Shielded cable and MIL spec connectors

[Arjay]

Hello folks,

I'm working on a long term cnc router build that I've been gathering parts for some time. My plan is to start with the z-axis and work my way 'backwards' to the x and y axises (ax-eye??). I picked up a few components like the ballscrew and linear rails, and just recently picked up an ATC 7.5KW Chinese spindle. My next steps are to pick up a VFD and servo motor for the z-axis. Once I get these parts, I'll start the z-axis portion of the build.

Well enough about that. Reading the recent thread about the 3.7kw spindle having issues that were solved by running shielded cable got me thinking about the wiring.

Rather than building one large wiring harness that contains the spindle cables, servo cables and other wiring, I would like to break the one large harness into several harnesses and connect them together with MIL Spec connectors. For example the z-axis would have its own harness that would plug into connectors fastened to the x-axis.

Even though this seems like a clean approach, I wonder if there are any drawbacks? For instance, can the 380v shielded spindle cable be broken into several lengths that are connected with the connectors? Similarly, is there a loss of shielding at the connector?

If using connectors in this manner are acceptable, is it okay to bundle the spindle power with the other electronics (such as servo wiring) in the same connector? My preference would be to have one 'z-axis' connector rather several connectors for the z-axis alone.

I would appreciate any input or advice.

Thanks,

Arjay

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[ger21]

Not sure what you mean when you say "harness", and why you want to break the cable multiple times.
All cables are typically individual cables for each device. 1 cable for the VFD. 1 Power + 1 encoder cable for each servo. If they are shielded properly, they can all run in the same e-chain.
The cables are usually only broken once, where they exit the control cabinet. So you have a cable from the VFD to the control cabinet connector, and from there, to the spindle. The shield needs to be properly terminated at the connector, so that it's continuous through the connector. Mactec posted some pics in another thread yesterday that showed this.

[Arjay]

Not sure what you mean when you say "harness", and why you want to break the cable multiple times.
All cables are typically individual cables for each device. 1 cable for the VFD. 1 Power + 1 encoder cable for each servo. If they are shielded properly, they can all run in the same e-chain.
The cables are usually only broken once, where they exit the control cabinet. So you have a cable from the VFD to the control cabinet connector, and from there, to the spindle. The shield needs to be properly terminated at the connector, so that it's continuous through the connector. Mactec posted some pics in another thread yesterday that showed this.

By harness, I mean a bundle of wires. In an automobile there are many such wiring harnesses that make up the overall wiring for the vehicle. For example, there might be multiple engine harnesses, an interior guages harness, a harness for the tail lights, and so on with each harness connecting to a bulkhead connector or another harness.

I have seen this approach (harness/MIL spec connectors) in use on commercial CNC machines, but it doesn't seem to be used on DIY machines.

I am wondering if there are technical reasons it isn't done in DIY (like noise, voltage drop, etc) or simply because of time/expense.

[Jim Dawson]

Time and expense are certainly considerations. But that is not the main reason. Unless the external device (motors, encoders, photo eyes, ect.) requires regular service or requires easy changeout for a quick turnaround, it is better to have to have one continuous cable between the device and the connection inside the panel. Each connection point becomes a potential failure point, and connection points should be limited to what is absolutely needed.

The reason this is done on commercial machines and automotive is for ease of installation, screw or push the connectors together and press go. Saves a lot of field wiring time. But does not make the most reliable system. With a few exceptions, everything I design/build has a continuous cable between the device and the internal panel connection.

[Arjay]

Thanks for the responses guys.