ER32's are prone to breaking either the cap, the seat or cracking the collet itself. Be careful how tight you make them. Over-tightening them WILL cause them to 1st, crack the seat, 2nd crack the cap-seat, and 3rd gouge/crack the collet.
My rule of thumb, (AFTER I've made sure I can snug the tool in the collet by hand, while not in the tool holder*), is to use a wrench with your hand as close to the nut as possible, and tighten as much as possible. (The further away from the nut, the more leverage, the more torque, the more likely to over-tighten).
I've scrapped thousands of dollars in holders/collets/caps because people use the gorilla-grip to tighten them.
* If you can insert the tool into the collet without force, and pinch it with your fingers enough to feel it 'grab', it's the right collet.
Matt
San Diego, Ca
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(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)