Determining the correct angles for the spindle holes in the seat blank and the top crest is an important step that impacts the aesthetics, comfort, and strength of the rocking chair. I plan to make a total of 14 (a pair for each of my kids and stepkids) of these rocking chairs over the years and production repeatability is another goal.
I used a diagram from the Thos. Moser web site to get an idea of the possible angles of the holes in the seat blank. I made a scrap top crest with the deisred curve. I then enlisted my wife’s help and held some spindles in place to confirm the angles. I then committed and drilled the middle six holes in the blank and the scrap top crest. This provided a means to hold the top crest in place while I fiddled with the more complex outer holes. At this point, I felt confident that I had the correct angles for the seat blank holes and drilled them (with a drill press) in the first seat blank. I then used that blank to build my seat template.
Trace the outline of the seat blank onto some scrap plywood. Screw some scrap 8/4 hardwood onto the plywood around its perimeter. (The harder the wood the better since you will use this as a drilling guide for future projects.) Clamp the bottom of this assembly to the top of the first seat blank. Use the holes of the original seat blank as a guide and drill through them, from the bottom, into the template. After drilling all 20 holes, you have a jig/template that will exactly duplicate the holes in your original seat blank.
Trace and cut out a second seat blank. Clamp the drilling template to the top of the seat blank and a scrap piece of plywood to the bottom. The scrap of plywood will minimize slintering as your drill bit passes through the bottom of the new seat blank. Label your drilling template in some way to ensure you always orient the contact face of the jig correctly to the top of the new seat blank. All holes are drilled through the top of the jig/template into the top of the seat blank.
Two of the spindle holes land near the rear leg hole. Use a piece of tape to stop the hole prior to entering the rear leg hole.
Here is the second seat blank with 20 spindle holes drilled accurately with only a hand drill and a clever jig! Many of these holes are compound angles. It is easy to get these angles drilled backwards when using the drill press on the original blank. It’s real mind bender! However, when the jig is clamped to the blank correctly, subsequent seat blanks are flawless and capable of being drilled in 10 minutes!
The procedure to drill the top crest holes is very similar to the seat blank. I used 2×4 scrap for experimental top crests. I determined the correct spindle hole angles on the fourth prototype. I tried different approaches and finally was able to visualize good compound angles for the outermost spindles.
The spindle holes in the top crest don’t pass all the way through. Therefore, the holes are drilled from the bottom of the crest. The jig/template is created by clamping the bottom of the prototype to the top of the jig/template and drilling from the top of the prototype into the top of the jig.
Drilling through the prototype top crest into the jig/template. For final cuts into the top crest, the top of the jig will contact the bottom of the top crest. I label that surface “contact” on the jig to avoid confusion. I will tape the drill bit since all of the holes are stopped. The three outside holes are compound angles and if drilled too far, will penetrate the back of the top crest.
Here’s how the spindles lay in a well-fit top crest.
Here’s a rough idea of how the spindles fit together from the seat blank to the top crest. Please note that the bottom spindles in this picture have undersized 3/8” tenons sitting in 1/2” holes. The final product will have more tension and curves in the outer spindles. Forcing slight curvature in the outer spindles creates tension in the assembly and keeps the back from flexing excessively. This tension is created by slightly misaligning the spindle holes between the seat blank and the top crest. The tension and the resulting curves in the spindles don’t appear in this photo since the undersized tenons allow the base of the tenons to straighten out a bit. I must turn new spindles with 1/2” base tenons to see the final result!
-- Mark, Minnesota