First few mm of prints missed on Prusa-Mendel
Posted by Peter Kinnon
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First few mm of prints missed on Prusa-Mendel January 22, 2015 01:45PM |
Registered: 9 years ago Posts: 3 |
Hi, I am a newbie so please excuse naivety.
I have acquired a Prusa Mendel. Produces very good quality prints except that the first (bottom) few millimeters are consistently missing. It is not simply a scaling issue (steps per mm) as horizontal holes are a corresponding distance off center.
Same thing occurs whether STL is downloaded from Thingiverse or prepared myself on Tinkercad. So presume problem not there. I have upgraded to latest versions of slic3r and Pronterface. All to no avail.
Can anybody diagnose/suggest remedy?
Edited 1 time(s). Last edit at 01/22/2015 01:51PM by Peter Kinnon.
I have acquired a Prusa Mendel. Produces very good quality prints except that the first (bottom) few millimeters are consistently missing. It is not simply a scaling issue (steps per mm) as horizontal holes are a corresponding distance off center.
Same thing occurs whether STL is downloaded from Thingiverse or prepared myself on Tinkercad. So presume problem not there. I have upgraded to latest versions of slic3r and Pronterface. All to no avail.
Can anybody diagnose/suggest remedy?
Edited 1 time(s). Last edit at 01/22/2015 01:51PM by Peter Kinnon.
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Re: First few mm of prints missed on Prusa-Mendel January 22, 2015 03:38PM |
Registered: 10 years ago Posts: 77 |
You first need to determine if it is something flaky in your G-code or if it is a printer hardware issue. Open something in pronterface and click the graphic then step up thru layers. Check that the Z dimension listed corresponds with the height of features you are expecting to see at that level. If that checks out, use a vernier caliper to measure the vertical height of your X carriage when the print starts and re-check it each time it goes up a layer. Pause the print with pronterface each layer so you can make the measurement. My guess is that you either have a backlash issue on your Z axis or your leadscrews aren't attached to the steppers tightly enough. The Prusa-Mendel usually has two nuts and a spring on each lead screw to control backlash.
Look up some simple calibration objects on thingiverse to find some quick objects to test with.
My Prusa Mendel i2 inspired Repstrap with welded steel frame: [youtu.be]
And my Smartrap derived Briefcase 3D printer: [youtu.be]
Look up some simple calibration objects on thingiverse to find some quick objects to test with.
My Prusa Mendel i2 inspired Repstrap with welded steel frame: [youtu.be]
And my Smartrap derived Briefcase 3D printer: [youtu.be]
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Re: First few mm of prints missed on Prusa-Mendel January 23, 2015 04:59AM |
Registered: 9 years ago Posts: 3 |
Thanks for your suggestions.
I will work my way through them. Not sure how to know at what point to pause Pronterface (eg at outline or at infill but guess I will figure it out..
Also not sure what is meant by back-lash, The slipping of the steppe rmotor drive shaft? The "jumping" of threads? I can't see why that would happen on both sides of the z-drive simultaneously.
But, then, I am pretty dim.
I will work my way through them. Not sure how to know at what point to pause Pronterface (eg at outline or at infill but guess I will figure it out..
Also not sure what is meant by back-lash, The slipping of the steppe rmotor drive shaft? The "jumping" of threads? I can't see why that would happen on both sides of the z-drive simultaneously.
But, then, I am pretty dim.
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Re: First few mm of prints missed on Prusa-Mendel January 23, 2015 05:10AM |
Registered: 11 years ago Posts: 1,592 |
Quote
Peter Kinnon
Hi, I am a newbie so please excuse naivety.
I have acquired a Prusa Mendel. Produces very good quality prints except that the first (bottom) few millimeters are consistently missing. It is not simply a scaling issue (steps per mm) as horizontal holes are a corresponding distance off center.
Same thing occurs whether STL is downloaded from Thingiverse or prepared myself on Tinkercad. So presume problem not there. I have upgraded to latest versions of slic3r and Pronterface. All to no avail.
Can anybody diagnose/suggest remedy?
What do you mean? If its missing the first few layers it would be printing in mid-air! Obviously not what you mean, so, perhaps, a photo might help?
_______________________________________
Waitaki 3D Printer
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Re: First few mm of prints missed on Prusa-Mendel January 23, 2015 04:22PM |
Registered: 10 years ago Posts: 77 |
Backlash is an engineering term used to describe the lost motion between moving a mechanism in one direction and then the other direction.
