Proper Grounding; How have you grounded your frame?

Kyle, as you obviously know, it is really important to ground the VFD and spindle to Earth. The Asian suppliers seem to let this slip far too often and ship spindles with power plugs with the ground not connected to the spindle. Not sure what spindle you have but that’s irrelevant since yours is grounded.

BTW, I assume you are using a 220V VFD/spindle arrangement.

Like you, I am worried about ground loops. So I personally keep my 110V devices on a separate circuit and they are separated physically from the Spindle/VFD’s Earth. So each respective circuit deals with its own grounding.

I haven’t experienced a need thus far to add any additional grounding to the machine’s carriage. I have assumed it is more important to keep the machine “noise” free.

I too will be looking at grounding. Did I read somewhere the touch pad wires might have to be reversed? or not?

Some users have reported when they ground their spindle and then attach the magnet for the touch probe it will complete the probe circuit and reversing the wires on the touch probe resolves the issue. Of greater concern IMO is that earth ground and the digital ground output of the controller seem to be bonded together somewhere that allows this to happen, these should be isolated and when their not it would be a direct path for EMI to the controller.

Only if it was fabricated incorrectly. There has been some incidence of that.

Kyle, I too have experienced the lack of HY grounding their spindles. Take it apart and add your own jumper to the spindle housing. When/if the spindle shorts to ground it is important to control the fault current giving it an easy path back to the VFD ground terminal. Purchase a small ground buss as you will have several other things to ground. The incoming ground, spindle ground, shields, and axis bonds all should be in a star configuration. Axis bonds should not be needed as we are only talking about the stepper drivers shorting through motors and that is less than 36 volts or your controller supply voltage.
However, think of the damage when the spindle shorts to ground. Personal protection aside, the fault current will find a path through the linear bearing and the controller destroying everything in its path.
Now that you have a dust collection system, ensure there is a path for static to discharge to earth as well. Grounding the flexible hose spring all the way back to the housing of the blower will ensure it does not go through the controller or you.
I have worked in the electrical industry for 30 years and now work in instrumentation/control in the oil fields. I have done troubleshooting most of my career and find if you design your system properly in the beginning, it eliminates a lot of money/headaches in the future.

I don’t have my rails grounded and see no reason to ground them. The black anodized aluminum parts would need to have the anodization removed to properly ground them. That includes where the tubes connect to them. The lead screw would also be difficult to ground. Going through the bearing is not the proper way to ground.

The spindle, OTOH, needs to be grounded (as others have said). My HY 2.2kW spindle came with the ground wire in the spindle. I have my VFD and all SSDs inside an old computer case with everything grounded to the metal case. The controller is mounted to the outside of it and also grounded to the metal case. Along with grounding ferrite beads should also be used.

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The grounded spindle will provide a ground bond to the rails and machine assembly as well.

Although not necessary having some path to ground via the metal chassis all be it poor (I measured about 25 ohms from the spindle mount through the gantry though the bearings to the foot) is better than none when it comes to ESD trying to find a path to ground, better a poor ground on the metal parts than ESD finding it’s way through the stepper motor wiring, probe block, controller etc IMO.

There are two topics getting intermingled when it comes to the discussion of grounding, one is for protecting personnel from contacting an energized part due to a “mains voltage” fault in the case of a spindle to protect the operator, grounding the chassis will help protect the controller from the EMI/RFI and ESD potentially generated.

So I just wanted to update this thread with the answer I found for grounding…

The answer is any points with more then 3 ohms requires a new ground. On my Journeyman, this means that the X & Y get grounded through a single ground. The Z/Gantry gets grounded through the spindle.

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Thanks for the update Kyle. Could you share your source for the answer?

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I emailed corvetteguy50 on youtube, who sells the wires for Spindles and offers tons of support on the topic. He stated that it should be 3 ohms or less for robotics. NEC calls for 5 ohms or less in circuits.

Either way the ohms between the Z gantry and the X is 120 ohms. So a single ground from the Y axis should be sufficient ground for the X & Y, while the Z can be grounded through the spindle so long as the spindle (or router) is sufficiently grounded.

I’d still like to know the sources, like the specific NEC article being referenced. I am an EE and retired electrician and have never seen reference to these values. I get the idea someone can speak authoritatively on a subject but that doesn’t mean it’s correct information. It may fall in to the category of “a good idea” or “in my experience” but that doesn’t mean it’s required per code.

FWIW The Makita router (as well as most every router) will not have a ground pin on it’s cord, they are double insulated and the manufacturer/certification agency is asserting it is impossible to have a ground fault or condition where a person could be subject to hazardous voltage and current.

A spindle is a bit different as it has the potential for a ground fault internally where the outer metal components could become energized - this requires bonding to a ground but doesn’t necessitate bonding all of the Onefinity chassis components to ground as well. That’s along the lines of saying my metal bench vise needs to be bonded any time I use a corded drill to drill a hole in a piece of steel it’s holding.

