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[CNC] Gounding Pros/cons Confusion

Discussion in 'CNC IN THE HOME SHOP' started by bpratl, Aug 24, 2016.

  1. bpratl

    bpratl United States Active User H-M Supporter-Premium

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    Presently trying to eliminate all NOISE so I have floating NEG PS returns, all inter wiring is shielded, grounded (EARTH) to a central point with only one end is grounded. The lathe, controller, VFD enclosure and computer is also grounded to the same point.
    1. Should the computer NEG's be connected to the stepper PS, 5 volt and 24 V NEG returns?
    2. Should the all the NEG's be grounded (EARTH) or floating?
    Running Mach3 Lathe, WIN 7 and now converting Mill to CNC
    Bob
     
  2. JimDawson

    JimDawson Global Moderator Staff Member Director

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    I normally ground my PS returns, including the computer PS. Everything back to a ground buss on the panel. Has worked pretty well for me so far. I also set up all of the inputs as active low, this pretty much eliminates the noise problems. This is especially important on the travel limits, that way a broken wire looks like a tripped limit sw.

    It sounds like you are doing it the same way I would.
     
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  3. John Hasler

    John Hasler United States Active User H-M Supporter-Premium

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    It is important to make sure that the power supply common is grounded at one and only one point, especially with computers, controllers, etc distributed around a machine often each with its own power supply. You don't want to have the computer ps common bonded to chassis ground in the computer and then also have a controller ps common bonded to its chassis ground on the other side of the machine. Bring all power supply commons together at one point and then bond that point to the green wire. It's best to do this even if everything is in one chassis.

    There are also arguments for isolating logic and analog grounds from earth ground.

    Active-low open-collector inputs also eliminate problems stemming from having the power supply in one chassis energized while the other is off. Differential signalling works well too, but it's much more complex.
     
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  4. bpratl

    bpratl United States Active User H-M Supporter-Premium

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    Thanks guys for your inputs as I was getting paranoid about grounding the NEG PS returns because I got FLAMED on another site for asking the same question.
    I was blatantly told not to touch any electrical components and seek professional advice so I don't hurt myself. :bonesrock:
    I converted this lathe to CNC a couple of years ago and it's been running great since, except for some sporadic noise effecting fine long threading, so I an now in the process of cleaning up all of the wiring, adding better encoders, to make it more reliable. Bob
     
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  5. countryguy

    countryguy United States Active User Active Member

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    It has been so many years for me in the EET world but I know sometimes that GND can be a derived point. Myself, My home is Gnd to Neutral bonded. I did pound in my own 8ft. pole for the CNC plasma setup, and the bigger machines get frame to pole wiring. But I often wonder about the house setup in general! if it would be better to get the Neutral line and Gnd line separated/isolated in some way when I power the equipment... Just wondering is all gang. I mean anything that hits the GND, Surge, mis-wire, lighting for instance, travels from Gnd which is Neutral, right into the gear right?
    (since we do not flame anyone ever). I just have not had time to read about beni's of neutral/gnd bonding -vs- unbonded and why that was all done in the first place. Everyone have a great week!
    Jeff
     
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  6. JimDawson

    JimDawson Global Moderator Staff Member Director

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    If your controller will accept differential encoder inputs then that is the way to go. Almost noise immune. Sometimes these are called line driver or RS422 output. The encoder outputs will be A, /A, B, /B

    Being flamed for asking a question is really counter productive on their part. That's why The Hobby Machinist is growing every day. 17, 000+ members and counting! :encourage: :)
     
  7. bpratl

    bpratl United States Active User H-M Supporter-Premium

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    I will be using a UC400ETH which will accept the AMT103 encoder inputs and hopefully it will allow me to do more precise threading. If it works out I will apply the same fixes to my mill. Bob
     
    Last edited: Oct 30, 2016
  8. JimDawson

    JimDawson Global Moderator Staff Member Director

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    That is a single ended encoder, so shielding will be helpful. I think you are moving in the right direction!
     
  9. John Hasler

    John Hasler United States Active User H-M Supporter-Premium

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    The green wire grounding conductor in a single building should be bonded to the neutral and to earth ground at the service entrance. The neutral should be bonded to earth ground at the service entrance and nowhere else. The exposed metal parts of your machines must be bonded to the green wire. This is an entirely different issue from the grounding of the power supply common for DC power supplies for computers, servos, etc.

    Normal load current flows from the line connection at the transformer out on the pole through all the switches and breakers, through the load, and back to the transformer via the neutral (yes, this is an oversimplification). The neutral is grounded out at the pole. Without grounding of equipment frames a fault such as a short from line to frame would still get "right into the gear" but it might get out again through you or through something that it could set on fire. Grounding results in fault currents getting out through the green ground wire instead.

    Lightning wants to get to earth ground and it *is* going to do so no matter what. Grounding minimizes the damage it does along the way.
     
  10. wswells

    wswells United States Swarf Registered Member

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    Nice post John,
    To expand on the subject, a true dedicated circuit should be considered for all CNC control equipment, which eliminates dirty looping grounds. I can't count how many "dedicated" orange circuit's i have repaired with loops and not grounded to lowest earth point in the box.
    Steve
     
  11. Metal

    Metal United States Active Member Active Member

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    As a note, I have no idea how electricity works.

    However on my cnc, seperating the VFD/conduit-its-power-runs-through ground from the rest of the system grounds seems to have entirely resolved any interference issues that I had.
    As far as I understands VFD's are pretty much the ideal noise generator
     
  12. Smithdoor

    Smithdoor United States Active User Active Member

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    pros No shocks
    Cons none
    I ground ever thing including the hot water heater Note more than plumber had a shock or very bad life
     
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  13. tq60

    tq60 United States Active Member Active Member

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    There are two terms that often get confused and that is ground and return.

    In automotive to save weight and cost the chassis is used as return and also is called ground.

    In reality "safety ground" is the bare or green wire and the simple part is everything that is metal needs to be connected or bonded to it.

    The wire size needs to be such that any power wire touching anything metal will trip the breaker.

    Shielding ground is nothing more than it states.

    It offers zero safety protection by design but it cab allow ambient noise a place to go.

    Look up generating electricity and simply passing a conductor past a magnet creates electricity.

    So a magnetic field passing a conductor generates electricity.

    A balanced line gets equal in both lines do no voltage difference.

    A single ended one picks up noise as that moving magnetic field so the shield allows some of the energy to be absorbed into the shield.

    Return is the path for hot power to return to the source.

    Keep ALL power which is both supply and return as well as all signal and control circuits isolated from the machine safety ground.

    There are snap on magnets or other devices that can also be used to isolate signals.

    If you have vfd keep the motor leads in a shielded cable or conduit and strap groubd the motor and vfd to the machine

    Sent from my SAMSUNG-SGH-I337Z using Tapatalk
     
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