Another Z banding problem with Prusa i3

So, I've got a Z banding (or wobble) issue that I can't figure out.

The printer is a China-kit but I've tried to measure everything to make sure the numbers are what they are supposed to be. From what I can tell, my Z-screws are 8mm with 2mm pitch ones. I've done the math according to the calibration guides online and from what I can tell, the steps/unit are fairly correct. I've ended up with 400 steps/unit for the Z-axis and 80 for X and Y.

Now, no matter what I do, I get pretty severe banding on my prints. Nice sine waves offsetting the plane along the Z axis. As you can see in the image attaced of my calibration cube (25x25x25mm), the edge is far from straight.

As this is a Prusa i3 with smooth guide rods, I'm not really sure how this happens. One theory I had, was that the X-axis was too tight or too wide causing wobble, but I just rebuilt it and the result is the same. The smooth guide rods for the Z-axis are constrained both top and bottom while the Z-screws are only constrained at the bottom. The connector to the stepper motors are metal and the rods to not go down too far, i.e. the screws can wobble as much as they like.

Another theory I had, was that the motion of the printer caused the acrylic frame to warp, but slowing the print speed to 30mm/s and holding the printer down while printing didn't make the slightest difference.

Attached are images of the calibration cube as well as the Z axis (top and bottom) and the X axis connectors.

Any suggestions for how to go about solving this would be appreciated.

What layer height are you using? Not all microsteps are the same size, so it's best to choose a layer height that is a whole number of full steps. If you have 400 microsteps/mm and 16x microstepping, your full-step distance is 0.04mm. So your layer height should be a multiple of this, for example 0.2mm or 0.24mm, but not 0.25mm.

This is especially important if you are driving 2 Z motors in parallel from one driver, because your Z motor currents are probably rather low. For most types of motors used in repraps, it's better to connect two Z motors in series, not in parallel.

Edited 1 time(s). Last edit at 09/10/2015 03:06PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Oh, sorry, forgot to include that info - I'm printing at 0.2mm layer height.

Connecting the motors in serial sounds awkward if you ask me. But I'm far from an expert on the subject. I guess I could rewire the motors like that if it helps though.

In most cases, connecting the motors in series will provide more torque, which in turn makes microstepping more accurate. But as your layer height is already a whole number of steps, the banding is caused by something different anyway.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

My printer is the same as yours. What material are you using? I had banding printing with .2. Went to .3 and banding was eliminated. This is with ABS at 240 degrees hot end and 105 bed temp. Also use a raft for better bottoms. Print speed is 30. Voltage on x and y drivers set at .75.

I'm printing using PLA at 190°C. I'll make a test print at 0.3mm and see if that resolves anything.

I'll be darned. Printing at 0.3mm layer height solved 90% of the problem. There is still slight banding, but now I have to look closely to see it.

I just don't understand how the heck 0.3mm can work better as all the numbers indicate 0.2mm should be working just fine.

I just had a very similar problem with banding on an i3 kit this week.

I did this to solve it :

1. check the wobble of the X carriage that the extruder is bolted to. I found slight play in the linear bearings and that was causing 75% of my wobble. I replaced the linear bearings with self-printed bearings ( http://www.thingiverse.com/thing:24990 )

2. check the bearing on the Y axis under the bed for the same problem.

3. replace the springs under the bed with fixed bolts. Once you have the bed level, you shouldn't need to change it often. The springs setup allows the bed to move very slightly, also causing banding.