UNF · cutting tap · nominal dimensions
9/16-18 UNF
tap drill size.
For 9/16-18 UNF, the reference drill is 33/64: 0.5156 in (13.097 mm).
The reference value is kept separate from the 70% and 75% calculated targets. Actual fit still depends on thread class, tool, material, runout, and the hole you really make.
Detail A · basic 60° profile
What the percentage describes.
“Theoretical full thread %” is radial thread height from nominal geometry. It is not axial engagement length and not a fit-class acceptance measurement.
Real drill alternatives
Neighboring sizes.
These are diameter neighbors, not silent recommendations. A positive delta makes a larger hole and a lower nominal theoretical percentage; a negative delta does the opposite.
| Bit | System | Diameter | Delta | Calculated full thread |
|---|---|---|---|---|
| 13 mm | metric | 0.5118 in / 13 mm | -0.0038 in | 70.2% |
| 12.9 mm | metric | 0.5079 in / 12.9 mm | -0.0078 in | 75.7% |
| 12.8 mm | metric | 0.5039 in / 12.8 mm | -0.0117 in | 81.1% |
| 1/2 | fractional | 1/2 in / 12.700 mm | -0.0156 in | 86.6% |
| 12.7 mm | metric | 0.5000 in / 12.7 mm | -0.0156 in | 86.6% |
| 17/32 | fractional | 17/32 in / 13.494 mm | +0.0156 in | 43.3% |
Why 3D is useful here
A thread is a helix, not a row of triangles.
Load a draggable cutaway to see the continuous internal thread. Exact diameter and profile comparisons remain in the 2D drawing above.
Evidence and limits
Why this row says cross-checked.
Tap-drill row: Rows tagged with this source were matched to a named 65% or 75% cutting-tap column. The discrete drill can calculate to a slightly different percentage from nominal dimensions.
60° geometry: Public primary reference for Unified and metric 60-degree thread geometry. The percentage-thread equation is a theoretical basic-profile calculation, not a fit or torque guarantee.
Open National Institute of Standards and Technology geometry source
The repository also records 11 source records and a dataset version on every page.