Sizing Steppers

[ArvidHj]

I have been designing and building a CNC machine to cut aluminum and some steel parts. Although I am not sure about whether or not to buy servo motors or steppers. I would like to buy servos since they all allround better in both accuracy and acceleration but I have gone way above my original price range. As the project stands now it has costed about 25 000 kr (roughly 3000 dollars). The cost has primarily been steel and the machining of the frame. I also bought a cheap kit of square linear rails and ball screws that I will upgrade since they were quite bad.

Cost now:
Material (Steel for the frame): 10 000 kr

Linear components (Bad): 5 000 kr
(https://www.ebay.com/itm/Square-Line....c100005.m1851)

Machining: 9 000 kr

Additional cost:
Precision epoxy grout (for align the machined X and Y assemblies to the frame): 2 000 kr

Spindle and accessories: 10 000 kr

New linear components from FBT China: 6 000 kr
(Ball screws: 20 mm in X and Y, 16 mm in Z. Double nut rolled C7 ball screws with 5 mm pitch. 20 mm linear rails with carriages (medium preload) (all axis), bearing blocks with P5 angular contact bearing and couplings)

Sidenote: Does anyone have any experience with this manufacture? FBT INDUSTRIES, located in Zhejiang, China. Chinese Ballscrew,Linear Guide,Linear Robot,Linear Module Stage etc.- FBT INDUSTRIES

Now the choice between servo and stepper motors is what I really wanna figure out. The machine is quite large and of the fixed gantry type.

The Y axis will be moving the table and workpiece, about 140kg. The ball screw is 20 mm thick and 935 mm long excluding BK-/BF15 end machining (TOT 1008 mm). The ball screw mass is 2,5 kg and Inertia 0,39 kg-cm^2

The X-axis will move the weight of Z-axis and adaptor plates, which is about 60 kg. The ball screw is 20 mm thick and 635 mm long excluding BK-/BF15 end machining (TOT 708 mm). Ball screw mass is 1,8 kg and Inertia 0,2 kg-cm^2

The Z-axis will be moving the spindle and Z-plate, about 40 kg (vertically). The ball screw is 16 mm thick and 335 mm long excluding BK-/BF12 end machining (TOT 397 mm). Ball screw mass is 0,6 kg and Inertia 0,01 kg-cm^2

The motors will be direct drive through a coupling. And as stated previously the pitch of the ball screws is 5 mm. Ideally I would like to use Mach 4 hobby (2 000 kr) in combination with Fusion 360 (free).

I will be using a 2.2kw water cooled 6000-24000 rpm spindle motor to cut aluminium. Sadly I am not sure about cutting speeds and feeds.

I have talked to DMM (Dynamic Motor Motion) and they recommended their 3 Axis 0.4kW DYN2 AC Servo CNC System. 15 000 kr. I have done calculations with their 0,4kW motors and they would be properly sized for my machine. Although I would have to buy a smoothstepper (2 000 kr) and also Mach 4 brining the price to about 20 000 kr.

https://store.dmm-tech.com/products/...c-servo-system
https://www.warp9td.com/index.php/order#SmoothSteppers
https://www.machsupport.com/shop/mach4-hobby/

As for stepper motors I quickly step out of my comfort zone. I was thinking about buying the following kit from Sorotec (See link below) for about 6 000 kr. I would still have to buy Mach 4 (2 000 kr) and some sort smoothstepper (2 000 kr) brining the price to about 10 000 kr. Half the price of servo motors. But I am not sure that the motors would be correctly sized for my application.

https://www.sorotec.de/shop/DriveSet...ital-7089.html

Stepper motor specs: http://www.sorotec.de/webshop/Datenb...88.42_V1.2.pdf

What calculations will I have to do to size stepper motors? I am concerned about stability of the system in regards to load inertia and what acceleration and speeds that would be possible. Do I need larger motors? 1000 kr =100 USD (roughly)

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[peteeng]

Hi Arvid - Seems you are quite capable of machine design. The steppers will be cheaper then servos as you have found. The calculations are the same for steppers and servos. Get the torque/speed charts from the stepper supplier and the inertial calcs are the same. You can also get hybrid steppers with encoders which are a halfway house frome steppers to servos. Comes down to your budget and performance required. If your running out of budget then slow the spec down to save money on motors. Peter