Quote Originally Posted by Boyan Silyavski View Post
i was trying to say / but as i was busy at workshop/ people typically use for woodworking machines xx10 screws on all axis and xx05 on Z, as this more or less gives compromise between speed, precision and step count/pulse train needed to generate movement


So say you have a servo motor that will spin to 3000rpm. From look of the servo it says 2500PPR/ pulses per revolution/ if i am not wrong. And ball screw moves 10mm per revolution.
Then ball screw needs 1/10 rev to move 1mm. If no geared then 3000/max rpm/x10=30 000mm/min max speed , like Gerry says thats your max velocity in Mach3. So from above it seems then you will need 250pulses per mm , so 1mm/250pulses=0.004mm resolution.

There is something important that Gerry told you but you missed his point. The servos should not be greatly OTT as you may think, they must be closed mutch to what effort they will do, otherwise youwill run in other problems. read literature about servo sizing. Its not a good thing to be much bigger than job they are meant to do.

Then read carefully page 14 from my build thread where Johnatan explained me very well how to calculate details and especially about snappiness.

And by the way i was suggesting 20t at the motor and 30t at the rotating ball nut, not vice versa. So you finish with slightly geared up machine, 0.7-1kw servos, and very fast acceleration. But just do your self the calc. My gantry is moved by 2x 400w servos, not 2x1.5kw ha ha
Yeah I think I did miss understand what Ger meant by OTT. Anyway Fred is holding the order for me so I can make some changes if I want.

Alright doing the math for the inertias of the system I did it based on a rotating ballscrew at the moment.

Some assumptions:
with 2 motors each motor on the X will handle 1/2 of the gantry inertia
Steel pulleys
2/3 ratio on the pulleys

Ballscrew inertia = 7.4E-4 kgm2
Gantry equivalent inertia (1/2 of total) = 3.44E-4
Pulley inertia Driven = 9.86E-5
Pulley inertia Drive = 1.48E-4
Total Inertia = 1.33E-3

Motor inertia for 1.8kw from BST page = 7.6E-4kgm2

So Inertia ratio with the 1.8kw motor = 1.75

I also ran Jonathans Matlab Script and got
Feedrate in m/min: [x y z]=[20.0 10.0 7.50]
Inertia in g-m^2: [x y z]=[1.56 0.96 0.80]
Torque in Nm: [x y z]=[4.29 2.63 2.27]
Inertia ratios: [x y z]=[2.06 1.27 1.06]

It looks like my X axis needs 4.29 Nm, how much above this number generally do I go?
These numbers show a slightly different inertia ratio for X but pretty close.

I know the general rule of thumb here is the keep the ratio under 10 but is there a lower limit as well? Ive searched to see if there is any information about that particular motor for inertia ratio but I havent found anything. Boyan I see your motors show an allowable of 12 but I dont see that for the motors BST sells (searched the model number online as well) so not sure how high I can go with them.

I guess based on these numbers the motors are probably oversized however by how much? These motors show 6Nm continuous, how much is that number trusted for the chinese motors? If I need 4.29Nm per the torque calculation?
Should I be closer to an intertia ratio of 5,6,7,etc?

Dont know if I know more now than I did or less...

Thanks for the heads up