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Making rectangular holes in project cases

my Cat Faucet has an IR sensor in a small plastic box which requires a small rectangular hole in the front face of the box. the below documents the best work process that I’ve been able to come up with so far for producing said small rectangular cutout.

I’ve tried using a ‘nibbler’ but the size of the required hole is too small for all the nibblers I’ve been able to find so far.

if anyone has a better way, let me know….

Plastic box, no hole
This is the initial plastic box, no hole, it’s about 1.25″ x 2″ and the rectangular hole I need is about 0.25″ x 1.125″.

Template, showing where the square hole goes
The first step is making a paper template and covering it with tape so that it snugly slides onto the box. It shows the 4 corners of the rectangle as well as the drill centers for the rough cutting out of the rectangle.

Transferring the template to the box
Using a sharp point, anything will do as long as it’s sharp. A pin, a nail, anything sharp that can withstand sufficient pressure to mark the plastic. Use that to transfer the 4 corners and drill center locations to the plastic box.

The transferred template, look close, 8 marks
It’s hard to see in this picture, but there are 8 small holes/marks in the plastic of the box. This will be the outline of the rectangular hole we’re going to make.

Connecting the dots
Now take a straight-edge and whatever sharp point you used to make the marks and “connect the dots” on the 4 corner marks. Does not need to be deep marks, just enough that you can see them. These will be the guidelines for actually making the hole. When you’re scribing the marks, make sure not to overrun the corner marks, this will result in scratches in the finished product.

Completed template transfer, rub with pencil
If you run the surface with a bit of pencil lead or ever your finger, generally you can get the transferred lines to show up a bit clearer. As you continue to work the cut, these lines will be your guide, so make sure you can seem them clearly.

Drill starter holes
Here’s a trick. Drill small diameter starter holes. The amount a drill “walks” when it’s started is related to the diameter of the drill. Starting with a small diameter drill first, improves the accuracy of the location of the holes.

Drill final holes
Now, open up the starter holes with a drill of sufficient diameter to mostly reach the edges of the rectangle. You don’t want the drill do big because there will be some variability in the drill location. Too small and you’ll have much more filing to do. Also, don’t think putting the holes closer together is a faster way to make the rectangle. If the drill holes overlap, likely the bit will just walk out of the current hole into the adjacent hole. So same with the spaces on the edges of the rectangle, close but not touching.

Roughing out the square with cutters
Use a part of cutters to remove the bulk of the material from the middle of the rectangle. This is a much more controlled way to effectively bring the drilled holes together than trying to just overlap the drill sites. Plus, if your cutters are sharp enough you can actually trim the peaks in-between the drill holes down to almost flat surfaces.

Roughing out the hole with a knife
If you want, use a sharp knife (a box cutter in this case) to get the edges of the hole closer to the scribed lines. If you’d rather not work with a sharp knife that close to your fingers, just go to the next step and use the file. The knife is just a bit faster, but a little less controlled than the file.

Rough hole, after knife work
here’s as close as I can get before the file work starts. Hole is mostly square and just a tad small.

The main work, flat file
The bulk of the finishing work will be done with a normal flat file.

Working the square hole with a flat file
Gently work the file side to side as you move the file back and forth. the wider the file blade, the easier it is to get straight edges. Try not to make more than one or two file strokes in the same spot, overlap them by a 1/2 or 1/4 file width. This also helps ensure straight edges. Take some off, trial fit the sensor, take some more off, trial fit and repeat. Work slowly…

Getting pretty close
Almost done…

Eyeball down the edges to ensure they#039;re straight
It’s pretty easy to eyeball down each side to see if you indeed kept the edges straight. If they’re not, typically it’s either a slight U shape, so you’ll need to take a bit of each end, or a slight inverted U, so take off the bump in the middle. Continue trial fitting during the whole process. Once you get a corner of the sensor in, try to slide one short side up and down the length of the rectangular slot looking for places where it’s too tight and then take a bit off in that spot.

A light touch-up
A light touch-up. Note holding the file at an angle into the hole rather than perpendicular. The more file surface you can get in contact with the box edge at once, the straighter your edges will be.

Trial fit
Trial fit. It’s pretty close at this point, the rectangular hole is the correct width, it’s just a tad short. I generally work the trial fit from the front side like this.

It fits!
the hole is done. the sensor fits from the front. Generally fitting from the back means simply rounding / de-burring the back side edges of the hole

Finished front
Here’s the correctly oriented sensor as seen from the front of the case. Nice smooth fit.

Finished inside
Here’s the view with the sensor inside. I just generally throw a bit of super-glue to keep the sensor in place after the press-fit. It’s a bit more solid that way than just with the press fit.