Confused about stepper sizing

[Jeff]

Hi

My mechanical build is almost complete 20mm HiWin rails and bearings on X & Y axes with 2010 lead screws X axis is 1500mm and Y axes are 1000mm. Total weight of my gantry including the spindle is around 40Kg and I've been trying to calculate what size motors I need for my axes, but both my own calculations, and the output from several online motor calculators are giving me results that I wasn't expecting.

For example on the Y axis with a total load of 40Kg and a cutting force of 5N my total required torque @ 80 rpm (800mm/min) is coming out with a figure of around 0.18Nm, this would easily be achievable with the 175oz Nema 23 stepper on the openbuilds store which should put out around 0.7 Nm @ 48v and 80 rpm.

My gut instinct was to go for motors in the 400+ oz range, which seems to be what everybody recommends for this sort of size machine. Are my torque calculations way off or is there another reason that I need to massively overspec my motors?

[Clive S]

Hi Jeff and welcome to the forum. In my opinion I would go with nema 23 3.1Nm from cnc4you and get AM882 drives from China and run them at 68V with a toroidal transformer power supply.

[Jeff]

Hi Clive, I had looked at the motors from cnc4you before I confused myself with my calculations. I think I know where I stuffed it up I was getting confused about feed rates inches/m vs mm/m the speeds I used in my calculations are way too low. I'll do them again at around 10000mm/min and see where that leads me :)

[Jeff]

Yeah that was where I was going wrong :). The 4Nm motors from cnc4you will give me cutting speeds of around 3000 mm/min and rapids around 10000 mm/min (which is as high as I can go due to the critical speed of the ballscrew), on a 48v supply with a 3x margin. 68V is way above the ideal voltage for these motors (3.0mH inductance) so I think I'll stick with 48V rather than turn my motors into workshop heaters :)

I'll probably get the CWD556 DSP drivers from there too.

[Clive S]

Yeah that was where I was going wrong :). The 4Nm motors from cnc4you will give me cutting speeds of around 3000 mm/min and rapids around 10000 mm/min (which is as high as I can go due to the critical speed of the ballscrew), on a 48v supply with a 3x margin. 68V is way above the ideal voltage for these motors (3.0mH inductance) so I think I'll stick with 48V rather than turn my motors into workshop heaters :)

I'll probably get the CWD556 DSP drivers from there too.

Jeff It is the norm to use 68V with the motors from cn4you 3.1Nm you won't have any trouble with them. You will stiffen the machine with only 48V. Your choice though>

[Ger21]

68V is way above the ideal voltage for these motors

The "ideal" voltage is the voltage that makes them perform the way you need them to.

If you'll be cutting wood, you'll quickly find out that 3m/min is quite slow.

[AlexDoran]

I'm using the same motors, am882 drivers and a 68v psu. Got the motors running at 12500mm/min at first testing.