Here are some details about the Y stage mirror mount. The mount is well designed and uses a method called the "flexure" mount. The flexure mount is nice because it is machined from one solid block of metal. The thinner section of metal allows the mirror section to be slightly adjusted as the metal "flexes". The flex allows pure X plane motion without any twisting that could cause a Y plane motion.
As I mentioned in an earlier post, the first corner lens is mounted in a box behind the engraving area. Neither it nor the laser will see the air environment of the cutting area because the laser passes through a window to the cutting area. The details of the following images should give you an idea of the adjustment capability.
Here is where we get the answer to that nagging question. The flying mass is 3.5 ounces. To this I guess you would consider the belt and belt friction to be another item to add to the total mass, but this is still very light indeed.
The wheels are made from a very hard plastic and they do contain small roller bearings. The drive belt tensioner and belt clamps are a pretty slick design.
Those little rollers are used to create a radius for the timing belt to wrap around without causing any stress concentrations and causing the belt to fail during repetitive operations. Not sure if the rollers can be adjusted since that would be a nice way to provide some belt tension as well.
You can see this in one of the pictures posted below.