Skipped Steps

So my printer is starting to skip steps on a lot of prints that previously came out fine. It's mainly the Y axis, but it looks like there may be a bit on the X axis as well. Here's a picture:



The Y axis goes from left to right in the picture and the layers where steps are skipped seems random. I've tried this print at least 5 times now and the layers height and number of layers with skipped steps are always different.

I've tried disassembling the Y carriage and lubricating the insides of the bearings and realigning the bearings with the bar clamps according to the procedure in the manual, but neither really improved anything. The skipped steps are nowhere near 2.5mm, they look to be .5mm at most. And I'm not sure if it means anything, but there seems to be a pattern after every layer with a skipped step. It looks like the layers skew in sets of 4. The first is only slightly shifted, the second shifts about twice that amount (about .5mm), third shifts the same as the second, and fourth shifts the same as the first. The print then goes on okay for awhile before doing the same thing.

Am I right in thinking my bearings have gone bad? Thanks for any help.

Does the carriage more freely when you have the motors turned off?

If the motor skips steps it will always be a multiple of 0.8mm. It can't skip 0.5mm. The only way that could happen would be a loose pulley.

---

[www.hydraraptor.blogspot.com]

Yea the carriage moves freely. But it seems like the friction isn't constant along the length of the rod, which is why I thought it might have something to do with the bearings. Especially because the positions where there's more resistance to movement seem to move around.

The 0.5mm was an estimate, I don't really have an accurate means of measuring it. I just checked both pulleys and they're both fastened pretty tightly.

I used to get random skippings, which screw up big prints pretty badly. But I managed to get rid of them by turning up the trimpot of the pololus by 1/8 turn.

I have had bearings fail after years of use but they jammed solid in one direction rather than creating small errors.

The pots on the Melzi are set to the optimum value so should be OK unless vibration has caused them to change. The voltage on the wiper should be 0.5V.

Digital callipers are pretty much essential for running a RepRap.

---

[www.hydraraptor.blogspot.com]

Does it happen when the extruder is making a bunch of rapid moves? If so then try lowering your acceleration in the firmware.