Best way to cut mounting squares out of plastic project boxes?

Question

My electronic kit-building to date has included pre-drilled kit boxes for mounting.

My latest purchase, however, has a plastic project box, and no pre-made cutouts.

I need a rectangular hole for a small LCD screen - and I don't want to mess it up!

How best to cut the hole? Tools to use?

Answer 1

If the plastic isn't too thick, then you could try this:

1. Cover the surface of the plastic with masking tape, so you don't scratch it.
2. Mark out the rectangle you want to cut out.
3. Using a very sharp knife, score the edges of the rectangle against the side of a metal ruler.
4. Cut deeper and deeper grooves with the knife, until you can eventually push out the rectangle.
5. File any rough edges, and remove the tape.

Answer 2

Drill a 6mm hole and use a step drill to open it out to at least 10mm. A step drill is the best way to make large holes in thin materials, it doesn't grab as much as a large twist drill will.

Then use a 'hand nibbler' to open it out to rectangular. You can drill more large holes with the step drill to reduce the amount of nibbling required.

I made the cutouts for this project with a nibbler:

Answer 3

The question is old, but I also had a lot of trouble to get this right, so it's worth answering.

1. Draw the rectangle with a pencil or a thin felt tip pen. You can clean it later. I sometimes draw the whole panel in milimeter graph paper and tape it on the plastic box, so things are perfectly aligned and distributed. Make sure you got the measurements right.

2. With a 1mm or smaller drill bit (you probably have one for drilling PCBs), make a hole on each corner of the rectangle. Using a ruler and the holes as a guide, draw an X across the rectangle, and make a 1mm hole on the center, where the lines intersect.

3. With a larger drill bit, and using the 1mm hole as a guide, make the central hole big enough to fit a jigsaw blade.

4. Using the jigsaw, carefully cut along the X lines, until you reach the 1mm holes on the corners. This will leave 4 triangles.

5. With a very sharp knife (I use a box cutter) and a ruler, carefully make an incision along the edges of the rectangle, using the holes on the corners as a guide. You might want to always cut from the corner to the center, half at a time, to avoid damaging the opposite corner. Make the cut deeper and deeper, while pushing the triangle, until you can break it off. If you can, do it on both sides, it gives a cleaner cut.

6. Finish with the knife and sandpaper.

It gives perfect cuts every time.

Answer 4

I use an 1/8th drill bit after drawing a rectangular template on the box. Drill holes close to each other along the outline of the rectangle. After all the holes are drilled, I cut each hole together and then push out the cut rectangle. Then I file the rough edges slowly and carefully, checking if the panel fits and where to file more. Once the panel fits you are done, as long as the line is all flat and smooth.

Answer 5

I've been battling this problem for 50 years some of the time as a professional engineer building prototypes.
It depends on the plastic the box is made of and if the panel is flat. The best approach I have found for many plastics is a laser cutter it produces precision straight sided cuts and a polished edge finish with perfect square corners. The next best choice is a vertical mill. Don't get carried away with the cut rate as many plastics will melt rather than cut. A two flute cutter seems to work best. Two passes produces the best finish; clockwise to rough out the hole, then anticlockwise (climb milling) to give an accurate quality edge. The rounded corners can then be filed out. If you have a 3D printer, just print an enclosure. Then it will make your project look less DIY and you can include so many extra features. If you don't have access to these machines: mask, mark up and drill the corners carefully. Then stitch drill all the way round leaving a small margin to file smooth later. I find if the drill holes leave a small web it can be cut or broken out depending on the plastic. I have tried the repeated knife cutting method but have found it slow and all too easy to slip and scratch the panel so cut from the back if you can.

Answer 6

This question is old but I wanted to revisit it in 2024 as affordability has changed and this is still a valuable and relevant query.

The best way to get perfect cuts is to get an entry level CNC machine; the generic '3018' models cost less than $150 now while offering sub-millimeter precision; they handle plastic project boxes easily. Use a one- or two-flute straight or upcutting endmill bit, 1/16" - 1/8" (1.5mm - 3.0mm) diameter. Fewer flutes means less frictional heating. A fast feed speed (1000-2000 mm/min) is best for soft plastic for the same reason, avoiding friction heating. This assumes a typical 8000-12000 rpm spindle on the machine. Take off 1mm of depth at a time.

Adhere the part to be cut to a spoilboard or piece of scrap wood using a sandwich of CA glue between two pieces of wide masking tape. Apply tape to the scrap board and to the project box, then apply CA glue to one tape and accelerator (even just a water mist) to the other, then press them firmly together for 30ish seconds following the directions for your specific tube of glue. Clamp the scrap board down, with the clamps well out of the way of the CNC machine's toolpath. When finished cutting, just peel the tape off the board and then from the project box.

On the software side, it is simple to do layouts using any vector imaging tool (Inkscape) or with the free-tier of Instructables' Easel, and simple to run the gcode using OpenBuilds Control or Universal GCode Sender. Nearly no expertise is required other than being able to use a rectangle drawing tool and typing in the dimensions.

