Tap Drill Size — Definition in CNC Machining
September 27, 2026 · Exactalloy Technical Team
Glossary entry for tap drill size, one of the terms that most often decide whether a tooling process runs at the expected cost.
Quick answer: tap drill size: Pre-drill diameter giving the correct thread percentage for a tap.
What it means
Pre-drill diameter giving the correct thread percentage for a tap.
Why it matters in practice
Getting this right typically changes tool life and surface finish more than any single parameter tweak.
How to apply it
Record the value for every job, and compare against the recommended band for the material group you are cutting.
Frequently asked questions
Where is this used most?
In production CNC milling, turning and drilling, and in any tool-selection decision.
Can I ignore it?
You can, but the cost usually appears as unpredictable tool life.
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