Thread: Earthing?

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  1. #11
    Trust me they don't come any stranger
    The more I know, I know, I know the less. (John Owen)

  2. #12
    After reading these post it's all becoming a little clearer, so many thanks.

    A few questions that may be a bit naive, but important for me to understand things better:

    -I'm in the process of putting all the elements into an enclosure. From a practical point of view, should I buy a plastic or metal enclosure?

    -Regarding the star ground connection, if the "star" point is a bolt making contact with the metal enclosure, to which all the grounds are connected, and the 3 drivers metal base are also touching the metal enclosure, does that affect the efficiency of the star connection in any way?

    -Also, my 68V linear PSU (Leadshine) the mains input Ac doesn't have an earth connection point so only the Live and Neutral are used, shouldn't the Earth wire be connected to something? When it finally goes into the enclosure the 240V mains enclosure input socket will be earthed to the star point, so I guess that's all that's needed as far as the drivers PSU?

    -On my vertical mill spindle motor, there is a ground wire bolted to the back of the motor. Why do you need this if the motor casing itself is already touching the mill metal body, which presumably is in turn also grounded somewhere. Is there a star arrangement within the mill itself?

    I can't think of anything else to ask...wait a minute, I notice in the schematic (Jazz) you have a fuse to the PSU, etc. Is this essential or recommended since the PSU is already connected to a fused mains socket?

    Last edited by Edward; 13-01-2017 at 03:47 PM.

  3. #13
    OK, I'll kick off but there will be plenty of opinions given to this list of questions! One point to remember is that sometimes you earth for electrical safety and sometimes for screening/electrical noise reduction. Good earthing practice via star point will generally address both concerns.

    Metal case - shielding comes for free that way. You will probably need a bigger box than you thought, and not sure about strength of larger plastic boxes.

    Driver enclosure is isolated from all internal electronics so doesn't matter whether it's connected to ground or not. Bolting an anodised enclosure to, say, an epoxy-coated steel box might or might not give a decent electrical connection but I wouldn't worry about it myself. I didn't in my own control box.

    I reckon that anything that has mains going in should be earthed, so if your PSU doesn't have a connection for mains earth, then the case should probably be earthed (via a wire to a mounting bolt? See comments about bolting to epoxy-coated steel) anyway, and that would go to the star point, where the mains earth is also connected.

    My first router was built from MDF so the spindle motor (usual 2.2KW water-cooled) was isolated from ground. Under certain circumstances, you could feel a slight tingle from it when you touched it while running, which was some kind of electrostatic or electromagnetic leakage from the power feed to it. Not dangerous but mildly inconvenient. Decent earth connection immediately stops this kind of thing. Also, if you are going to use a touchplate for tool height setting, the spindle must be earthed. It might be that your mill builder took a belt-and-braces approach to earthing, but it might be because it's better to avoid relying on a metal-metal contact in sliding surfaces which, theoretically, are held apart by a tiny layer of lubricant. My new all-steel router uses earth wires running through the various cable chains to make sure that there is an earth wire between all pairs of moving parts to avoid relying on contact via the profile rails and bearings. Like that, there is a solid electrical earth run from spindle platform right back to the main router frame which is in turn earthed inside the control box by a wire from a mounting bolt back to the star point.

    I use a 10A (I think) MCB in the control box as the mains enters. It's not clear to me what use further fuses are as any average electronic item will blow in a thousandth of the time it takes a fuse to react; fuses do not protect electronics, faulty electronics blow fuses (occasionally). Fuses sit there saying, "I wonder where all that magic smoke is coming from? Good job I'm still working!" Primarily, they might stop a fire if there is a major wiring fault causing a sufficiently high current short.

    One man's view...
    Last edited by Neale; 13-01-2017 at 03:31 PM.

  4. #14
    Ok A metal enclosure is best by far just make sure it is big enough 600 x 600 x 200 is popular.

    The star point is where you connect all the cable screens to including the mains in earth.

    Fuses I personally rely on the MCB in the box for the power supply fuse But it does no harm to fuse the smaller PS ie 5V or 24V desperately (good to get din rail type for these)

    Regarding the mill the earth to the mains frame is probably for the electronics in the speed controller that might be on a swivel type mounting. You would be powering (mill) this from a separate 13A plug.
    Last edited by Clive S; 13-01-2017 at 03:34 PM.
    The more you know, The better you know, How little you know

  5. #15
    Thank you for the helpful answers:)

    However, could I ask further about the PSU earthing? This is the PSU which I bought from Zapp ready-made for convenience:

    As you can see, it has no metal case, it all sits on the pcb and I can't see an earth point. Maybe this one has internal earthing, if such a thing exists?

    I see you are both using MCB's that's cool, I'll have to investigate this. At the moment, the limit switches (micro type) IO's are fed by the 3.3V supplied by the Kflop board so I don't need additional low DC supply. However, I am going to use proximity sensors instead, and that will require a 24V supply. So as I expand and start to use relays to trigger this and that, I will need an enclosure that allows for this expansion, even though at this stage it may look huge and rather empty.

