by the same amount.

 

use rise and run to calculate the drilling angle

To begin, I need to calculate the height of the riser block for the drilling jig to account for the splay in the rear legs. To do this I need to know the length of the two rear rungs and the distance between them (from the story stick). Then I can draw a trapezoid and record the angle using rise and run. This angle will be the splay (side-to-side angle) of the rear legs and for this chair it is 3/8″ rise over 4-5/8″ run. 

To more accurately draw this angle I can quadruple both dimensions from 3/8″ rise over 4-5/8″ run to 1-1/2″ rise over 18-1/2″ run. The run of my drilling jig for drilling the rear rung mortises into the rear legs is 12″ long. To raise the jig to the correct angle I simply need to measure the rise at a 12″ run, which is 1″.

 

the drilling jig

This is the same jig that I used for drilling the front rung mortises into the front legs, except that I’ve replaced the 5/8″ riser block with a new 1″ riser block to account for the greater amount of splay in the rear legs compared to the front legs. With the rear legs held in the jig I can accurately drill angled mortises into a pair of rear legs with a splay angle of 3/8″ rise over 4-5/8″ run. 

 

Rotation of the rear legs

The 1″ riser block on the rear leg drilling jig only accounts for the splay of the rear legs. But I also have to account for the rotation of the rear legs by the slats. The tenon ends of the slats arc in an angle of roughly 25°. When the slat tenons are placed into the slat mortises in the rear legs they rotate the legs 25°. In the drilling jig the goal is to rotate the rear legs so that, after assembly, the rear rungs will be parallel to an imaginary cord line drawn between the shoulders of the slat tenons.

 

Place the legs in the drilling jig

During this process the back, top of the legs are a reference surface that will contact the bench top. It’s important that the rounded backs of the legs be as consistent and similar to each other as possible. For that reason I wait to do the final shaping of the ears at the top of the rear legs until after the rear rung mortises have been drilled.

To begin I place the rear legs in the drilling jig as shown below — with the bottom of the legs extending roughly 4-5/8″ from the low end of the jig and the backs of the legs touching the bench top. Then I use a story stick to transfer the location of both rear rung mortises onto each leg. I always measure from the bottom of the leg. The story stick has five marks on it — three are labeled “S” and are for the side rung mortises, while two are labeled “F/R” for the front or rear rung mortises.

The initial setup is shown below. The process may seem overly complex but in practice is simple, efficient, and accurate. Paying particular attention to the following aspects of this setup assures accurately drilled rear rung mortises:

  • Place the long side of the drilling jig parallel to the long side of the bench top
  • Place the legs in the jig as shown. They must extend beyond the low end of the jig equally — 4-5/8″ is a good distance. This distance allows access to the mortise locations on either side of the hold-down and keeps the curved portion of the legs off the jig
  • The backs of the rear legs must contact the bench top
  • Place the bottom slat in the slat mortise on one leg. The slat must be positioned so that it accurately reflects the final position of the slat in the finished chair, meaning it shouldn’t be tilted towards the bottom or top of the leg
  • Place a strip of blue tape around the exposed slat tenon at the shoulder. This will provide a clear reference point for aligning the slat tenon shoulder to the framing square
  • Use a single color backdrop to make it easier to see the reference points on the slat, particularly if the area behind the backdrop is cluttered
  • Clamp a wooden hand screw to a framing square to hold the square vertically. Position the framing square as shown below, parallel to the short side of the bench top
  • The hold-down should be loose so that the legs can rotate 

Now I am ready to check the vertical alignment of the slat in the left leg. Typically the slat is not initially vertical and either leans to the left as shown below or leans to the right. I begin by aligning the framing square to one of the slat tenon shoulders. If the slat is leaning to the left as shown below, I align the framing square with the slat tenon shoulder at the point that it meets the leg and sight up. If the slat is leaning to the right, I align the framing square with the top slat tenon shoulder and sight down. Then I compare the edge of the framing square, which is vertical, to an imaginary cord line connecting the slat tenon shoulders. It’s obvious in this case that the slat is leaning to the left. 

Once I’ve determined if the slat is out of vertical towards the outside or towards the inside I can adjust the vertical alignment of the slat:

  • The backs of the rear legs must continue to stay in contact with the bench top
  • The hold-down must be loose to allow the legs to rotate as the drilling jig is raised
  • The legs should continue to extend beyond the low end of the jig equally
  • If the slat is out of vertical towards the outside (as shown in the illustration above), I slip a thin wedge under the low end of the jig. I continue sighting down the framing square towards the slat tenon shoulders and push the wedge towards the jig. As the wedge raises the low end of the jig, the legs will rotate in unison, which in turn rotates the slat. I stop when the imaginary cord line between the slat tenon shoulders is parallel to the edge of the framing square
  • If the slat is out of vertical towards the inside, I slip a thin wedge under the high end of the jig. I continue sighting down the framing square towards the slat tenon shoulders and push the wedge towards the jig. As the wedge raises the high end of the jig, the legs will rotate in unison, which in turn rotates the slat. I stop when the imaginary cord line between the slat tenon shoulders is parallel to the edge of the framing square

The same set-up as viewed from the front is shown below. Before moving onto to the next step:

