One of the things that intimidates people new to chairmaking is the thought of drilling compound angled mortises into curved, bent chair legs. Although it may seem a difficult thing to figure out and execute, the principles used to determine each angle and the jigs used to drill each mortise are actually quite simple.
To determine drilling angles I use a method called rise and run. If you have ever built a roof or a stair case this term and method of work will be familiar to you. Using two known dimensions — the distance one end of a line is raised above a horizontal surface (rise) and a given distance (run) — I can figure out every angle in the chair and make drilling jigs to accurately drill each mortise.
Use rise and run to calculate drilling angles
One of the beauties of rise and run, as compared to using degrees, is that there are an infinite number of ways to express the same angle. For example 1″ rise over 12″ run, 1-1/2″ rise over 18″ run, and 2″ rise over 24″ run all describe the same angle, and are just three of an infinite number of ways to describe this angle.
Being able to express an angle in more than one way comes in useful when building jigs for holding parts for rung mortising — rise and run dimensions can be taken from an existing chair or from chair parts and expanded or contracted to any dimension suitable for a jig.
For instance, using the known length of the two front rungs and distance between them (from the story stick) 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 front legs and for this chair it is 1/4″ rise over 4-5/8″ run.
To more accurately draw this angle I can quadruple both dimensions from 1/4″ rise over 4-5/8″ run to 1″ rise over 18-1/2″ run. The run of my drilling jig for drilling the front rung mortises into the front 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 5/8″.
Build a drilling jig
Then I can build a jig with a 5/8″ rise over a 12″ run by simply attaching a 5/8″ riser block to one end of the jig. With the front legs held in the jig I can accurately drill angled mortises into a pair of front legs with a splay angle of 1/4″ rise over 4-5/8″ run.
A really nice thing about this jig is that if I need to drill at a different angle I can simply calculate the rise and run and make a new riser block. I will use this same jig for drilling the rear rung mortises into the rear legs. The splay of the rear legs is greater than the splay of the front legs so I will replace the riser block with one that is 1″ tall.
Place the legs in the drilling jig
If you remember from the discussion of grain orientation and wood movement in the previous blog post it is necessary to rotate the end grain of the front legs 45° relative to the front and side rungs. This locks in the relationship between the end grain of the front legs and the long grain of the front and side rungs and is an important component of making a strong joint.
Beyond the need for controlling grain orientation for a strong joint I like to pay attention to a couple of things that are primarily aesthetic. First is the direction of the end grain when viewed at the top of the front legs. Structurally, for a strong joint, the 45° angle of the end grain can point towards the front and sides or towards the middle of the seat.
I prefer Option 1 with the end grain pointing towards the front and sides. When looking at the tops of both legs at the same time this orientation frames the front of the seat and mimics the grain in a tree, as if you were putting the tree back together. It’s a subtle detail that I think is worth doing.
Once I have decided on Option 1 I have a second decision to make — to choose which of the tangential surfaces on each leg face outward at the front corners of the chair. The tangential surface is characterized by the cathedrals or chevrons created by flat sawn grain. I often prefer the pattern on one surface over the pattern on the opposite side. First I place this simple jig in the vise. Next I take one leg and choose the tangential face that I like best. Then I place the leg in the jig with the best tangential surface facing out and at a 45° angle relative to the front of the jig. I repeat this for the second leg.
This jig simulates the orientation of the two front legs in the assembled chair. The inside face of each leg is the surface that will receive the rung mortises and should be facing up in the drilling jig. I label each of these inside surfaces with a piece of blue tape so that when I transfer the legs to the drilling jig I know that the blue tape should be facing roughly up. I also label “Front” on one leg. Later on in the mortising process, when I am drilling the side rung mortises into the front panel assembly, I want to make sure I drill into the back of the assembly so I put the side of the assembly with the blue tape labeled “Front” down on the drill press table. All of these things ensure that my choices for best tangential faces will be on the outside front corners of the chair.
When I transfer the legs to the drilling jig I put the surface of each leg with the blue tape facing up. I also rotate the legs so that the growth rings are at a roughly 45° angle relative to the bench top.
Using a story stick I mark the location of both front rung mortises. 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.
At this point I am ready to lock the legs into position in the drilling jig. Before tightening the hold-down I make sure that all the mortise drilling locations will be accessible and I rotate the legs so that the end grain is exactly at a 45° angle relative to the bench top.
Finally I mark centers for each mortise. 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″.
It’s 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.