MANUFACTURING – CONVERION & MANUFACTURING CONTANT CALCULATOR
Drill Tap Size
A precise tool.
What is the Drill Tap Size & How does it work?
When tapping a hole, the drill size is not the same as the final thread’s major diameter. The drill must be sized to leave enough material for the tap to cut the threads while avoiding excessive torque or thread deformation.
For metric threads the common practice is to use a tap‑drill percentage (often 75 %). The drill diameter is calculated by subtracting a fraction of the thread pitch from the major diameter.
For Unified National Coarse (UNC) threads the same principle applies, but the pitch is derived from the threads‑per‑inch value. The formula works for both systems; only the way the major diameter and pitch are extracted differs.
D_{text{drill}} = D_{text{major}} – P times frac{%}{100}
Ddrill = drill diameter, Dmajor = thread major diameter, P = thread pitch, % = tap‑drill percentage
Parameters
Result
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Frequently Asked Questions
What is the purpose of a tap-drill percentage?
The tap-drill percentage determines how much material to remove with the initial drilling step, allowing the tap to cut threads without excessive force or deformation.
How do I calculate the drill size for metric threads?
Multiply the thread major diameter by the tap-drill percentage (usually 75%) and subtract the result from the major diameter.
What is the difference between metric and UNC threads in tapping?
Both use a similar principle of leaving enough material for the tap to cut threads, but UNC threads require specific pitch considerations.
Why is it important to get the drill size right when tapping?
Incorrect drill sizes can lead to excessive torque, thread deformation, or failure to create proper threads.
Can I use a tap-drill percentage for both metric and UNC threads?
The concept is similar, but the specific calculations differ between metric and UNC threads due to their unique pitch systems.
What should I do if my drill size calculation results in an impractical value?
Re-evaluate your thread specifications or consider using a different tap-drill percentage that better suits your material and application.
Results are for informational purposes only and do not constitute professional advice.