Tortuga - A sand drawing robot

So, I figure this is as good a place as any to start. My background has always been in hardware, going back to a very early and very brief career in engineering, hardware has always just made sense to me. I think its because to me, it's easier to quantify the physical as opposed to the software side which has been more of an abstract to me, I need to understand how X has affects Y for me to physically land it in my head. I digress. 

Now I've got that off of my chest, let's turn the attention towards the actual design and concept. I wondered, how then should I go about drawing in the sand itself? To my mind, anything sharp or the potential to harm, whilst pretty cool sounding isn't generally what I'd consider to be accessible. So on that note a blunt and rounded off blade was selected as the probable design, printing has yet to begin as I am waylaid by other commitments.