JoeM
Master Poster
Joined: 09 Jul 2004
Posts: 122
Location: Jackson Hole, Wyoming USA
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 Posted: Thu Mar 17, 2005 11:35 pm Post subject: Proper grounding of a CNC machine/DNC computer |
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Most CNC manufacturers recommend a copper ground stake through the floor, with a large diameter stranded cable tying the ground stake with the frame ground lug inside the CNC cabinet. This short, low-impedance ground wire is best for attenuating RFI (Radio Frequency Interferrence) and EMI (Electro-magnetic Interferrence). It also provides a safety ground for AC current that may leak to the frame of the machine from a shorted coolant pump, shorted limit switch, etc. Without a ground of some kind, and operator can receive a shock by touching the machine and touching another grounded device. Your machines can also be grounded with a "4th wire" through the 220 or 440 AC buss duct, and that would be an OK safety ground, but it wouldn't be as effective for EMI and RFI as a separate ground stake.
As for the DNC system, we always use fully shielded cables with the shield tied to the connector shell (which is grounded to the frame of the CNC) A shielded DNC cable where the shield is simply tied back at both ends is useless at preventing noise on the serial data lines. The shield must be grounded somewhere (at the PC end or the CNC end doesn't matter).
A DNC system MUST NOT permit a "ground-loop" between the grounds in your 110v wall plugs and the CNCs ground stakes. We prevent this by grounding all the serial devices to the CNC controls, and NOT PERMITTING the serial device from getting a ground from the wall outlet. If you have an external serial device, like an Ethernet serial hub or a USB serial hub, they are usually powered by a DC power cube, so there is no potential for a ground loop. AC powered serial hubs or PCs with Rocketport cards should NOT be grounded to the walloutlets. Instead, let the DNC cable shields ground the PC to the CNC controls and use a 3-to-2 prong adapter on the PC, the PCs monitor, printer, etc.
If some of your machines have ground stakes through the floor and others have a "4th wire" ground, BEWARE of a machine-to-machine ground loop where a short circuit on one machine seeks a path to ground on another machine through the DNC cables. This is a good reason to invest in opto-isolators in the serial lines, or to bite the bullet and ground ALL your machines in the same way.
If the PC's and CNC's ground references are not nearly the same you could burn out ports or worse. If you suspect a problem, measure with a voltmeter between the machine frame and the CNC cabinet to see if there is any signicant (more than a few millivolts) potential difference both AC and DC wise. If so check all grounds. I have found for example a machine with a ground rod into the watertable whose connection to the CNC cabinet frame was with lug to a painted surface.
Hope this info is helpful.
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Thanks.
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