Thread: Earthing?

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  1. #21
    Hi Dean,
    Yes, I can designate them as Home or limit switches, or both. I think Mach3 works similarly. With Kflop for instance, you assign a Home button to execute a C file for homing that basically reads the sensors as home switches, homes the machine, then pulls off a little, zeros the coordinates and then turns the sensors into limit switches.

    The mill is a Sieg2.7 conversion, which is just a little smaller than an X3, but with better Y range and I think a similar brushless motor, 750W. It's massively better than the SX2 in every sense, for those who want to know. I haven't actually cut anything with it yet, though everything moves as expected. I want to get the electronics in a box to get them away from the muck. In terms of accuracy, well, I am not expecting great miracles, but it's as neat as one can do...for the money spent. We shall see what the pieces look like, the time it takes to produce them and so on, it's my first foray into CNC.

    I have been playing with the backlash compensation, and it's pretty unbelievable what software can do! The compensation happens in milliseconds, and using a dial gauge, the backlash "removal" is bang on. I don't know how good it will be in real life, cutting a circle, changing direction, for example.

    Edward
    Last edited by Edward; 13-01-2017 at 06:35 PM.

  2. #22
    Quote Originally Posted by Edward View Post
    I have been playing with the backlash compensation, and it's pretty unbelievable what software can do! The compensation happens in milliseconds, and using a dial gauge, the backlash "removal" is bang on. I don't know how good it will be in real life, cutting a circle, changing direction, for example.
    Backlash Comp is a fudge at best, better if can remove any slop.! . . Not seen one yet that will get of rid the chatter. . .Lol

  3. #23
    If you have tight Gibb strips then you have backlash. No Gibb's then what you have is slop

    If you have a circuit going through a hole in a metal box, I suggest you make sure that both wires go through the same hole.

  4. #24
    If you have a circuit going through a hole in a metal box, I suggest you make sure that both wires go through the same hole.
    Is that because you will get a ground loop
    ..Clive
    The more you know, The better you know, How little you know

  5. #25
    I cannot pretend to understand metal boxes and the strange things that happen when you drill holes and run wires through them. Putting the whole circuit through a single hole sounds like a good idea, balance things out, like an earth leakage trip.

    OTOH, sounding like a good idea is not always enough. Recently there was a lovely picture doing the rounds of a plated steel nut and bolt holding an earth loop to a copper bus bar. Should have used copper, they used to use copper nuts and bolts in that golden age of understanding when I were a lad. In the picture dissimilar metals had formed a thermocouple. The earth loop probably only gave a fraction of a Volt but the current was enough to heat the nut red hot

  6. #26
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 11 Hours Ago Has been a member for 5-6 years. Has a total post count of 1,156. Received thanks 209 times, giving thanks to others 5 times.
    Electric current through a wire creates a magnetic field around it. Current flowing in opposite direction creates an equal and opposite field. Keep the wires close, ideally twisted together, and the magnetic fields cancel out, don't transmit interference, and are resistant to outside magnetic fields for the same reason (rather simplified but the principle is true). Pass wires through separate holes and not only do you lose that "cancellation" effect because the wires are further apart, but you now create a magnetic field between the two wires that can induce (= create) a current in the metal box between them, which is exactly what you don't want to do. I can bang on about differential signalling and common-mode rejection, but the simple model is pretty sound to understand what's happening.

  7. #27
    I can't see why anyone would want to separate wires of a circuit through different holes or not....

    A practical question about wiring the 8 wires of a stepper in parallel:

    For short run desk tests I normally join and solder each pair length as supplied to go to the respective A+,A-,B+,B- twisting it together along the way, as Neale mentioned above. But now I have to do it properly using 4 core screened CY cable, 1.5mm2. which I think is rated at a whopping 18A. Should I cut the motor cables as close to the motor source as possible, solder each pair to one of the four cores of the CY cable and use heatshrink on each joint, and then a bigger dia heatshrink to hold together the lot? This bulky point where all the cables join at the motor side, and the screening ends, isn't it a possible source of interference?

    If so, is there a recommended best way to do this?

    In terms of magnetic fields and interference, is it better to instead use 8 core CY cable all the way to the driver but of a smaller cross sectional area?

    Edward
    Last edited by Edward; 15-01-2017 at 11:56 AM.

  8. #28
    Quote Originally Posted by Edward View Post
    But now I have to do it properly using 4 core screened CY cable, 1.5mm2.

    Why is that "properly"? I like big fat wires, as big as will fit in the screw terminals on the driver. I run my big fat wires through flexi-plastic conduit to keep it tidy.

    Why would you want screened? Armoured would be more understandable.

    If you are worried about interfering with your high speed computer connections, you could screen those perhaps.

  9. #29
    Robin, now you are confusing me:)

    As far as I understand, "armoured" hardly offers any electromagnetic containment. All signal cables will be screened. I will use flexi conduit for the motor wires too. Do you mean you run 8 separate fat wires inside the conduit?

    Edward
    Last edited by Edward; 15-01-2017 at 12:50 PM.

  10. #30
    Quote Originally Posted by Edward View Post
    Robin, now you are confusing me:)

    As far as I understand, "armoured" hardly offers any electromagnetic containment. All signal cables will be screened. I will use flexi conduit for the motor wires too. Do you mean you run 8 separate fat wires inside the conduit?

    Edward
    Edward just do as you suggested chop the wires off about 250mm from the motor and then join them to the CY cable personally for the motors you are using I would go with 1mm size. If you want to be posh you can get an end cover for the motors and take the cy cable into that.
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    ..Clive
    The more you know, The better you know, How little you know

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