Hamond Industries Ltd - the Slide Forming Specialists
A Tiny Tutorial on designing for Tapped Holes
A screw and nut together are quite a complicated machine. 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 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 and 10.
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 (see our reference list of Common Screw Sizes ).
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.
Pitch and Stampings
An important thing to check, when designing a formed tapped hole in a stamping, 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?
If this were a #8-32 screw, 32x0.055=1.76 turns. For most applications, this would not be adequate thread depth. What to do?
Extrusion to the rescue!
If you are trying to thin the material for cost reasons and still maintain adequate turns, there is a way. For most common metals, it is possible to double the effective thickness of the material (and therefore the number of turns) locally in the area of the thread. We do this by extruding the material downwards (into a kind of a tube) before it is tapped. This means that your part can be made out of thin material and still capture and adequately hold the screw. The tooling to do this is a bit more expensive, but in reasonable quantities, the savings in material thickness are substantial and outweigh the additional tooling cost.
In some cases, extrusion can make walls up to 3 times the material thickness. This requires more expensive, less common material. This technique is generally not used as a cost-cutting measure, although it may be used to solve other problems, such as space constraints.
Other less often used measures of screws
Another measure of screws is fit. No one makes a batch of screws (or any other part) that are completely identical. Generally, people want the largest screw to fit inside the smallest hole (so that every screw goes into every hole). The converse situation, the smallest screw in the largest hole, will leave you with a loose, wobbly screw. Just how much wobble is permitted in that worst case scenario can be an important specification for some customers.
Another measure is thread completeness or thread depth. A screw is made of hills and valleys. A nut is made of matching valleys and hills. The pointiest hill has to fit into the most filled in valley if the screw is going to turn. In a perfect world, all hills and valleys would be perfectly sharp. This perfect world is called 100% thread depth, and is seldom achieved in commercial practice. Thats probably just as well, since such screws would be sharp, and youd cut yourself when handling them.
Why are we writing this?
We are slide formers who can also tap our formed, stamped parts. If you're a designer, you may be able to save a fair bit of money in your product and reduce part count and assembly time by incorporating tapped parts into your design. See our article How to reduce part count and product complexity by incorporating Tapped or Threaded Parts into your design for more details.