TL;DR version: Had a random clunk and occasional bind on Z axis. One of the screws holding the top Z axis lead screw/threaded rod bearing had worked loose a small amount. It was barely rubbing the bottom of flex coupler from time to time. Tightened and Loctited that screw. Problem solved.

I have an auction site 3040 Chinese CNC router/engraver. I've been experiencing a somewhat random "clunking" sound in the Z-axis off and on for awhile now. From time to time the z axis seemed to bind ever so slightly. It would even occasionally miss steps but that happened very rarely. Shorter runs would often turn out fine but things seemed to run a bit more slowly and you could hear that everything seemed to sound a little slower and just slightly more labored at times. At first glance things looked fine and I couldn't pinpoint the cause.

When turning the Z axis by hand it would spin smoothly at times but at other points in the Z axis travel it would feel like it would bind very slightly with resistance. Not a very serious bind but the motor would requiring a little more force than normal when hand rotating the axis. At these points the axis travel felt more "crunchy" then the X and Y axis when I'd rotate. The odd thing was I could feel and hear the clunk only at certain points along the Z axis but the bearings etc were clean and freely sliding.

The other day the clunking got slightly worse and more persistent. I decided to tear the Z axis down and look for the issue. At first I was suspecting a cracked or loose flex coupler since everything else looked ok. I did find one of the set screws on the flex coupler a little loose but not loose enough to cause any problems yet.

Then I noticed the head of one of the Philips head machine screws holding the top Z axis lead screw bearing was lifted off the bearing mounting plate a small amount. Not something that you'd notice at first glance, but the screw had definitely worked loose over many hours of use and vibration. That screw (the rear most one furthest away from the spindle had backed out just enough to slightly rub the flex coupler. The clunk I often felt or heard was the screw head barely scraping the bottom surface of the flex coupler mounting the stepper motor to the Z axis threaded rod/lead screw. You would think something rubbing like that would be less random and more repeatable but It would only do it at certain points and only ever so slightly. However it was just enough to cause the axis to bind at times. It doesn't take nearly as much as you'd think to knock an axis out of whack and cause issues.

Long story short, I removed the screw, put some loctite on it and screwed it back down. I cleaned everything up, lubed it all and put it back together. I've got about an hour on the machine after that and the Z axis runs buttery smooth now. No clicks, no clunks and no binding.

I went through all the rest of the fasteners on the machine and that screw was the one and only one that was loose. Easy to forget how much vibration and abuse the Z axis takes over time! I just thought I'd share this in case anyone is ever trouble shooting a weird, seemingly random clunk or bind on an axis.

This is also a good reminder to just periodically go through the machine and check fasteners to ensure everything is still tight. I got lazy about it and just periodically checked the tightness of stuff I could see and reach easily. At some point I should just go through and loctite or lockwasher things so I don't have to worry as much.