If you’re cutting internal threads, you need to drill a pilot hole first. This is where a tap drill size chart comes in.
These charts tell you exactly what size drill bit you need for different thread types and sizes, ensuring a clean, accurate thread every time.
Understanding Thread Types
Before you dive into a tapping size chart, it’s important to know a little about the different types of threads you might be working with.
Metric Threads
Metric threads are measured in millimeters, along with their pitch. To calculate the tap drill size you’ll need, subtract the pitch from the major diameter.
Imperial Threads (UNC/UNF)
Imperial threads are defined by the number of threads per inch.
- UNC (Unified National Coarse) is your standard coarse thread.
- UNF (Unified National Fine) is your standard fine thread.
How to use tap drill size charts
If you’re looking for an accurate tap drill, especially for larger imperial threads, you may want to consult a tap drill reference chart.
Typically, these charts list the thread size and the corresponding tap drill size to use. Some charts also include the pilot hole sizes for different materials.
As a general rule for imperial threads under 1″, you can aim for 85% of the major thread diameter for coarse threads and 90% of the major thread diameter for fine threads.
How to calculate tap drill size
Finding the right tap drill size doesn’t have to be complicated.
Metric calculation
To calculate the tap drill size for metric threads, subtract the pitch from the major diameter. The result is the core or minor diameter.
Imperial approximation
For imperial threads, a common rule of thumb is to use a percentage of the major diameter to estimate the tap drill size.
In Closing
Choosing the right tap drill size is essential for making strong, accurate threads. Tap drill size charts can help, and understanding thread types can improve your chances of success. Just be sure to double-check the chart, and think about the material you’re tapping before you start.