Quote Originally Posted by Tenson View Post
I'm sorry to be persistently stupid but I still don't understand the desire for 10mm pitch.
It's all about the speed vs torque. Remember with steppers as the speed rise's the torque drops away.
So with a 5mm pitch screw and usable max torque for cutting feeds translated to rpm is around 1000rpm with nema23 motor will give about 5mtr/min if your lucky . Taking into account the safety margin for twin direct drive screws the safe area will probably be around 4-4.5mtr/min(Obviously this will depend on mass and friction of each axis). Now at these feeds you'll have low torque so any deep roughing or hard material will require even lower feeds.

With a 10mm pitch the same 1000rpm gives you 10mtr/min with a similiar amount of torque. So obviously at the same 5mtr/min the motors are spinning half the speed so far more torque available so deep roughing cuts are no problem if the spindle and machine can handle them.
The other reason as Jonathan pointed out is 10mm pitch means for the same speed the 10mm pitch screws are spinnig half the speed of 5mm so less chance of whip.
Thou to be honest I don't hold much stall by this and Jonathan and I have debated this before.!
While in principle he's right in physics terms but in practice it doesn't quite work that way and my machine is living proof as it use's all the things Jonathan disagree's with like: twin 20mm x 5mm pitch geared 1:2 with nema 34 and non AC drives on 80V.
It will rapid at 12mtr/min if need be thou I restrict it by tuning to around 7mtr/min to save on screw wear as I don't require high rapid feeds and this allows higher acceleration which is much more usefull.
It will happly cut at 7mtr/min and if you check the vertical post I put up you'll see it cut 10mm deep 1 pass in dense resin inpregnated MDF with no problem, it even cut full depth 13mm for half the length untill the very very worn 6mm cutter snapped and even then the machine didn't stall.
So the critical speed issue is a non starter to me and I'm sure by the very fact of human nature most won't have the will to resist using all the available speed the 16mm x 10mm will give them so whip will be an issue because there's no strength in the screw. With 20mm you have more meat so better able to handle the slightly higher spinning if geared like I do.

All that said if you don't require high resolution then I would still advise you to go 16mm x 10mm pitch as it's far more versatile and cost affective. Rotating nut, direct drive or connecting belts is an individual's decision.!! . . . All I'll say is you still have the sync issue whether rotating or spinning screw.!!. . Thou Jonathans obviously then man to tell you exactly how it differs to direct drive spinning screws.