There is always going to be a max acceleration that is possible no matter what, it will depend on the rigidity of the machine. There is also the fact that Mach is basically bang bang with the acceleration, full accel, full decell. If it had S Curve then you could likely get much higher accel and things would stay smooth.
Regarding milling, well accel is as important in milling as anything else, the fastest you can get, whilst still being smooth, the better. Most parts will run quicker with the higher accel and no problems with corner rounding or such like.
On my Bridgeport, when it had steppers I actually got a boost in performance by lowering the Velocity as it allowed me to increase the accelerations, parts ran much faster.
I think it was something like dropping the Velocity from 2500mm/min down to 2400mm/min allowed me to increase the Accel from something like 40mm/s/s up to 120mm/s/s. although I cant be certain of these numbers as it was probably getting on for 8 or 10 years ago.
As previously said my Beaver NC5 runs servos with Vel of 8m/min and accel of 800mm/s/s (just looked to confirm that, first time it has been fired up for a long time
![Cheesy :D](https://www.machsupport.com/forum/Smileys/default/cheesy.gif)
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The 8m/min is just what I limited it to as I reckoned for old iron it was fast enough, I did have it cranked up to 20m/min (same as the Chiron) but it was way too fast for that style of machine.
Hood