From WoodworkersZone WoodWiki
I make my own, I can custom cut them for the arch required. If no radius is specified I try to make the arch a 2" rise in the center across the width of the part. The jig here is representative of what I make. I use a router and a trammel base to cut the radii. I mark a center line on the jig before cutting the radii, This gives me a reference to locate parts accurately It also allows me to add stops to accurately locate multiple parts quickly. I try to make the jig at least 4" longer on each end for starting and stopping cuts. The top arch is the depth of the sticking (3/8" in this case) larger than the lower arch so the margin stays equal on the rail and the panel.
This is the piece to be cut in the jig.
Next I turn the jig over and mark a pencil line to roughly define the material to be removed.
I then remove the piece and cut on the waste side of the line using a bandsaw. A saber saw will work as well just be sure the cut is all on the waste side or the cut will not be smooth once complete
The cut made ready to install in the jig
Now we can shape the part
The attached side guides are set tightly to the part to me shaped and in this case an upper backing block is installed
Here is the jig ready to cut a part.
A pressure bar as a hold down
Disclaimer this is not the same cut as I am describing above and below these two pictures.
this is a friction fit setup the part has to be tapped into the jig. If there is any potential for movement with the part use toggle clamps or a pressure bar to hold the part.
The side view: the gaps are there to hold the part securely. It is very important that the system used is designed to promote your safety. If the part can move at all the potential for kickback is much greater. That is not a good thing in cuts like these.
Using a starter pin
You also should use a starter pin for your own safety. I prefer the pin to the outfeed side nearly always
It is so much safer to use a starter pin to give you a point of contact as you feed the jig and stock into the cutter head.
This helps keep the part from grabbing the knife and a potential kickback situation
This is the pin contacted by the jig and the ready to feed into the cutter against the bearing
into the cut: note I am moving away from the starter pin
it is not necessary to keep the jig in contact with the pin in fact it can be a problem if you try
partially cut note how it is flush with the jig under it Ignore the gap that is from a different operation done on this jig.
the finished cut