[jimthurgood]

Would you expect a 1/8" tool in a 3mm ER11 chuck to be OK?
With the chuck lightly tightened the tool is straight, but when fully tightened the tool tip wobbles. Am I tightening it too much or is it the size which is the problem?

[Edit] Or is the chuck faulty?

[Superman]

No....
the shank is larger than designed for the collet bore,
- unless the shank goes thru the entire collet bore, the outside collet taper will NOT match the holder's inner taper....this shows up as runout of the tool

ER collets can only clamp on the range they are specified for,..... ideally, it is best to use the upper collet size, matched to the tool shank size
---- for a 1/8" shank, get a 1/8" ( or a 3.5mm ) collet..... it will run true when clean & clamped

[jimthurgood]

OK, thanks. At least the cure is simple if not desperately cheap!