Dropping Y steps

I have been very happy with my prusa since it has entered it's second generation with improved parts, but recently two prints failed because of dropped y steps. One was a small print - not so bad, but the other was on layer 122 out of 132 of a very large print. I wasted a lot of plastic on that one.

My feed rate setting in Skeinforge is 40 mm/s, and my Y axis firmware (Marlin) settings are max speed = 500 mm/s max acceleration = 9000 mm/s2.

In both cases, the failure happened as the print head was entering an area where is was going to do a VERY short (in the Y direction) solid fill. In other words, it was jumping into an area where there was going to be a lot of vibration. Both times I heard a loud click, and the print was off from that point on.

The first time this happened, I bumped up the pololu trimpot a bit, and I can certainly do that again, but I'm also considering the following:

- slow it down. I don't like this idea because I don't think I'm going very fast at all right now, and this happens so infrequently that I'd hate to slow down the 99.9% of the time that things work well. Right now I just dial down the speed a bit when I get near the critical areas, but that takes baby sitting.

- reducing the acceleration maximum. I don't know if the acceleration number is too high or not, but 9000 mm/s2 seems outrageous compared to the other numbers here. I think this is the default value, though, and I don't know what effect reducing it would have.

- reducing the tension on the y axis. This is the top idea on my list. When I rebuilt my machine, I was striving for rigidity. Maybe I went a bit too far. I can see how over tightening can cause missed steps.

Anything else I'm missing? Any other suggestions?

Thanks

You care about the default acceleration or the number stored in the EEPROM not the max accelleration in Marlin.
You also care about the JERK setting, that's the maximum instantaneous change in velocity.

I'd try turning both down somewhat, For point of reference I run 1500mm/s2 on my Mendel Max, and my Y table is not exactly light.

Just so there's no confusion, the values I put in above ARE the values from memory/EEPROM. When I type in M503, here is what I get:

Sending: M503
Steps per unit:
 M92 X44.44 Y44.44 Z2560.00 E699.90
Maximum feedrates (mm/s):
 M203 X500.00 Y500.00 Z5.00 E45.00
Maximum Acceleration (mm/s2):
 M201 X9000 Y9000 Z100 E10000
Acceleration: S=acceleration, T=retract acceleration:
 M204 S3000.00 T3000.00
Advanced variables: s=Min feedrate (mm/s), T=Min travel feedrate (mm/s), B=minimum segment time (ms), X=maximum xY jerk (mm/s), Z=maximum Z jerk (mm/s):
 M205 S0.00 T0.00 B20000 X20.00 Z0.40 E5.00
PID settings:
 M301 P23.82 I1.87 D75.73

When I put in M501 and then M503 again, I see the same values.

So if I understand you correctly, My Y acceleration is set to 9000 mm/s2 where yours is set to 1500? Also, my Y jerk (XY jerk actually) is set to 20.0 mm/s. You don't say what yours is, so I can try to reduce this a bit, and also reduce the 9000 to 1500 or so?

If I am understanding Marlin your accelleration is set to 3000 ( the M204 setting) and a X/Y jerk is set to 20 (the M205 setting).
I'd try reducing both to half, and if that works try raising them a little.

Thanks - I'll give it a try. It's like a lightning rod though - you never know if it's working until it doesn't