Reference guide
Sources and Methodology
How this site stores chart values, checks assumptions, and separates reference values from job-specific engineering decisions.
How chart values are handled
The site keeps drill, tap, clearance, and pilot-hole values in a central data model instead of scattering numbers through page copy. That makes corrections easier and reduces the risk that one page is updated while another page keeps an old value.
Reference hierarchy
Each data family has a named method, publisher or internal owner, destination, and access date. Production work must still be verified against the drawing, a recognized standard, or the tooling supplier.
| Data family | Source or owner | Reviewed | How it is used |
|---|---|---|---|
| unitConversion | SI Units - Length National Institute of Standards and Technology | 2026-07-23 | Decimal-inch values are converted with the exact international-inch relationship: 1 in = 25.4 mm. |
| unifiedCuttingTap | Tap Drill Sizes UNC / UNF Guhring | 2026-07-23 | Named UNC and UNF drill sizes are maintained as common cutting-tap reference values and checked against manufacturer ranges. |
| metricEstimate | Tap Drill Calculator Guhring | 2026-07-23 | Metric values use the visible major-diameter-minus-pitch estimate unless an alternate chart assumption is displayed. |
| formingTap | Drill Size for Thread Forming at 55% Thread Guhring | 2026-07-23 | Used to explain why forming-tap holes are not interchangeable with the site's cutting-tap starting values. |
| nptReference | NPT tap-drill working dataset Drill Size Chart Tools | 2026-07-23 | Stored as a separate reference family. Values must be checked against the selected pipe-tap maker and applicable thread requirements before production. |
| clearanceReference | Clearance-hole working dataset Drill Size Chart Tools | 2026-07-23 | Close-fit and free-fit values are starting points; the drawing or specified fit class controls final selection. |
| woodPilotReference | Wood-screw pilot-hole working dataset Drill Size Chart Tools | 2026-07-23 | Softwood and hardwood values are practical starting points, not a standard for every screw root diameter or wood species. |
Why assumptions are shown
A tap drill answer changes when percent thread, material, hole depth, coating, cutting tap versus forming tap, and fit class change. The pages state those limits because a fast answer is useful only when the user knows where it stops being enough.