So I found NEC calls for 25 ohms, and on Flukes website they state that others with specialty equipment call for 5 ohms. When googling it I found 5 ohms repeatedly, but none talk about robotics.

But again either way, if you are sitting on 120+ ohms that seems totally out reason for a ground.

the 25 ohms for the NEC is in reference to a ground rod, there is no specification on impedance limit on a grounding conductor, since the grounding conductor is usually the same gauge as the grounded and ungrounded conductors the resistance and load carrying capabilities will be similar so it doesn’t need to be individually specified.

There is no requirement in the NEC or elsewhere for the chassis of the Onefinity to be grounded regardless of the measured resistance or impedance. It is an electrical system operating at less than 48 volts. You can ground it if you like to but there is no requirement to do so. I have mine grounded for the purposes of giving ESD a path to ground that doesn’t go through the controller.

To elaborate on the 5 ohm recommendation it really comes down to the overcurrent protection for a typical circuit, at 120v 5 ohms of resistance would draw 25 amps which is sufficient to trip a 15 or 20 amp circuit breaker. Too much more than 5 ohms and you could have a ground fault that doesn’t trip the over current protection (7 ohms at 120v would be 17 amps) . For reference 14awg wire has a resistance of about 3 ohms per 1000 feet, and is realistically the longest distance you can reasonably use 14awg wire due to voltage drop - different but related topic.

In the case of the Onefinity operating at 36v this would not occur, the controller electronics would give up and let the smoke out long before the circuit breaker would trip.

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Hey Kyle,

not that I want to participate in the 396403rd revision of this topic, but just as an example, a problem like shown on the picture below, which shows a short between stepper cable and CNC chassis due to an assembly fault, did only produce the correct Motor Error on Controller Display when chassis was grounded. Or when magnet of touchprobe was brought into contact with the rail. Without this, it remained undiscovered.

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What if the entire CNC table is made out of wood? My spindle is internally grounded but I’m still getting lots of static buildup and my touchscreen flickers when the spindle gets near it. Sawdust wants to stick to the dustshoe/hose and linear rails…

Hey Jerry,

did you ensure that your whole setup (rail’s feet, controller, VFD, dust collector) are electrically bonded and then grounded? If so, the electrostatic effects should not appear, even with a table made of wood.

PS: By the way, nice table with integrated technics and its red elements

Thanks Aiph5u!

I’ve since purchased a bus bar and some copper grounding strap. I mounted the bar inside the cabinet and connected it directly to the ground in my garage’s electrical panel. I then grounded the rear two Y-axis feet, controller enclosure, and the shielding going to my spindle (on the VFD side) using separate grounding straps (star). Before I mounted the bus bar/straps today, the shielding was connected to the same terminal as my spindle ground at the VFD. It’s now going straight to the bus bar.

So I guess I have three grounding unknowns:

  1. Should I ground the other end of the shielding (spindle end) to one of the top bolts of the spindle or not connect it to anything like it is now?
  2. Should I wrap a wire spiraling around the outside of my dust collection hose/separator and run it to ground? What about the vacuum itself (it isn’t pictured, but it’s in the same cabinet)?
  3. How do I ground the VFD? The two screws visible at the bottom of the VFD pic don’t have any continuity to ground or to each other…

I haven’t fired it up yet since I installed the bus bar.

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Hey Jerry,

I would also ground the X and Z axis. As you can read in this spindle manual:

Ground the machine in accordance with the regulations!

The machine in which the motor spindle is installed must be grounded separately for each moving machine component. Even if, for example, the moving axes of a milling machine are electrically conductively connected to each other via ball screws and linear bearings, this electrical connection is insufficient for the protective earth connection and must be supplemented by a separate earth connection for each axis in accordance with VDE regulations. A protective conductor test must be carried out individually before commissioning.

– Source: Mechatron HFS Series High-Frequency Spindle Operating Manual

Some of the questions you ask suggest that you haven’t read the VFD manual all the way through. I admit, not every cheap Chinese spindle has as good a manual as the Omron MX2 (PDF) / Hitachi WJ200 (PDF) VFD manuals, but I’ve looked at all the Chinese VFD manuals I’ve come across, and grounding was always described.

I also see that your VFD is not installed in a grounded control cabinet and in addition is lying on its back. The VFD manual should state that the VFD should not be operated lying down for ventilation reasons, and that the unit is intended to be installed in a grounded control cabinet, e.g. like said here in my VFD manual:

:warning: Caution! The equipment is intended for installation in a cabinet. The end application must be in accordance with EN 60204-1. Refer to the section “Choosing a Mounting Location” on page 29. The diagram dimensions are to be suitably amended for your application.



– Source: Omron MX2 User’s Manual

Also, a loose VFD means that the cable ends are subject to movement, and this is not good because there is normally no strain relief. After all, such devices are intended for fixed mounting. I see you fixed the cables to the VFD at some points, so it’s not loose, but I would not consider this really being a good location for mounting a VFD. I would expect it to be mounted at a height where I can read the displays and operate the device when standing in front of the machine. Also usually in the door of the control cabinet, your find the Power Switch, the Emergency Stop knob and the Reset knob of the installation at that height.


