Grounding, Noise, and Power Filtering

Grounding of your CNC machine is very important, for safety reasons and enhanced reliability.  Unfortunately, most kits on the market completely ignore grounding of the machine.  This can create safety hazards and intermittent interruption of machine operation due to static buildup and electrical noise.

Also, since your machine is generally operated in an electrically noisy environment, around lots of other motors and high draw machines, noise can cause problems with the machine and wreak havoc with the motion controller.

This blog addresses some of those issues and poses some tested solutions.

Grounding

Grounding all parts of your machine is a must for safe operation.  Grounding provides several benefits:

  • Safety AC ground – The machine frame should be grounded in order to safely route any fault currents to earth ground (third prong on AC cord) in case a component or cable fails and shorts to the chassis.  This could make your chassis live and be extremely dangerous if it is not properly grounded.
  • Drain static buildup – A CNC router can build up significant static charges through the rubbing action of the bit on a dry material such as wood.   This can build up dangerous static charges that can cause malfunctions in the controller, pose shock hazards to the user, and even ignite dust in the atmosphere causing an explosion or fire.
  • Shunt electrical noise away from the electronics – CNC machines use motors and high frequency signals that may cause interference with other devices in proximity.  Noise may affect the controller, the computer, or other devices causing them to lock up or cause errors in operation.  Proper grounding shunts the majority of this noise harmlessly to ground, preventing most of these issues.
  • If your machine has a VFD spindle or is a Plasma cutter, grounding is even more important.  These devices generate huge amounts of electrical noise, and unless care is taken to properly shield and ground the systems, noise will cause performance and reliability issues.

Installing a proper ground system is relatively easy, and we will go through some of the steps here.

First thing to do is install what is known as a “SPG or single point ground”.  This is a screw stud or terminal bar where all the individual ground wires from all parts of the machine come together at a single point, which is tied through the incoming power cord to the building safety ground.

It is necessary to ground each of the moving parts of the machine separately to this SPG point.  The reason for this is that a moving part, such as the Z axis, may be electrically isolated from the rest of the machine by its motion components.  For instance, on an OB kit, the Z axis is mounted on plastic wheels that provide no conductivity through to the X axis, or any other part of the machine.  This is pretty much true of all the axes of the machine, they are isolated from each other because of the insulating motion components.  This is less of an issue on machines with linear bearings, since there is some metal to metal contact, but a good ground connection is still needed.  Bearings are not the greatest conductor, and they may offer only intermittent contact.  Also, fault currents or leakage flowing through a bearing may destroy it through corrosion or arcing.

So, a separate ground wire connected to the frame of each axis should be run back to our single point ground.  The separate wires provide a low resistance path for current which eliminates any insulating or intermittent mechanical connections.  This ground provides a path for fault currents, should a short occur, and also drains away static buildup and electrical noise.

It is important that good electrical contact be made at each connection point.  It is imperative that any paint or coatings be removed at the contact point, and that a good star washer be used to penetrate any coatings or oxides.  I usually connect the axis ground right on one of the motor mounting screws, using a ring terminal and star washer.  This ensures a pretty good contact point for the axis, and serves to make the connection  the best point to shunt noise to ground at its source.  You will need to run a separate wire from:

  • Z axis motor mount
  • X axis motor mount
  • Y axis motor mount
  • Main chassis (part of the machine that does not move, may or may not be Y axis)
  • moving table (if equipped)
  • power supply main input ground (power cable third prong)

The main ground coming in from the power cable (green or grn/yel wire) should go directly to the SPG ground bar or stud.  This connection should be the first wire on the stud, and must be separated from all other wires by a separate nut on the stud.  (See Below)  This insures that if the machine needs to be serviced or disassembled, this connection normally does not need to be removed.  This ensures that you do not forget to reconnect it after.  All other ground wires should be placed on top of this connection, with a star washer between each wire terminal to ensure good contact.

Note that additional wires to the stud shown below will be added above the second nut.  The top nut should either be a double nut or some type of locknut to prevent loosening.

If a grounding bar is used instead of a stud, it needs to be mounted firmly to the main chassis, again using star  or toothed washers under it to insure good contact.

 

Note that proper ring tongue terminals and star washers should be used on both ends of each ground wire if using the ground stud approach.  Ensure that these terminals are good quality, properly sized for the wire used, and crimped on correctly using the proper crimping tool.  Do not connect bare wires to the studs.  Bare wires, however, can be used on the ground bar.

Ground wires should generally be 14 gauge green or green/yellow wires so they are easily identified.  14 ga. is likely larger than the supply wire, but going a size larger is much better than a size smaller.  The ground wire must be at least the same gauge as the incoming power cable ground wire.

