Well, most of us would rather eat sand than cut foamboard. I, like many others, had a very hard time getting a 90 degree edge when cutting circles.
Earlier I bought an X-Acto product touted for making up to 9/16" 90 or 45 degree cuts through foamboard. Pffff. It did not make cuts that deep, not even 1/2' deep. However, with practice, it did make 90 degree cuts, nice and straight, just not through 1/2" board.
The knife has double blades; we only need one, so I removed the left one so I could see my mark as I cut right-handed. I also switched the screw that holds the blade to the right side for a better view. One must concentrate on keeping the blade buried and the "foot" of the knife pressed firmly against the board as the cut is made. The foot is what keeps the blade at 90 degrees.
All that is fine and good, but it still didn't cut through the 1/2" board. I tried using a #2 X-Acto knife and a couple of other blades to finish the job, but always butchered the straight cut the first blade started.
Today I used a hot knife to finish the cut the special X-Acto blade started, just taking my time and going very slowly, allowing the heat to finish the cut. I turned the board over and made sure the paper was cut through, then popped the circle out. The end result was a nearly perfect 90 degree edge! And no scraggly pieces sticking out either! See the blurry close-up photo below.
http://www.dickblick.com/products/x-acto-board-cutter/#description
Wow thats great.. where do you purchase something like that.. I have been using a band saw (only used for cake boards.) works great just have to trim a bit of paper edges off not bad though and my BIL is making me a jig for circles.. that will be really nice.. but always striving for that nice clean edge.. Thanks for sharing
Congratulations! Thank you so much for sharing you technique!
Here's the hot knife http://www.dickblick.com/products/x-acto-soldering-iron-and-hot-knife/
bcake1960, both a hotknife and the special X-Acto knife can be bought at www.dickblick.com. There is a link to the X-Acto in the first post.
One can use the hotknife by itself, but I'm unable to get a 90 degree edge freehanded. Starting with the X-Acto really helped me with that.
Did it fail to cut the top layer of paper, or the bottom?
I cut the top with the X-Acto, and the hot knife cut almost all the way through the bottom layer. I just had to work loose a few tiny spots it missed.
Maybe the blade on your hot knife isn't large enough to cut through? The blade in mine is about the size of an X-Acto #2.
Or if it's not cutting through the top, then, maybe the hotknife doesn't get hot enough, or isn't sharp enough, or defective in some way? Even with a hot knife, I still have to push the blade through the paper to get started, but it's really very easy to get it through.
I'm baffled!
I have the worst time cutting foamboard!!!
I watch Sugarshack in her DVD's, and she just zips right through it. Pfffttt, I thought being a nurse had something to do with it. 
Anyway, my new mission is finding a hot knife at Michaels, and trying that. Has to be better then what I'm doing....
I traveled within a 35-mile radius of my hometown and could not find a frickin' hotknife in any store. I finally bought a Walnut-Hollow Versa-Tool at Joanns online. It was on sale and I had a coupon, so I got the thing for about $14 plus shipping. I like the Walnut-Hollow brand because their blades are large.
I already had another brand of hot tool,geared mostly toward sewing, and it had a blade attachment. It was useless. The blade was small like a #1 and it just did not get nearly as hot as the Walnut-Hollow. It didn't even get hot enough for the intended purpose, which was pressing fabric.
But anyway, I attribute the straight sides to both tools, the special X-Acto which starts the straight cut and the hotknife which finishes it.
Funny how methods and tools work great for some, not so great for others! I could swear I'm holding whatever implement straight and true, just sure I'll get a nice straight edge - seldom happens!! Same way I guess when I was a kid and saw that badmitton (sp?) birdie coming my way... I got it! I got it!! I GOT IT!!! swoosh... right by me...
Funny how methods and tools work great for some, not so great for others! I could swear I'm holding whatever implement straight and true, just sure I'll get a nice straight edge - seldom happens!! Same way I guess when I was a kid and saw that badmitton (sp?) birdie coming my way... I got it! I got it!! I GOT IT!!! swoosh... right by me...
Well, I don't worry about a nice straight edge, though. I cover my cake board with fondant all the way down the sides and wrap a ribbon or a boa around it 
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