Building a Circular Window Jamb

Constructing the circle from overlapping half-circle segments promotes stability and accuracy. December 24, 2012

I have been asked to make a circular window (exterior). The diameter of the stained glass (by others) is 36" and the wall thickness/jamb is approximately 10" deep. Constructed from mahogany with a painted finish. Fixed window with an integral stop.

I am fairly comfortable laminating radius work with thin strips and Unibond 800, but the full circle and the seam that will have to exist somewhere on the skin has me concerned. Also a little worried about the 10" depth. Some of you must have experience with this type of thing?

Forum Responses
(Architectural Woodworking Forum)
From contributor B:
Make 4 half circles at 5" plus each. Bring 2 of the half circles together to make a full circle. This will most likely be somewhat out of round, so take some bar clamps and pull it to round. Then take the third half circle and edge glue it to the completed full circle, staggering the joint 90 degrees. As soon as it is clamped in place, do the same with the final component. Again, use long clamps to pull it into round.

With the spring back from the two 5" wide layers being offset 90 degrees, the final 10" wide full round jamb will be round. We use this technique on a regular basis and it always produces a good final product.

Theoretically you could do a continuous wrap of 10" strips around a form to build out to jamb thickness, but I don't find that a reasonable solution.

From contributor M:
Alternate the seams on each strip, randomly.

From the original questioner:
Thanks - extremely helpful. For the composition of the 4 parts, do you recommend several layers of 1/8" +/- solid wood or skinning some type of kerf core material?

From contributor B:
Our typical 3/4" jamb is made up as follows: 3 layers of 1/8" resawn solid wood on the inside, then one layer of 1/4" bending ply and finally a backer layer of 1/8" solid wood.

You could do it anyway you wanted, though. We use this layup because the 3/8" of solid wood on the inside accommodates a typical 1/4" reveal to the casing while the 1/8" solid backer makes for a nice looking product. The 1/4" internal bending ply layer makes it a little less rigid for moving into place.