Question about shielding of cables

Hi all. I hope this is posted in the correct section - I did look at the descriptions of the other sections and this seems the most accurate (I am using a UC300ETH controller). I just wanted to know people's thoughts. I am almost at the end of my first CNC mill conversion and have just this morning heard about the plague of EMI on CNC machines.

I have a small mill (about X2 size but made by Real Bull rather than Sieg). I have a UC300ETH controller and breakout board, DM556 stepper drivers and Nema 24 566 oz.in steppers. I have ME-8108 limit switches on each axis. Each stepper driver will have its own power supply (36v 5a for X and Y axis and 48v 5a for the Z axis). The controller will use a repurposed computer PSU with a boost converter that I put inside it to get 24v. This is (probably obviously) not anything other than a hobby so things are done on a budget.

The mill is on a movable table, metal framed with a metal coolant tray under the mill. The motor is a brushed motor, controlled by the controller provided by the manufacturer - I think it is a universal motor. All the other electronic hardware is underneath the mill in a wooden drawer.

I hadn't considered any requirement for shielded cables or any suppression. I had planned to use unshielded throughout. I wont be making aircraft grade parts or anything of the sort. But do you think I could be opening up a can of worms if I dont use shielded? With no VFD or any high frequency motors, do you think I can get away without shielding? If the answer is no or not likely, what is the bare minimum I should shield?

Thank you very much for any answers / recommendations you can provide.

Hi,

Not shielding is like kicking a sleeping dog, at some point it's likely to wake up and bite you.! For the sake of a little wire, it's not worth not doing it.

The main areas to use are Stepper cables and limit/home cables or any cables that carry low voltage signal cables.

Here are a few tips to help you avoid EMI :

#1 Star grounding is a MUST DO, this means all Gnd's and shields, etc go to a single Earth point.
#2 Keep low voltage signal cables, like step/Dir, limits, etc away from Mains voltage wires.
#3 Use shielded cable and only ground one end of the cable at the control box end.
#4 Earth the frame.
#5 Use 24vdc for signals like E-stop, limits, homes, etc.

Now you say you are using a booster of the PC PSU, this is a bad idea and could cause problems because you are tieing the 24Vdc Gnd to the PC Gnd which can cause ground loops. For the sake of £10 do yourself a favor and buy a dedicated 24vdc PSU, you only need one 3a at max for the low voltage side.

Thank you for the excellent reply. Well, excellent except that I have more work to do...

I thought I would just clarify something about the computer PSU. I am using it because I need 5V for the controller as well as an Arduino, I want 12v for the fans and I figured it had more than enough juice to give me 24v through a boost converter. Just in case you thought I was actually using it in the PC which will feed the controller - I am not; I will be using a separate laptop. I do have a 24v PSU but I figured the controller only wanted 500ma and seemed overkill having another supply. All the grounds of 5v, 12v and 24v would therefore be the same. Does it still sound like a bad idea? Should I have separate 5v, 12v and 24v supplies?

Thank you for your input

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Originally Posted by JAZZCNC

Hi,

Not shielding is like kicking a sleeping dog, at some point it's likely to wake up and bite you.!

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That's a tag-line waiting to happen.

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Originally Posted by JAZZCNC

Now you say you are using a booster of the PC PSU, this is a bad idea and could cause problems because you are tieing the 24Vdc Gnd to the PC Gnd which can cause ground loops.

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Generally agree - but think the OP is using a repurposed PC PSU as a stand-alone 24V supply.

OP: You're using mechanical limit swithes, into the BoB?, you don't mention the nature of the BoB, but I'm assuming the common Chinese flavour which has opto-isolated inputs. That should provide a bit of EMI resilience (noise picked up in the cabling on the machine is unlikely to have enough energy to generate a spurious trigger on the BoB input). A brushed motor is likely to generate a bit of HF noise - usually suppressed on the motor with a cap filter).

Ultimately the proofs in the pudding. You might find that your machine works well. Or you might find a spurious trip - usually half way through a job that ruins the workpiece. What Jazz says above is generally good advice.

Thanks for the response. I will be using the UCBB breakout board paired with the UC300ETH. I bought them in a package from CNCDrive in the hopes that it would make compatibility easier. Does that change anything with your answer?

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Originally Posted by masdo

Does that change anything with your answer?

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Not mine - though it does address my assumption that the board has opto-isolated inputs.

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Originally Posted by masdo

Thanks for the response. I will be using the UCBB breakout board paired with the UC300ETH. I bought them in a package from CNCDrive in the hopes that it would make compatibility easier. Does that change anything with your answer?

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The UCBB is a good board with nice fast opto's and you won't have any troubles with it.

I did miss understand the 24V PSU and thought you were using the control PC to provide. So Ignore what I said, how you have it will be fine. But I would still use Shielded cable and do all that I said regards Star Gnd etc.

Thank you for all the feedback and information. So much for a nice easy: 'No, everything will be fine'. Oh well, that's what learning is about...

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Originally Posted by masdo

Thank you for all the feedback and information. So much for a nice easy: 'No, everything will be fine'. Oh well, that's what learning is about...

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It very well could be fine and you could take the line of leave it alone and deal with it later if it does cause trouble. However, I find it's always easier doing it right the first time and very often works out cheaper, esp if you factor in lost work time and snapped cutters, material, etc when the Sleeping Dog wakes up.!:sorrow: