Some prints vulnerable to shifting

This problem has been plaguing me for some time now. To be honest I am not entirely sure if it is a software issue or not. It seems like it cant be. I get print shifting on some of my prints of up to a cm off. Sometimes it is just a few mm. Sometimes it happens just a few layers in and other times is comes after hours. For the ones that happen after hours they are usually really small so I am thinking that it is just a small miss in steps from something like the belt. The thing that I am most worried about is prints that start off bad and can't be fixed. I spent a lot of time making sure my x axis belt was perfect and I do get some great prints. For some reason on some files there is always shift. My Y axis never had problems before but for some reason some files will make my y axis shift. I don't see how it can be mechanical. It is never both axes that shift. When it happens on the y axis I go into Slic3r and rotate it. Then it happens on x. So for that reason I am starting to think it is a software thing. It just seems so unlikely for the software to make mistakes. The prints don't shift in the same way every time and I don't have another printer to test them on. Some prints end up perfectly and other shift. I also think that creating large flat plates with full infill is causing shifting. At this point I am still working to try to figure out what exactly is causing the shifting by experimenting with variables and testing things out. I will get back to you on these.

PS
I am posting this in general b/c I don't know if it is a Slic3r problem or a printer problem, nor if it is specific to my printer. I have a prusa i3

Thanks a lot everyone

Shifting is either caused by mechanical issues, such as a belt that is too loose, or from dropped steps. Since your problem shifts between x and y when you rotate, I'll rule out mechanical.

So now we have the theory that it's from dropped steps. Dropped steps can come caused by 1) mechanical issues such as an axis that binds which is unlikely in this case, 2) improperly set stepper drivers which seems less likely since the problem shifts between axes, or 3) you're printing too fast for your printers mechanism. Acceleration and jerk settings are very important. If your carriage tries to change speeds too rapidly, it can drop steps. I notice this especially when doing infill in narrow spaces - about 1 or 2 mm. The head moving in short quick bursts can cause this issue if the speed is too high.

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jbernardis
Shifting is either caused by mechanical issues, such as a belt that is too loose, or from dropped steps. Since your problem shifts between x and y when you rotate, I'll rule out mechanical.

So now we have the theory that it's from dropped steps. Dropped steps can come caused by 1) mechanical issues such as an axis that binds which is unlikely in this case, 2) improperly set stepper drivers which seems less likely since the problem shifts between axes, or 3) you're printing too fast for your printers mechanism. Acceleration and jerk settings are very important. If your carriage tries to change speeds too rapidly, it can drop steps. I notice this especially when doing infill in narrow spaces - about 1 or 2 mm. The head moving in short quick bursts can cause this issue if the speed is too high.

Also to add stepper drivers overheating, which can happen even if they are adjusted right. Do you have a fan on your electronics?

Also you say you're using Slic3r, which version? And how are you getting the gcode from slic3r onto the printer? I.e. using Repetier or something to print over USB or load it onto an SD card? Also, what electronics do you have? I'm assuming as it's an i3 that you're using Marlin firmware on RAMPS 1.3 or 1.4, but it's not certain.

To rule out software, try using Cura. It manages the slicing and printing without reference to any of your existing toolchain, so if that makes the problem go away you know it's software. I'm guessing it's hot electronics though.

Thanks for the help,

Im using RAMPS 1.4 with Marlin with everything cool to the touch, even the steppers. My drivers have heat sinks and a fan. I spent a long time dialing in the steppers just right and I'm pretty sure they're good based on what I can tell. I am doing a Cura print right now to see if I get anything different. I create the gcode directly onto an sd card and use my Discount smart controller with built in sd. I'm using Slic3r 1.1.7. Running a print with Cura 15.01, lets see how it goes!

I'm beginning to suspect that the shifting comes from print speeds that go too fast in tight areas of infill. It also has to do with the general shape of the print. Something that has no infill and is just a repetitive rectangle shape with an open bottom and only several layers thick will not shift. An example of this is a gopro case. But when I go to make a phone case the large amount of infill on the bottom layers and the tiny fast motions of filling up the side walls will throw the printer off. I'm about to start a print with the infill speed setting very low. Then im going to change the firmware. Any recommendations on the proper jerk/ max accel.? I have looked into this a lot from when the problem first started and I have very modest values.