I had occasion to need 5 23” diameter circular pieces of plywood for some garden planters. The first one I cut freehand with my new Ryobi Saber Saw after drawing it on the wood with the old pencil and string trick. This sort of worked but it took longer than I would have liked and resulted in a ragged edge. I got to thinking – always dangerous – and remembered that the saw had come with an edge guide. I thought that maybe I could use that as the basis for a circle cutting jig. The saw and edge guide are shown here:
And here is the saw with the edge guide installed:
As you can see this is a very simple attachment – the bar of the edge guide slides through two slots in the base of the saw and is secured by a thumbscrew in one of the slots. Here’s a closeup:
So the circle cutting jig practically designed itself – just need a bar the right size to fit these slots with a pin in one end to act as a pivot. As these pieces of plywood were to have a center hole of about 5” diameter I could just drill a guide hole for the jig without causing any problems. For a disk without a center hole I suppose I can use a pivot block stuck to the surface with double stick tape – if I want to make a table top for instance.
I had hopes of finding a piece of aluminum the right size to slide into the slots on the saw so I measured the edge guide’s bar. It is about .338” x .125” in cross section – more than 5/16” and less than 3/8” wide but exactly 1/8” thick. Measuring the slots in the saw showed that 3/8” would be too wide and 5/16” would allow too much slop for my taste.
Here’s how I measured the bar – I know I know this is overkill but it’s easy enough and I have micrometers anyway.
So off to the metal store I went, hoping to find a piece of aluminum or maybe even stainless of about the right size. No luck, everything they had was cut into 12” pieces and I wanted to make the jig a little over 24” long so I could use it to cut 48” circles, the largest I can cut out of standard sheet goods. I had to go to the big box store and buy a piece of 1” x 1/8” x 36” aluminum for about $12. They had 1/2” x 1/8” but no 3/8” and certainly no 5/16” – no big surprise there. Took it home and ripped a piece .340” wide with my table saw – after taking off my Forrest blade and installing an old Freud blade – which is still good but it’s no Forrest. I first made a test cut on a piece of plywood I found on the shop floor – it was about 1/8” thick and about 1/2” wide and about 2 feet long. Procrastination in floor sweeping pays off again! This is the setup for the test cut:
If you cut metal on your table saw wear safety glasses AND a face shield AND stand off to the side – the chips coming off the blade are hot and sharp. I cut about 30” into the 36” piece of metal and stopped the saw, then used a hacksaw to cut off the piece as shown here. This reduced the kickback possibility as there was no cutoff piece to worry about and a single pushblock was enough to hold the metal in place until the blade stopped turning.
A little file work on the edges and the bar was ready to have a pin installed. I had some 1/8” hardened steel dowel pins left over from a previous life as a toolmakers’ apprentice so I used one of those – one inch long. Drilled a .116” hole in one end of the bar and pressed the pin into it using my bench vise – here’s the drilling setup:
And here’s the setup for pressing the pin into place – the nut is there to allow clearance for the pin – I wanted it all the way through the bar.
All that remained was to try out the new jig. I measured over 11.5” from two adjacent edges of a piece of plywood and drilled a pivot hole for the pin. Then I inserted the jig into the slots of the saw and measured 11.5” from the blade to the pin and tightened the thumbscrew. Here’s the initial setup with the saw in place ready to cut:
And here’s the result – a “perfect” circle – 23” around.
It took about fifteen minutes to cut four of these – much quicker and much more accurate than the freehand method. I found that this jig didn’t work so well for cutting the 5” diameter center holes – the blade didn’t want to track for some reason – I’ll have to investigate. I cut the center holes freehand.
Well that’s my invention for the week.