They can multiply the power of a small motor making even a tiny motor lift a lot of weight.
Here is one of my designs. This is the second of this model I have made. It is still a little rough looking but it moves well.
I have since replaced the scratched plate that is missing some bolts with a new 12mm thick clear acrylic part and shiny new zinc chromate coated bolts.
The stepper motor is 2 amps and the black box under the motor is not a reduction gear, rather it houses a flex coupler. That little box is 3D printed. (using machines to make more machines)
This will hold the little trim router going into this particular machine and provides the up and down movement.
Since it is up and down movement I can get away using a threaded rod and nut without backlash
I have machined these as anti-backlash nuts, however the long nuts work just as well and I have yet to experience backlash with them even with months of use. The thread rod is m8 stainless. This moves the machine 1.25mm per turn. The white part is 3D printed plastic.
I prefer stainless metric thread rod over English measure for my CNC machines mainly because the math comes out even. I use mild steel nuts as stainless to stainless parts tend to stick together.
With English measurement rod, the fractions come out often times to for example 100.xxx steps a stepper motor can not move in numbers that are not whole. For example 10.5 steps.
This is still a bit of a prototype design. I am looking for a way to join the corners of the frame in a more reliable fashion. Maybe by the use of corner brackets.