The spindle cable shield should always be connected at both ends. At the spindle end it should be connected to the connector housing internally, and the connector should not allow it to be connected any other way. Here you can see how the cable shielding is connected inside spindle connector (figure 3, see “PS2”).

:warning: I would not recommend anyone to connect a spindle and a VFD themselves who has not acquired at least the part of an electrician’s knowledge necessary for this purpose.

Furthermore, I would not recommend anyone to connect a spindle cable without having read this document carefully:

Even electricians usually have not this knowledge since it deals with shielded power cables.

If the hose is plastic, this is useless. See here:

I’ve linked the moment in the video where the author gets to the summary, but of course it’s worth watching from the beginning.

If you get electric shocks from static electricity from your dust hose, you can however cover the areas you usually touch with copper tape or aluminim foil and ground it, so you don’t get the shocks anymore, but basically you can’t ground any non-conductive material itself. All the instructions about this you find in youtube are nonsense.

Here you can see that even though the PVC pipe is wrapped with grounded wire, it still attracts paper scraps when it is statically charged.

I don’t know which screws you exactly mean, but how to ground the VFD should be described in the manual of your VFD. I can’t insist enough on this. Read the fine manual! :slight_smile: Here I have covered the topic again.

:warning: Under no circumstances should you connect a self-wired VFD/spindle assembly to the mains without it being properly grounded! An electrician should be consulted for this.

In many cases, the grounding of the VFD is connected to the metallic part of its housing, but you have no guarantee that this is the case. Usually the ground symbol (⏚) is the grounding point of the VFD.

A VFD is designed to be installed in a metallic grounded control cabinet, such as the one shown below. Such a cabinet has a steel mounting plate that you can remove and mount the VFD on, and the whole enclosure is grounded.

Usually it’s not only the VFD that finds its place here, but also many other things you probably want to use.

How the VFD is connected to ground is described here in Figure 2. There you also find the answer what to do with the VFD end of the spindle cable shield: It should be clamped to the mounting plate with a cable clamp or grounded with a grounding cable gland.

Basically, all the ground wires should converge in one point, and that’s in the control cabinet usually at the one screw of the metal housing part of your VFD, which is also marked accordingly (⏚).

If the metal part of the housing of your VFD is not connected to its ground terminal internally, then you should run a ground wire from this ground terminal (⏚) of the VFD to the steel mounting plate by screwing in a ground connection lug there as close to the VFD as possible, and run all the ground wires together there. From the ground at the VFD on the mounting plate, the ground wire in the power input cable goes to your wall box or your room’s electrical panel, and it’s also from the ground at the VFD that the ground goes to the spindle (the yellow or yellow/green PE wire) as seen below.

Note that even if PE wire and cable shield are both connected to ground, they serve two different purposes. PE is crucial for electrical safety, and shield cannot replace the PE wire.

If you bought a cheap Chinese spindle, you should also be sure that in the spindle the PE (fourth) pin of the spindle socket is actually connected internally to the spindle housing, as seen here. You can check this with a multimeter (set to ohms/resistance, the resistance between the PE wire and the housing of the spindle must be close to zero).

Did you buy a ready-to-use spindle cable? Isn’t the shield already connected inside the spindle connector?

Then it is likely that your spindle or your spindle cable is not shielded sufficiently. The VFD, the spindle and the spindle cable are sources of heavy EMI, because the currents the VFD produces are not nice sine waves. Therefore it is necessary to enclose all of them into something that shields (shielded cables, grounded control cabinet, connecting spindle’s housing to ground). Be sure that the cable shield of the spindle cable is connected to the connector housing internally at the spindle end, and at the VFD end, the cable shield is connected too. Refer to the document mentioned above (Proper VFD Cable Termination).

I hope this is of help to you!


So informative! Thank you once again. My next CNC build will involve several changes after reading up on all this. It’ll have a metal table, better quality VFD/spindle, a proper external dust collection system, and all shielded cables and cabinet(s). For now though, I’ll make my current setup more safe and reliable.

I checked and my PE ground wire is indeed connected internally to the spindle.

For this current CNC setup, here’s my plan:

  1. Replace current cheapo communication cable from Onefinity controller to VFD with a shielded one.

  2. Get a metal control cabinet for my VFD and mount it higher up in the CNC cabinet (and vertical).

  3. Mount the bus bar inside the control cabinet, as close to the VFD ground terminal as possible.

  4. Run a ground wire from the bus bar to one of the ground terminals my garage’s electrical panel.

  5. From the bus bar, ground the ground terminal on the VFD, Onefinity controller box, PE ground wire in spindle cable, spindle cable shielding, upgraded controller => VFD cable shielding, ALL AXES of the CNC frame, and the metal dust separator bin/lid.

  6. Properly terminate the shielding at the spindle end. I may even just buy a pre-assembled cable from Vince (Corvetteguy50 on Youtube).