There are a number of different methods for cutting a circle, and depending on what diameter is required will often dictate which method is used.
For very large diameters (and we are talking in the order of 1m or larger) the router is a very useful tool.
Get Woodworking in Williamstown (2/ 239 Kororoit Creek Rd Williamstown VIC 3016, ph (03) 9399 1963) have come up with the Rout-A-circle ($27.50), which attaches to the fence of your router to make cutting very large circles easy.
The Rout-A-circle has instructions for attaching to the fence of Hitashi, Makita and Triton routers. Not being in a position to try it on the other two, I did give the Triton a go, and it was a breeze to attach, and cut the circle.
The largest circle it can manage is approximately 2600mm diameter, which isn’t too shabby. Given how easily it fitted to the Triton fence, you would almost swear it was made with the Triton in mind.
The original Triton fence (which attaches via the quick-release coach bolts) has a circle cutting capability, although it is very limited to about 300mm diameter. Pictured here is the triple-fluted Carb-i-tool spiral router bit which is excellent for this sort of operation.
One of the times it is very handy having more than one router – also means I have more than one fence, so I am able to upgrade one with the Rout-A-circle, and leave the other in its original configuration. Not that changing back and forth takes more than a few seconds.
The first step is to remove the non-required componentry. The bolt, washer and butterfly nut are reused to hold the Rout-A-circle in place.
The Rout-A-circle attaches very easily using that existing bolt and nut, and it fits neatly in the slot in the bottom of the router plate.
Here you can get a better idea of just how large a circle this jig allows. You can’t see it particularly well in this photo, but at the other end of the jig, there is a raised portion. It is this end that is used when attaching to the Makita or Hitashi routers.
Along the length of the jig, there are holes to take the supplied screw, which is screwed into the workpiece at the radius required. The Triton fence then allows this radius to be fine tuned if a hole isn’t perfectly located for the job.
This is the first pass – I took a number of passes mainly because I didn’t bother trimming away the excess material with a jigsaw or similar before starting the routing. If I was working in a more substantial material, trimming away the excess is definitely recommended.
This final shot shows the piece cut away. The block of wood underneath is there to allow the router bit to fully penetrate without cutting into the table. Given the table in this case is cast iron, that is rather advisable!