It's a gear box. I am building a larger CNC machine. The one I build is not big enough any longer...
With that larger motors come along. For the heavy axis I use two motors in union. One runs right and the other left.
In the middle is a pulley. Which uses a timing belt to drive the larger Pulley. With the 1:3 ratio the RPM are in the speed range of the machine and the torque is tripled as well. One cannot have enough torque! More when the bugger is running.

I made all parts myself. The screws are available as Library. But I have many in my shop and I am not sure if they are fitting the model. So I make them myself. Screw takes about 30 seconds to make and the coupli9ng a minute. Being lazy... I neglect the thread on the screws. It is more important for me to see how long they protrude in the work piece so I can instantly see if two or more screws would 'hit' each other in the middle. That would be a bad surprise after milling all the parts and then having to figure out that some element cannot be fastened, screws have to be cut, and all becomes a sea of pain.

The Machines i use to make the parts are pre WWII machines (early thirties) ... Once in a while they need a kick and a tweak, but in general they behave better than the cheap modern copy from China.

Cheers
Lemo