Thursday, May 21, 2020

Child's Sunday School Chair - Part 2: Design Considerations

Last week's post described the little Sunday School chair that I picked up.  I used that chair as a model to make my own version.
The original chair
There were some things that I changed for various reasons.  First, I don't have steam-bending capability, so the arched rail was out.  I decided to keep the three interior spindles and add a thicker spindle on each side of the three interior ones.

I kept the middle three spindles in about the same locations as on the original chair.  For the outside two, I wanted to space them out roughly equally along the back curve of the seat, and away from the curve of the back by about 5/8".  Sketchup helped a lot with this.

Up top of the five spindles is a crest rail.
Partial Sketchup model (stretcher rails in undercarriage not shown)
I used the original chair to estimate the curvature - about a 25" radius.  It is 1" thick and 2 1/8" tall.  For the length of the crest rail, I used the original chair for guidance - the widest part of the arched back was 13 1/2", so I wanted the crest rail on my chair to be something similar.  Looking at some chair pictures, the crest rail is commonly wider than the seat.  I settled on 14 1/2" (which became a little less after shaping).

To determine how far back the crest rail should be, I looked to the original chair.  The top of the arch is set back about 1 1/2" from a line vertically perpendicular to the seat at its back edge.  I used the same for my chair.  For a crest rail about 13-15" higher than the seat, this gives a seat-back angle of about 10-11°.

For the undercarriage, I wanted to keep close to the original.  But I thought the original's leg mortises were a little close to the edge of the seat.
Underside of original seat
In the picture above of the original seat, the front leg mortises are to the right; the rear leg mortises are at far left (the other two holes were through mortises for the arched back).  The nearest edge of the front leg mortises was only about 1/2"-5/8" from the side of the seat and about 7/8" from the front edge.  The original chair did not have through leg mortises (they were about 7/8" deep in a 1-1/16" thick seat), and maybe that was a factor in the builder's choice of mortise location.  Another factor could have been that the original chair seat was made of cherry or some other hardwood.
A 7/8" deep leg mortise on the original seat
My chair is being made from poplar - seat, legs, stretchers, spindles, crest - all poplar.  Don't ask - it's what I have around the shop.  And with a weaker wood, I wanted a bit more meat around the mortises so that the seat wouldn't blow apart when driving the wedges or when someone sits on the chair.

So I moved the tops of the front legs toward the center of the chair about 1/2" and towards the back by about 1/4".  I kept the bottoms of the legs in the same position to give the chair the same stability as the original and then used my resultant and sightline calculator to figure out the new rake and splay angles.  The original front legs had 2° rake and 4° splay and the new chair has 3.4° and 6.7°.
Leg mortise locations on underside of the chair
I was also concerned about having the front and rear legs along the same line of the seat's wood grain.  So I made sure the rear legs were not directly behind the front legs.  However, I also needed to take into account the two back posts (outermost two spindles).  These were going to have their own mortise holes in the seat.  I don't show it here, but they ended up being almost in line with the front leg mortises.  C'est la vie.

I ended up moving the tops of the rear legs 1" forward and 3/8" towards center (might have been 1-3/16" and 1/2" - didn't document it properly).  The original rear legs had 14° rake and 6° splay and the new chair has 18.6° and 8.2°.

Next up: the chair build begins!


  1. For a simple piece of furniture, a board with holes on it to stick pieces of wood in it, there are a lots of design and technical considerations in chairmaking. Never cease to amaze me. Were Art meet Science

    Bob, were we had 26c yesterday, frost warning overnite and high of 14 today...

    1. Agree with you, Bob. I tend to get bogged down with stuff like that - maybe I'd get more woodworking done if I didn't think so much ...

  2. Matt,

    Good start, will the M/T be tapered or straight? Poplar should work with no problems, I've made a couple of seats out of Poplar, it will split if you use tapered M/T's and go all Conan driving the leg home so do not believe those photos of folks with hammer or mallet above their head while setting legs.


    1. Ken, I used (it has been complete for a while now) tapered m&t. I got a nice fit with the tapered reamer and tenon cutter, so I didn't need to pound them in, but I did seat them completely. We don't get big humidity shifts here in CA, but I'll keep an eye on it over time to see if anything changes. Hopefully an expanding tenon won't split the seat. I expect it'll be fine.