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Author Topic: GROUNDING - what is the correct way to ground a CNC machine?  (Read 46921 times)

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Offline simpson36

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Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #10 on: November 22, 2008, 10:08:29 AM »
I appreciate the replies, but I still don;t think I have an answer.


I get the chassis grounding and protecting people from high voltage, etc.

I get that the DC common is 0v and should not be connected to earth ground. All of the 0v (common) wires from the Geckos go to com terminals on the BOB.

I have a 5v (to the BOB) and 12v (fans and relays) supply coming from an old PC power supply. Each is only two wires. I am not taking any power from the PC and it is connected by a shielded,  all wires connected parallel cable.

Here is the question as succinctly as I can put it:

Are there any earth (or otherwise) grounding requirements that must be done on the electronics side i.e BOB, indexer board, charge pump, spindle controller, PC, etc that I need to specifically address?

Sorry to be a pest, I just don't want to fry anything else due to my lack of electrical knowledge.
Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #11 on: November 22, 2008, 11:20:20 AM »
I appreciate the replies, but I still don;t think I have an answer.


I get the chassis grounding and protecting people from high voltage, etc.

I get that the DC common is 0v and should not be connected to earth ground. All of the 0v (common) wires from the Geckos go to com terminals on the BOB.

I have a 5v (to the BOB) and 12v (fans and relays) supply coming from an old PC power supply. Each is only two wires. I am not taking any power from the PC and it is connected by a shielded,  all wires connected parallel cable.

Here is the question as succinctly as I can put it:

Are there any earth (or otherwise) grounding requirements that must be done on the electronics side i.e BOB, indexer board, charge pump, spindle controller, PC, etc that I need to specifically address?

Sorry to be a pest, I just don't want to fry anything else due to my lack of electrical knowledge.

Just as for the power ground, all your power supply commons should be connected together at a single point.  You are probably accomplishing this, sort of, through the BOB, but that's really not ideal.  You should bring the commons from all supplies to a single point, and bring a dedicated wire from each device (BOB, drives, etc.) to that common point as well.  You can determine whether the mounting lugs on the BOB are tied to common by using an ohmeter.  Your supply common may, or may not, be bonded to the power ground.  I would suggest not.  Since it sounds like everything is mounted to a metal plate, that metal plate should be bonded to the supply common.

Regards,
Ray L.
Regards,
Ray L.

Offline simpson36

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Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #12 on: November 23, 2008, 08:37:33 AM »
Thanks Ray,

Based on the advice I got here, the metal plate that the electronics are mounted on now has a separate wire running directly to earth ground via the ground pin on the wall outlet.

The only shielded wire I have is to the indexer sensor and it's shield is connected to common on the controller board. It came wired that way.

So any boards that are grounded thru their mounting holes will be grounded. Everything is working fine, so I guess I'm good to go.
Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #13 on: November 23, 2008, 04:08:10 PM »
Simpson36,

That's good news! Now the debugging starts. I usually  put a screen over the stepper driver wires and earth them only in the control box at the star earth point.  This is because the signals in the wires are relativly high power and high frequency and can cause odd induced voltages in other control wires.
Most DC SCR speed controls have their speed pots connected to the neutral line, so unless you have opto isolation in the control or the BOB you are going to have the problems that you had.(Put it down to the learning curve.)
By the way I like your simple 4th axis solution in the other thread.
I hope to clarify earthing the 0 volts star point with Jeff see my later post.

Regards,

Iaqn
Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #14 on: November 23, 2008, 04:18:10 PM »
Jeff,

I wrote :-

"and you connect all 0 volt lines to a star point which is earthed," and I gave my reasons for doing this and regard this as an essential safety requirement.

You wrote :-
This is absolutely wrong (not to step on anybody's toes).

I am always interested in improving my knowledge, can you give your reasons for making this statement please? ( Even at my age I am still light on toes ;D, I might be able to keep them out of the way!)

Regards,

Ian

Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #15 on: November 24, 2008, 06:04:29 AM »


I am always interested in improving my knowledge, can you give your reasons for making this statement please? ( Even at my age I am still light on toes ;D, I might be able to keep them out of the way!)


Ian, I think Jeff explained the reasons in his last post.  He gave one example with the
switching power supply common.

