Made from alloy spring steel and hardened to HRC 44-48
All fractional size is true imperial size, not a metric conversion size
All collets are checked its runout individually on 4 contact points by a high tolerance bench marked testing rod
On size 3.0mm and larger the runout is .0003"; under size 3.0mm the runout is .00059"
Each size of ER20 collet has collapsing range of .039" (1 mm); the size of the collet is the largest size it can hold and able to hold anything smaller within the collapsing range
Conform to DIN6499B
This collet size is true metric size and the surface on this collet has been specially treated to increase its tool life.