CNC stepper motors etc, help

Mickyas said:

Hi guys, apologies if I've posted in the wrong location or if it's been covered umpteen times already.
I have absolutely no CNC experience whatsoever, but I'm a tinkerer and love trying to make things just for the hell of it which has led me here.
I am attempting to make a CNC (for wood and aluminium) measuring approx 600mm x 600mm. I have had a look over a few threads trying to gather info, but I'm struggling a bit.
I have plans for the machine, but after reading about the stepper motors the plans recommend, they could well be too 'underpowered' and are miles apart (specs wise), they are:-
1st recommended is a Nema 23
2.3 v
2.8 A
0.83 ohms
2.2 mH
The 2nd is a nemá 23
3.08 v
3.0 A
1.1 ohms
3.6 mH
I'll be using 4 motors and am thinking of using a DM542 stepper driver for each (as opposed to the TB6600 suggested in the plans) and an Arduino uno 3 (or raspberry pi3??), there's also 2 x SFU1605 ballscrews.
I have researched other motors and found the following:-
1st choice nema 24 2 phase
2.5 v
5.0 A
0.5 ohms
2.2mH
2nd choice nema 24 2 phase
1.4 v
4.0 A
0.35 ohms
1.3 mH
...my questions are...
1. Are the nema 24 overkill?
2. What size power supply should I be looking at for 4x nema 23 or 4x nema 24?
3. What driver should I use...DM542, DM556 or R60???
4. Does the inductance really matter that much?
5. What current, inductance etc should I be looking at?
Going off a calculator in one of the threads, am I right in assuming the rpms are (respectively), using a 36v power supply...
1st nema 23 = 876 rpm
2nd nema 23 = 500 rpm
1st nema 24 = 490 rpm
2nd nema 24 = 1038 rpm
Any help would be much appreciated, please bare in mind I know nowt about CNC etc, so layman's terms please. Thank you.
Mick

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Hi Mikyas,

Confusing topic isn't it? There are so many options and each one is expensive and you want to make the best decisions you can.

I just finished a router build that has a 30" (762mm) wide by 48" (1219mm) long cut size. The machine I built uses belt drive for X and Y and I use NEMA23 and have it running at 1000 inches per minute on rapid travel.

In my case Nema23 was a perfect match for this router. They are fast enough and powerful enough. I can cut 3/4" plywood in one pass with a compression bit at 25ipm which is good enough for me. VCarving cut speeds at 50ipm which is fun to watch!

If I was just starting out, I would go with the highest torque NEMA23 with dual shaft you can find on line. eBay and amazon have kits. Then I would go for a USB interface card like the RNR Controller (AliExpress) and associated controllers (I used Gecko but there are lots out there). You want enough power to run all the motors. In my case I have two power supplies, a 24VDC supply for the RNR and Z axis, and a 36VDC supply for the single Y and double X motors. I used large capacitors on the outputs of the supplies to provide a bit of extra capacity.

The easy math for power supply calculation, is the motor currents all add up. So if you use a 3A stepper motor for everything and there are four motors, you will need 4*3A=12A. You want the highest voltage possible to give you the highest speed from the steppers, 36VDC is good, some systems use as high as 80VDC. Since the four motors will require 12A (assuming they all are running at the same time), you would need at least 15A for a bit of safety margin. AliExpress, eBay or Amazon are good places to find power supplies.

Now one thing that most people do not think about is the cabling. CNC Machines are electrically noisy. The motors radiate Radio Frequency Interference (RFI) and Electromagnetic interference (EMI). I would go with 18 AWG (the wire size) two pair, shielded wiring (you may want three pair depending on the motors and controllers you pick). The shield is only grounded in the control box not at the motor end or you may end up with a ground loop. I used Aircraft Connectors (Amazon) for the connections at the controller. While you are thinking about the wiring, remember you will need LOTS of it. One wire for each motor, that are long enough to move to the far ends of the machine. You need some slack, and room to work on the control box as you fiddle with it. While you are wiring the machine, do not forget the cabling for the limit switches for this, I used 3 wire 18 AWG shielded so I could use proximity style limit switches (AliExpress). Again ground in the box on one end only. You will also need wiring for the spindle, and make sure you future proof it with a relay controlled AC outlet in case you want a router rather than a DC Spindle. AliExpress also has 24VDC relays that are useful for turning on/off the DC Spindle power supplies, as well as other things like LED lights for illuminating the cutter area for bit changes. You will also want a pair of wires for the auto Z zero touch plate.

