This topic has come up quite a few times - reference collet tightness from Nov 2022.
I’ve chewed into my spoilboard more times than I care to admit - pretty much every time when using a 3/8" endmill (usually an upcut). Never with smaller end mills. This has frustrated me to no end. I even went so far as to purchase the Amana torque wrench - which was NOT the solution. For me it was not a problem of having the collet “tight enough”.
For me the problem was the Amana bit(s) I was using have a wonderfully helpful triangle/line on them telling me how far into the collet to insert the bit. I have loved this feature - made it feel idiot proof.
However on my 3/8" endmills the “idiot line” telling me how far to insert the bit - was making the endmill stick out above the top of the collet; this was up inside the spindle so I never noticed this issue. [I usually had the collet loosely threaded onto the spindle when I jammed the bit up inside.]
So, it wasn’t until I watched this video (https://www.youtube.com/watch?v=UoHSk9dO6oM), and specifically tip #2, that I realized the Amana reference line (which I loved) was biting me in the butt on my 3/8" endmills. The suggestion is to always leave the endmill 2-3mm below the top of the collet.
Not sure if this matters for everyone, but this was definitely my issue (on my long list of issues/learning from my mistakes).
Hope this helps someone else !
-scott