Techniks part # 22917-4.63 CAT40 1/2" Solid end mill holder with 4.63" gage length and balanced to 15,000 RPM at G6.3 with the taper ground to AT3 for maximum spindle contact. The ID bore is also precision ground to H5 which is the industries best. Techniks is known for its high quality tool holders for a very economical price and have been a proven brand for decades.