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Author Topic: GROUNDING - what is the correct way to ground a CNC machine?  (Read 13050 times)
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simpson36
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« on: November 19, 2008, 06:36:23 PM »


I need some advice or direction on the correct proceedure for grounding my CNC conversion.

There seems to be a lot of potential for problems if the mill is grounded to the computer to the breakout board to the 5v source to the stepper power supply, etc , etc.

For example, what goes to earth ground?
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Hood
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« Reply #1 on: November 19, 2008, 06:44:23 PM »

I am certainly no expert on the subject, hopefully Jeff Birt will look in as he seems to know what he is talking about with grounding procedures. Heres my take on it and so far following this I have never had any issues.
All of your shielding wires go to ground(earth) and only one end of them seems to be the preferred method. All of your ground wires should go to ground at one point if possible. You should not confuse a 0V with a ground and you should never use a Ground as a 0v.

Hood
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Hood
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« Reply #2 on: November 19, 2008, 06:50:47 PM »

Another thing I should add is if at all possible try and use 24V on all of your limits, home switches, external buttons etc, it will mean you need to convert them to 5V for interfacing with the PC but it will help eliminate noise in these wires as they are usually long runs and often pass high voltage wires or potentially noisy equipment such as VFD's. All of the I/O on my Lathe and Mill are 24V and the reason 24V is better is the difference between Hi and Low is much greater where 5v it is much closer so a slight amount of noise can be seen as a signal.

Hood
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HimyKabibble
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« Reply #3 on: November 19, 2008, 07:25:08 PM »

The most important thing is to HAVE a ground, and only one.  The body of the machine should be securely bonded, through a dedicated wire, to the power ground of your building.  In the US, this is the GROUND terminal present on any self-respecting outlet.  You want to have a single ground for the electronics, and connect everything to this point.  This can be a buss-bar, heavy terminal strip, a metal chassis, or something on that order - something with essentially zero resistance between the many connections.  NEVER ever daisy-chain grounds.  For shielded cables, ground the shield at only one end, usually the power source end.  So, for example, encoder cables should have the shield grounded at the end opposite the encoder, where the power supply is.  NEVER use a shield as a power or signal return.  Always provide a dedicated power or signal return wire, or you render the shield largely useless.  You never want to setup a situation where you can have current flowing through a shield.  You never want to have a situation where a signal wire can serve as a power return.

Regards,
Ray L.
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Regards,
Ray L.
Jeff_Birt
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« Reply #4 on: November 20, 2008, 01:55:35 PM »

I copied most of the following from another post of mine on the forum:

OK, 'grounding'...'(earth)ground IS NOT equal to DC common. They do not mean the same thing, unfortunately we all throw around the term 'ground' very loosely. 'Ground' is this context refers to 'Earth Ground' which is a safety device. The incoming mains voltage is referenced to ground (through the 'ground' rod(s)) and so is your equipment. This provides a path of least resistance to shunt the voltage away from important things like people in case something goes wrong. 'Ground' SHOULD NEVER carry any current, it IS NOT a common return path for all circuits. That is the job of DC common, and your system may have more than one DC common which is not a big deal. Generally speaking you should not tie your DC common(s) to (earth) ground anywhere. Some power supplies may do this internally with a small resistor to pull the DC common up off the ground plane noise.

The problem is that folks think 'ground' is this universal reference for EVERYTHING in a system and try to measure voltage to it from any given point, which is wrong. Think of it this way, if I were to nail three pieces of wood together at odd angles and ask you to measure their length how would you do it? Would you pick the bottom of the closet piece of wood and measure from there to every other piece? Or, from the floor (ground) to each piece? Nope, because that would not tell you a thing. You would run your measuring tape from end to end on each piece of wood (so your reference is the beginning of each piece of wood and you are measuring from that reference to the end of the wood to find its length.) Measuring voltages is the same idea, you are measuring from a reference point (common) to some other point in the same circuit (same piece of wood).

It is very important that you tie your machines (Earth)ground to your mains earth gound, even if you have a local ground rod. This prevents nasty shocks from things being (earth)grounded at diffrent physical locations. A differnece of 100' could create a 50V potential difference.
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Happy machining , Jeff Birt
 
Ian Ralston
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« Reply #5 on: November 20, 2008, 05:46:48 PM »

This is how I do it for UK use. We have Live(L), Neutral(N) and Earth(E). This Earth is either a local earth rod or Supplier provided earth. All domestic systems have an earth leakage detection system fitted, an RCD - residual current device (detector) typically 30mA with a 40ms tripping time so that any mains fault to earth breaks the mains circuit. This is also called an earth leakage circuit breaker (ELCB)

If I need say a 12 volts DC, L and N are fed to the primary of an isolating transformer, the case or laminations of which are connected to E. The secondary voltage is fed to a rectifier and smoothing capacitor and the 0 volts line has a star point to which all 0 volt connections in this 12 Volt circuit are returned. The star point is in turn connected to E. This is to stop the secondary voltage floating above E, for example, the secondary could be at 200/212 volts above earth and the circuit would still work but would be an electrical hazard. All machines, computers, cases etc. have a direct connection to E.

This system has served me well in many control circuit installations and is very robust, with no earth induced problems.

I should point out that I am not a qualified electrical engineer, only a Fitter/Turner apprenticed in the days when it was a requirement that safety in the use of electricity should be understood. So, you cannot take any of the above as a recommendation for use in your circuits. Any qualified quys out there prepared to comment?

