Router Circle Cutting Jig

Circle Cutting Jig

I needed an 18" circle cut, what's the best way to do it? A router jig is my answer. I used scrap wood and a few screws.

Required:

• Router
• 3"x18" piece of scrap wood for the main base, size can vary
• 3"x3" scrap stop block for the router, size can vary
• 2"x2" corner brace, this provides a rigid edge to help guide the router
• (4) 1 1/2" coarse bugle head screws - 1 for the center point, 2 for the stop block, and 1 for the steel corner brace

Tool List:

• a saw if you're cutting scrap, I didn't have to cut anything
• ruler & pencil
• drill
• drill bits to pre-drill screw holes (3/32") and a drill bit the size of your router bit (1/4")
• router bit(s)

Introduction
I'm making a Thien baffle for my dust collection system. Being a round shape, I needed a way to cut circles. A jig saw or a band saw following a line wasn't going to be precise enough and I also need to cut an internal curved slot. With a few bits of scrap wood, I built a jig for my router.

Procedure
The long piece of scrap is the main pivot with a screw at one end. The smaller scrap block is the stop. The corner brace screwed to the stop block provides an edge for the router. Without it, the router will cut through the scrap block with the resistance of cutting. While I just tried to hold the router in place on the first few cuts, it's easier with a guide.

The pivot screw of the jig is screwed into the center of the piece to be cut. It doesn't have to screw in far. Just enough to hold it. Two screws hold the stop block in place to prevent it from pivoting.
The corner brace gives you an edge to hold the router against. Without it the router will want to walk as you cut, enlarging the hole. The corner brace frame helps prevent that.

I marked the center of the scrap long ways and drilled a hole for the main pivot screw.
For the router bit hole, you'll need to account for the width of the router bit in your measurement so you don't cut too wide or short.

When you start the cut, drill a pilot hole first, as close to the length of the cutting bit. I would lower the router in the hole, but make sure it didn't bottom out, before switching it on. When you lower the router into the hole blind as it's spinning, you need to hold it steady. If not it can eat into the hole and make it much larger. That first blind cut is just hitting your depth. The guides help steady the router and make the first blind cut precise.

Most of my cuts were with a 1/4" cutting bit. I marked my line from the center, offset a line 1/8" to account for the thickness of the 1/4" bit and drilled a 1/4" hole.
Pay attention to whether you are cutting inside or outside your lines.

I used the same piece of scrap for multiple holes/cuts. I marked my line and moved the stop block as needed. If I needed a new hole that was too close to an existing hole, it's easy to just pick a new center pivot. This jig also allows me to make circles exactly the same across multiple pieces of wood.

Finish
While I could toss this makeshift tool and make another as needed, I will keep it. Everything is in place and I can always replace the main arm once it gets too many holes in it.