Method & Sources
Standards
| Chart | Follows |
|---|---|
| Tap drill & clearance (SP-TD-01) | ASME B1.1 (unified inch threads), ISO 261 / ISO 262 (metric threads), ISO 273 (metric clearance holes) |
| Drill bit sizes (SP-DB-01) | ASME B94.11M (twist drills: number, letter, fractional and metric sizes) |
| Thread pitch (SP-TP-01) | ASME B1.1, ISO 261 |
| Bolt & socket sizes (SP-BS-01) | ASME B18.2.1 and B18.2.2 (hex bolts and nuts), ASME B18.3 (socket head screws), ISO 4017, DIN 933, ISO 4762, ISO 7380, ISO 10642 |
| Resistor color code (SP-RC-01) | IEC 60062 (marking codes), IEC 60063 (preferred values) |
| SMD resistor codes (SP-SR-01) | IEC 60062, EIA-96 |
How Each Table Is Checked
Every value is checked against two independent published references before a chart’s revision is released. On top of that, the build refuses to publish if the data fails any of these checks:
- Number drills shrink with every number, #1 to #80; letter drills grow from A to Z; every fractional and metric size matches its name exactly.
- Every inch tap drill leaves between 60% and 85% of a full thread, by the formula below.
- Every metric tap drill is within 0.06 mm of major diameter minus pitch.
- Every clearance hole is larger than the thread, and free fit is larger than close fit.
- Numbered machine screw diameters follow 0.060″ + 0.013″ × the number.
- Preferred-value tables have the right number of values, in order.
Formulas
Percentage of thread for a cutting tap, with major diameter, drill and pitch in the same unit:
% thread = (major − drill) ÷ (1.299 × pitch) × 100
In inch units this is usually written (major − drill) × TPI ÷ 0.01299. A metric tap drill of major minus pitch leaves about 77% thread.
Revisions
Each chart’s title block shows its revision letter and date. When a value changes, the revision goes up. If you find an error, please tell us.