It's obvious when you see it.....

 It's funny how even the simplest, most obvious things can sometimes catch you off-guard. Like this belt-driven "race track" for our Full Tilt game remake.

To date, we've stuck with acrylic for fixing the stepper motor and the bearing down and laying things out. It's not the cheapest material to use for "iterative design" (read try something, cock it up, put it right, try it again, etc. ad infinitum). MDF would probably be cheaper and a more appropriate material for prototyping and getting the layouts right.

But acrylic has one thing in its favour - you can make your holes for your bolts ever-so-slightly-too-small (e.g. 3.5mm for a 4mm bolt) and they take a threaded tap really, really well. So where we've got upright "posts" for our bearings to sit on, by using acrylic, we can drill these holes and tap them, and screw the bolt straight into the thread in the acrylic.

It's possible to tap into MDF but after a little while, the threads tend to work loose. 
If ever you put a bolt through a hole in some MDF you almost always end up putting a nut on the other side to keep the bolt in place. And we don't want unsightly nuts everywhere, because that would raise the bearings up in the air, and make the belt ride higher than we'd like.

So we're sticking with acrylic. 
Because laser-cutting holes then tapping them to take our M4 bolts is really neat.
But there's something we overlooked...

When running our belt backwards and forwards, the belt kept "creeping up" towards the top of the bearings (if left unchecked, it would work itself up and over the top of the bearings and effectively just fall off). It was as if the belt wasn't travelling perfectly horizontally after all.....

And that's because.... it isn't.

One thing we'd overlooked is that if there's any tension in the belt at all, it will be pulling against the acrylic base. And with nothing to support it, the long, thin base is simply warping.

Before we go much further with this, we're going to have to build something to give that piece of acrylic some reinforcement!