The Historic District

Have just the simple fir window and door casings - flat 3/4 thick, 5 inches wide with rounded edges. I cannot for the life of me figure out which size of roundover bit to get - what the radius is of the curve? How are you supposed tp know? If I buy it then it would be easy to see if it fits the curve of my existing trim, but would rather know before I order. Looking at it and putting a ruler up to it, it could be anywhere from 1/8 to 3/8 depending on where Im looking. Was wondering if anyone out there had similar trim and could go measure theirs? Maybe I'll just take an average of all measurements and go with that.

THANKS!!!!

Buy one of these then draw a quarter circle on a piece of paper then cut it out. Try different sizes against the trim till you find the right one.

http://www.michaels.com/10012623.html

a 1/4" roundover bit is what I use for that, it should be right. if you have a router you can probably get a set of bits with all the profiles quite cheaply anyway. just check the shank size to make sure they fit your router.
you need one with the guide bearing , most should have one unless you are using a router table with a fence.

I keep a spare router mounted under the side table of my tablesaw usually with a 1/4" quarter round in it. and whenever I do roundovers I use that along with the fence already on the saw. unless it is something permanently attached or really long boards. the fence just helps if there is some roughness along the edge of the board but even if you have a little of that some sandpaper will usually hide it.