This is a mini-instructable as it will be useful for some other instructables that I will post which require square holes in ABS! I thought I would post how I do it - as it took me a VERY long time to find a satisfactory solution. I am still experimenting with this method but so far it seems to be the fastest way to cut into plastic for making square holes. I have found so far that the smaller the hole the narrow your blade needs to be to avoid plastic bubbling over the line - with larger holes it is not so much of an issue. ( Thanks to Iliah Borg for pointing me in this direction )
Step 1: Tools Needed
The other item you will need is some kind of grinding kit. I recommend the Rotary Craft mini rotary tool. This contains a selection of grinding tools. I bought a grinding/cutting addon for mine. This is actually what I attempted to use to cut the square holes to start with but it didnt work out - very hard to control the circular blades and pretty useless for small holes. Do a search on ebay for them.
The grinder I use is shown below. You can see two tips. The one attached to the rotary tool I used to polish up the metal after grinding. The pink tool is what I used for grinding - worked very well.
Step 2: The Hot Knife Tip
The first tip shown here is half way done. The next two show the complete(ish) job.
This is good for large hole but for small ones you will need to grind down the blade thinner.
Step 3: Setting Up the Plastic
I actually recommend using a very small diameter drill bit to drill out markers for the corners.
Once you have the shape in the plastic fire up the gascat with the new tip. You will need to practice quite a few times. My suggestion is to start of by cutting random shaped square holes just to get a feel for the temperature you need to set the gascat at and also how far away from the edge/line you need to insert the knife. The thinner you can get your blade the better - even thinner than the one shown in the image. If the tip is too thick then a lot of excess plastic will bubble up which will need sanding away at a later point.
The knife should go through the plastic like a knife through butter so be carefull how much pressure you apply - several times in my first attempt then knife went through too far!
Another point is make sure you keep the flat unground side towards your line - this will result in a smoother/straighter line. Unless of course your grinding is perfect :)
After the hole is made you will need to do some small amount of sanding to get any plastic which has bubbled down smooth.
WARNING - make sure you do this in a well ventilated place - the fumes are toxic and can be harmful if too much is inhaled.
To make the hole look even better cut out a small rubber sleeve for the hole. Get a peice of thin rubber material and cut out a square slightly larger than your hole ( this will cover any imperfections). Then cut out the centre so whatever it is which want to poke through can. I use this method for holes for cat5 keystones. I actually cut out the centre of the rubber square in such a way that it leaves a small sleeve around the keystone. ( More on this in another instructable! )
I am still experimenting with mine attempting to get the perfect finish but it still seems to be the fastest way to cut plastic. The gascat has been a boon in more than one way for me - a decent temperature controllable soldering iron and a way to cut plastic.
I have a few Gascats for sale at my ebay store.