Tapping chart.

In a flat sheet, a tapped thread is only as long as the sheet is thick. Its strength is how many threads fit in that length — and whether the sheet around it is stout enough for the screw.

Two rules decide it, and a size has to pass both. Threads: thickness ÷ pitch must be at least 2; below that the thread strips under any real torque. Diameter: thickness must be at least half the screw's major diameter; below that the joint tears out through the sheet before the thread strips — 0.090″ sheet clears two threads of 5/16-24 on paper, but that joint fails.

On the chart each row is a thread. Amber is where it holds (two to three threads), green is where it is solid (three or more). Vertical lines are the gauges we stock in the grade you chart; the dot where a gauge crosses a row says whether that thread works at that gauge.

Holds from = the larger of 2 × pitch and ½ × diameter; solid from = the larger of 3 × pitch and ½ × diameter. Tap drills are the standard pick and the thread percentage they give.

Not tapped: AR500, Spring steel 1075 / 1095 — hardened grades destroy taps. Full drill sizes are in the tap drill size chart, and the trade-offs between tapping, hardware and other joints are in threads in thin sheet.