Identify DC Motor from Dremel
You will burn out the motor brush armature interface using 12V on a 4.8V motor with excessing arcing.


  This is not a good idea. 


Motor RPM increases with voltage and starting current which at full voltage is >10x rated current.

With 4.8V giving 14,000 RPM , 12V would try to go up to 35,000 RPM with a starting current also 250% more than original.  Eddy current losses and armature losses with much higher currents result in 2.5x higher arcing voltage on the armature resulting in bridging between armature copper contacts and effectively causing shoot thru on the power supply or arcing short circuits which will burn out the rotating contacts raising the temperature greatly and reducing the MTBF many orders of magnitude to burn out in a minute rather than a month or more of steady operation.

If you want a 35 kRPM Dremmel DIY design, use a better motor and do not exceed rated voltage significantly. 

FWIW Dentists use 400k to 800k RPM with a properly designed system.