Grounding a cnc machine

[golash]

Hi
I am reworking my control box for a 3 axis cnc machine. It seems emi or noise can be a major problem
for reliable machine performance. I`m confused between the grounding of the 115 VAC and the various
dc power supplies. Its my guess that anything that has shielded wires goes to a separate grounding location
from the AC buss bar. From my reading this is setup in a star configuration.
In my setup I have a few din rail dc power supplies. They have a ground not just + and - . Do these also go to
the star grounding location. Any help is greatly appreciated. As you probably guessed I`m flying by the seat
of my pants.

My new control

https://groups.yahoo.com/neo/groups/cnc4pc/photos/photostream/lightbox/1807665960?orderBy=mtime&sortOrder=desc&photoFilter=ALL#zax/1807665960

[Tandy]

I expect you have one of the many no-name Chinese CNC machines that are all over eBay these days. They are notorious for being problematic.

It can be a bit confusing as terms are often used interchangeably when they shouldn't be.

The mains EARTH connection is literally a wire that is connected into the soil and its primary use is to provide electrical shock protection. Exposed metal parts that could possibly come into contact with high voltages such as the metal box that the power supply is inside or if the spindle on your machine is a high voltage rather than low voltage DC type then the frame of the machine should also be earthed. This is so that should a fault cause a high voltage to come in to contact with metal parts the earth connection provides a lower resistance path the earth than your body would. This should ideally cause a fuse or other circuit protection device to shut off the power.

The Earth connection is often used for other secondary purposes such as static discharge and as a way of discarding over voltages from power surges. It is however not intended for screening EMC although frequently both the Earth and power supply ground are connected.

On a DC power supply you typically have the positive and negative terminals from the power supply (ignoring negative voltages) with the negative terminal being at 0V. This is usually used as a common ground for the entire circuit. To reduce EMC interference one end of a screened cable is connected to the 0V line to provide a way to reduce the amount of interference that can reach the signal wires inside the cable.

A common modification to these machines is to replace the supplied cables to the steppers with a flexible screened cable. Make sure you get a cable that has multiple fine strands so that it is flexible. This is the main reason why these machines fail
because the cable is not flexible enough and physically breaks and the lack of a screen means that pulses from one stepper can interfere with another.

Having said all of that the controllers in these CNC machines are extremely poor quality being built very cheaply. It is not just EMC interference that causes problems. The controller ICs are often unable to work at the required speeds causing dropped steps. Also the parallel interface to the PC is a major pain as it often can not supply correctly timed pulses to the controller especially if the PC starts doing something else like scanning for viruses.

[golash]

Hi Tandy
Thanks very much for the comprehensive answer. For better or worse the machine is not chinese. It is
a homemade machine.


http://i391.photobucket.com/albums/oo351/bgolash/CNCFinishedLarge_zps59e6690c.jpg

[Tandy]

Looks like a lot of effort went into building the machine. Is it dropped steps that you are truing to solve?

[Richard Crowley]

It is not clear whether you are trying to solve some specific problem, or whether you are asking generic questions about shielding and grounding?

You appear to be constructing your power/control circuits on a large metal plate.  I would certainly connect that plate to the green-wire safety ground of the mains power supplying the unit.  That would provide not only safety ground (in case of failure of any of the supply or control circuit elements), but it would also provide signal shielding when connected properly. 

You don't necessarily need shielding on the output wiring going to the stepping motors, etc. But the controller may be sensitive to interference from transients caused by switching large power loads (i.e. feeding the stepping motors).

Generally, things with a separate ground terminal should be connected to the large metal plate that forms your "chassis".

It is common to find the controller (Arduino board or whatever) in a separate (and separately shielded) enclosure to protect it from transients. And it is also not unusual to see optical coupling used between the controller and the motor drivers. That decouples any large switching currents from possibly getting back into the controller.

But you may want to simply finish the construction and test the system to see you actually have a problem or not.  With that nice big ground plane, you may not have a problem at all.

[golash]

Hi
I originally designed the machine with steppers. I could never get the machine to function as I liked.
I then upgraded to dc brushed servos and the performance was vastly improved. I had a problem
also with using the wrong linear motion bearings on the x axis. The axis would shake when the
machine came to a sudden stop. I overcame that problem with a more rigid linear motion bearing for
the x axis. Before a few of these changes were made I contemplated selling the machine and starting
a new machine. Now with these changes I am happy with the machine performance.
The control box was always a rats nest of wires. I decided to spend the time researching control wiring
and then upgrade the control box. This is now the current project. The one change is a 72 vdc power supply
that produces 20 amps. I`m hoping with this change I can get faster 3d machining times. I`m interested in violin
making and have done a top plate on the machine. The machining times were too long. 7 hours to machine the
top. I`m hoping with these changes I`ll be able to run the machine faster. But also minimize servo faults.

[Richard Crowley]

With a machine that big, you could carve fronts and backs for double bass!
There is probably a tradeoff between that size and mass vs. speed and accuracy.
I am in final stages of deploying my new CNC machine which I built from a kit...



http://www.pdjinc.com/

[golash]

Hi Richard

That machine looks very nice. You`ll  have many hours of enjoyment with the machine. In 2004 when I
built my machine not many kits were available. I now have enough spare parts to make another machine.
DIY can be expensive when your upgrading parts as I did.

[golash]

Hi

With the new control panel I`m attempting to minimize emi interference. I`m attempting to learn the
correct grounding to eliminate any potential noise issues. Both the stepper and the servo had noise
issues . I spent hours with alligator clipped cables trying different grounding configurations with the shields
on the wires . After much work the machine has been much better.  But I want to attempt to do even better
with the new control box.

[Richard Crowley]

With something running such high power, I would very seriously consider isolating the controller in a separately-shielded enclosure, and optically-coupling the control wiring.

It just strikes me that a machine that large and massive may be overkill for something as small and delicate as a violin.
And starting and stopping are typically ramped up (and down) specifically to avoid the sudden starts/stops that cause that kind of shaking.

I was about to build a machine from scratch, but I found this vendor who actually lives just over the hills from where I am. And he has all the details worked out already so I can get on with USING the machine (vs BUILDING the machine, as much fun as that is).

It looks like you have some interesting experimenting to do there! Are other people carving traditional instruments like this?  Are you making your own patterns? Has anyone digitized the profile of a classic Stradivarius?

[golash]

Hi

The machine was designed as a general purpose machine. It was designed to machine metal. It has a 1 hp Dc
motor controlling the spindle. It has machined cast iron, brass and aluminum so far. You are correct I believe.
A large machine with a lot of mass would tend to shake. With the upgrade to more precise linear bearings the
system shakes much less.
Yes, other people are using CNC machines to mill the violin parts. I had a professional CAD designer produce the
top and back plates in Solidworks for me. He did a wonderful job. Its design is derived from a Strad.