Ok lets bring this back to real world use and implementation.?

While the motor calcs like Irvings are great they are really only good for giving a ballpark figure IMO.!
Unless every aspect of the machines friction and resistance to movement etc are known, which can vary wildly dependent on design, build quality and component quality then it's very difficult to accurately choose the perfect and ideal motor.
Also like Irving pointed out it's very difficult to get real useful motor torque curves graphs from manufactures.? When dealing with cheap motors this is no accident either because they would show that they are really quite below what people might expect.? Only the better motor manufacturers tend to give them and these cost far more money than the average DIY user would want to spend.!

In real DIY built in the shed, garage, basement or coal house and soon to be back of a boat.!! . . things don't often work out like the Calcs or graphs would have you believe so some (plenty) sticky wiggle room needs to be factored in.
Then there's electronics side and in-balance that using different sized motors brings to the control box and it's power requirements.?
Lets take this machine or system for instance.!
If ball-screws are used then it may be possible to use 1.85Nm motors which will run on 40-45V using 50V drives for 2 axis but the 3rd twin screw setup will need at least larger 3Nm motors and 65-70V and 75V drives. This means 2 different PSU's costing extra money and space.
The only advantages to the smaller motors will be they spin slightly faster which really in all probability won't be required, they will accelerate slightly faster but again the little extra will be negligible in real use for this machine, the total cost will be slightly less.
The while on paper the larger motors will accelerate slower and spin slower in real world use the difference won't make much if any difference to this machine other than cost. This is offset by the extra PSU required and not so massive.
The larger motors on the other hand will give great over build "wiggle room" while not having any real detrimental affects to machine performance.
Yes While it's very true Bigger is not always better in this case the difference won't be so much it affects performance to be worth bothering about.!

Then there's the longevity factor.? An over spec'd machine tends not to be so stressed and can run well within it's limits giving long life to both motors and drives.

That's my view on the motor choice obviously others have there's.!


Regards the twin screw and 3.1Nm then yes if you use ballscrews with 10mm pitch and gear either 2:1 or 1.5:1. This will increase torque and resolution while still keeping the motors in a nice RPM range well below the corner speed while cutting. The sacrifice will be rapid speed which to quote John S only any good for "dick slapping".

Like Jonathan I urge you to use ballscrews and the difference in cost if bought from china won't be very much and really lead screws can't be compared with even cheap ballscrews unless high quality lead screws which cost more than ballscrews.!

Use 16mm as well not 12mm.

Also if you use the 90x45 profile from KJN then the BK/BF blocks fit the slots and can be bolted straight on. Also the BK/BF blocks from china are ridiculously cheap and I see no point in making your own.?