Hey Randall, hey Bill, hey Mike, hey all,
are any of the other connectors getting hot? Or only certain connectors, e.g. at the extensions? At the controller, at the X axis input (especially since they are subject to permanent motion and are lacking strain relief), on the front at Y axis inputs?
Randall, you show extensions which are probably 3rd party, are these the only ones that get hot?
Some background: In principle, since they are rated 8/9 A, they should not get hot, but the specification is of course only valid if they are crimped correctly (see here for instructions of how to crimp them), which means with the matching crimping tool. If you consider the fact that the correct crimping pliers cost about $300 for the Molex version, about $750–$950 for the Amphenol version, I’m not sure if the Onefinity manufacturer, but especially the 3rd party supplier for the cable extensions, really uses the correct crimping pliers. And only if you use the correct crimp pliers, you have the specified current rating valid. I have seen people soldering(!) such connectors. The cheapest way to have the correct crimping pliers I have found is to buy the Knipex 97 49 26: Crimping die for connectors in the Mini-Fit™ series from Molex LLC (16-24 AWG model) for the Knipex 97 43 200 A Crimp System Pliers for exchangeable crimping dies (version with case) (version without case) pliers (I own the pliers, but the insert for these Molex is about $200! I assume it’s because of patents, since all other dies for these pliers are at about $50.
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See here for about to crimp them.
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See here for making your Onefinity cables/extensions yourself.
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See here for what I think of these connectors.
Stepper cables are power cables.
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In the industry, you find a wide choice of circular connectors for power applications.
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See here what circular connectors I would use for connectors for open-loop stepper motors.