What does BT40-ER20 mean on a tool holder?

Q: What does BT40-ER20 mean on a tool holder?

Short answer: `BT40-ER20` is a BT40 machine-taper holder fitted with an ER20 collet chuck; it holds tools from 1 mm to 13 mm shank diameter. The BT40 end goes into the spindle (7:24 taper, JIS B 6339, pull stud M16), the ER20 end grips the tool. Collet range is set by DIN 6499. A 12 mm end mill needs ER20 or larger; an ER20 will not close on a shank outside 1–13 mm.<

Question

Accepted Answer

Short answer: `BT40-ER20` is a BT40 machine-taper holder fitted with an ER20 collet chuck; it holds tools from 1 mm to 13 mm shank diameter. The BT40 end goes into the spindle (7:24 taper, JIS B 6339, pull stud M16), the ER20 end grips the tool.

Collet range is set by DIN 6499. A 12 mm end mill needs ER20 or larger; an ER20 will not close on a shank outside 1–13 mm.<