Thread-Forming Screws

Around here, no detail is too insignificant or obscure to go unremarked upon, so here's a little item about thread-forming screws you may not be aware of.

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Thread-forming screws are widely used in the chassis of printers wherever things are fastened to plastic. They don't 'cut' a thread, they form a thread by displacing and compressing the plastic material as they're first run in at the factory.

You may have noticed when reinstalling such screws that you've removed, that sometimes they run back in easily; sometimes not so easily. There's a reason for that.

It's too fine a detail to photograph well, but for what it's worth, here's a shot of two typical screws from a Lexmark top cover.

If you examine the threads closely, you'll see that the threads are double helices, aka 'two-start' threads. I suspect that there's a limitation inherent to the screws' manufacturing process, because the threads are never perfectly symmetrical and uniform.

The upshot of that is that when you happen to start a screw with the same orientation it had at the factory when it was first installed, the screw will run in easily. When you happen to start a screw 180° around from its original orientation, the screw will meet resistance.

Try this the next time you start in a screw and get resistance -- carefully back the screw out until it 'clicks over' to the other one of its two 'starts'; the screw will then likely run in easily. The difference can be quite pronounced.

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