I just wanted to post my experience and pitfalls using rotary tools when glass engraving.
Hopefully those who are looking to start may find this info usefull
I started out using a dremel fitted with a flex shaft.. at first i found this very useful but upon closer examination of earlier works of mine I noticed I was getting unwanted shades where I wanted it just a single shade..
After engraving a few pint glasses I noticed that it was the dremels speed that was causing these issues.
At random intervals the motor would change speed (mainly speed up) this would change the shade of the engraving on the glass. My father in law also has a dremel so I tried his and experienced the same issue!
I then decided to invest in a micromotor and handpeice (as used in dentistry).
This had many advantages over the dremel such as lighter in the hand, more comfortable and natural to use, change in motor direction (im left handed) the speed control for me was the biggest advantage
as you can slow the motor right down to a stop or to 35K rpm.
The micromotor i use is called a "Strong 90" which also came with a foot pedal which again was a massive advantage over the dremel.
I have been using this now for about 12 months and it is very reliable and does not speed up or slow down on its own like I found with the dremel.
The only down side of using a micromotor over a dremel is the shaft size.
The micromotor handpeice takes a thinner burr shaft then that of the dremel.
I also use a FG To HP Shank Converter Bur Adaptor Mandrel 2.35mm to 1.6mm so I can use very small diamond burrs for my glass engraving.
Hope this is usefull to anyone looking to start glass engraving.