Thread Pitch Vs. Threads Per Inch
There is a considerable difference between the thread measurement system used in the United States and that used in countries that use the metric system. In this article, we will discuss the difference between thread pitch vs. threads per inch, why understanding threading is important, and how to measure threads properly. It may facilitate various projects you may have in the future, so keep on reading to learn more!
What is Threads Per Inch (TPI)?
Threads per inch, sometimes incorrectly called thread per inch, is the way different fastener types are calculated in the US. As the name suggests, it is simply the number of threads within an inch of space on the fastener.
In the case of a fastener that is at least an inch in length, you can simply count the number of threads occurring within an inch of space.
When it comes to shorter fasteners, like quarter-inch fasteners, simply count how many threads are within a dividend of one inch and multiply that by the appropriate amount. For example, a quarter-inch fastener with ten threads would have a thread-count of 40 threads per inch since there are four quarter-inches in one inch.
Threads per inch can be easily converted to thread pitch by dividing 25,4 (the number of millimeters in an inch) by the TPI. For example, a fastener with a TPI of 10 will have a thread pitch as follows:
25,4 (MM per Inch) / 10 (TPI) = 2,54 (Thread Pitch)
What is Thread Pitch?
Thread pitch is the means of thread measurement used in the UK and other metric-based countries. If you’ve gone fastener-shopping and found yourself wondering, “What is a thread pitch,” here’s what you’re looking at.
Metric thread pitch is a measurement of the distance between two thread peaks. So, if you want to know how to measure thread pitch, you simply need to measure the distance between one thread’s peak and the next using a millimeter-based caliper.
Considering how small the spaces are between threads, it’s generally easier to measure the distance between 10 threads, and then divide the result by 10. For instance, if there’s a distance of 22,5mm between the ten threads, the thread pitch will be 2,25mm.
Converting thread pitch to TPI is relatively simple. All you need to bear in mind is that there are 25,4 millimeters in an inch. When you divide 25,4 by the thread pitch, you’ll get the number of threads per inch.
If you have a thread pitch of 1.25mm, you can convert it to TPI with the following equation:
25,4 (MM per inch) / 1,25 (Thread Pitch) = approximately 20 TPI
Differences Between TPI and Thread Pitch
The differences between these measurement systems are minimal. The main difference is that threads per inch are used solely for American fasteners while thread pitch is used for metric fasteners.
Either one can be determined using a thread gauge, and the formulas mentioned above make it relatively easy to convert one into the other.
Fine vs. Coarse Threading: When to Use Each
There are significant differences between fine and coarse threads. Finer-threaded fasteners generally have a much tighter structure and are much less pronounced. Coarser-thread fasteners have a much more pronounced thread, and are more forgiving in terms of damage to the threads. Most fasteners are available in both standard and coarse versions.
Fine threads are best suited to applications where you require precision and control. They are generally paired with precise pilot holes to ensure that the fastener fits perfectly. Coarse threads are better for applications where speed and ease of installation are prioritized over precision.
For instance, fine threads may be used in fine woodworking or machine components, while coarse threads may be used for bulky farmyard construction or in less precise construction work.
Why Understanding Threading Matters
The main reason why understanding threading is so essential is that it allows you to ensure that any nut and bolt that you want to pair will couple appropriately. When the threading on these two couplers don’t match, it will lead to either seizing or stripping of one or both parts, resulting in an unstable connection.
Tools for Measuring Thread Pitch & TPI
We’ve already mentioned how to measure thread pitch and TPI using a caliper, but how does that correlate to fine or coarse threads? To explain the answer, we need to look at the fastener’s size representation.
In a US fastener, you’ll see the size represented in the format ½”-20 or ½”-28. The first number represents the fastener’s length, and the second represents the TPI value. In this case, the fastener with 28 TPI has a finer thread. The higher the number is, the finer the thread.
However, in the case of thread pitch, the size is represented as M4-2x40mm or M4-1.5x40mm. The first number refers to the fastener’s diameter (4mm, in this case), the second number refers to the thread pitch, and the third number refers to the fastener’s length.
Since the thread pitch measures the distance between threads, the lower this number is, the finer the thread (because it has a higher TPI). So, when searching for a fine thread, look for a higher TPI or a lower thread pitch, and do the reverse if you need a coarse thread.