Power Supply NEG returns questions

[bpratl]

I'm in the process of rewiring my Mach3 Lathe controller due to noise problems and I have a few questions:
1. Should I connect the 5 Volt and the 48 Volt negative returns together or leave the outputs floating?
2. Should the negative outputs be isolated from equipment grounds, like Pc, Lathe and controller which are grounded to the GREEN AC ground.
3. Is it better use an external 5 volt power supply for the BOB or is the USB input OK, PROS & CONS?
4. Is it wise to add EMI/EFI filter and if so where should they be installed.
Bob

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[A_Camera]

I'm in the process of rewiring my Mach3 Lathe controller due to noise problems and I have a few questions:
1. Should I connect the 5 Volt and the 48 Volt negative returns together or leave the outputs floating?
2. Should the negative outputs be isolated from equipment grounds, like Pc, Lathe and controller which are grounded to the GREEN AC ground.
3. Is it better use an external 5 volt power supply for the BOB or is the USB input OK, PROS & CONS?
4. Is it wise to add EMI/EFI filter and if so where should they be installed.
Bob

It is very difficult to help somebody who lack a very basic understanding of electricity and electrical wiring principles. I think you are better off if you contact someone locally who can help you out with this, but some words of warning:

Regarding your question 2:

Do NOT under any circumstances connect the GND on DC side with the protective earth (PE), which I believe you wrongly call "GREEN AC ground". That would be purely hazardous and very dangerous, not only for your equipment but also for yourself. Protective earth is NOT the same as GND.

Your question 1 is not possible to answer without more information because it depends on what the 5V and the 48V is used for.
Your question 3 is not possible to answer without more information because it depends on your BoB. If it is made for using an external 5V than it is better if you don't connect it to your PC USB port. Remember that not all PCs have 5V on the USB, some have only 3.3V.
Your question 4 is easy to answer. EMI filter is always a good idea and can never do harm, only good. An EMI filter is normally connected to your mains input, after the mains switch, before the power supply. Here is a picture of how mine looks like.



Why are you rewiring your system without knowing what you are doing? Are you just guessing that it will solve your problems? Many things can cause noise, and it is not clear if you mean electrical noise, which you can't know anything about unless you measured, or noise generated by your motors which actually can come from a faulty driver or just a simple lose wire. Start with fault analysis because rewiring may result in a lot of time and money spent without real benefit. OK, it may solve your problem because in that process you will tighten the right screws, or change a partially broken wire, but it would be cheaper and quicker to just identify the problems first because you might find the faulty wire or contact and could fix that without changing everything else around. In the end, if it turns out to be a motor or a driver, the whole work may have just been wasted money and time. For all we know, it may even be your PC which is causing the problem, with noise like symptoms.

[bpratl]

The reason for the rewiring was to replace all the input and output wiring to the drivers, limit and home switches with shielded cables and ground the shield to a central point.
The suspected noise, we think, is causing a very random jumping of the X axis during multi passes in threading fine long threads. The PC, stepper and driver was swapped with no improvement.
In scoping the power supplies, stepper pulses, grounds in respect to DC returns did not show up any transient noise. There was noise coming from the VFD which was causing bogus index pulses and was terminated by proper shielding of the motor cables and controls. The Lathe, PC chassis and CNC controller are well grounded to "Earth" ground (green AC ground) with no measureable differences of potential between them. Every combination of motor tuning velocity and acceleration was tried to include current limiting resistors to the steppers with no improvement. Mechanical binding was ruled out because everything is very smooth with the ball screws unattached. EMI filters will be installed when they arrive.

[Al_The_Man]

There are two schools of thought on earth grounding DC power supplies, for example your P.C. P.S. common is connected to earth ground.
Many systems and myself included prefer to earth ground all power supplies to the common service star ground point.
Ensure motor frames such as spindle etc are equi-potential bonded to the star point.
Al.