[Crimson11]

Good Evening,

I am totaly a newbie to CNC world.

I bought a CNC without its electronics. It is 5' x 3' (150cm x 100cm) and has steel gantry of 60lbs (27 kilo) which moves on ball bearings.

The size has to be Nema 23.

I was wondering if 439 oz-in.(3.1nm) will be enough or should I go for 566 oz-in.(4.0nm) versions?

Thanks a lot in advance.

[awerby]

You're focusing on the wrong numbers. Those figures are about holding torque - how hard the motor holds still. This is not as important as how well it moves. That's determined by other things, like inductance (lower inductance means higher speeds without faulting) and voltage (more voltage makes a motor go faster, up to a certain point). The formula is: square root of the inductance times 32 equals optimum voltage to be supplied. So if you want the motor to go fast, you need to use a driver that will handle enough voltage, and give it an appropriate power supply. In practice, the sweet spot is around 50 volts for most NEMA 23 steppers, which works out to about 2.3 milliHenries of inductance. The running torque of a stepper depends on how much current it can handle. In a 23-frame motor, that will vary between about 2.5 to 4 amps per phase. But the inductance goes up along with amperage, so you have to balance all these factors. Look at the torque curves that should be listed with the motors to find one that maintains good torque at the speed you think you'll be running them, since torque falls off with speed, sometimes drastically.

[Crimson11]

You're focusing on the wrong numbers. Those figures are about holding torque - how hard the motor holds still. This is not as important as how well it moves. That's determined by other things, like inductance (lower inductance means higher speeds without faulting) and voltage (more voltage makes a motor go faster, up to a certain point). The formula is: square root of the inductance times 32 equals optimum voltage to be supplied. So if you want the motor to go fast, you need to use a driver that will handle enough voltage, and give it an appropriate power supply. In practice, the sweet spot is around 50 volts for most NEMA 23 steppers, which works out to about 2.3 milliHenries of inductance. The running torque of a stepper depends on how much current it can handle. In a 23-frame motor, that will vary between about 2.5 to 4 amps per phase. But the inductance goes up along with amperage, so you have to balance all these factors. Look at the torque curves that should be listed with the motors to find one that maintains good torque at the speed you think you'll be running them, since torque falls off with speed, sometimes drastically.

Thanks a lot for the reply.
I was just wondering what inductance means as I was reading the threads...

I have no idea in reality about what speed that I need, but as I was going through some sites etc. lets say, it could be around average of 16 inches/min as suggested on one of the sites. Or should I be looking something with more speed?
I will be working mostly on wood and perhaps some aluminium.

The kits that I found are 4.2 ampers for motor with either CW5045/DM542
or
tb6560 for 3.0A motor with 3.5A drivers and 36V power supplies.

[aarggh]

Thanks a lot for the reply.
I was just wondering what inductance means as I was reading the threads...

I have no idea in reality about what speed that I need, but as I was going through some sites etc. lets say, it could be around average of 16 inches/min as suggested on one of the sites. Or should I be looking something with more speed?
I will be working mostly on wood and perhaps some aluminium.

The kits that I found are 4.2 ampers for motor with either CW5045/DM542
or
tb6560 for 3.0A motor with 3.5A drivers and 36V power supplies.

The DM542's are a great driver and pretty bullet proof, I would suggest don't even bother wasting your time on anything TB6560 based, you'll be bitterly disappointed and after spending weeks of time trying to turn that pigs ear into a silk purse, will go and buy better drivers.

A 48VDC PSU would be much better than a 36VDC PSU in this case.

cheers, Ian

[Crimson11]

The DM542's are a great driver and pretty bullet proof, I would suggest don't even bother wasting your time on anything TB6560 based, you'll be bitterly disappointed and after spending weeks of time trying to turn that pigs ear into a silk purse, will go and buy better drivers.

A 48VDC PSU would be much better than a 36VDC PSU in this case.

cheers, Ian

Thanks a lot for the reply.

Here are more likely options that I have:

(they have single shaft version for the same price)
IT FREE,3 AXIS Nema 23 Stepper motor 4.2A 425oz-in 8mmshaft 4-Lead 115MM CNC HOT | eBay

or

Nema23 Stepper Motor Kit 4Nm x 3 Axis + Mounts CNC Parts Mill Router DIY Hobby | eBay

[aarggh]

Thanks a lot for the reply.

Here are more likely options that I have:

(they have single shaft version for the same price)
IT FREE,3 AXIS Nema 23 Stepper motor 4.2A 425oz-in 8mmshaft 4-Lead 115MM CNC HOT | eBay

or

Nema23 Stepper Motor Kit 4Nm x 3 Axis + Mounts CNC Parts Mill Router DIY Hobby | eBay

I wouldn't consider either of those two to be very great kits, one has steppers with 9mH inductance and is supplied with a 36VDC PSU, the other has 12mH inductance if you wire is series, and 3mH in parallel, but at double the current load. And again supplied with a 36VDC PSU.

The DM542's or equivalent, a 48VDC PSU, and steppers as close to 2.3mH or so inductance for any small to mid (or even large) machine will usually be the sweet spot and most optimal config.

The rough but tested way to size a stepper's PSU is 32 x square root of the inductance, so as an example a 2.3mH stepper works out to around 50VDC, so a standard 48VDC PSU is just perfect.

cheers, Ian

[ger21]

I have no idea in reality about what speed that I need, but as I was going through some sites etc. lets say, it could be around average of 16 inches/min as suggested on one of the sites.

You want at least 200-300ipm, if possible.


How is the machine driven? Screws, or rack and pinion? What are the specs of the drive components?

[Crimson11]

You want at least 200-300ipm, if possible.


How is the machine driven? Screws, or rack and pinion? What are the specs of the drive components?

Thank you for the reply.
Leadscrew of 25mm diameter / 3mm.
Don't know what other specs you need?

The leg of the gantry slides on 2 triangular profiles.