The faster you go, the less resolution you'll have. 1/2-8 2 start will give you twice the resolution of 1/2-10 5 start. Twice as many steps/inch. And, depending on motors and drives, 1/2-8 2 start is still capable of giving you up to 150-200ipm speeds.I followed the previous post talking about the issues with 2 & 5 start screws trying to find out what would be good precision screws without going bankrupt so I wanted to create a separate post just dealing with precision instead of speed.
If you have CV turned off, and you circles start and stop, it sounds like the g-code is made up of multiple arcs for each circle, or the circle is made up of multiple straight lines. Ideally you'd want it to be a single G2 or G3 line of code.My second issue would be the movement of the machine when it's cutting a curve or a circular pattern. This does not happen with small radius patterns but on the larger ones the machine does not follow the curve cut in a smooth motion and it travels in waves as it receives the information from Mach3. It does not have the constant velocity on curved patterns (diagonal straight cuts are fine) and I know there is a setting for CV in Mach3 but if I set it on then my sharp corners come up rounded.
One thing you can try with CV mode. Go to the settings page, and make sure CV distance and CV Feedrate are turned OFF. Then, go to General Config and uncheck all the CV settings except the last one. "Stop CV on Angles>". Check this and set it to 89°.