# Cutting a circle with my router?



## siouxdawgs0409 (Dec 5, 2009)

I have to cut a circle in a 3/4 inch piece of plywood. How would I make a simple jig to do this? My router is the Ridgid R2900.


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## gerrym526 (Dec 22, 2007)

What diameter circle?
If small, you'd make a template out of 1/4 masonite with the hole cut into it, and use a top bearing straight bit.
If large, you take off the baseplate of the router and replace with a plywood base with a pivot hole in it (think of a big compass with a router on the end)-with the distance from the pivot hole to the outside edge of a straight cutting bit equal to the diameter of the cirlle to be cut.
Hope this helps.
Gerry


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## sandt38 (Jun 9, 2010)

I use a scrap piece of luanne (some people recommend plexiglass, as you can see right through it) and screw it to the router
. I measure 1/2 of the necessary cutout diameter, and set it from the OUTSIDE edge of the bit to the marked point. Now, I drill a 1/4 inch hole through the luanne and the workpiece. Cut 1/2 an inch through the wood, place tape over the cut side, flip the board and cut the rest of the way through.


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## dbhost (Jul 20, 2009)

What the above listed lacks is adjustability. An inexpensive and functional setup is the Milescraft #1203 edge guide and circle cutting jig.

I have one for my Hitachi routers, and it works GREAT. A caveat though, The OE knobs are worthless and will let your jig slip. Get some 1/4" threaded knobs from Rockler to replace the stockers with and you'll be fine.


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## siouxdawgs0409 (Dec 5, 2009)

6 inch diameter hole.


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## sandt38 (Jun 9, 2010)

DB, he did ask for a *simple* jig


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## siouxdawgs0409 (Dec 5, 2009)

The issue here is the hole size. I am not sure that if I can do a 6 inch diameter hole with the type of jig sandt38 has made. I have a jigsaw that I could use by just using a good ole compass and draw a line and cut it out. But I figured with a jig on my router I could get a smoother, rounder hole with out all the sanding.


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## JJohnston (May 22, 2009)

The method and jig described by Gerry and shown by sandt38 are the same, and they will work perfectly for the hole you want. Here's a hole 6.25" in diameter in 3/4" plywood I made just last night for a downdraft table, using the commercial version of sandt38's jig as shown. It's adjustable in 1/8" increments. You'll need some kind of sacrificial pad under the ply (I use Homasote) to allow the bit to go all the way through, and to provide some friction so the center of the hole doesn't shift as you cut it free right at the end of the cut.


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## siouxdawgs0409 (Dec 5, 2009)

Are you using a straight bit to cut all the way through a 3/4 inch ply? In the picture shown of your commercial model, does the pivot shaft go through one of those many hole that are in the plate? From the photo they dont look like holes but I believe they need to be in order to work like I am thinking it works.


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## dbhost (Jul 20, 2009)

Huh, I guess you are right… Never mind…


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## JJohnston (May 22, 2009)

The jig is designed to work with a 1/4" straight bit. I plunge in several small increments, and go around several times. Yes, they are holes. Here's a better shot of the underside. You drill a 1/8" hole at the center, then stick the rod into the chosen hole in the jig and down into the work. As you can see, the holes are arranged by the whole diameter in inches along the bottom axis, with the fractional part on the vertical. The holes for each whole inch are arranged in a spiral instead of a circular arc, so each one is 1/32" farther from the bit than the last. For the smaller holes that fall inside the base plate, the pivot rod just bottoms out against the baseplate. Also shown is the slug that came out of this hole. And I guess I misspoke - it's adjustable in 1/16" increments. The numbers shown are the diameter, so the actual distance from the pivot to the far edge of the bit is half that.


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## sandt38 (Jun 9, 2010)

JJohnston has the real deal. The Jasper Jig is very nice, and one many speaker builders like to use. But TBH I like to go to my buddy's house and use his circle cutter drill bit for holes up to 7 inches. you can find one just as nice at Harbor Freight for a lot less. While the router jig, both my home made one and the Jasper jig, work well, they do leave a little room for error when you reach the end of the cut, which is why I recommended going 1/2 way through the workpiece, taping it and flipping it over and finishing the cut. The center pivot point will not move as much, or as abruptly as the portion you are resting your router in will not fall through to the floor, which can throw off the end of your cut. I guess I should add that when I cut the second 1/2 of the hole, I place it firmly, and flat on my workbench.

Of note, I am a speaker builder and I do cut a lot of holes. I just have a few jigs made for specific size subwoofers, and use the drill bit for my smaller ones, like midrange and smaller midbass drivers, and forstner bits for most tweeters, of course using the circle cutter bit for some odd sized tweets.

Just some food for thought.


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