While it is possible to drive a system with a gear directly mounted on the motor shaft, there are two severe limitations this introduces.
Torque First and foremost, without a reduction (like that in our belt drive systems), you will lose the mechanical torque advantage these systems provide. As an example, for a motor putting out 300-oz*in of torque, with a 1 inch pitch circle gear, a direct drive system will put out about 37 pounds of force, compared to over 100 pounds with our 3:1 belt drive reduction. Stepper motors do lose torque as they spin faster, so reductions much beyond this are not recommended, but with no reduction much of the motor's usable power is not effectively leveraged. A smaller pitch circle gear can provide higher forces, but gears smaller than our current pinion do not effectively keep two teeth engaged with the rack, leading to backlash and lost motion.
Resolution With a 1 inch pitch circle gear, without a reduction the linear motion per rotation is pi * diameter, or 3.14" per revolution. With typical stepper motors that have 200 steps per revolution, this leads to an effective motion per step of 0.015" (0.4mm), which is quite coarse for CNC applications. While 10x microstepping reduces this to 0.0015", microstepping can't be relied upon for accurate positioning between steps, so a direct drive system will leave visibly "stepped" motion. In contrast, with our 3x reduction, these steps are significantly reduced, and because there is less load on the motor, microstepping is also more effective, leading to smoother, higher resolution, and significantly more accurate motion.