Now, thinking about all the wiring you are putting into the box, along with the controllers, power supplies, relays, etc. Try and find the largest control box possible. It is no fun jamming all this stuff into a small box. I found it easiest to mount the components onto a board that was slipped into the box once the majority of the cabling was completed on the bench.

Good luck with the build.

How do you eat an elephant? One bite at a time. A CNC build is a big project, but each part is relatively simple, so start with one part and keep going and it will be done in no time.

Remember to document as you go, you are going to be learning a lot quickly. You will forget it just as quickly and next year you will say to yourself, now how did I do that..... I use Google Drive and just keep adding information to my build document. As it gets larger I format it so it is organized for easy reading and reference later. Once the router was built, I printed it out and put the "manual" in a booklet and leave it by the machine. All the schematics, product info, and notes are all in one place.

Regards,

Steven

1. Are the nema 24 overkill?

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Short answer is no. I've never met or heard from anyone that wouldn't like their machine to be faster.

2. What size power supply should I be looking at for 4x nema 23 or 4x nema 24?

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For voltage, the highest your drive can safely use. Speed (rpm) will be roughly proportional to voltage.
For amps, take 2/3 of the motors rated current, and multiply by the number of motors. I'd add 1-2 amps for a little overhead. For four 4 amp rated motors, that's 2/3 of 16 amps, or 10.66 amps. I'd get a 12 amp power supply.

3. What driver should I use...DM542, DM556 or R60???

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The only difference between a 542 and 556 is the max current. If the motors are rated less than 4.2 amps, both will perform the same. Not familiar with an R60.
If you want even better performance, you could go with a DM860 and use a 60-72V power supply.

4. Does the inductance really matter that much?

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Yes, it's really the most important factor in how fast a motor will spin. The lower the better. But it is somewhat related to other motor ratings.
For example, a lower inductance motor will also have a higher current rating and lower voltage rating. All else being equal, you'd want a motor with a higher current rating.

5. What current, inductance etc should I be looking at?

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As I mentioned above, the lowest inductance, and highest current.


There's a proper way to size stepper motors, but almost nobody does it.
You need to figure out the mass of the moving components, the required acceleration rates and velocities, and the forces required to achieve those accelerations and velocities. Then select a motor (and drive) that meets those requirements.

I'd go with the 4 amp Nema 24's, if it were me.
Also, you do NOT want to use 1605 ball screws. They require higher rpm's to achieve higher speeds, and steppers lose torque as RPM's increase.
1610 screws will give much better performance.

You have gotten some good advice above. I will provide a bit more about figuring out how to determine the optimum way to size voltage and current for steppers. The inductance of the stepper is the key. Those voltage ratings you listed have nothing to do with operating voltage, that is just the voltage drop across the coil at full current. The ideal operating voltage is given by V = 32 x sqrt(inductance) . Lower inductance means that the operating voltage of the stepper can be lower. So while a stepper with an inductance of 2.5mH can operate well at 48V, to get the same speeds, one with an inductance of 5.6mH would need 72V power supply. Many stepper drivers cannot operate at the higher voltage. No Nema23 sized steppers will operate at peak efficiency at 24 or 36V. Now, the power of the stepper (what actually creates movement), is given by P=VxA, voltage times amps. So lets say your stepper is rated at 3A. Running it at 24V, you can get 72Watts of power. Running the exact same stepper at 48V and you now get 144 watts of power. The higher voltage (and wattage - power!) makes it twice as powerful and much less likely to "lose" steps. It will also have nearly double the speed. Voltage affects speed, current (amps) affects torque.
One other thing to consider, is that the "oz in" (or Nm newton meter) torque ratings are at "stall", 0 rpm. Pretty useless for CNC, you want the torque while rotating, which is less than the stall torque. The better stepper sellers will have a "torque vs. rpm" chart or table for their steppers. That way you can calculate the actual Force that the stepper can generate across the speed. I made a little spreadsheet when designing my machine, helped me determine the max federates (where there was enough force to overcome cutting forces). I have actually replaced the 1605 ballscrew I started with for Z-axis with a 1610 for more speed. Ballscrews are incredibly efficient at converting torque into force.


Here is a more detailed explanation:

Stepper motor basics