Quote Originally Posted by 200k View Post
Addendum: I just had a thought. Here I am talking about the extra mass of an ER20 spindle and yet the same effect will be present using an ER11 adapter on a CT spindle, i.e. extra spinning mass on the flex shaft and cut motor. Of course any time I use the ER11 adapter I would be using small bits and by simple physics, the stresses would be smaller than using my Freud 1/2" shank, 1" diameter, 90° V-bit (one of my favorites for Centerline Text). So, though this may be a valid thought, I'm not sure how relevant to any machine damaging concerns it might be.

200k
The addition of the ER11 adapter is no big deal. It is not the mass that is so important, it is the rotational inertia. The rotational inertia is proportional to the mass, true, but also proportional to the radius squared. Mass out at a large radius is what is hard to spin up.