So recently while working on a project as seen in my technical gallery, (the link below), I had to modify a small plastic enclosure as seen in the above photo, to allow the installation of an APRS TNC dongle.
The enclosure needed a rectangular opening on one end of the enclosure to allow the USB connector located on one end of the T3-Micro to protrude out the end of the enclosure, once the T3-Micro was placed in the enclosure.
There are various ways this can be done, including marking out the location of the rectangular opening, and then daisy-chain drilling around the inside perimeter of the required opening, and following this up by using a small file to finish the opening to size.
Now there is another way if you happen to have a shop that includes a vertical milling machine as I do in my shop.
With the dimensions of the USB connector jotted down, and the plastic enclosure mounted in the milling machine vise, and with a 3/32" slot-drill (type of milling cutter used for plunge cutting) mounted in the milling machine collet chuck, it as simple as dialing in the measurnments with the X and Y micrometer hand-wheels located at either end of the milling machine table.
When machining composites (plastics), whether it be with a drill-press, milling machine, or with a lathe, make sure and run your machine at the proper spindle speed required for the material choice, material thickness, and finish required.
Most guys when machining composites, tend to run their machines (drill press) set to slow, and it makes for a possible inaccurate machining job with a poorly finished surface. If you're not sure what spindle speed is required to run your drillpress at, that's when you reach for a reference book like the "Engineer's Black Book" as seen here.... https://photos.smugmug.com/MachineShop/Machine-Shop/i-Nww6kCR/0/XL/1-6N4A3629-2-XL.jpg
Mind you, and depending on the material, an example being black Delrin, you can get in to difficulties with driving the temperature of the material up to a critical point, where it becomes toxic, as black Delrin can give off Phosgene Gas if over heated, something to be aware of.
I have machined black delrin and had the temperature reach a level where you could smell unpleasant odors coming off of the material being turned in the lathe, time to dial it back.
Click on the photo for a closer look.....
The project as finished.....
Machining black Delrin in the lathe.....