I have done a lot of research in the Dremel range .
This is what i have come up with.
The Dremel 300 when you turn it on full speed and put a say a eze lock.
The unit will make a load noise and vibrate this is because they have machined the armature to close to the field they do this so they have to make the speed they requirer.
You see the old Dremel had a nylon colet half way between the armature and the sharft thats why the unit lasted longer and there were no vibrations or clicking it was a good thing because if you ever over done the unit it would just break the nylon colet.
Dremel knows the new 300 would make noise and vibrate because if you look closely at the new 300 and the old one they have made 2 indentations in the the shaft towards the top and the bottom half of the case is holding this they know its going to vibrate and click ect .
The one before the 300 had a strate shaft with a break in the middle and a nylon colet .
ITS ALL ABOUT COST CUTTING
I have machined the 300 armature just a fraction of a mil and all problems solved but the speed went down about 500 rev.
Know if Dremel did this they would not be able to claim that speed cuts not torque.
The 4000 is made just to get it out in the market it has so many problems from the variable speed control to the bushes arcing and the front end getting to hot to hold.
The electonics that control the speed is very cheap as to just make it work.
You can buy a speed control kit for $15us and it will perform 100 times better than the one in the 4000.
When you turn on the Dremel 4000 and is set to 5000 rev its so unsteady then you slowly turn up the speed it will fluctuate up or down know i tell you even a person who knows nothing about Dremel or electronics will ask what the '' '' is wrong with the unit.
As for the 8200 its also have bad speed if turn up the speed slowly it will pulsate in your hand this is because the electronics is switching the speed control on and of very very quicklyto make the seed control work this is because its how its made.
I say again its all about cost cutting they are only selling the units because of the name and the blue and grey color.
If they would only listen to the message board a lot of problems would be solved!