Not sure if this has been shared before.
When holding things in an ER series collet, where possible, I get piece of plastic and put a hole it the size of the round bar part the collet is holding. The outer I make to cover the diameter of the collet nut. So for example, in an ER40 collet holding a piece of 20mm bar, I use old grease caps, to make these washers from. I punch out a 36mm diameter ring, can be bigger . Then punch the centre to match the bar being held. It is thin enough to stay close to the collet nut, and stops the greater majority of chips getting into the collet.
On the smaller ER11 collet, I use a 10mm punch for the outer, and drill the hole for the part being held. Even plastic milk bottle tops work fine.
Neil