The reduction should be determined by your max blank diameter and what resolution you require. For example, 6:1 yields .002" step resolution (1/8 microstepping) on a 6" diameter blank. More than ample for rotary wrapped machining up to that diameter. That said, if you are planning on indexed machining, especially off axial center, then the 20:1 may be a better choice. The 20:1 will reduce max rapid speed to around 50-60 rpm, if that is a factor and you may not be able to position it by hand. Are there backlash specs for that 20:1 reduction?
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