I am still continuing to improve my "snakeline" programming approach which I first used for the plastic springs of the flexing battery holders and then for the treble clef. In the meantime, I have enhanced this technique to use OpenScad's child mechanism, so this is one more project trying out this technique in practice...
When my daughter, who is teaching German to refugees, saw me doing some test prints with letters based on the snakeline approach, she thought that such letters would be very useful to her teaching, but she would need them to be magnetic, so that they would stick on the white board. Thus, I added conical magnet frames at the rear of the letters, which could be redrilled after printing with a 3mm drill bit, and cylindrical 3x3mm neodymium magnets could be pressed into the holes.
This project contains STL objects for capital letters (A-Z), numbers(0-9) and some special characters (@#$%^&*+-!?.,)which are about 10cm high and up to 6cm wide. The fully parametrized SCAD source files are also included snakeline.scad contains the snakeline library and sletters.scad contains the character definitions.
Update 2015-10-10: I have now also uploaded the STL files for the lower case characters a-z.
I printed these objects from 3mm ABS on my DIY box-frame Prusa i3, using a 0.40mm nozzle with 0.25mm layer height and 0.56mm extrusion width.
I always print on a clear mirror without Kapton tape, but thoroughly cleaned with acetone and double concentrated lemon juice.
After printing, I redrilled each conical hole at the rear of the letter with a 3mm drill to about 4mm depth. I then removed any drill brim and pushed a cylindrical 3x3mm neodymium magnet into the hole. I did this by taping a bigger magnet onto the working table, putting a 3x3mm magnet on top of it an pressing the letter down on it, using a rubber mallet to make it go in it all the way. Note that it is important for the magnets to sit firmly in the hole, so that it cannot fall out or be pulled out, even with a very strong magnet. This prevents magnets from being lost, but even more importantly, it makes sure that no loose magnets will get swallowed by small children - which is very dangerous!
There is no need to fill all holes with magnets. Using 2 or 3 magnets for each letter is quite enough, using more make it just more difficult to move or remove the object from the magnetic surface.