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underextrusion at same height on every print

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Hi, I've had this problem, a couple of times. I think I actually have is fixed now. Turn everything off and raise and lower your bed manually. Make sure you bring it up to the nozzle. When you lower it, if you notice that it sticks, place your print on the table and compare if it sticks at the same height as your missing layer. If so, your going to have to get some lube and take some stuff a part. Check the rod to see if there's anything on them that may cause it to stick.

Take the bearing covers off and tape it to the top. Next take the tread rod nut and unscrew it and then rotate it to the top. Make sure this doesn't stick as well. Now you should be able to freely move the table up and down just on the bearings. Try lubing (Use sewing oil, but I ended up using car bearing grease) to lube the rails and move the bearings up and down until smooth. If that doesn't work, you'll have to find the sticking bearing by removing the screws from them and moving them up and down the rail to see if one sticks and the other doesn't. Just remember when putting everything back together that as use tighten the bearings you move them up and down the rods and that they move freely, no binding.

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This is a common problem. It could be your model (check it in Cura layer view very very carefully around 12mm height) or more likely it is Z issues.

There are known Z issues with the Z driver in the PCB where recent boards put out too much current and it gets too hot and fails intermittently and the bed falls about 1mm and recovers. These tend to be almost always the same height but a little different each time.

Then the most likely cause - something with the two Z bearings. Apparently you shouldn't put oil on them as it gets in the ball bearings and can gum them up. Maybe remove the Z rods, remove the Z bearings and clean the bearings very well - I would probably try WD40 and then dry them completely of all oils and cleaners and then assemble it all back together. Maybe one drop oil on each Z rod but no more. Probably residual oil from WD40 is enough.

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You can do two things here. Feel both rods to see if you can figure out which one is sticking at that spot. If not, you'll have to take each bearing off the bed to find the problem. Or, you could just grease the heck out of both rods and slide it up and down until that spot is gone.

If you remove each bearing, and find the sticky one, lube it and slide it up and down until the spot is gone.You may want to take some very fine sand paper or tin foil and sand the rod at the sticky spot (very slightly) until the sticking is gone.

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A bearing is when any two things slide across each other in one or two directions but keeps the thing from moving in one or two other directions. On the bed there are 2 vertical 8mm rods that pass through 2 8mm cylinders. Those 2 cylinders are the bearings and inside they have ball bearings. Those are a specific type of cylindrical bearing called a "linear" bearing that does not rotate around the rod well (twisting has friction) - but slides along the shaft with very little friction.

Whereas the cylindrical bearings inside the print head have no ball bearings and instead are simple cylinders that allow sliding *and* twisting motion.

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I suspect it is due to oil on the Z axis pillars.

The Z axis bearings are ball bearing races and I think that oil on the pillars causes them to slide and not roll, making a jam.

I had this problem for some time, new bearings and no oil fixed the problem. As an experiment, I oiled the pillars again, promptly creating the fault. Cleaned off the oil and everything was OK again.

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