The voltage used is relative to the motor running the machine. Going under 5v with a 55t running the lathe makes HORRIBLE cuts as the copper will not chip properly.
"regardless of what anyone else wants to say....." The face speed is more pertinent to getting proper cuts. Voltage is merely a secondary factor determined by comm diameter and driving motor speed.
Soft copper machining is generally around 200 feet per minute face speed. HSS bits will typically be used from 150 to 300fpm rates, and carbide and diamond are used with rates of 400 to 600fpm. Diamond also works well at slower speeds, but I have found carbide to be difficult to work with at slower rates as it is not as sharp. A 10mm diameter comm converts to .39 inches roughly. Multiply times Pi and we get 1.2246" circumference. That is close enough to .1 foot, so we can turn the 10mm comm at 2000rpm for starters. 7.5mm comms will need to spin 33% faster for the same face speed. Light cuts can be made at faster speeds, and sharper bits (diamond) allow for slower face speeds and deeper cuts. Very basic machining principles. I will always choose a faster and lighter cut. Bits last longer and each cut can be fed quicker. It also allows for better cuts with a machine that is less rigid, as many of the toy motor lathes tend to be quite flexible.
I use HSS and diamond bits depending on what is handy. Both can do a superb job. My little motor lathes have roughly a 3 to 1 geardown from motor to armature. Turning a 55t on 3s lipo gets around 9k loaded, a 27t motor would be ran at 3.7v for the same speed. This gives me an arm speed of 3000 and face speed around 300fpm, which allows me to take very light cuts of .0001" at a time on my big lathe and .0005" (radius) on my motor lathes. I can also switch to 7.5mm comms and the face speed is 225fpm, still in a good zone without having to change the voltage. If I tried to run the 55t on 5v it would take forever to complete the job, and the finish would not be satisfactory with a face speed of 135fpm on a 10mmm comm. I tried, it sucked. The copper tears and doesn't chip off properly. A laser tach aimed at the flat spot on the armature shaft being cut is an easy way to find the face speed of any setup. Divide RPM by 10 for 10mm comms and divide by 13.3 for 7.5mm comms. Keep it between 200 and 250 for easiest and most consistent work.
Because of the V blocks on the little lathes it is better to run them a bit slower if you are worried about the tool life. As long as the face speed of the copper doesn't fall below 150fpm and doesn't go above 300fpm you are in the best zone for cutting copper with either HSS or diamond tooling. With a 27t motor timed close to zero and 3:1 reduction you could go down to 1.6v with a 10mm comm or 2.2v with a 7.5mm comm and still get a fine cut. Once the face speed falls below 200fpm an inexperienced user can start to get tool grab and copper tearing, normally caused by taking too deep of a cut or a tool that is not rigid.
My $0.02.
I think one of my first experimental winds will be a 55T lathe motor. I figure that would be the safest application for a motor that most likely wont be perfectly balanced.
