What does BT50-ER25 mean on a tool holder?

Q: What does BT50-ER25 mean on a tool holder?

Short answer: `BT50-ER25` is a BT50 machine-taper holder fitted with an ER25 collet chuck; it holds tools from 1 mm to 16 mm shank diameter. The BT50 end goes into the spindle (7:24 taper, JIS B 6339, pull stud M24), the ER25 end grips the tool. Collet range is set by DIN 6499. A 12 mm end mill needs ER25 or larger; an ER25 will not close on a shank outside 1–16 mm.<

Question

Accepted Answer

Short answer: `BT50-ER25` is a BT50 machine-taper holder fitted with an ER25 collet chuck; it holds tools from 1 mm to 16 mm shank diameter. The BT50 end goes into the spindle (7:24 taper, JIS B 6339, pull stud M24), the ER25 end grips the tool.

Collet range is set by DIN 6499. A 12 mm end mill needs ER25 or larger; an ER25 will not close on a shank outside 1–16 mm.<