Hi. Don't have a mill but have a Sherline lathe. Expensive but a very awsome and capable bit of kit and have turned SS and cast iron with ease. The 3 jaw chuck is only about a 1/4 thou out and now 10 years use. Being made in the US they are imperial, check the lead screws, they do metric versions but imperial is most common which would go a way to explain 5.1 (5.08) if the conversion is 2.5mm per 1/10th inch instead of 2.54 (just an idea?). Anyway, definately the best small lathe money can buy.
http://www.sherline.com/CNCfaq.htm
stuck, have you asked them? I have found their team obliging and helpful gentlemen.
Tom