Just like with the plumb cut it is easier and more accurate to use the bottom rail as a guide to cut the arch rail. Here I use the bottom rail as a support for the swing arm to cut the curve in the arch rail. I start by lining up the two stiles with the rails so they are parallel. I have already marked a centerpoint on the bottom rail and positioned it 30” or so from the top of the top rail. Since I’m using the Milescraft circle guide it is very easy to reposition the swing arm so that the bottom or inside of the router bit cuts the maximum arch on the outside of the top rail and stiles.
I have found that even a shallow cut with the router puts significant pressure on the bit and the whole setup which raises the possibility of a slip which would then have to be fixed. Since I’m using 2” thick stock I decided not to get greedy with the router and settled on cutting a channel that I can then use to guide a flush trim bit on the router table to finish the cut (see below).
I usually cut the outside arch first then while the top and bottom are securely clamped, I reposition the swing arm to cut the inside or bottom arch with the outside or top of the router bit and move the stiles aside (actually I use one of them for support) and cut the inside arch. Remember you want to use the same pivot point for both arches so the inside radius will be a little steeper than the outside radius. See the blogs on Gate I or II or search ‘segmented arches’ to get whole enchilada.
I use a jigsaw to cut along the inside of the channel then trim the pieces up using a large flush trim bit. Works great and you get a smooth and accurate result. It is a little extra work than trying to cut through with the swing arm setup, but I think it is worth it.
-- tim hill www.newcalshop.com