Here's a really poor instructable on how to make a circle jig!! (i forgot take pictures during the making process!)
Ive made up some CAD drawings, showing the steps in a little more detail... check them out!!
Step 1: Lets Get Cracking!
So i'll start by apologising for the lack of pictures! but this is my first go, so i'll let myself off this one....
First of all, figure out what material you wish to use to make your jig, i went for 18mm MDF.
Start out by working out a decent length you're going to be able to work with. I went for about 600mm.
Now you need to get a width... i sat my router onto the MDF and drew around it, so i could see how wide i needed my jig. (around 200mm).
So i jumped onto the table saw (not literally) and ripped down my MDF to 600 x 200mm.
On the opposite end to where i had sat the router, i paced a can of WD40 as it was a pretty decent size circle in comparison to my router.
so now you've got you're set sizes for either end.
Using a straight edge, i transferred a line between the two circles, giving me a cam shaped image on my workplace. kinda like this --> o O (just imagine the lines between the two)
I didn't have an angle sled for my table saw at the time (that'll be my next instructable) so i just kinda winged it along the pencil lines!
Now that i've got my kid cut out, i took the face plate off of my router, and placed it where the router would sit in the jig. Transferring the screw holes onto the jig so that it'll screw into the existing holes on my router...
as for the other end, as of yet i haven't finished it...
with regards to making out etc. i wasn't keen on the slide scale that is commonly used for measuring the size of the circle to be cut. so I've got some brass collets to press into the jig, so that I've got a scale of average sizes i can just pin into and cut!
From the second picture you can see, that for the mean time, i just used a small drill bit, drilled through the jig, and into the new workpiece, and left it in.
I'll update this intstructable once i've pressed in my brass inserts for constant sizes!.