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Author Topic: Steps to making mill bulletproof  (Read 3855 times)

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Offline BarryB

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Re: Steps to making mill bulletproof
« Reply #10 on: January 13, 2015, 12:08:28 AM »
Hood, Mach 3 would freeze which in turn would stop the mill.  This would happen within milling for five min.  This was super annoying.  I'll replace all the motor wires, limit switch wires, use cable, and add chokes on all the cabling.  Then I'll test again.

Thanks mrprecise, I'll get 18 awg for the motors and 22 for the limit switches.

Btw, my mill build is on this site.  I've always had issues with it, so happy with the help.

Garyhlucas, I won't be putting in a ground rod, but will run a common ground.  I'm in a live/wirk loft and don't have the option to drop that outside.  I'm hoping it's the shielding that will do it.

Barry
Re: Steps to making mill bulletproof
« Reply #11 on: January 13, 2015, 07:36:39 PM »
John,
No they are not crazy, they are just wrong.  Having been an industrial electrical contractor for about twenty years in my first career, and having built control panels for almost 50 years now I have never once found the need to install a ground rod at a machine of any kind. I have seen lots and lots of machines that have had noise problems and when you investigate closely it is never a surprise why that is the case.  In case of home built machines I see I am more amazed that they run all rather than whether they have an occasional problem!

Electronic engineers are a big part of the problem.  They just don't seem to grasp that the grounding done for shielding purposes is completely different than than the grounding done for safety purposes.  Grounding for shielding is all about making sure there is one and only one path from every point to ground.  Grounding for safety is all about making sure the voltage difference between any two metal parts on a machine is as small as possible, and you want to create as many paths to ground as you can.  In the case of shielding noise your are trying to keep out very small currents at possibly high frequencies.  In safety grounding you are trying conduct away what may be many thousands of amperes of current from a fault in motor, heater, etc.



So back to that ground rod.  It is driven into the earth. What is the resistance of that earth?  Well I can tell you that it is so high that it cannot conduct away a large current surge from from a faulted motor.  If the fault lasts the ground will be warm around the rod, and the rod will be at a voltage that will kill you, and yes I have actually seen this happen.  I figured it out when I kneeled down to look at something and the concrete was hot!  That rod is also a second path to ground, the other is through the safety ground wire that comes in with the power conductors.  Ohms law teaches that the current in a circuit is the same everywhere, while the voltage varies with the resistance.  So that means that leakage from motors, transformers, solenoids etc. cause all kinds of cray voltages to flow on the ground path, and you have two paths to complicate matters.

So a little checklist for you home builders:
Shielded wire on all sensors that connect to the CNC?
Shielded cables on motors that generate noise servos, steppers, VFD controlled?
All shields connected at only one point on the whole machine?
Ground wires from all electrical control devices installed and connected to just one point?
You checked with an ohm meter to make sure the shield isn't connected to the case of any device with a metal case mounted to the metal frame?
Power wires and control wires routed in separate raceways?
Control wires cross power wires at right angles and don't run parallel?
You are using a PC with a communications cable. Is the shield in that cable tied to chassis of the PC and then connected to ground on the card in the machine? OOPS that is a ground loop!

Let there be chips!

Offline dude1

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Re: Steps to making mill bulletproof
« Reply #12 on: January 14, 2015, 12:31:19 AM »
You are using a PC with a communications cable. Is the shield in that cable tied to chassis of the PC and then connected to ground on the card in the machine? OOPS that is a ground loop!

Let there be chips!


that's a good one to know I will be changing tomorrow
Re: Steps to making mill bulletproof
« Reply #13 on: January 14, 2015, 07:56:37 AM »
A couple of other thoughts for you home builders:

If your wiring looks like a bowl of spaghetti you are bound to have noise problems
"I'll" neaten it up later" doesn't work, later you find out that some wires are too short and you won't rip them out and lengthen them.
Tie the wires down as you go, use cheap ties and cut them off when you add more wires, finish with good ones.
Once you strip off the shield twist pairs together as far you can. Twisting is an effective shield.
Wires that are not in cables that are pairs (to a limit switch for example) should be twisted when possible.
"Bury the grounds, and neutrals) my dad taught me.  Grounds are very important and never change, the same with neutrals.  So wire them first so that they are at the bottom of the wiring bundle, not on top of wires you may need to change.
Think about you wiring layout before you mount components. Always try to layout components for the least amount of wire and to minimize wires going in opposite directions in a bundle.
Draw a schematic or wiring diagram before you start.  You will be amazed at how quickly you will get confused if you don't, and how many wires there really are!
If want help, and you don't have a drawing you are pretty much on your own, and electricians bill by the hour!

Offline BarryB

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Re: Steps to making mill bulletproof
« Reply #14 on: January 14, 2015, 11:00:08 AM »
It's funny, that when I built the control box I followed all these steps of using shielded cable, keeping high and low power away from each other and crossing only at 90 degrees.  I made a visual schematic before I wired it, and then I tied everything down as I went along too.  I didn't do that will the mill proper though.  I'm not sure what I was thinking back when I wired that portion, although, at least the spindle does have shielded cable.   The wire will be delivered this weekend, but I've scheduled a solder party with some buddies, so we can solder all the cables in a couple hours at the end of the month.

Barry