Grinding Z-motors and uneven extrusion during layer switch

I'm in the process of calibrating my prusa mendel, currently trying to get the thin-wall cube from here to print correctly. I'm running into a problem with uneven plastic distribution while switching layers. The moment the extruder jumps to the next layer, a glob of plastic is dumpled, resulting in a bulge at that spot (point A in the attached image), and the printhead then zooms over to the next vertex (point B ) much faster than it should, resulting in very little plastic being aplied to the length of wall between A and B.

I found the reducing the travel speed helps reduce this effect (why is the layer-switching speed controlled by the general travel speed? Seems like it should be a separate setting). However, if I set the travel speed too low (below about 50mm/sec), after about four layers the z-axis motors start making grinding noises instead of moving. I don't have this problem if I control the motors manually from the Pronterface interface, no matter how low I set the movement speed.

So I guess there are two separate issues here:
- Z-axis motors grinding at low speeds
- Bad gcode generated by slic3r, making the printhead move too fast after a layer switch

My setup:
RAMPS 1.4
Sprinter firmware
Pronterface + Slic3r
J-Head 4B hotend

are you using retract settings on the extruder? Possibly pots set too low for the Z motors

---

__________________________________________________________________________
Experimenting in 3D in New Zealand

first of all, it looks like the layers haven't had enough time to cool before the next one is extruded. Try putting a fan pointed at the print (this may cause other issues like the need to insulate the hotend). as for the bulge at point A, try increasing your max z feedrate in firmware and the slicer. mine is set to 5mm/s with a 4mm/s² acceleration. This will shorten the time it takes to jump from one layer to the next and minimize the ooze. The extruder shouldn't retract in this short space.

I'm not using retraction. I don't think the pots are the problem, since I can move the motors just fine my sending commands from pronterface, but I'll try to set them higher.

Does slic3r have a setting for Z feedrate? If so, I can't find it.
I see the max Z feedrate is set to 50 in the firmare (no mention of units, is it in mm/s?). My experiments so far show that a slow Z transition is actually better, I think if it jumps too fast, the PLA gets jolted out, if that makes sense. I will play around with the setting though. How did you set the acceleration?

I used to have a fan, but removed it because I though it was causing problems. I will add it back once I have the other issues worked out.

I'm using marlin firmware and skeinforge for slicing. In the firmware, under the configuration tab you'll see this:

// default settings 

#define DEFAULT_AXIS_STEPS_PER_UNIT   {160,160,2267.72,998.88}  // default steps per unit for ultimaker 
#define DEFAULT_MAX_FEEDRATE          {500, 500, 10, 45}    // (mm/sec)    
#define DEFAULT_MAX_ACCELERATION      {9000,9000,100,8000}    // X, Y, Z, E maximum start speed for accelerated moves. E default values are good for skeinforge 40+, for older versions raise them a lot.

#define DEFAULT_ACCELERATION          3000    // X, Y, Z and E max acceleration in mm/s^2 for printing moves 
#define DEFAULT_RETRACT_ACCELERATION  3000   // X, Y, Z and E max acceleration in mm/s^2 for r retracts

// 
#define DEFAULT_XYJERK                30.0    // (mm/sec)
#define DEFAULT_ZJERK                 4.0     // (mm/sec)
#define DEFAULT_EJERK                 5.0    // (mm/sec)

so for the z my max feedrate is 10mm/s, my max acceleration is 100mm/s² and my speed at the beginning of acceleration is 4.0mm/s

I don't see an option in slicer for z-feedrate so maybe it relies on the firmware to set it.

I switched to Marlin, and the results are definitely better, even without cooling (attached). I also noticed that my X and Y distances were off, which was contributing to some of the other problems I was having. There is still some troublle with the infill now, so I'm going to try to figure out how to use skeinforge, maybe the additional control it provides will help.

Thanks for the help.