Let’s consider the gear drive.
What is the error caused by play between gear teeth?
It is almost nothing. One complete turn of the leadscrew is .1” travel so with 80 teeth, each tooth is .00125” so being off a whole tooth isn’t much. Using a real world number of .003” for gear clearance, the error is 0.00003”.
The load on the gear is pretty low. The motors don’t have to generate much torque to move the table. Think how little effort it takes to crank the handles on even a large cut. Probably the highest load would be to spin the handles manually. Some CNC retrofits use motors as much as 1600 oz-in of torque. This much torque has the potential for damage if it runs out of travel or something interferes with the handle or any similar event. It turns out that being able to overpower the motors by hand is an important feature if the cutter veers off in the wrong direction.
What about wear?
The design uses a combination of Delrin and aluminum. Plastic to metal is ideal for gearing. The theory says, during the break-in period, plastic is transferred to the metal and fills up the microscopic pits on the aluminum gear and afterwards the wear is extremely low. However in this application the PV value, that is, pressure x velocity is very low and it may not even be enough to initiate the bedding process which means there isn’t even initial wear. In engineering terms the life is infinite. There has never been a report of gears wearing out.
Using plastic against metal teeth drops noise considerably. In most cases the gear noise is hidden by the sound of the stepper motors ringing. My videos tend to amplify certain frequencies in the camera’s attempt to capture voices. Nonetheless, a belt drive is usually quieter than gears.
Advantages of gears
- They are less expensive than two pulleys and a belt
- Gears place the motor and driven shaft close together at a fixed distance. Belts usually have a much larger pulley to pulley distance. Being able to change center distance without affecting ratio is sometimes an advantage of belts. In this application this is not useful
- Larger ratios are available with gears. A belt drive with a large ratio has a small pulley that must be spaced at a distance to get enough wrap around the pulley. A gear set can reduce speed considerably and still be tucked up close
- Belt drives need some sort of way to adjust belt tension. Tension places side loads on shafts and bearings that are far greater than driving the load. Lack of a tensioning feature on gear drives makes the assembly more simple and compact
- The tight package of the gear assembly allows it to be rotated 90 or 180 degrees if desired