  • Confirm that the backs of the legs are in contact with the bench top
  • Make sure that the legs extend beyond the low end of the jig an equal amount — 4-5/8″ is a good distance
  • Lock the legs into position by tightening the hold-down
  • Mark the position of the jig on the wedge

Next I evaluate the vertical alignment of the slat in the right leg as shown below:

  • Without moving the jig or the wedge, I carefully remove the slat from the left leg, move the blue tape around the slat tenon shoulder to the opposite tenon, and place the slat in the right leg
  • In this example the slat is leaning toward the inside so I align the framing square with the top slat tenon shoulder and sight down
  • The imaginary cord line between the slat tenon shoulders is not parallel with the framing square, with the gap at the bottom

Next adjust the vertical alignment of the slat in the right leg as shown below:

  • Loosen the hold-down so the legs can rotate
  • To rotate the slat, lower the low end of the jig by moving the wedge away from the jig
  • As the wedge moves away from the jig continue sighting down the framing square until the imaginary cord between the slat tenon shoulders is parallel to the framing square
  • Confirm that the backs of the rear legs are in contact with the bench top
  • Make sure that the legs extend beyond the low end of the jig an equal amount
  • Lock the legs in place by tightening the hold-down
  • Mark the new position of the jig on the wedge

Since the legs are always rotated in unison, adjusting the legs so that the slat in the right leg is vertical will force the slat in the left leg out of vertical alignment.

Now I need to find a compromise position that will result in both legs being out of vertical alignment an equal amount. To do this I draw a “split the difference” line on the wedge that is halfway between the lines for a vertical slat in the right leg and a vertical slat in the left leg.

To set the jig to the final position for mortising:

  • Loosen the hold-down so the legs can rotate freely
  • Move the wedge so that the low end of the jig rests on the “split the difference” line
  • Confirm that the backs of the rear legs are in contact with the bench top
  • Make sure that the legs extend beyond the low end of the jig an equal amount
  • Check the vertical alignment of the slat in the right leg. In this case there should be a gap at the bottom of the slat. I like to assign a measurement to the gap — in this case I would call it 1/4″ — so that I can remember the size of the gap
  • Tighten the hold-down to lock the legs in place
  • Without disturbing the position of the jig or wedge, carefully move the slat back to the left leg
  • Check the vertical alignment of the slat in the left leg. In this case there should be a gap at the top of the slat that also looks to be about 1/4″, which is equal to the gap in the right leg 
  • If the gap at the top of the left leg is equal to the gap at the bottom of the right leg the setup is complete and the legs are ready for mortising
  • If the gaps are unequal, start the entire process over

This is just one of many possible scenarios when rotating the legs into position for mortising, but the principle is always the same — rotate the legs in unison to adjust the rotation of the first leg so that the slat is vertical; rotate the legs in unison to adjust the rotation of the second leg so that the slat is vertical; and find a compromise “split the difference” position where both slats are out of vertical alignment by the same amount, one at the top and the other at the bottom.

Once I am happy with the rotation of the legs in the drilling jig I am ready to mark centers at each mortise location. Using a pencil lead, I rub along the top of both legs simultaneously in the area over each mortise position.

 

Drill the mortises

Before drilling the mortises I always check the drill press table for square since my drill press has a tendency to go out of square. This is particularly important if there are any square assemblies such as the front panel of the arm chair or rocker. If the drill press is out of square it will be impossible to get a square assembly — it will always result in a parallelogram. I place a 1/2″ diameter x 12″ long piece of drill rod in the chuck and use a combination square to check for square from several points around the drill rod. My drill press does not allow for fine adjustments to the table, so I tighten everything up and use a dead-blow to hammer one side or the other until the drill rod is square to the table.

I am now ready to drill the mortises. My favorite drill bit is the 5/8″ premium brad point available from Lee Valley. It is an expensive bit — around $28 at the time of this writing — but worth every penny. I am not just drilling a hole. I am drilling a precise mortise which is an important component of making a strong joint. This bit drills a clean mortise at exactly 5/8″ diameter. I only use it for chairmaking. With a Sharpie I put a mark on the bit at the full depth of the mortise, in this case 1-1/4″.

I always ask my students if we should take the wedge that we used to rotate the legs over to the drill press along with the drilling jig. This is somewhat of a trick question and only about half the time do students answer correctly. The wedge is only used to rotate the legs in unison in the jig while adjusting for vertical. If I kept the wedge under the jig while drilling the mortises it would change the angle of the legs relative to the drill press table, and consequently the splay angle of the rear legs would be wrong. The correct splay is built into the jig with the 1″ riser block. So, no wedge at the drill press.

It’s also very important that both ends of the drilling jig contact the drill press table. Most drill press tables are too small for this. An easy solution is to clamp a larger table to the existing drill press table. 

For each mortise I line up the tip of the brad point with the center mark. Next I slowly lower the bit until the mortise is fully scored. Finally I increase the downward speed and drill until the mortise is full depth on the low end of the leg. This ensures a minimum 1-1/4″ mortise depth. When drilling at an angle like this, the depth of the mortise at the high end of the leg will be greater.

In the next post I will explain how to drill the side rung mortises into an assembled front panel.

Jeff Lefkowitz

 


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