Firstly… acknowledgements and thanks to Jlsmith for his response and link to Gary Katz's Sketchup Video on segmented arches. It was very helpful in my understanding of the topic of concentric circles, arches, and such.
I divided the arch cut in to three parts. The first was to cut the outside arch. Second was the inside arch. Third was to complete each cut using a flush trim bit to reference off the first cut for each. I set up the door frame on a large piece of MDF on my (new) assembly table and affixed a box down the centerline upon which I could mount a trammel arm. I cut the outer curve on a 27 1/2” radius and the inner curve on a 22” radius; the difference of 5 1/2” being the width of the stiles. Since I was using a 1/4” bit I had to compensate for the fact that, relative to the pivot, the outside curve was measured from the inside of the bit while the inner curve used the outside of the bit.
After making the outside cut I moved the stiles out of the way to make the inside cut. These cuts left me with a channel 1/4” wide and about an inch deep. I used a jigsaw to cut down the channel to free the piece. I then used a flush trim bit referencing off the previous cut to complete the arch. I clamped scrap pieces to the outside rails to prevent tearout.
Coming up next, cutting the mortises and tweaking the dryfit.
-- tim hill www.newcalshop.com