In this case the Z axis nuts have some necessary clearance on the threaded rod. Depending on how the nuts are retained, they may also allow some axial movement relative to the carriage. These clearances mean that after you have auto homed the Z axis, it might need quite some degree of rotation of the Z steppers in the other direction before the carriage starts to move upwards. That is why many designs include two nuts on each screw with a spring in between. You could test for backlash by just zeroing the Z axis then measuring to a vertical reference point. Then motor upwards say 0.3mm and take another measurement. If you see the steppers move but the second measurement isn't 0.3mm different then that is your problem. If you don't have a vernier, use a few business cards to check the clearance between the nozzle and the bed. A business card is about 0.3mm thick. If you do find that you have an excessive amount of backlash, you should be able to fix it by adjusting your Z endstop to be slightly lower and then compensating for that error by putting a fiddle factor in the Z offset under general printer settings in Slic3r. This will only work if your bed has the flexibility to tolerate a momentary downward bend in the zeroing corner. For example, if you find that you have 1mm of backlash, set your Z endstop 1mm lower and make the Z offset 1mm in Slic3r. This will make the Z axis push the bed down by 1mm then come up to zero level before starting the print, taking up the backlash in the process. You can fine tune the exact zero level in Slic3r. Note that this fix for a backlash problem will only be effective if you only ever feed the Z axis downwards at the start of the print. If you ever evolve your printer to doing wild things like dual extrusion where you might want to lift the Z axis between extruder changes, then your physical backlash will cause problems.
It won't really matter where you pause pronterface to make a measurement. Pausing in infill won't do so much damage to the surface finish of your print but then there can be a slight delay between pressing pause and it actually pausing so it might be a bit hit and miss where it actually pauses. You should even be able to take a measurement while it is running if you pick your measurement point carefully (e.g. between Z stepper and carriage). Of course if you check for backlash as I have described above, you can do it while it isn't printing.
My only other thought is that if your bed is mounted on very weak springs, the nozzle might be pushing it down on the first few layers. Or there is excessive friction on your Z axis and the steppers skip trying to move it for the first few mm.
Good luck
My Prusa Mendel i2 inspired Repstrap with welded steel frame: [youtu.be]
And my Smartrap derived Briefcase 3D printer: [youtu.be]
In this case the Z axis nuts have some necessary clearance on the threaded rod. Depending on how the nuts are retained, they may also allow some axial movement relative to the carriage. These clearances mean that after you have auto homed the Z axis, it might need quite some degree of rotation of the Z steppers in the other direction before the carriage starts to move upwards. That is why many designs include two nuts on each screw with a spring in between. You could test for backlash by just zeroing the Z axis then measuring to a vertical reference point. Then motor upwards say 0.3mm and take another measurement. If you see the steppers move but the second measurement isn't 0.3mm different then that is your problem. If you don't have a vernier, use a few business cards to check the clearance between the nozzle and the bed. A business card is about 0.3mm thick. If you do find that you have an excessive amount of backlash, you should be able to fix it by adjusting your Z endstop to be slightly lower and then compensating for that error by putting a fiddle factor in the Z offset under general printer settings in Slic3r. This will only work if your bed has the flexibility to tolerate a momentary downward bend in the zeroing corner. For example, if you find that you have 1mm of backlash, set your Z endstop 1mm lower and make the Z offset 1mm in Slic3r. This will make the Z axis push the bed down by 1mm then come up to zero level before starting the print, taking up the backlash in the process. You can fine tune the exact zero level in Slic3r. Note that this fix for a backlash problem will only be effective if you only ever feed the Z axis downwards at the start of the print. If you ever evolve your printer to doing wild things like dual extrusion where you might want to lift the Z axis between extruder changes, then your physical backlash will cause problems.
It won't really matter where you pause pronterface to make a measurement. Pausing in infill won't do so much damage to the surface finish of your print but then there can be a slight delay between pressing pause and it actually pausing so it might be a bit hit and miss where it actually pauses. You should even be able to take a measurement while it is running if you pick your measurement point carefully (e.g. between Z stepper and carriage). Of course if you check for backlash as I have described above, you can do it while it isn't printing.
My only other thought is that if your bed is mounted on very weak springs, the nozzle might be pushing it down on the first few layers. Or there is excessive friction on your Z axis and the steppers skip trying to move it for the first few mm.
Good luck
My Prusa Mendel i2 inspired Repstrap with welded steel frame: [youtu.be]
And my Smartrap derived Briefcase 3D printer: [youtu.be]
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