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I need to create a six inch hole in some plywood boards (making a cornhole game for my son).  I'm having a hard time, using jigsaw, rasp and sandpaper, getting it perfectly round.  I know I've read somewhere on this forum a process using a bearing guided router bit (mortising bit?), starting with a hole and then making it bigger by degrees using the bit.  If I can just get one good pattern then I can use it with a flush trim bit later.  Ideas?  I thought of making a center point and using my router like a circle cutting jig but since it would be inside the hole I'm concerned about what happens when I'm 3/4 of the way done and the center starts flexing.  Ideas?  

Thanks for your help guys!

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I've made these before and used a circle cutting jig for my router. Just take multiple passes and go slow on that last inch or so. I did mine with the plywood standing one end holding the router sideways. Probably not the safest but I had zero issues. If you want to buy a 6in hole saw like the previous poster said that would be easier. I just can't justify it unless I was making multiple sets. 

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To enlarge an existing hole by using a router, you need a bearing-guided rabbeing bit, followed by a flush trim bit. Dril the starter hole, the take a pass with the rabbeting bit. Switch to the flush trim, and run the bearing along the rabbeted edge to cut away the lip. Rinse and repeat until the desired diameter is achieved.

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That technique works much better with 2 routers !  As your woodworking addiction progresses the router collection grows....

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Tiods I agree that would be easiest but I don't think my benchtop drill press will have much success cutting with that large a saw. And I certainly wouldn't attempt it free hand. 

Steve that makes perfect sense the way you explain it!  I'm gonna try that. I've got two handheld routers so that will simplify the process. Thanks!

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5 hours ago, wtnhighlander said:

To enlarge an existing hole by using a router, you need a bearing-guided rabbeing bit, followed by a flush trim bit. Dril the starter hole, the take a pass with the rabbeting bit. Switch to the flush trim, and run the bearing along the rabbeted edge to cut away the lip. Rinse and repeat until the desired diameter is achieved.

Also, to make it easier to hit your final size, subtract the size of the rabbet from your final size repeatedly until you get to a drill bit size you do own.

For example, you want to make a 6" in hole, and your rabbeting bit makes a 3/8" rabbet.

6", 5 5/8,5 1/4, 4 7/8, 4 1/2, 4 1/8, 3 3/4,  3 3/8, 3, 2 5/8, 2 1/4, 1 7/8.  Hey I have a 1 7/8 forstner bit, so start by drilling that hole into your piece. 

Then using the rabbet bit, cut a little over half the thickness of the piece with the router.  Flip the piece over and do it again.  Keep flipping the piece over and rabbeting until the largest circle is your final size.   Then swap out your rabbeting bit for a flush trim bit, and get rid of the smaller circle. 

It's a little more time consuming than a hole saw, but if you don't have a hole saw, it's a lot cheaper.    And I find it's a bit therapeutic to be hogging out that much material on the router table.   And you get a dead nuts perfect circle. 

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Prov,

I have made a couple of corn hole boards and all have come out nicely with the 6 inch hole saw.  Since it has a pilot bit, you don't have to worry about much, but I made sure After I went half way through, I flipped it over and finished from the other side. I did it with a regular drill.  I think it cost me about 35-40 bucks for the hole saw.

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2 hours ago, Cliff said:

I got one of these. I highly recommend - if you think you'll use it often enough. So far I have used it once, but since it was like a 30" hole, it was well worth it.

 

*edit* Well, nevermind. I think it's 7" minimum cut. :(

https://www.amazon.com/gp/product/B00009K77C

312274QGQ7L.jpg.39d7f216868bd141e97f141a87a510ac.jpg

Rockler sells a smaller version I made one out of 1/4" BB 

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