I need to produce a very slow rotational speed for a disk -- 0.25rpm, or 1 rotation in every 4 minutes. The disk is nominally 12" in diameter. It will be driven by a DC gearmotor, through a worm and gear with a 1:20 drive ratio. 

[![enter image description here](https://i.stack.imgur.com/VOdyt.jpg)](https://i.stack.imgur.com/VOdyt.jpg)

The slowest gearmotor that I can find (within my budget and space parameters) is the Servo City 12rpm motor here: <https://www.servocity.com/motors-actuators/gear-motors/heavy-duty-gear-motors/premium-planetary-gear-motors>

But even at that slowest speed, and with the 1:20 worm gear, that still only gets me down to 0.6rpm, more than twice as "fast" as I need.

The options that I can think of would be an additional pair of gears being driven by the worm gear, or a planetary gear reduction unit between the gearmotor and the worm gear. 

A few more parameters:

* All of the components I'm already using for the mechanism are based upon .25" dia. driveshafts (most are from Servo City)
* I need to go with off-the-shelf components; nothing fabricated (modified is OK)
* It must be long-wearing and precise (no plastic or cheap alloys)
* There's limited space between the worm gear and the disk
* My budget for whatever solution to reduce the speed is =/< $50

I had been considering some of the gearboxes from AndyMark ( <https://www.andymark.com/Gearboxes-s/55.htm> ) but frankly, I'm put off by the fact that they don't even list the shaft diameters for their gearboxes; most of them seem to be proprietary sizes intended for use only within specific systems.

Any ideas would be appreciated!