Thread: Continuously Adjustable Circle Cutting Router Jig

Hello people,

I've been a lurker on these forums for a few years now, picking up many words of wisdom along the way, and thought it was time to give something back. So here goes...

About a year ago I set out to make a pair of speaker stands for my DIY home theatre rear speakers. What I had in mind was a couple of aluminium tubes (yeah, not wood but...) glued between a wooden base and wooden platter. Easy (or so I thought).

First problem: how to cut the holes for the tubes? I made it harder for myself by using one 100 mm and one 50 mm tube. Next problem: even if I could cut a 100 mm hole, it would be too snug to fit a 100 mm tube into it anyway. So I needed to be able to cut a 100.5 mm and 50.5 mm hole. Strangely, you can't buy hole cutters this size! I tried to make a trammel using MDF but found it was impossible (for me anyway) to drill a pivot hole for a brad accurately enough to give me exactly the right sized hole - given the tools at my disposal: a router, a circular saw, an electric drill and a pair of kitchen scissors.

Solution: the (unpatented) Continuously Adjustable Circle Cutting Router Jig...see attached pics.

The basic idea was to make the pivot hole in a piece of hardwood that slides in a dovetail-shaped channel in a piece of 12 mm MDF. The dovetail shape stops the slider from falling out. The pivot hole can get really close (about 2 mm) to the cutting bit, so the radius of the smallest circle you can cut with this jig is in theory the diameter of your cutter + 2 mm. For a 6.35 mm cutter, this gives a minimum radius of 8.35 mm or a diameter of 16.7 mm. The biggest radius you can cut is limited by the length of the jig - which you could make extendable with a bit of thought.

I attached a bit of scrap perspex to the slider and scribed it on the underside to act as a gauge. Then I bought the cheapest 30 cm rule I could find (about $4 I think) and recessed it into the top of the MDF along the slider channel. The position of the rule needs to be adjustable by the half the width of the biggest cutter you are likely to use - i.e. the length of the recess needs to be slightly longer than the length of the rule. My 30 cm rule had a handy hole in one end and so I could fix the rule in position using butterfly nut and a bolt in a slot.

To cut a circle you will need to drive a brad (I just used a 2 mm bullet head nail that I beheaded and rounded off with a file) into the centre of the piece being cut and then lower the jig onto it so that the brad slips into the pivot hole on the slider. Tighten the butterfly nut on the slider and start cutting. I found that hanging the router power cord from a rafter was helpful at this stage.

To calibrate the jig for a particular cutter, all you have to do is cut a test circle of any diameter (a shallow one will do) and then measure the outside diameter as accurately as possible. Divide that diameter by two to get the radius and then adjust the position of the rule (not the slider) until you can read off the same number through the perspex gauge. Once calibrated you can set the slider to the radius of any circle that you want to cut - as long as you use the same sized cutting bit.

As a bonus, the el-cheapo rule I bought had imperial measurements on the other side, so I can literally flip between metric and imperial measurements.

I gave the whole jig a couple of coats of white shellac and polished it with wet-n-dry. This gives a tough glass-like finish that slips easily with minimal scratching over the piece being cut.

Even after all that, it was still quite hard to get exactly 100.5 mm circle (given that the gauge had to be positioned within 0.25 mm for that) so a few experimental cuts were necessary. Some sort of vernier adjustment would be a nice improvement.

Hope this is useful to somebody. BTW, the last pic is the speaker stand base.

Cheers,
Andrew.