Arched Door Jamb Joinery
From contributor D:
With a 39" radius your maximum width for the entry will be 39", and that would be technically a roundtop, leaving only 19" or so for your active door in a double entry. Just an observation.
Anyway, we laminate the header from 1/8" veneers. We use a radius rf press, but used to use a male and female form and cold press them. Two parts, one for your jamb thickness (most architects spec 3/4"), and one for your filler (most all are 1/2"). The filler is then ripped to the spec'd width but left long and a 1/8" x 1/2" rabbet is routed around the edge. This will be the weatherstripping kerf.
Then we laminate the two together. We use a box joint where they meet at the springline, but if you don't have a CNC machine this could be pretty time consuming. I recommend it anyway, as you could do a decent job on the bandsaw or a perfect one on the router table, depending on how much time you want to spend. I've never used dowels, but they would be a quick method. We use Titebond 2 both in rf and while cold pressing. The pressures vary for species, and our library of tweaked programs that account for springback is vast, based on thousands of door and window jambs laminated over the years.
From contributor D:
The benefit to a box joint is no clamping. You just press them together once and shoot finishing nails through the ends to hold it in place while the glue dries. It is strong and withstands shipping and installation stresses well. We often ship units whole and prehung, and the extreme weight can result in handling mishaps - usually at this joint.
From contributor Z:
We do them the same way - leaving the tails long. I've seen another cabinet shop brick them up, but with the same rabbet-type joint at the tangent created by the longer tails.
From contributor C:
Leaving the glue up long after the radius is absolutely necessary. I have always made a two sided jig (inside radius and outside) or a wide ratcheting strap works as well the applied stop is laminated after in two stages and screwed from the back of the jamb with a kerf for weather strip built in.
As far as joining to the legs I miter the head joint and rabbet away the stop and carry the integral stop of the legs into the head rabbet. Stainless screws and good glue have always worked better than trying to dowel such a joint. The real trick is creating a jig that counters the stress of the glue up so the resulting radius head has net zero tension before joining. Make the door after the jamb if you don't feel confident in your jig.
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