I've been working on making a ships ladder
for my granddaugther's fort. For the hand holds, I am making templates for using a circular saw for the straight parts of the cutouts and a router for the radii at the ends and for finishing the sides and edges. The router template needs to have a large radius for the router base, to create a template radius of 3.394" which will, in theory, create a radius of 1.5" at each end of the hand holds.
I was thrashing about trying to figure out how to make a circle cutting template for my router (Bosch 1617). I was in a woodworking store recently and saw some really cool circle cutting templates and normally I wouldn't hesitate to buy one. But, money is tighter right now than usual, so I refrained. The jigs I looked at in the store rely on using a 1/4" router bit for establishing the radius that the jig will cut. The jigs had series of radiating 1/16" holes allowing radii to be set at increments of 1/16". One reason I refrained from buying one is that I remembered I'd bought the Bosch edge/circle guide jig (RA1054) about the time I bought the router. I realized I didn't need to buy anything and digging it out when I got home proved that correct.
The Bosch jig has a cool little circle pivot point tab for inserting a 1/4" dia. rod located either in the workpiece or by using a provided suction cup with a 1/4" pin protruding. The radius I need puts the pivot tab too close to the router base to use the suction cup, so I needed to come up with a different method. Either way, I needed a way to set the radius of the router/jig. I reasoned that a 1/4" pin securely mounted in the pivot tab could be used along with calipers and a similar 1/4" pin in the router chuck for setting the radius, subracting half the pin diameter of one of the pins. I set about making a somewhat fancy one, but I realized that a partially threaded 1/4" bolt could do the job satisfactorily. I tried it and it does, but made the fancy one anyway.
Bolt Pivot Pin (head would need to be cut off and the shaft shortened)
Brass Pivot Pin
Bolt pivot mounted in the jig
Brass pivot mounted in the jig
Setting jig for 3.394" finished template radius.
It appears that the bolt would work every bit as well as the fancy brass pin, but I do like the brass one better. I haven't tried either one out yet and I expect to encounter some glitches, perhaps in how well the caliper does in setting the right radius. It will likely take some trial and error, sneaking up on the desired setting. Even with a bit of fiddling, if it works, I'll consider it a success. I expect to set it shy of my final desired dimension for roughing it out and then setting it for a final light pass.