[OXO cube]

well designed sturdy looking piece of kit Greg , i especially like the printed adjustment dial , very handy 
one thing that occurred to me , because there is no type of reciprocation ( either manual or mechanical side to side movement ) any high spot on the sand paper tubes grit would transfer to the tyres , leaving grooves 
i know this because i built a 3dp Hudy  style truer/grinder using these same sand paper tubes , which just like the real Hudy doesn't reciprocate at all and left grooves in the tyres 
not sure if it was the grit grade or quality of the tubes i used , but even when i tried changing out tubes the grooves were still present 
i looked at designing an oscillation mechanism but other projects took over
and is the reason i designed the reciprocator for the tyre razor type machines
not a criticism at all , or even if it is a concern for the casual racer ,  just an FYI from my own experience 
Kev