VFD and Plasma Grounding

VFD spindles and Plasma torches provide special challenges for noise and grounding.

A VFD spindle create a huge amount of electrical noise and transients which may affect your machine operation.  It is important to follow these rules when wiring a VFD

  • Use shielded wire for connecting the motor to the drive controller.  This will go a long way towards minimizing the any interference.
  • Ground the shield of the drive cable only at the VFD controller end.  Do not use the shield as a chassis ground conductor.  Ensure the cable has a separate safety ground wire, and use this to ground the spindle to the incoming power ground of the controller
  • Do not run the VFD spindle power cable through the same cable chain as signal, limit switch, or other low voltage wiring.  Induction can cause hard to troubleshoot noise problems.  Run the cable as far away from other wiring as possible.
  • Use a good power line filter on the incoming AC to the machine power and computer power lines.  Plug the VFD spindle into a separate AC circuit if possible.

Plasma torches are also a source of heavy electrical interference.  The plasma cutting action generates huge RFI ane EMI spikes which need to be contained at or near the torch.

  • Again, keep the plasma torch cable well away from all control or low voltage wiring.
  • Do not run the plama torch cable through a cable chain with any other wiring.
  • Ground the plasma torch holder, Z axis, and any other components near the torch with a suitable separate ground conductor to the SPG.
  • Use a good power line filter on the incoming AC to the machine power and computer power lines.  Plug the Plasma power into a separate AC circuit if possible.  This is usually not an issue since the Plasma power supply is 220V.

Following these simple rules will ensure trouble free operation of the system.

Cable Routing

Cables from noisy devices like plasma cutters, VFD spindles, and brush type universal AC motors like the ones used in router motors are prolific sources of AC noise and radiated EMI/RFI emissions.  It is always prudent to run the power cables from these devices well away from any low voltage or control wiring on your machine.

I always recommend, if possible, to plug these devices into a separate electrical circuit from your machine electronics and computer.   This isolates the noise further, and reduces the possibility of interference.  Note that a different outlet does not necessarily mean it is on a different circuit.  A different circuit is defined as an outlet that is on a separate breaker in the main panel.

Ferrite beads can also be used on the power cables of these devices to further reduce noise.

Dust Collection

Another problem area with these machines is not with the machine, but with external dust collection.  Moving dry dust and air at high speed through a plastic hose generates huge amounts of static electricity, which can arc over, causing EMI/RFI which can affect the operation of the motion controller or computer.  It also presents an operator hazard, as the discharge can startle you and cause injury.  Also, arcing from static can ignite fine dust in the dust collection system, causing fire or explosion hazard.

Grounding your dust collection is a must, especially if there are plastic hoses involved.  All metal piping and parts should be grounded by proper wires to a nearby grounded outlet box or something similar.  Any plastic hose or tubing used to connect to a machine should be spiral wrapped with a well grounded wire.  If the hose has a metal spring wire embedded into it, connect this to ground at one end.  Dust collection grounding kits are available at most woodworking stores and should always be installed on your system.

Noise Filtering

Noise is generally the enemy of control circuits and microcontrollers.  Noise can cause intermittent operation, crashing of the computer or motion controller, unintended Limit trips, and many other hard to diagnose problems.  It is generally better to do some pre-emptive work to prevent this from happening before it becomes an issue.  Nothing is more frustrating than an 8 hour CNC carve dying 20 minutes before it finishes from unknown causes.

Several things can be done to eliminate these problems:

  • eliminate the noise at the source – observe good grounding, shielding, and wire location practices to minimize the causes of interference
  • Use a good quality power line filter on the incoming AC lines for the machine and the control computer.
  • Use ferrite beads on any cables that are suspect for picking up noise.

A good power line filter/surge suppressor is a worthwhile investment.  Something like the one shown below:

These power line filters are not just surge suppressors.  A standard surge suppressor power bar has no noise filtering whatsoever.  Devices like the Isobar series have a large filter system built in, consisting of capacitors and inductors which significantly reduce noise incoming and outgoing on the AC line.  They may cost a bit more, but are well worth the added piece of mind.

Ferrite beads are also effective tools to help suppress noise.  The are available in clip on types, and some cables come with them already attached, such as USB cables and power leads.  They are available in many sizes and shapes for various cable diameters, and are easily clipped on to cables at one or both ends.  The larger the ferrite bead size, the better it’s suppression, and the lower the frequency at which it is effective.  They are available at most electronics supply outlets, as well as on Amazon, Ebay, etc.

 

Conclusion

Hopefully, the information in this blog will keep you safe, as well as help troubleshoot noise or interference problems.  Proper grounding of any CNC platform, be it commercial or kit, is essential for personal safety and reliable operation.