paper model

Stop trying to cut it with one pass...  Make several light passes instead..

Hi ROSHELL; I believe that you do have a situation. The advice about the angle and newness of the blade is excellent. Thing is When you use that metal straightedge make sure the ruler or device does NOT have any substance fastened to the bottom. Use a small stainless steel rule.Hold it steady and go for it.   tankerbuilder

Use a punch set to take of the majority of the window first, then trim away the access. You can find inexpensive punch sets at most craft stores.

I did an post on just such a set. Round Disc Tool

MAke sure you're using a new blade.  Then cut on something hard (a sheet of glass, is usually best).  And use a straight edge (metal ruler) as a guide.

If all else fails, micro-mark carries a corner punch (usually used for making dollhouse windows).  It's a sharpened, beveled punch, with a 90 degree angle - made specifically for cutting out windows.

-Fred

If the paper is tearing with an Xacto knife, then either the blade is dull, or you are using too high of an attack angle. Try lowering the blade so that is is around 22 degrees above the horizontal. You want to shear the paper, not just cut it. A higher angle will pile up the paper fibers before it shears, and that's when the tearing starts.

Is it built yet? Must be or else you'd slice and dice. Never underestimate the use of a good pair of scissors, which have the advantage of self-anviling. I use them a lot on card models. Since you don't have interiors, right?, I would not bother with smaller than 1/4" square, and no they'll never be "perfect". Good luck!