back draw end mill extension holder, you can get higher precision:1.BLC20-150L, front diameter 14mm, shank diameter 20mm,2.BLC16-150L, front diameter 14.8mm, shank diameter 16mm,3.BLC12-100L, front diameter 12mm, shank diameter 12mm,They are used for 3mm, 4mm, 6mm carbide end millsThese extension ho