Primary sources first
- Makers’ tables: drill and tap charts published by the tool makers themselves, such as Starrett, and fastener makers’ specifications.
- Standards and law: U.S. law where it defines a size (15 U.S.C. §206, the U.S. Standard Gauge for sheet and plate iron and steel), and the standards bodies and industry groups that maintain sizes, such as the GalvInfo Center for galvanized sheet.
- Government research: the USDA Forest Products Laboratory’s Wood Handbook for wood properties, lumber measure and fasteners in wood.
- University references: machine shop and engineering course charts published by universities, used to cross-check the makers’ tables.
Two sources, and the math
Every number in a chart is checked against at least two sources. Where a size follows from a formula, such as the percentage of thread a tap drill leaves, a board foot, or a gauge’s thickness from its weight, we calculate it and compare the result with the published tables. When the sources disagree, we say which we used and why.
One example: one widely shared university chart lists a #22 drill for a 10-24 tap at 75% thread. The thread formula gives about 61% for a #22, and Starrett’s chart and the formula agree on a #25, which gives about 75%. We use #25.
Drawings to scale
Our drawings are made from the same numbers as the charts, at a stated scale, so a hole, a board or a sheet is shown at its true proportions. They are our own drawings, not copies of catalog art.
Safety and your own parts
Tolerances vary by maker and material. Before drilling a part that matters, check the size against your own fastener or a test hole in scrap. Follow the instructions and guards that come with your tools, and wear eye and ear protection.
Corrections
If you find a number that doesn’t match a source, tell us. We recheck it and update the page and its date.