Regards,

Ian

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simpson36
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« Reply #6 on: November 21, 2008, 04:04:48 AM »

Thanks for the info, guys.

I was looking for a little more specific 'how-to' for CNC rather than general rules. You know "Grounding for Dummies" type of help.

I am adding a fourth Gecko 203V to my control box right now and I want to know that I have the grounding set up correctly. Gecko says to ground their driver frames and I assume that means to earth ground, even though the are not at mains and have a 5v side. I have them all mounted on a .090" aluminum plate as a sink and I plan to attach that plate to earth ground via the unused ground wire in a 16/3 power cable coming from the 36vDC  power supply's earth ground terminal which goes directly to the earth ground center prong on the wall plug. I know the ground is good in the outlet.  Gecko says to feed each drive separately all the way back to the power supply so I've added the necessary cables to do that.

Also the system has a minaric variable speed DC drive which is installed in an separate aluminum box along with it's associated relays, pot and a CNC interface board that (supposedly) produced the control voltage for the drive to follow. The box is fed 115vac from the wall outlet and also 12v for the relays from a two wire 12v outside source and 5v which is supplied via the breakout board in the control box directly to the CNC speed controller. hot and common from the 115VAC goes to the board terminals and the green ground wire is bolted to the chassis and these run in a 16/3 cable to a grounded outlet.

So far, with the exception of the Gecko's, which are isolated, I have kept the 12v and 5v stuff off of the earth ground as I'm imagining that a short to ground on the 115 side would fry any 12v or 5v (including the laptop) that was on the same ground. I don't think this is correct and I'm wondering what the correct procedure is to ground the laptop and the breakout board, or even if they should be grounded. The geckos are all opto isolated, and the breakout is opto isolated on everything except the specific gecko connections (it is a gecko specific board).

So I take it that all of the 'big' stuff just goes to earth ground individually. My remaining question is how exactly do I ground the 'electronics' side. There is no terminal or screw that says 'this one goes to earth ground' and that the level I need help on, unfortunately.
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Ian Ralston
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« Reply #7 on: November 21, 2008, 05:58:57 PM »

As has been said below as long as you do not use earth as a return supply line and you connect all 0 volt lines to a star point which is earthed, you should be OK

I would suggest that you leave the motor speed control in manual mode until you actually need Mach to control the speed. There are a lot of posts on this site discussing speed control - do a search. Most DC speed controller speed pots are not isolated from the mains supply and you need a good breakout board to make sure that the control voltage to the pot is safely isolated, ask your BOB supplier, most like CNC4PC have diagrams on their website to show how connections should be made.
You mention a laptop; most people use a standard cased PC for Mach because they have found that laptops have issues involving power saving problems and on board video card interrupts that can cause Mach to seem unreliable.

Regards,

Ian
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simpson36
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« Reply #8 on: November 22, 2008, 08:16:28 AM »

Thanks Ian,

I read all of the posts carefully. I have all the power grounds done and tested. I put a ohm meter between the machine frame and the ground in the actual wall outlet and got 100%. Did same for control box and spindle control box (both aluminum). So that's done.

The 0 volt thing is what I am still unclear on. There are no grounding terminals that I can find and no instructions on grounding for the electronics that I can find.

It occured to me that the BOB is probably already grounded as it is mounted via aluminum standoffs to an aluminum plate which is grounded to earth. But I don't know if the mounting pads on the board are connected to ground. There is no separte specific ground wire between any of the electronics boards that I know of.

As to the laptop, I already had the issue with power saver feature. On this particular laptop, it is defeatable in the bios. I've done that and also I run it off wall current thru the charger, so it is not succeptable to the battery going low. It is an el cheapo laptop that I bought only to use with tuning and diagnostic software for an '82 Vette and I don't particularly care if it gets ruined by chips and oil.

I'm just in the process of building my first CNC. I'll put a permanent PC in place one everything is final and I have appropriate protection (including proper grounding) for it.

The spindle controller is already connected and working. It just does not provide a very consistent speed and I have not looked into that yet.

As to 'isolation'. There was only passing mention of it in both the BOB and the DC drive docs. I had no idea what 'isolation' was and consequently fried two DC drives and the BOB. After I fried the first DC drive, I explained in detail to a Minaric rep what had happened and he sold me a replacement unisolated drive . . which of course also fried and again took the BOB with it. Minaric made good by selling me new isolated drives for only the difference in retail price, so I can't complain too much.

But now you see why I am a little paranoid now about grounding.










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Jeff_Birt
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« Reply #9 on: November 22, 2008, 08:38:08 AM »

Quote
and you connect all 0 volt lines to a star point which is earthed,

This is absolutely wrong (not to step on anybody's toes).

DC common (i.e. OV) is not the same as earth ground. You should NOT connect your DC commons to earth ground. As I have mentioned before many (most) switch type DC power supplies will bond their DC common to ground internally through a small resistor. This pulls the DC common up off the low level noise a 100mV or so. If you run around and bond the DC common to ground you have defeated this and will cause yourself problems.

Anything marked with an earth ground symbol should be bonded to a single point (star ground), you should run a large cable from your star ground to your main ground and an external ground rod (if you have one), any shield braids should be grounded at one end only (usually the controller end).
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Happy machining , Jeff Birt
 
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