I do a lot and I mean a lot of parts construction using screws and adhesives.
For screws, I would say that looking at my toolbox, M3 screws are probably good for 95% of my work.
Here are some things I learned over time.
Heated inserts while obviously the best of the best, are unnecessary unless you’re fastening large plates together that are over 150mm. Self Tapping screws will work just fine. Or better yet, machine screws in a tapped hole.(more below) Sure, if I am making a part that I know I will be disassembling frequently, yes, then I use brass heated inserts but for everything else, I rely on tapped reinforced holes.
Did you know that if you go through the trouble of creating a set of filament profiles with calibrated X-Y hole adjustment, you can create a hole large enough for a screw and then finish the threads with a Tap and it will be nearly as strong as brass heated inserts. The trick? Just increase your wall thickness on print to ensure that you have at least as many walls surrounding the hole as the hole is wide.
Start off with a series of sample holes. I used ONSHAPE as my CAD tool but it is possible to do this right within the slicer using the negative part cylinder primitive albeit, far less accurate.
Once I had my test plates set up, I cloned them and marked them using the Text tool in the slicer in order to test the accuracy and vary the X-Y hole compensation.
Then I eyeball it and increase the number of wall loops so that the outer edges of each hole as enough wall thickness to allow my tap to bite into. This also will show me when I have my holes too close to the edge of a part.
Then I use a tap to create the thread and I have found that this is as strong as brass inserts without the hassle of heating up the solder iron.
This is the best style tap as it has a flat bottom and is perfect for plastic. BTW: It’s also the same thread that is used in 90% of the screws on a BL printer. So if you have this around and somehow can’t get a screw in, or if you’re adding a part to a previously unused factory hole, this will clean out the thread and make it perfect. It’s very handy item to keep in one’s tool kit.
BTW: This is also very forgiving method because if you make the hole too small, simply enlarge it with a small drill held be a Spindle vise, then tap it.
But for brass inserts, I learned the hard way as to which one’s you do NOT want to use.
Straight inserts that do not stay easily put when pressing in. The tend to wobble.
Fluted inserts that have a taper at one end which seat nicely and do not wobble when you apply heat
But you know what also works well for putting to pieces of PLA, ABS, PETG etc?
I swear buy this whenever I have to split an object for printability and use dowels for aliment. This is very runny so I found it works best when squirted into a small disposable cup and applied with a disposable tiny paint brush. You’ll get better control that way. It is a welding agent not a glue. It melts the two materials together and the resulting bond that I tested, is stronger than the surrounding plastic. That’s because the layers of filament that come in contact are chemically fused into a single bond.