When this question was younger, end mills were financially unjustified if you only wanted to cut project boxes. As prices fell, I expected the bottom tier of machines to be unsuitable, but eventually discovered they were in fact remarkably capable. Now I wish I could have saved myself the hassle of all the other options I tried, every one of which I found unsuitable:

• Razor scribing - using a plastic scribing tool or a scalpel/hobby/xacto knife repeatedly cutting an ever deepening groove. Hard to make corrections, difficult to get straight lines without a hard straightedge involved, basically impossible to do curves, risk of marring the exterior surface if the blade slips and skids out of the groove, high risk of injury when applying too much force due to impatience, very slow
• Chain drilling - repeated holes drilled with an ordinary power drill, then cutting with razor the pieces between. You end up with a wavy sinusoidal edge, not a straight cut, so it requires a bezel. Still very slow and tedious, moreso if you want to file the sinusoidal edge flat, and you need both a coarse file and a fine file or you will have problems with your file jamming full of material. Finding miniature files that cut with any speed is also its own difficulty - Swiss jeweler's files are suitable but can cost as much as $30-$60 apiece.
• Dremel/rotary cutoff wheels - very melty, difficult to get perfect corners in thicker boxes because of the curvature of the wheel, very difficult to do cuts inside deep boxes because of the geometry of the tool, difficult to get small cutouts for e.g. USB ports because of the size and geometry of the wheel
• Dremel/rotary tool with an end mill bits - still very melty, but overall a better experience than cutoff wheels. Difficult to get straight lines as it requires lots of pressure in thick sticky plastic. Ruins bits as plastic cools and adheres.
• Sheetmetal nibblers - most can do 1.5mm at maximum, while most project boxes I work with are about 3mm thick. Minimum size hole to pass the tool through that must be drilled first. These may work well in sheet metal boxes or altoid tin builds but are simply incompatible with the thickness of most plastic boxes.
• Jigsaw - only suitable for cutting very large holes. Very difficult to see precisely because the large metal shoe obscures the entire box. Difficult to mount box securely to make jigsawing possible at all, and the plastic walls of the box tend to snag even on dedicated plastic cutting blades - the ensuing vibrations can make a very ugly cut.
• MicroLux sword saw or gourd/cross saw - miniature reciprocating saw or miniature jigsaw, as expensive as the CNC machine once you add the special DC power supply, grossly underpowered to deal with thick plastic, melty, slow, difficult to cut precise straight lines
• Ultrasonic cutter - (no personal experience, comments welcome) These are substantially more expensive than entry level CNC machines, at $250-$400 for the cheapest models I can find in the US or EU. Effectively a razor blade that vibrates ultrasonically; appears to be substantially faster than cutting by hand but otherwise still seems to face the same line-following difficulty of any other hand cutting method. Unsuitable for metal boxes.
• 3D printing - out of scope for the question, which is about cutting holes in existing boxes, but has merit as an alternative. A good idea for open-sourced designs with custom PCBs and such, but for quick personal projects I find designing and printing an enclosure to be a substantial investment of time compared to picking an appropriate sized box off a virtual shelf and cutting holes, even when using the suboptimal techniques in this list.

Answer 7

1. A small pair of (mini) side cutters
2. Melt them with the tip of a smallish soldering iron
3. Dremel
4. Carefully, with a jig saw (electric or hand, though you would have to start the cutting with a larger hole in the waste area)
5. Use your own brain to work through the process.

Answer 8

If you have a 3D printer, or access to one, printing two drill templates for the outline holes has worked for me. There are two templates because one is for the odd holes, another for even holes. The template should be rugged, more perimeter layers than usual.

The first template should have a bottom plate that extends over the box and locates it firmly on the box. The 2nd template can be printed smaller and located on the existing hole pattern using 4 spare drill bits as locating pins, or similar. Or it can have the same locating means as the first one. Tradeoff between ease of use and print time.

10% overlap between the odd and even holes gave me a detached cutout. Then it's a simple matter of cleaning it up with a flat file, and corners with a square file. Or, for circular holes, with a half-round. Having medium and fine files helps, especially on metal boxes.

Before printing the entire template, I printed parts of it to ensure a tight fit of the drill bits (but not so tight the friction will melt the template), and a tight fit of the locating rim.

I have made a bunch of such drilling templates for various hole-making jobs, and they make the job stress-free and accurate. E.g. I find it not feasible to drill large holes in steel or even aluminum front panels using a stepped drill bit, with nothing but hand tools. So, even for round holes, drilling a bunch of small holes on the perimeter using a template keeps the panel from warping and can very accurately locate the cut-outs.

For drilling in steel, I recommend to have an extra "stage zero" template that's only used to center-punch all the holes. The template can be fairly thin - e.g. 3mm, and the spring-loaded center punch can fit quite snugly. It will center itself well, and will push any interfering plastic on the template hole walls out during the strike. You can of course also use a spring-less punch and a small hammer. The template should stay put without having to hold it down. Whatever wood you use under steel best be hard to prevent local warping. Scraps of oak worked for me.

Also, once you choose a drill diameter - say 1/8" - you'll want to buy a couple dozen of bits. They get dull eventually, and having many bits encourages the use of technique for all sorts of "cutouts" that were unwieldy before.