Understanding Thread Types and Tap Sizes
Understanding the two main thread systems used in engineering—metric threads and imperial (or inch-based) threads—is crucial before getting into the technicalities of selecting a thread tap size.
Metric Threads
The diameter of the thread and the pitch (the separation between threads) are the two factors that constitute a thread in the metric system. MxP is the most widely used metric thread format, where “M” stands for a metric thread and “P” for the pitch in millimeters.
Something like an M8x1.25 thread has a standard width of 8 mm and a pitch of 1.25 mm.
Imperial Threads
In places like the US, imperial or inch-based threads are common. They are marked with the letters UNC (Unified National Coarse) or UNF (Unified National Fine). The width of the thread is given in inches, and the pitch is generally shown by the number of threads per inch (TPI).
With a 1/4″-20 UNC thread, for instance, the thread is 1/4 inch wide and has 20 threads per inch.
Tap Sizes for Different Thread Types
Once you know how the thread type you’re working with is put together, you need to choose the right tap size. The tap size tells you what size hole you need to drill in the material before cutting threads with a tap. The hole size should be slightly smaller than the thread’s outside diameter so that the tap can make threads that fit the fastener snugly.
Using a Thread Tap Size Chart
A thread tap size guide is an important tool for anyone who works with fasteners that have threads. For different threads, these charts show the suggested tap drill size (the hole size for tapping) in both metric and English units. Here are some tips on how to read and use these charts correctly.
A simple example of a metric tap size chart is shown below:
Thread Size (mm) | Pitch (mm) | Tap Drill Size (mm) |
---|---|---|
M3 | 0.5 | 2.5 |
M4 | 0.7 | 3.3 |
M5 | 0.8 | 4.2 |
M6 | 1.0 | 5.0 |
M8 | 1.25 | 6.8 |
For the imperial system, the chart would look something like this:
Thread Size (Inches) | TPI (Threads per Inch) | Tap Drill Size (Inches) |
---|---|---|
1/4″ | 20 | 0.2013 |
5/16″ | 18 | 0.2720 |
3/8″ | 16 | 0.3125 |
1/2″ | 13 | 0.4490 |
These guides will help you pick the right bit size for your tap drill, making sure the hole is the right size for the tap. As a general rule, the tap drill size should be a little smaller than the thread’s outer diameter. This depends on the pitch. So, the drill size for an M8x1.25 thread, as shown in the figure above, would be 6.8 mm.
How to Read the Chart?
In each row of the chart, there is a “thread size,” a “pitch” (in the metric chart), or a “tap drill size” (in the imperial chart). This is how you use the chart:
- Figure out your thread size. Choose the thread size you want to use, such as M8 or 1/4″-20.
To find the right pitch or TPI, do the following: If it’s a metric thread, find the pitch, which would be 1.25 mm for M8. To find the TPI for imperial threads, for example, 20 TPI for 1/4″ - Find the drill size: Once you know the right thread size and pitch (TPI), you can find the tap drill size that goes with it. You need to drill a hole this size before you can tap it.
Step-by-Step Guide to Choosing the Correct Tap Size
Although it is important to know how to read the thread tap size chart, there are other things you should think about when choosing the right tap size for your job. Use the step-by-step instructions below to help you pick the right tap size for your needs.
Step 1: Determine the Thread Type and Size
Prior to starting, you need to understand what kind of thread you have. “Imperial thread” or “metric thread”? To be sure, look at the part’s specs or call the maker.
Step 2: Measure or Identify the Diameter
Next, find out what kind of thread it is by measuring or identifying the screw or bolt’s diameter. This is a good starting point for choosing the righttap size. Using a M8 screw as an example, the width is 8 mm.
Step 3: Find the Pitch or TPI
You will need to know the thread’s “pitch” if you are working with metric threads. The pitch is the space between the threads. A pitch tool can be used to find out what the pitch is or to get information from the maker. In the case of an M8 thread, the width might be 1.25 mm.
The most important number to know about imperial threads is the TPI, which stands for “threads per inch.” This number tells you how many threads are in one inch. A 1/4″-20 UNC thread, for instance, has 20 threads per inch.
Step 4: Consult the Tap Size Chart
The thread tap size guide will help you find the right tap drill size once you know the thread type, size, and pitch (TPI). If you’re not sure about any of the steps above, check your measures again or look them up in a book.
Step 5: Select the Tap Drill
You can start drilling the hole to the right size once you’ve found the right tap drill size on the chart. Use a drill bit that is the same size as what the chart shows. Like, you’ll need to drill a 6.8 mm hole for an M8x1.25 thread.
Step 6: Tap the Hole
Use the right tap to make the threads after making the hole. Taps come in different types, like hand taps, machine taps, and spiral point taps. The type of material and use determine the choice of tap.
Common Mistakes and How to Avoid Them
There are some common mistakes that people make when they are trying to choose the right tap size. These tips will help you choose the right tap size without making the following mistakes:
Incorrect Tap Drill Size
Not using the right size tap drill will make the threads too tight, which will make it hard to screw in bolts. In contrast, if the drill bit is too big, the threads might not form correctly, and the screw might not fit firmly. Check the numbers on a reliable tap size chart before you start.
Misunderstanding Pitch
It is very important to know the difference between pitch (in metric threads) and TPI (in imperial threads). If you mix these up, the threads will be off and the fit will be bad.
Choosing the Wrong Tap for the Material
Different types of materials need different kinds of taps. For instance, aluminum or other soft metals might need a different kind of tap than steel or other harder metals. To keep the threads from getting damaged, make sure you pick the right tap for the job.
Not Accounting for Tolerance
Tolerances in threads mean that they can be slightly different from one another within certain limits. When you choose a tap size, make sure that the range of the tap fits the thread’s requirements.
Conclusion
A very important part of accurate cutting and threading is choosing the right thread tap size. You can be sure that your threading is accurate and reliable if you know about the different types of threads (metric and imperial), how to use a thread tap size chart, and the steps for picking the right tap size.