When you look at a screw, the most obvious dimension is the outer diameter of the threads. Most of the time a ¼″ screw fits into a ¼″ tapped hole or nut. Sometimes, however, this doesn’t hold true as the male (screw) thread doesn’t match up with the female threaded nut or hole. Most likely they can be forced together for a turn or two, but the results are disastrous for your fasteners. What’s going on here?
It’s possible that your thread pitches (the distance between threads) don’t match up, or even that you’re trying to match English and metric threads. Read on to find out more!
Threads are specified not only by diameter, but by threads per inch. A ¼″ diameter bolt, for example, can have a ¼–20 UNC specification, which means there are 20 threads per inch. It can also have a ¼–28 UNF specification, which means there are 28 threads per inch. UNC and UNF stand for Unified National Coarse and Fine threads respectively
Stated another way, screws with –20 threads take 20 turns to go into an inch-deep threaded hole, or 10 turns to go into a ½″ threaded hole. Screws with –28 threads take 8 more turns for an inch of depth. Just because a screw measures roughly the same diameter as a hole doesn’t mean it will necessarily fit.
Here’s a chart of ANSI (English) thread sizes for use in your designs.
Further complicating matters is the fact that, in addition to English threads, metric threads are also in use. Although they are less common in North America, they are far from rare in the US and are more prevalent than English threads in other parts of the world. Like mismatched thread pitches, if a metric screw is threaded into an English-tapped hole, things will not go well. If possible, it’s good practice to use only one system or the other in a design to avoid mix-ups. For that matter, I try to use as few screw sizes as possible, since this can make assembly a little easier.
Metric screws are specified by the diameter in millimeters followed by the distance between each thread (pitch), also in millimeters. So an “M6 × 1” screw would have a diameter of 6mm and a distance between threads of 1mm. This system is probably easier to understand than the English version, but I don’t see either system going away any time soon.
Here’s a chart of metric thread sizes.
Other Types of Threads
If the metric/English thread “argument” wasn’t complicated enough, there are also left-handed screws which tighten counter-clockwise, or opposite of the “rightie-tighty-leftie-loosie” method that you are probably familiar with. There are applications for this, like when a wheel might loosen something by itself in normal use. They’re not that common, but you may encounter these at some point.
There are also a few oddball standards, like Acme, and Whitworth threads. You’re, however, unlikely to encounter these when setting up a 3D Printer or putting together a remote control plane.
Wood screws and sheet metal screws could be more broadly considered “fasteners,” and adhere to somewhat different standards. Fortunately, they are able to form their own threads, and only require a pilot hole to get them started. Pipe threads are a different animal as well, generally following along with the pipe sizes used. See this post for an introduction to PVC sizing.