Scott

Offline Jeff_Birt

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Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #16 on: November 24, 2008, 10:17:46 AM »
Ian, my primary purpose for the warning is to get folks to stop and consider what they are doing before wiring everything that we typically call 'ground' up to earth ground. Switch type power supplies are a prime example, for instance see: http://www.astrodyne.com/smartcat/pdf/AD155drl.pdf, notice the PS has both ground and DC common terminals. Internally they are most likely bonded with a small resistance to 'pull' the DC common up off the ground plane noise a bit. externally bonding defeats this (and most probably the way the switching regulator was designed to operate.) The 'proper' way of bonding DC common to earth ground is largely dependant upon the design of your system (and the components therein). Most folks will get a bit concerned as with a floating DC common you cannot measure from chassis ground to DC components and they interpret this as a problem; of course the only problem is that they are trying to measure a voltage by using the improper reference point.

Here some good references that briefly cover some of the issues involved in proper grounding:

http://ecmweb.com/mag/electric_resolving_grounding_issues/
http://www.ese.upenn.edu/rca/instruments/misctutorials/Ground/grd.html

While it is common (he, he...electrical joke) to earth ground DC common some thought needs to be put into doing so. Consider how your DC power supplies are wired ( is the DC common bonded internally to earth ground?), and how it might effect isolating your controls from noisy equipment, such as VFDs.
Happy machining , Jeff Birt
 
Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #17 on: November 24, 2008, 06:56:12 PM »
Jeff,
(With appologies to Simpson36 for hijacking his thread)

I think we have some common ground in this discussion. :) and I thank you for your considered reply.

I agree with you that the case should never be considered as a reference point to measure circuit volts - a hangover from the days of early radio and television where the chassis was used as a common signal return path. Much confusion exists with regard to using the terms ground and earth interchangeably. One even sees the earth symbol used in automotive applications!

Your references were very interesting but I do not see the statement that under no circumstances should the 0 volt line be connected to earth. My problem is your adament statement that connecting the low voltage DC 0 volt line to earth is "absolutely wrong." Your third reference Figs.7 and 9 actually show this connection. My guru in this aspect of circuit design was a M.I.E.E and as far as I was concerned his word was law.

Practical experience, designing and building switch mode power supplies, a touch sensitve electronic piano and power audio amplifiers, taught me the advantages of star earth points and earthing the 0 volt star point from several internal power sources, but the overriding point is safety, the transformed, rectified voltage should never be allowed to float up and this is avoided by connecting it to earth. Floating secondaries are allowable where the equipment is double insulated or, as one of your references stated, the circuit is designed so that finger contact is impossible, not really practical with the circuits we construct to drive our CNC machines.

Bernard Babani publish many books in the UK for the amateur circuit builder and one their most prolific authors is R.A.Penfold. (Never sure if he actually writes all of the books attributed to him!) in his "Power Supply Projects", without exception, all transformer DC 0 volt lines are connected to earth.

You are quite correct in emphasizing the importance of correct ciruit design and the isolation of control signals from noisy wires like VFD's and even microstepping power lines to stepper motors. Metal cases and braided tubular copper shielding for control wires should be considered essential to minimise these problems.

You raised some valid points and interesting references but we really could do with an Authority or Electrical Regulation that would give people reading this thread the confidence to build circuits that are safe. (Not me, I am only a impecunious machinist!)

Ian












Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #18 on: March 17, 2009, 06:10:48 PM »
I whole heartedly agree with Ian on the subject of grounding all systems to a common earth point, after implementing industrial PC based systems for over 25yrs using this procedure with positive results, I am not ready to change now.
Not so much the safety aspect, with low voltage systems, but the avoidance of spurious noise problems.
Siemens also has an excellent publication on their site on Equi-potential bonding in control system installations.
Max.
 

Offline Rimmel

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Re: GROUNDING - what is the correct way to ground a CNC machine?
« Reply #19 on: January 12, 2014, 06:30:02 PM »
Hood, you mention that you convert 24v to 5v for interfacing with a pc. How exactly are you doing this?

Thanks
Rimmel
« Last Edit: January 12, 2014, 06:32:59 PM by Rimmel »