No Need For Inserts If You’re Prepared To Use Self-Tappers
As the art of 3D printing has refined itself over the years, a few accessories have emerged to take prints to the next level. One of them is the threaded insert, a a piece of machined brass designed to be heat-set into a printed hole in the part. They can be placed by hand with a soldering iron, or for the really cool kids, with a purpose-built press. They look great and they can certainly make assembly of a 3D printed structure very easy, but I’m here to tell you they are not as necessary as they might seem. There’s an alternative I have been using for years which does essentially the same job without the drama.
Enter The Self-Tapper
When we think of screws or other fastenings, if we’re not a woodworker, the chances are that it’s a machine screw which comes to mind. A high-precision machined parallel thread, intended to screw into a similarly machined receptacle. Where this is being written they’re mostly metric, in fact I have a small pile of M3 bolts on my desk as I write this, for mounting a Raspberry Pi LCD screen. These are what you would use with those heat-set inserts, and they are generally a very good way to attach parts to each other.
However good an M3 bolt is though, I don’t use them for most of my 3D printing work. Instead, I use self-tapping screws. A self-tapper is a screw with a wide tapering pitch, designed to cut its own thread into a soft material. Most wood screws are self-tappers, as are many screws used for example with aluminium sheet. The material is soft enough for a reliable enough coupling to be made, even if repeated use or over-tightening can destroy it. It’s easy to make 3D prints that can take self-tapping screws in this way, I find it reliable enough for my purposes, and I think it can save you a bunch of time with heat inserts.
How To Make It All Happen
Designing for a self-tapping connection in a 3D print is simplicity itself: a suitable hole for the screw thread to pass through is placed in the upper side, while the lower side has a smaller hole for the thread to bite into. The size of the smaller hole can vary significantly without penalty, but I normally make it the diameter of the shaft of the screw without the thread. A simple example for a 3mm self-tapper in OpenSCAD is shown below, along with a render of the result.
//Screw head end
translate([0,0,20]){ //Move upwards to see both parts
difference(){
cube([20,20,4]);
//screw thread
translate([10,10,0]) cylinder(10,1.5,1.5);
//screw head
translate([10,10,2]) cylinder(10,3,3);
}
}
//Screw thread end
difference(){
cube([20,20,10]);
translate([10,10,0]) cylinder(10,1,1); // For the screw to bite into
}
Assembly follows construction in its simplicity; simply line up both holes and screw the self-tapping screw into them. It should be obvious when the screw is tight enough. Mashing upon it, just like with any other self tapper, risks stripping the thread.
Everyone makes things in their own manner, and it’s likely that among you will be people who might decry the use of self-tappers in a 3D print. But I have found this technique to be a simple and cheap time saver for as many years as I’ve been 3D printing. I hope by sharing it with you, I’ve given you a useful tool in your work.