Quote Originally Posted by martin54 View Post
For plastic and ali I would go for the 5mm pitch screw to keep the torque advantage. You can always gear up if using belts. Diameter is dependant on length/speed but 16mm should be ok.

Ross obviously I got this one wrong, still learning myself & haven't started machining yet other than a few test pieces. Reason I had said 10mm pitch screws was because I was under the impression that you had to use fast speed & feed rates for cutting plastics. Have I got that wrong or would you just gear up 2:1 for that.
No not wrong at all. I just think that for Ali and plastic 5mm would be safer as the Cutting force is higher with Ali than plastic. If the speed is an issue then yes it is a simple job to gear up as plastic is lower cutting force and the steppers are 3.5N

You could take the other approach of gearing down a 10mm pitch which gives the same speed/force but the 10mm ballscrews are less rigid than the 5mm so you would lose out there.

Theres no real right or wrong way but I figure it is best to make it as mechanically as strong as poss and then pick/upgrade motors later.