NMRV30 Worm Gearbox Speed Reducer NMRV030 Worm Gear Reducer Speed Ratio 20:1 RV30 Square Type for Nema23 Stepper Motor Output Hole 14mm Shaft NMRV-030

This unit is super solid and is a great price for what you get. Gearbox comes fully lubricated and is factory sealed. so don't expect to open it easily (not that you'd need to). It's full of oil and you'd need to remove the rubber gasket without damaging it somehow. The biggest issue I had was finding the appropriate hardware for driving the input side of the gearbox. The main input has an odd inner diameter of 11mm (7/16") with a 4mm wide key and I underestimated how hard it would be to find appropriate components to drive this with a larger shaft. With that said, the manufacturer included coupler that adapts this down to an 8mm d-shaft and that fits standard sized servos traditionally used for this.

As others have said, the mounting holes on this gearbox DO NOT LINE UP WITH NEMA 23 STANDARD HOLES. I bought 4 of them hoping it would be OK. Also, the movement of the gears is quite stiff; definitely use with beefy steppers/servos if you can figure out how to get them to fit. Will be returning these for sure. Absolutely unusable.