    Another question regarding proximity sensors, I ordered a couple of the cheapo ebay ones for less than 10 quid each to play with, a bit bulky, but are they any good? I notice that some proximity sensors can cost a small fortune. For our type of usage, what does an expensive sensor offer that the cheapo ones lack?


  6. #16
    Hi Edward.

    The PSU will need mounting either directly to the back plate using stand offs or on separate backplate(with standoffs) which then mounts to enclosure backplate or Cabinet side. My prefered way is to mount on separate plate then fasten this to cabinet side as it saves space.
    In either case if Metal enclosure you won't need Separate earth as the Whole Box will be Earthed. However if you want to bolt the gate so to speak then run separet wire from Star point to one of the holding boltings. !!!!!! WARNING !!!!! DO NOT put Earth wire to the Bolt which holds transformer.!

    If Plastic case and PSU isloated then Good idea to run Wire from Star point to the point plate.

    The main thing is you don't have Wires Earthing to different points, if running earthing wire then always to Star point.
    Personaly Even with metal back plate and the component does have Earth connection I always take it from the Star point with wire and never rely on the back plate for Earth.
    Same with Shield cables, while technicly could connect to back plate I don't. Always back to Star point. This way I never have any issues or chance for ground loops to occur.
    Proud to say never have any ground faults or false trips on my machines and these little details are mostly why. These and good components.

    In general you want to Fuse all AC devices with MCB ie: Main PSU, 24Vpsu, VFD etc. The DC devices like Drives, fans then I use Glass type fuses.

    Regarding the cheap Prox switch then depends on what usage and accuracy/repeatabilty your chasing.?
    If just limits then yes they are fine. If Router/Plasma etc then they are also fine for Homing. But if it's Mill and your chasing V-High repeatabilty on Home Sw then may pay to buy better quality.
    Last edited by JAZZCNC; 13-01-2017 at 04:49 PM.

  7. #17
    I am using one of those power supplies myself, and I have just bolted it to the case on stand-offs. Works fine for me. Doesn't need earthing as there is no external metalwork that is likely to come into contact with mains voltages.

    I have also just come in from the garage where I have been setting up my new machine. As part of squaring the gantry, I have been checking repeatability of position of the gantry using a dial gauge. I'm using cheap proximity switches (mine cost something like 20 for 10 when I bought them a couple of years back but they are a bit suspect so I've just bought some more at about 10 for 4 although I haven't tested these yet). You can't expect wonders at these sorts of prices, but doing a set of runs where I move the gantry down the X rails a distance, then home it, I'm seeing something like 0.02mm repeatability in the homed position. However, I haven't gone round and checked things like the ballscrew bearing nuts since I first put it together and there might be a bit of play as the bearings settle in after a little bit of use. There's also a bit of uncertainty in the position as my dial gauge isn't ideally positioned and isn't as solidly mounted as it might be. Nevertheless, that's probably acceptable for a machine primarily intended for wood.

  8. #18
    Thank you Dean, for the clear advice and for the warning about the transformer! I've learned so much from this forum.

    A metal enclosure it will be, more expensive, but worth it.

    I think I will try the cheapo prox sensors first, as I am more interested in them acting as limit switches, at least at this stage. I'll keep you posted.

    Oh, a last question unrelated to this. What is the average or expected backlash on bog standard ballscrews C7, chinese. My backlash is between 0.045mm and 0.054mm for each axis, i.e. including the bearings, etc. Is that reasonable?

    Last edited by Edward; 13-01-2017 at 05:28 PM.

  9. #19
    Hi Neale,

    Good to know about the PSU. I will use standoffs too.

    The prox sensors I ordered are these:

    Maybe yours are something similar. I will play with them when I receive them before I commit. If I get 0.02mm repeatability I will be very happy:)

    Last edited by Edward; 13-01-2017 at 05:26 PM.

  10. #20
    Quote Originally Posted by Edward View Post
    I think I will try the cheapo prox sensors first, as I am more interested in them acting as limit switches, at least at this stage. I'll keep you posted.
    They will be fine but bit of advise Fit Home switches ASAP. Infact if I could only choose one it would be Home every time. Far more usful than Limits and can use Softlimits to put you one until Limits are fitted.

    Quote Originally Posted by Edward View Post
    Oh, a last question unrelated to this. What is the average or expected backlash on bog standard ballscrews C7, chinese. My backlash is between 0.045mm and 0.054mm for each axis, i.e. including the bearings, etc. Is that reasonable?

    About good as it gets for Chinese C7. Acceptabe or not again depends on usage. For Router yes more than enough. For Mill then might want to chase higher but again depends on what your doing and prepared to accept.!

    Didn't see you say what type of machine.?
    Last edited by JAZZCNC; 13-01-2017 at 06:02 PM.

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