On Sat, 21 Feb 2009 12:06:07 -0500, you wrote:

>I'm trying to find a way to cut neat rectangular holes in small plastic box 
>lids for mounting LCD displays for hobby work.
>
>In the past I've drilled a series of small holes, then used a coping saw to 
>cut out the piece. Then file the edges smooth. This is not very neat - the 
>edges are never straight lines. 
>
>When I do something like this in woodworking, I use my router to make the 
>rectangular hole, then clean up the corners. But a router on a small (e.g. 
>4"x6" box) piece of plastic isn't easy.
>
>I do have a Dremel tool, but I don't have the router attachment for it - that 
>may be an option, since it is a lot smaller.
>
>What do others out there do? Using a commercial service for a one-of box is 
>probably rather expensive.
>
>Any ideas?
>
>Thanks
>
>Larry

With care it can be done very  neatly, but it does take patience.  
Most important is to mark accurately - a super-fine point permanent marker is good as it doesn't rub
off, but can be taken off with solvent afterwards.
For black cases, carefully put a layer of brown parcel tape on the panel to ease marking and reduce
risk of scratching.

Drill the corners (well inside the line)
 Fretsaw (like a coping saw but finer blades, use a lot of tension) or Dremel-type jigsaw as close
as you dare to the line. wIth practice this can be <1mm). Use a fine wood blade, essential that it
is very sharp.
If using a jigsaw take care to avoid the sole plate scratching - some tape on the sole or the box is
good. 

Here's the important bit - use the widest flat file you can get to finish - the wider the file the
easier it is to get a straight edge. Cutting at an angle also helps lengthen the cut. 
MUST be nice and sharp to cut cleanly, and fairly fine (A few teeth in the space of the plastic
thickness). You should grind off the sides of the file so it doesn't matter if you hit the sides
when doing the corners. This makes a big difference!
Do the long sides first, then the short ones, and then finish the corners with a small square file. 
Use a sharp knife or fingernail  to clean the last bit of file swarf off the edges. 

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