Print size is inaccurate (x and y)
Hello I have built my first printer, an MK3. I had no problems printing. But.. the scale is slightly off.
I made a circle with an outer diameter of 20mm, inner diameter of 13mm and a thickness of 0.8mm.
When i print this the actual measurement is 20.05 x 12.70 x 0.8mm
I have no idea where to begin what is causing this. Please keep the reply simple because i’m new to printing.
Re: Print size is inaccurate (x and y)
You may be slightly over-extruding, that is putting down more plastic than intended. There are several factors that affect this, but here's the basic idea. The printer pushes plastic into the extruder (and out through the heated nozzle) based on a calculation of how much plastic is needed. The way it controls the amount going in is by generating a certain number of steps on the extruder's stepper motor for each millimeter of distance the nozzle moves in the X and Y directions. Of course the calculation takes the layer height into account.
So how does it calculate the right number of steps, and how can you "fix" these?
1) Actual filament diameter. If it's larger than the nominal 1.75mm, then you'll be getting proportionally more plastic. Obviously you have a precise digital caliper, so you can use it to measure the filament you are using. I find it easiest to cut off a piece of filament about 2cm long for easier handling. I put the filament between the knife-edge part of the caliper jaws and I rotate it to find the maximum and minimum "diameter" (because it can be oval). I then use the average diameter. This is a setting in your slicer, on the "filament" tab.
2) Extrusion multiplier. This is a handy way to tweak the quantity of plastic extruded. It's also found on the "Filament" tab of your slicer. Nominally 1.0, you can tweak it up or down a little - maybe in increments of 0.05
3) Extruder Calibration. This one is a little harder to adjust, and probably doesn't need to be adjusted on the MK3, but just so you know, there is a place in the firmware's EEPROM where there is a factor that says X number of stepper motor steps are required to push 1mm of filament from the spool into the hot end. If you really want to work on tweaking that one, there are instructions on the Prusa web site.
All that said, your results really aren't bad at all. the surface of a printed object is never perfectly smooth, and your caliper measures from high point to high point. And if you require really tight tolerances on interior diameters of holes less than 15mm diameter, just be prepared to print them a bit small and drill them out, or else experiment and find a "design" diameter that results in the desired diameter. I learned this watching a training video for one of the commercial $100K printers!
There's a quick test for extrusion on Thingiverse. Print two of these and see how they fit together. I do adjust for filament diameter, but I haven't touched either of the other settings, and my two little test pieces mesh perfectly - just a little force, about enough to lift the second piece while holding the first one, but no sense of forcing the two things to fit. https://www.thingiverse.com/thing:1622868
Re: Print size is inaccurate (x and y)
Thanks for the quick reply. I’ve made a few test prints where i changed ‘default extrution with’ but no luck. I’ll keep trying when i have some time.
With bigger prints the outside diameters are perfect.
Re: Print size is inaccurate (x and y)
Default Extrusion Width is not the same thing at all. That's setting the NOMINAL width of your trace. The path of the nozzle is calculated to be 1/2 of the Extrusion Width inside of the desired surface of your print. It's Extrusion MULTIPLIER, Filament Diameter, and Extrusion CALIBRATION that you use to tweak the actual Width of your trace to match the desired Width (whatever you set with Extrusion Width).
Re: Print size is inaccurate (x and y)
Hi Rams,
honest enquiry...
what is the working tollerance for your design? is 5/100 of a millimetre that much of an issue...
if you measure the same part, the next day. is it still 5/100 of a millimetre oversize?
we are using heat fused filament deposition (Posh glue gun) to make these parts if you measure them, "hot off the Printer!" they may well measure a different size to the size when they are cooled to ambient temperature.
See Coefficient of thermal expansion https://www.simplify3d.com/support/materials-guide/properties-table/
maths was never my strong subject
if temperature of the model when measured, was 20 degrees above ambient or design temperature then I think that the size when warm, is likely to measure
design size + ((coeff. therm. exp. x original size in metres ) x temp difference) or
20mm + (( 0.068 x .02 x 20 )mm = 20.0272mm
even if it was exactly the correct size at ambient temperature or design temperature...
bearing in mind we are effectively squirting molten glue ontop of semisolid glue in order to create these items, it's amazing that they are so close to design size!
ooooh and... is your measuring device actually calibrated?... or is it a consumer level guessing stick 😯
of course I might be totally wrong
but I don't get particularly hung up about it...
regards Joan
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK
Re: Print size is inaccurate (x and y)
Hi Rams,
honest enquiry...
what is the working tollerance for your design? is 5/100 of a millimetre that much of an issue...
if you measure the same part, the next day. is it still 5/100 of a millimetre oversize?
Joan,
I think you misread something. The 0.05mm is not an issue. The 0.3mm is. Every hole, opening or whatever is 0.3mm smaller when printed. I can work with it to modify models to make them 0.3mm bigger but getting the right settings is more efficient.
Re: Print size is inaccurate (x and y)
That's a known issue with FDM printing in general. You're squishing hot layers of glue down on other layers, so there's some bleed into any openings. Filament pulled around the circumference if a circle will tend to pull into it. Material varies and shrinks.
[...] I think you misread something. The 0.05mm is not an issue. The 0.3mm is. Every hole, opening or whatever is 0.3mm smaller when printed. I can work with it to modify models to make them 0.3mm bigger but getting the right settings is more efficient.
Slicers have different tricks for dealing with this, and some are better than others. If you're specifically concerned about hole accuracy, Here's Slic3r's take on it. In short, you can tweak some settings in software, but much will ultimately come down to how much time you're willing to spend calibrating your hardware and buying quality materials.
and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan
Re: Print size is inaccurate (x and y)
Default Extrusion Width is not the same thing at all. That's setting the NOMINAL width of your trace. The path of the nozzle is calculated to be 1/2 of the Extrusion Width inside of the desired surface of your print. It's Extrusion MULTIPLIER, Filament Diameter, and Extrusion CALIBRATION that you use to tweak the actual Width of your trace to match the desired Width (whatever you set with Extrusion Width).
I succeeded. Thanks for sending me in the right direction. I’m going to play with these settings. Overall i’m very pleased with the print quality. Better than expected.