As a beginner playing with plastic fabrication on a 3D printer, I hadn’t known about heat-set inserts for putting durable and reliable threads in plastic construction. In all my projects to date, I tapped threads into the plastic directly and made sure to be careful when tightening a screw threaded into plastic. The inserts look like a much, much better solution and they are easily available from hardware vendors like McMaster-Carr.
Before I put in an order, though, I wanted to do a quick experiment. I salvaged some M2.5 heat-set inserts from the dead Dell laptop, and I laser cut holes of various diameters into a scrap piece of 3 mm acrylic. When the hole is too large, the result seems to be obvious: insert will be unable to grip tightly. It’s less obvious to me what happens when the hole starts becoming too small. Recognizing the symptoms will help me determine proper diameter for future applications.
For their M2.5 inserts, McMaster-Carr recommends drilling a hole .152″ in diameter. This translates to about 3.86 mm. The largest hole in this test piece is nominally 3.75 mm, but with laser kerf will end up closer to 3.91 mm. The hole labelled 3.7 would, after laser kerf, end up right on the dot at 3.86 mm.
The experiment showed that they will all suffice to hold the insert into the acrylic, so in practice there is some amount of tolerance for the diameter precision. As the holes got smaller, more heating is required to install the insert, and more acrylic is visibly distorted around the insert due to the additional heat. Fortunately optical clarity seems to be mostly preserved, the distortion is barely visible in the above picture.
Once I got down to around “3.5” (actually ~3.66 mm with kerf) I started seeing the insert pushing plastic out of the way during installation. This results in a small ring of excess plastic around the base of the insert, which is undesirable. This is a good enough marker for “too small” and I stopped there. The holes smaller than “3.5” remain unused.
Experiment complete: In the future, the combination of optical distortion and excess plastic at the base will serve as my first warning sign that I’m installing heat-set inserts in too small of a hole.