Elliptical Jamb Extensions

      Pros puzzle out how to make extension jambs for elliptical windows. January 9, 2008

I need to make up some elliptical jamb extensions on a job in order to bring it flush to the face of the drywall. I asked the contractor why he didn't have the window company supply them and he said that it had to be done when the windows were ordered. Anyhow, I've done many arches and elliptical jambs in the past, but never one that was a full ellipse. They are approximately 35" X 23" and 5.5" deep. I generally vacuum bag my laminations, but in this case it would be tricky as each layer would have to be cut perfect for a seamless splice. I figured I'd use 2 layers of 3/8 bending ply with 3 layers of veneer, 2 on the outer sides and one in the middle. I could also use the form and do it layer by layer by hammer veneering it, although I've never used bending ply that way. I could go full laminated wood, but I'd rather not for the expense and time. Any advice would be welcome.

Forum Responses
(Architectural Woodworking Forum)
From contributor B:
We run our oval and full round jambs in two pieces and then bring the halves together. I think you'll find that is how most people do this task.

However, just a few weeks ago I decided that we should try a single seam full circle jamb. It was only about 16" in diameter by 8" deep, so seemed like a good one to try this on. Like you, we use a combination of solid wood and bending ply. On this jamb one edge was buried while the other was fully exposed. So, we only used a single 1/16" layer of mahogany on the inside of the jamb and the rest of the layers were bending ply. We capped the exposed edge with 1/8" thick segmented sections of mahogany glued to the edge and then flushed on the shaper.

We made a full round form and then carefully test fit an inner 1/16" mahogany layer to make a good tight single seam. That was put on the form with only one outer layer of bending ply glued to it. The bending ply was about 1/4" short of completing the circle, which allowed for it to be pulled really tight with strap clamps. A strip of plastic packing tape on the form where the 1/16" inner layer ends met kept glue squeeze out from sticking the mahogany to the form.

Then we added a second layer of bending ply after inserting a strip to fill the first layer of bending ply gap, again leaving the length short of completing the circle. When this dried we did the same with a final layer, making for a 3/4" thick jamb. Leaving the bending ply layers short is key to pulling the whole thing tight. We had almost no gapping between layers because of this.

The whole thing worked like a charm... albeit a bit time consuming. The only problem was that I forgot to take a photo before the pair of them got shipped out!

From the original questioner:
Thanks for the reply. It's funny, I slept on it and came to the two halves solution, then read your post. As to a seamless oval, your idea of leaving a gap and filling it later is something I hadn't thought of.

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