Cutting styrene

Antinice wrote:

What sort of tools are available for cutting styrene sheets (1 to 2mm) accurately? Do you cut this stuff with the garden-variety hobby knife?

Use a steel rule/straight edge and the back of a #11 Xacto knife.   Measure and mark your cut.   Hold the straight edge tightly to your marks and draw the back of the point of the #11 blade along the straight edge.   The backside of the point plows out the plastic  instead of cutting (it can skip and/or raise furrows of plastic when the cutting side of the point is used).   Make several passes with increasing pressure.   Snap off the plastic at the cut.

Antinice wrote:

What tools are available for scribing a 60 degree angle, for example?  I would wager there is something more commonly used than a protractor.

I use a draftsman's 30-60-90 drafting triangle.   A 45-45-90 triangle is also made.    If you check the technical drawing section at artist supply shops you will find these as well as other useable tools (T-squares, french curves, flexible rulers to name but a few).

Antinice wrote:

How about an inside cut, like this: ------+       |    +--+    |    +--+       | ------+ My inclination would be to drill holes in the inside corners, scribe and snap as you described, then file to the line. Sorry for the basic questions, I just don't want to re-invent some 50-year-old technique.

What side are you trying to save, the one with the X or the other side of the line?

0+------+1
        |
    2+--+3
 X |
    4+--+5
        |
7+------+6

It just takes a little planning. 

If you want to save the side with the X, I think I'd cut from 1 to 6 and snap it off.  Then I'd cut from 2 to 3, 4 to 5 (cut from 2 toward 3 and 4 toward 5 so the knife doesn't slip inside the X-area.)  Make these cuts a little heavy.   Then  score from 2 to 4 and snap it off there. 

If you want to save the other side,  cut from 1 to 0, 3 to 2, 5 to 4, and 6 to 7.   Then cut your 2 to 4 line extending each side of the cut to the 0 to 1 and 6 to 7 lines.  Snap off that piece.  Then cut 1 ro 3 and 5 to 6 and snap off those pieces.

Either way, your cleanup with a sanding stick will be minimal