What is the right stepper motor for the laser table?

Hi, my laser machine has “only” a manual controlled, driven laser table (Z-axis) with the Up and Down buttons, and I want to change that. The table has four threaded rods connected with a chain, driven by a typical standard motor (with a gear on the top).

I read some posts that others installed hybrid stepper motors, and I found lots of them, but I have no idea how strong the motor should be 3, 6, 8, or 12 Nm. What is enough, and what is the minimum? I know the strongest is always ok, but I think why should I buy, for example, 12Nm when 8Nm is perfectly adequate.

Has someone of you an idea what type of stepper motor I need and how strong it should be?

In addition to that, I saw in some youtube videos that they sometimes use the existing power units of the x-/y-axis for the motor of the z-axis. I think it is better to install a separate power unit for the Z motor.

What is your opinion?

Thanks in advance

This is a newton torque wrench. ( https://www.amazon.com/Neiko-03727A-4-Inch-Torque-Wrench/dp/B01FMXEWQC/ref=asc_df_B01FMXEWQC/?tag=hyprod-20&linkCode=df0&hvadid=309763890402&hvpos=&hvnetw=g&hvrand=4340086480963430426&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9015618&hvtargid=pla-572951906812&psc=1 ) hooked to whatever you want to measure (the chain and 4 lead screws), watch the needle and see where it starts to move. if it move starting at 4 newtons then double that number for good measure (4 x2 = 8). Then a 8 newton stepper or bigger will work.

Do you have a limiting factor in the physical size of the motor? Cost probably isn’t a big deal.
NEMA34 is an option.

Current is not likely an issue. Presuming you’re using modern stepper drives, a “3amp” motor is the winding current and that is not a 3A load on the power supply, it’s a wattage load. Wattage is low while the motor is static and just holding. The Z will never be in motion and creating a large load when the XY motors are running and creating their loads. Dynamically, like a “3A” stepper motor might be a 1A load on a 24V supply and a 500mA load on a 48V supply.

First of is statics- what’s the pulley ratio, the lead on the screws, and the weight of the table. Then a question of how much acceleration you need.

Best to NOT use separate power supplies for these motors. You NEED the z supply there to act as a constant load to help bleed off back EMF from the XY motor’s braking torque. That puts current back into the supply which the supply can’t regulate, and it can overvolt it. But more loads makes that unlikely.

But, ok, simple answer. 600mmx400mm bed, NEMA23 1.9Nm is fine

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