Just as an extra point from Don and Pix, it's apparently not best practice to use spur gear combinations where the numbers of gear teeth can be evenly divided into each other such as 14/28 (or 12/24, 12/36 etc). David Eveleigh brought this up in the VAG as apparently gear wear becomes exaggerated though this may only affect models run for considerable (ie exhibition) distances.
Pix makes a good point about clearance for the m*ff. Failure to allow for this, CAD or paper sketch, can result in much wasted work. Alternative split axle designs will mean some serious micro engineering needing machine tools etc.
It's both amusing and sad that a ladies hand warmer, that I can only recall from one of the 'Little Women' films, has now become unmentionable.
Michael