How to cut foamcore at a nice, right angle?

I usually cut the foam core so that my blade is not straight up and down, but rather off to a bit of angle, and the resulting cut is not square.

When I am cutting, I *think* I am cutting square, but I am really usually cutting at angle.

Any suggestions to ensure I cut square every time?

I have had good success with using one of those retractable extending craft knifes with the snap off blades.

I used a thick piece of baton wood as a guide to keep the knife at 90 degrees on the top of the material and then extended the blade for the required length of the thickness of the foam.

When I am cutting thin wood and foam, I'll glue/affix the blade to a block of wood which I know to be square. I then just have to keep the block flush to the surface. Works well enough.

Building on Caleb's advice above, I wonder if it would be feasible to rig a set of very small bolts (or a bolt and a tab) through an adequate size of wood block like this in order to affix X-Acto blades (using the existing hole in the blade shaft) temporarily but securely? (Or am I over-thinking and would be better off purchasing one of those pre-made cutters?)

In the short term: Foamcore Rabbet Cutter

In the longer term: learn to user your knife properly. Seriously. At the start of a session, get some spare foamcore, RELAX, and cut some strips. Examine them. Most people will tend to lean one way or the other. When you see how you're leaning you can endeavour to compensate. If you do this at the start of every session, for several sessions, you'll see an improvement as the compensation becomes an automatic thing.

Note that it's important to be sitting or standing comfortably, working at a convenient height, etc. If you're all hunched up, reaching and/or twisting your arm/wrist/hand at funny angles you won't hold the knife upright.

Last edited by AndySlater on December 6th, 2014; edited 1 time in total

I'm not a really great cutter with my knife, and I struggle with this same thing, but when you really relax and focus you can cut really straight. I think that the more this is practiced with a good, sharp knife (and polystyrene will really dull your blade) it'll become easier and easier.

I guess I wanted to think that as long as I lay a straight edge with a t square I'd suddenly have a nice straight cut without effort, but it still is an 'art skill' to do so.

One major thing that helped me out was to do a very, very light score along the straight edge with the knife first, making sure to keep it perpendicular to the floor and pressed against the straight edge. Then make a second cut a bit deeper, this helps keep the blade on the path without diverting off to the side or angling away.

If i'm really nervous about getting the cut straight I just keep making slightly deeper and deeper passes. If you go in really deep it is very easy to angle the blade away and extend the cut out away from the straight edge, or dig in at a side angle.

I know I kept seeing all these videos where people will just swipe their hand and they magically have a perfect cut in all sorts of things, saws, x-acto knives etc, but when I got to doing it myself I found it was anything but easy and requires a bit of practice for those clean results.