Cutting a large circle

[Dave Novak]

Good morning, hope everyone had a joyous Christmas.

I need to cut a 42" circle in some 6/4 red oak. My plan is to screw my plunge router to a long piece of plywood, attach the plywood to the center of the oak roughly 21" from the bit and cut roughly a 1/4" deep at a time with a spiral bit. Any flaws in this plan or suggestions/modifications based on your experience?

[Bill White]

That's the way I'd do it. There are a lot of fixtures like you described for sale in the catalogs, but I made mine. Works well and, since it was made from scrap 1/4" fiber core ply, the price was right.
Bill

[Ben Hatcher]

That's the way I've done it. The grain changes direction on you a few times on a cut like this. You might want to do your cut in segments to prevent chip out and the router from climbing into the cut.

[Prashun Patel]

Tear out is a big risk here. The spiral bit will help with this, but still, take very shallow passes each time. Move swiftly (as swiftly as is comfortable and controllable) to avoid burning.

I've also done this by bandsawing and flush trimming (when the curve can't be cut with a router/trammel). The problem - should you choose to do it that way - is cutting close enough to the line so you're not trimming too much off in the final pass.

Also, beware of climb cuts with a trammel. The router can leap out of control on you.

[Jeff Heath]

As stated by others, switching grain is going to be your enemy while doing it this way. I use my bandsaw to do this with a full support table and a pinned jig. You can simply use a 1/8" steel pin in a jig 21" away from the edge of your glue-up, and start spinning. Take your time with a sharp blade, and you can achieve a very clean and accurate cut. No issues with an out-0f-control ticked off router spinning at 21,000 rpm's, either.

Jeff

[Rick Christopherson]

You will have difficulty with this cut. You're going to need to take shallow passes and split it up in 2 parts. Start in either the 1-o'clock or 7-o'clock quadrants labeled "Either" in the image below. Move backward into the climb-cut zone, and then move forward into the push-cut zone. Change your depth and repeat. Then move to the other (opposite) quadrant and do it again.

[Mike Cutler]

Do you have a bandsaw, or a jigsaw?
My first step would be to cut that circle with a bandasaw, or skill saw, and get it "close to the line". Then I'd do the router on the trammel and just clean up the edge. My preferred method would be a full sized template, but that's me.
The sprial flutes will help you avoid tearout, but you will still need to follow Rick's advice.
The biggest radii I recall doing was 37", and it was time consuming.That's a lot of material.

[Dave Novak]

Thanks for the thoughtful advice - You've scared me quite a bit. I'd love to cut this on my band saw but I'd have to build an awfully big auxiliary table first. Spiral upcut or downcut?

[Greg Portland]

You will have difficulty with this cut. You're going to need to take shallow passes and split it up in 2 parts. Start in either the 1-o'clock or 7-o'clock quadrants labeled "Either" in the image below. Move backward into the climb-cut zone, and then move forward into the push-cut zone. Change your depth and repeat. Then move to the other (opposite) quadrant and do it again.

As Rick mentions, the cut direction will cause problems. I've cut some large circles like this and use my Bosch jigsaw + a good blade. I cut the saw into the trammel (to act as a zero clearance insert) & then mount it. I then measure the distance I need & make the center point (on the trammel). Using this method gives me a clean edge that's ready to sand.

[Chris Tsutsui]

Just an idea to throw in there but can you make a 3/4" MDF template table first and have that table a couple inches undersize. Then you can use double sided tape to attach it to your top and then use a router using a combination of climb/push?

One time I had to cut arches in 2.5" thick green douglas fir. No matter which jigsaw I used and a brand new blade there was too much deflection and the cut was painfully slow.

I used scraps to build a large trammel / compass to draw a pencil line of the arch on 3/4" MDF template sheet. Then I used a decent quality jigsaw to cut very close to the pencil line. Then you can use the cutoff to make a simple sanding block and you can sand the inner arch smooth.

Finally I used that arch as a template with a 3.25HP porter cable fixed base router and it took 2 passes with a 2.5" long 1/2" diameter straight bit.

I cut 32 arches this way and it turned out fine.

[Richard Coers]

Way too many warnings on this. Use a brand new quality bit, and it will go fine. I hate downcuts, unless I'm cutting melamine. Saber saw first? Yeah, if you want to have a nice wavy cut that is not perpendicular to the surface. The only time I get creative, is if the material is thicker than the cutter length.

[Prashun Patel]

Dont be scared. Just do it and beware of the tearout zones. You will be fine.

[Bruce Wrenn]

The band saw is my first choice. Using a router this is how I would do it. I would route one pass about 1/4" deep. Then using jig saw, cutting along OUTSIDE of routed groove, remove waste. Switch to a pattern trim bit (1/2" cutter length) route one pass, lower bit and do the same again. Then for third pass, turn top over and switch to flush trim bit to finish job.

[Rick Fisher]

I just did a 31" circle yesterday in some BB to use as a template. Basically did what your describing but did it in a couple of passes.

I used a 1/2" Spiral drill bit .. If the router is secure, its pretty straight forward.

[John Piwaron]

I do this all the time - BUT:

I make an MDF template first. Long ago I made a couple of trammels to guide a router in a circle. I use the trammel to make an accurate template in MDF.

Then I trace that template onto the work with a pencil. I rough cut with a jig saw or bandsaw to somewhere near the line. Then use good double stick tape to attach it to the work. After that I guide my router with a ball bearing guided bit around the template. Climb cutting or push cutting as needed. You'll know what's needed. Look at the grain. Are you "petting the cat" the right way? Or the wrong way? If the wrong way, the cat might turn and hiss at you.

Right now my template cutters are all straight bits, but I see that Whiteside has recently released a set of awesome looking solid carbide ball bearing guided spirals for this use. It won't be long and one or more of them will be mine. They're making them in 3 styles. Woodcraft is selling them, there's probably others too.