What causes a y offset to occur in the middle of a print?

[akkrolnik]

I have been printing a scale model of the floor plan of my house for my son who modeled it in a 3D software package. These are large sections that fill the majority of the build platform. In other posts I've gotten assistance related to first layer problems due to Z-height and bed leveling issues and also lifting up of the edges due to poor adhesion. I have gotten those issues under control, but it is still hit or miss if the full print completes properly. The issue, which has happened twice now, is that the print will get 60-70% completed and suddenly a y-offset will show up. The first time (on a print that went overnight) I had three offsets that randomly showed up. I threw that part out a while back. However, last night I was printing one of the more complicated sections of the floor plan and it happened again. Luckily my son saw the offset and stopped the print so I did not waste any more material.

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Picture 1

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Close-up of corner

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Anybody know why I am randomly getting y-offsets in my prints? The x direction is perfect.

I am printing at a fairly high speed 150-180mm/s on the exterior and 200 mm/s on the infill. I'm also using a 0.3mm layer thickness. I'm pretty sure these settings are why I'm getting the poor quality corners. I'm okay with the corners for these parts because I don't want them to take 10 hrs to print. The corner quality is a side topic, but if someone knows what can be done to improve it without significantly increasing the time I wouldn't mind hearing about that. As it is, it takes around 5 hrs for one section.

Regarding the y-offset, I have only seen this on parts that take more than a couple of hours to print, and when I am not present during the print. Many times I'll be printing while I am using my computer to work on a new design for something. In those cases, I have never seen this issue. I am printing over USB directly from Simplify 3D. I've turned off any sleep modes on the computer to prevent the print from stopping in mid-print which happened once. Could this issue be related to something the computer is doing when it is unattended? Unfortunately when I bought my new computer (prior to the M2), I did not get a media card reader because I never really used it with my old computer. I was thinking to try and print directly from the SD card reader and see if that helps, but it is definitely not convenient for me at this point. It also seems strange that the offset is only in one of the directions if it is a computer communication issue. Ideas?

[jsc]

Odds are you are over-extruding and at some point the head catches the bulgy top and skips steps. Two things will help:

Carry out an extrusion multiplier calibration: viewtopic.php?f=3&t=1964

Enable a small amount of lift on retraction, .10-.20mm. That will cause the extruder to raise a small amount before travel moves, hopefully avoiding contact with the object.

You can improve the corners somewhat by lowering acceleration, which has only a limited effect on print times. In your startup g-code, add:
M201 X1000 Y1000 ; lower default accelerations

Reducing it below 800 has diminishing returns.

[akkrolnik]

Odds are you are over-extruding and at some point the head catches the bulgy top and skips steps.

If the head is catching, why do I only see the offset in the y direction? I've done the tests with the square boxes to check my settings and they seem to be good. Unfortunately I do not have a very good calipers so I am not highly confident. I do plan to get a better one so I can have more confidence in my calibrations. Any suggestions on brand/model? Anyway, if you think contact between the nozzle and the part is the problem it is more likely due to oozing. I've noticed trails of material left every time that it travels. However, I am still perplexed by the fact that it is only offset in the y direction.

Enable a small amount of lift on retraction, .10-.20mm. That will cause the extruder to raise a small amount before travel moves, hopefully avoiding contact with the object.

I've looked through the settings in Simplify 3D and I found a similar feature under the ooze control settings. It's called retraction vertical lift. Does this sound like the right place for it? I was thinking that it wouldn't only be associated with ooze control, but I didn't find anything else that looked the same. Oozing definitely seems to be an issue with this particular filament. I just started a new roll and am seeing streaks of material deposited as the nozzle moves across the part. I hadn't really notice much of this with the last roll. Would any of the other ooze control settings help? Coasting seems like it could help some.

I'll also try lowering the acceleration. Is there any drawback to this? I was assuming that the default settings were already fine tuned for this machine so I don't want to create any other issues.

Thanks for the reply. I appreciate it.

[jsc]

Retraction vertical lift is correct. Add a little bit, that will help prevent any model contact on travel. If you are getting strings, try dropping your temperature by five degrees until it stops.

Another possible culprit, since it is only happening on the Y axis, is that it is skipping steps due to inertia. You are printing very fast, and the Y axis has a lot more mass to move than the X. Try either backing off on the printing speed, or make sure your rails are lubricated very well (white lithium grease, you should have gotten a little tub with your printer).

Calipers: Mitutoyo is a well regarded brand, but expensive. You can get away with using cheap plastic ones, though; I got by for a year using a $7 caliper from Lowes.

[Dale Reed]

Radio Shack (where they are still around) has a digital caliper for under $30 US. Another source, if you have one in your area, is a Harbor Freight store. I have the RS caliper and it works great and seems accurate. I got a digital micrometer at HF specifically for measuring filament, and it was probably about $25, and it's certainly good enough. Both are selectable for inches or mm.

Dale

[Slipshine]

I have had my m2 since october 2012. Last night I had my first part shift in the middle. The part looks fantastic other than that.

I have recently completed my last round of upgrades which included a new rambo board which was recommend due to a fuse problem after switching to 24v and v4 hotend. As a benefit of the board change I now have 1/16 micro stepping in stead of 1/8. (The sound is different now. I miss the old tone) . Since they are now dividing the same magnetic fields in to smaller slices is it possible there is a loss of holding torque in the stepper motor.

Any Thoughts?

[jimc]

well im no stepper guru for sure. i can say that im a late summer 2013 model and always had 1/16. i had a problem with shifting now and again when i got the printer. even swapped steppers. what cured it for me was lowering my acceleration from the default 3000 to 1000 and also in s3d lowering my xy travel speed from the default 18000 to 9000. i have never had a skip since.

[insta]

I have had my m2 since october 2012. Last night I had my first part shift in the middle. The part looks fantastic other than that.

I have recently completed my last round of upgrades which included a new rambo board which was recommend due to a fuse problem after switching to 24v and v4 hotend. As a benefit of the board change I now have 1/16 micro stepping in stead of 1/8. (The sound is different now. I miss the old tone) . Since they are now dividing the same magnetic fields in to smaller slices is it possible there is a loss of holding torque in the stepper motor.

Any Thoughts?

This is a thing, more steps is lower torque, but it's also smoother motion. You can change the microstepping in the firmware if you really want to, it's in configuration_adv.h

[Slipshine]

I was guessing that might be the case thanks for confirming.

Since the xy resolution at 1/8 step is .00089 I have no fear of the quality as I ran that way for almost 2 1/2 years. I feel the extra torque will serve me better than
the extra .00044 Inch accuracy.

Do you know if you can change it from the command line? That way a guy could easily switch back and forth.

Also can you change it for each axis separately or is it all at once.

Thanks.

[Dale Reed]

IIRC, there's not a G-code (or M or whatever) to interactively change microstepping. There is a G-code (M201, IIRC) to change acceleration -- you can enter it in the communication line in S3D, for example. M500 saves the acceleration setting to EEPROM so it survives a power cycle, and you don't have to re-enter it (or put it in your starting G-code). Speeds are changed in the slicer.