Start with the holder
Identify the ER holder family: ER8, ER11, ER16, ER20, ER25, ER32, ER40 or ER50.
Match the tool shank
Select a collet size that matches the tool shank and avoids relying on excessive collapse.
Define performance requirements
Confirm runout target, coolant need, inspection method and whether a high-precision option is required.