About

BT Engineering is a free machining reference. Calculators and shop data you can pull up at the machine, on your phone, without a login.

I'm Bradley Taylor, a production engineer in precision machining. I work closely with machinists every day, and this site exists because the reference material we use at work is spread across wall charts, old handbooks, and websites buried in ads. I wanted one clean place with the common calculations done right.

How the numbers are made

Every calculator computes from published standard formulas, the same math found in the standard machining references and the governing ASME and ISO standards. Each one has an automated test suite that checks its output against known values, over 150 checks like 25 H7 = +0.021/0 and a #7 drill giving 75.4 percent thread on a 1/4-20, and the test file itself is public so you don't have to take my word for it. Cutting data is aggregated from general industry practice and leans conservative on purpose. The full policy is on the About the Data page.

Use it with judgment

This is reference information, not a substitute for your certs, your gauges, or the governing standard. Machines, tooling, and setups vary. Start conservative and verify anything critical before you cut.

If you find a number that looks wrong, tell me. Corrections come first.