How to Cut Square/triangle Holes in ABS Project Boxes




Hi All,

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

You will need the Gascat 75P gas soldering iron OR something similar or with similar tips. This kit has 5 tips. A soldering tip, a hot knife tip, a flame tip, a hot air tip and a deflector tip. The particular tip we are interested in is the hot knife tip.

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

Upon viewing the hot knife tip you can see that it is wedge shaped. This isnt quite what we need. By using the grinding tools however we can grind down the wedge as shown in the first photograph until we have a flat blade. We keep the pointy end of the wedge as it is useful when making the first insertions into the plastic.

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

To set up the plastic you will need to score in using a stanley knife or other sharp implement the shape to be cut. If the plastic is too small to plot out the square using a ruler and knife you can print the diagram on a sheet of paper to size, tape it to the box in the right place and then drill out the corners and if you can do it score out the lines through the paper mask.

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.



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    10 Discussions


    3 years ago on Step 3

    draw your rectangle. drill four holes in each corner of the rectangle.

    use coping saw/hacksaw/rotary tool/cutters to cut straight lines between the holes... job done.. square hole. No special tool, no toxic fumes, no grinding down molten plastic.


    7 years ago on Step 3

    Hello, once I've seen this image in the net.Maybe it helps.


    8 years ago on Step 3

    I've also been looking for a good solution for square holes. Something just occurred to me. What if a square (hollow center) hot air rework nozzle was sharpened at the (inner) edges and was heated up and pressed through the plastic. The square would need to be the exact size or smaller than the desired hole. A method of quickly removing the nozzle may be needed so air isn't trying to blow onto the center of the hole as rework air cannot be immediately stopped.
    I'm in the market for a rework machine so this may be interesting to try.

    It would be nice if there were some pictures of the actual box being cut.


    12 years ago

    I usually just drill a hole and file the corners down. But that's a good technique if I ever need to make a slot. (although I'd probably just torch an x-acto knife blade and melt through)

    1 reply

    11 years ago on Introduction

    Is it possible to cut the exact right shape holes by heating up the (metal) part you want to insert? I'm thinking D-sub connectors (plastic removed)


    11 years ago on Step 3

    Thanks for the post! This is way better than my technique of drilling big gross holes and filing them into the shapes I need.


    12 years ago

    I've always used a combination of these three: (1) A drill press, possibly with a router bit, (2) A coping saw, and (3) A nibbling tool. This has always worked pretty well for me.


    12 years ago

    I've found my gas torch to be quite useful. I hadn't though about filing it down, but it is obivious when you think about it. If doing a large hole, start your cut in the middle, and move to the edge, then go around. It leaves a better finish.


    12 years ago

    nice lightbox