Aiph5u
(Aiph5u (not affiliated with Onefinity))
21
Hey Tom,
Yes, usually it’s a disease, that’s why I rarely approach things this way. Usually I buy literature used for engineering studies. My approach (in every matter) is to understand things. I achieve this mainly by rtfm rather than by visiting web fora. But web fora are nice if you meet someone that is exploring the same thing at the same time!
That is a common phenomenon with a lot of topics: Everyone says something about it, regardless of whether he or she has studied it or just heard something about it.
I’ve been dealing with grounding for a long time and I would say, it is not rocket science.
When I said above that it could introduce a ground loop, I am aware that it may not always be relevant. A relevant ground loop is if your cable TV provider grounds its cable in his distributor board out there a few miles down the road and then you ground it again at your cable TV wall outlet. It can produce a lot of hum that deteriorates the signal.
Even if the housing of the spindle may be grounded via the bearings on the rails of the CNC, I would not consider this as to be the reliable way of grounding it. I would think that there must be an intentional electrical connection of correct dimension. Protective Earth (PE) has to carry any current that can appear erratically at any point on the machine.
Also there is a difference between circuit grounding, shielding and protective earth. Usually shielding and protective earth do not participate in the operational current flow. There may be assymmetrical signal or power cabling where shielding is at same time functional grounding, but to avoid ground loops, usually digital and analog signals use symmetrical (balanced) connections with separate shielding (or twisted pair if one wants to avoid shielding).
A 3-phase spindle is operated with three wires using a delta circuit, there is no Neutral conductor that has to be connected to ground. The fourth connector on the spindle, to which the spindle housing and the cable shielding are connected, is protective earthing.
" However if the spindle housing has contact to the 80 mm mount which has electrical contact to Z Slider which has contact to X Axis which has contact to Y Axis which has contact to machine’s feet which could have been grounded, it would introduce a ground loop. Is the machine grounded at the feet?"
Isn’t this ‘contact’ that you speak of only available through the Z and X and Y bearings? So not a reliable electrical connection. I know it’s not a path you are wanting but it seems it could be an intermittent path and hence difficult to troubleshoot if there were ground loop issues.
Aiph5u
(Aiph5u (not affiliated with Onefinity))
25
Hey Bob,
yes, you are right, I came to this point in what I wrote later (above): The fourth pin (Protective Earth) of the spindle does not participate in operational current flow, so the issue of a potential ground loop is neglectible, but as being part of protection, it is clear that there must be an intentional electrical connection of correct dimension. For the intermittent path through the bearings this is not true. The instructions of the manufacturers mentioned above also are very clear about that:
On the frequency converter side, the cable shield must be laid as large as possible (using a suitable cable clamp) on the grounded plate (usually the base plate of the control cabinet) on which the frequency converter is mounted.
It is important here that if shielding serves as protective earth here, when choosing a cable, it must be dimensioned in such a way that it can divert any possible fault current that may occur on the machine.
Also for safety, I always plan to use a RCCB (in fact, a RCBO, a combination of RCCB and MCCB) in my control cabinet.
1 Like
Aiph5u
(Aiph5u (not affiliated with Onefinity))
26
Hey Alex,
what I like about your setup is the combination of simplicity and effectiveness. You have installed the dust collector at an ideal point in the room to hang the hose at only one point. I assume that the hose will also work with the Journeyman X-axis. You built the table so big that the Journeyman will fit on it. I always try to do everything so perfectly, but I always have to realize that of course I can’t get things done very quickly. With your setup, I immediately think about the fact that you already had a lot of fun with it because it is already running. And the cable at the other suspension point on the ceiling, it seems to me it is a cable and a transparent hose? What is the box for to which you attached the Onefinity Controller? It looks like a PC. Or is it a cooling station?
Aiph5u
(Aiph5u (not affiliated with Onefinity))
27
It’s an old PC case. Inside it is the VFD, power supply, and the solid state relays. It’ll be mounted to the side of the table out of the way. I was going to mount the controller inside it as well and move both the power switch and the E stop button but decided not to for now due to possible electronic interference. I also ran 240v power to the case and split it up for the various things that need power (like the VFD and the SSRs for the water pump and dust collector). I have the two water lines and power cable for the spindle bundled together away from the servo cables. Like with most of us here it’s a work in progress and will change with time.