Page 72 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 3 Screw It or Glue It: Fastening and Joining Parts 53

When first building a project, you can bet you’ll need to take it apart at some point.
Nonpermanent joints make this easy to do.

NOTE From Velcro to magnets and cable ties to hose clamps, there are
dozens of ways to fasten parts together. Finding the perfect solution is less
important than finding something that works for your particular project and
the intended life of your mechanism. More fastening techniques and
components are entering the market daily, so there’s a very good chance
you’ll find something that works for you.

Screws, Bolts, and Tapped Holes
For the purposes of this book, the words screw and bolt are interchangeable. Some
people use bolt when the component is paired with a nut, and screw for a
component that threads into a tapped hole in another piece instead of using a nut.
However, the distinction is not generally agreed upon and is not important here.

Major Diameter and Threads per Inch
Figure 3-2 shows the characteristics of a screw. The most important ones are major
diameter (outside diameter) and threads per inch. The diameter can be measured in
inches or millimeters. In the inch system, screws with a diameter of 1/4 in or more are
labeled with the diameter first, then the threads per inch. For example, a 1/4-20 screw
is one with 1/4 in major diameter and 20 threads per inch of the screw shaft. For
some reason, screws with a diameter less than 1/4 in are given a number. For
example, a 4-40 screw has a diameter of 0.112 in. To convert from screw number to
decimal diameter, use this formula, or refer ahead to Table 3-1.

diameter (inch) = (screw number × 0.013) + 0.073
Most screws come in a standard (coarse) pitch as well as a fine pitch (more threads
per inch). The pitch is the distance between threads. In the metric system, screws are
labeled with the diameter first, then the pitch. So a 3mm diameter metric screw with
a standard pitch of 0.05mm is called an M3×0.05. We’ll focus on the imperial system
in this book, but the metric system is good to know when you come across
components (like some motors) that have holes in metric sizes.