Tapped holes are formed threads in sheetmetal used to thread a screw or bolt through to hold a component or attached the sheetmetal part to another part. There are a number of specifications you have to keep in mind.
In the American system, a screw thread is designated by two numbers called the screw size and the pitch. For instance, #4-40, #8-32, 1/4-20.
The first number refers to the size (and therefore strength) of the screw shaft; the second to the pitch (number of threads per inch) on the screw. So a #4-40 screw has 40 threads per inch, a 1/4-20 has 20 threads per inch, etc.
A #4 screw is a smaller diameter (and therefore weaker) than a #6, etc. Common sizes are 4, 6, 8, 10, ¼”, 5/16” and 3/8”. For each shaft size, there are only 2 common usage thread numbers. For example, #8-24 & #8-32 are the common sizes for #8 screws.
The two pitches for any size are often referred to as "fine" and "coarse". The fine one will need more turns to tighten up, but will hold more securely in the face of vibration and other factors. The coarse one will be faster to attach but will not hold as well.
An important thing to check, when designing a formed tapped hole in a part, is "Will it be strong enough? How many screw threads am I going to get?" Here's how to figure it out. The pitch is the number of turns per inch. So, if you have a #4-40 screw in 0.055” material, there will be 2.2 turns or threads (40 turns per inch times 0.055 inches). As a rule of thumb, you need 2->2.5 turns to hold well. Depending on your application, the strength of the materials involved, the amount of vibration the part will be subjected to in use, the use of lockwashers, etc, 2.2 may or may not be adequate. It might be wise to try some destructive testing before the design is considered complete.
What if it's not enough? Options to consider:
Insertion Nut – this is what amounts to a nut that is actually pressed into the material with knurled ends so that it forced into the steel or material.