I’ve been having an intermittent problem with 1/8” diameter end mills slipping out of the Dewalt 660 chuck. As best I can tell, the chuck is tightened down on the bit. I primarily see this issue when milling aluminum. I’ve been using trochoidal milling to cut out parts from a 3/16” aluminum plate. Feed rate is about 400 mm/min. I think the issue may be related to the bit heating up but it’s hard to know for sure. If I use WD-40 periodically to lube the bit, it doesn’t seem to happen as often. Has anyone else had an issue like this?
In that case check there’s no debris in the slits, you can use a piece of paper to clean them. The other thing to check is your bits are actually 1/8 and not 6mm. Amazon seems especially prone to describing one as the other.
It might also be worth pulling the router out and giving the internal cone on the spindle nose a good clean… Sometimes stuff can get wedged in there pretty good, especially if the collet has been tightened into it, packing it in place.
How are you tightening the bits? Using the spindle lock or using 2 wrenches? For high-load stuff I tend to use 2 wrenches and position them such that I’m tightening them by squeezing them together with one hand. If you’re using the lock you might not be getting all that much torque on it. You shouldn’t need a whole lot, but that depends on the load and how good your collet is, etc.
I also give my collets and the router spindle nose a good spray with WD-40 when I clean it, then I try to dry it as much as possible with a cloth.