Hi,

I don't know if this subject is posted here before... (sorry) but I try to print a design with holes. The material i use is PLA and I end up having a spot-on outer dimension but always 0.5 mm smaller hole dimension. I use Slic3r and Cura. My nozzle is 0.5mm PLA 3mm and I calibrated the PLA diameter and extruder flow.

I assume a STL model dimension is the real printed dimension. Is it really possible to print accurate? What am i missing...??

I have a K8200 also knows as 3DRAG.

Thanks!

Br,
Webmug

I have this happen a lot too.  From what I know there is a couple reasons that happens.  One thing to consider is when a circle is drawn in a 3D modelling program like Sketchup, it is made up of a bunch of straight lines.  Sketchup calls them faces.  So the smaller the hole and a lower amount of faces could result in an inaccurate printed hole because each straight line is basically cutting into the true arc of the circle.  The other problem is our nozzles are rather wide at 0.5mm.  Imagine the printer using a fat felt tip marker instead of a plastic extruder. Now if we draw a circle with say 10 faces, at each corner the half of the tip of the marker that is closest to the center of the circle is travelling at a lower speed than the other half. Being that it is a marker, and ink only flows as it is needed, we don't get any extra build up on the inner diameter of the circle.  Now on the other hand with a plastic extruder, both the inner and outer half of the 0.5mm nozzle are flowing a the same rate.  The inner half could use a little slower flow rate than the outer half.  This can cause a build up of extra material on the inner diameter.   

One thing you can try is checking the diameter of your filament with a calipers at different spots, and at different parts of the circumference at the same spot.  Average the measurements, and put that number in your slicing program's filament diameter.  If that number was way off during calibration, you may need to recalibrate afterwards.  That setting will increase or decrease flow rate of the nozzle.  Decreasing flow will help inner hole dimensions but at some point will take away from the outer hole dimensions.

Usually small holes I always figure I'll need to clean them out with the correct sized drill bit.  When I design parts to fit inside each other, I usually give a measurement of 0.4mm tolerance between them.  They usually fit together with out any filing.

Hope that helps.  :)

Nate

Quote from firepinto on February 2nd, 2014, 07:40 AM

I have this happen a lot too.  From what I know there is a couple reasons that happens.  One thing to consider is when a circle is drawn in a 3D modelling program like Sketchup, it is made up of a bunch of straight lines.  Sketchup calls them faces.  So the smaller the hole and a lower amount of faces could result in an inaccurate printed hole because each straight line is basically cutting into the true arc of the circle.  The other problem is our nozzles are rather wide at 0.5mm.  Imagine the printer using a fat felt tip marker instead of a plastic extruder. Now if we draw a circle with say 10 faces, at each corner the half of the tip of the marker that is closest to the center of the circle is travelling at a lower speed than the other half. Being that it is a marker, and ink only flows as it is needed, we don't get any extra build up on the inner diameter of the circle.  Now on the other hand with a plastic extruder, both the inner and outer half of the 0.5mm nozzle are flowing a the same rate.  The inner half could use a little slower flow rate than the outer half.  This can cause a build up of extra material on the inner diameter.   

One thing you can try is checking the diameter of your filament with a calipers at different spots, and at different parts of the circumference at the same spot.  Average the measurements, and put that number in your slicing program's filament diameter.  If that number was way off during calibration, you may need to recalibrate afterwards.  That setting will increase or decrease flow rate of the nozzle.  Decreasing flow will help inner hole dimensions but at some point will take away from the outer hole dimensions.

Usually small holes I always figure I'll need to clean them out with the correct sized drill bit.  When I design parts to fit inside each other, I usually give a measurement of 0.4mm tolerance between them.  They usually fit together with out any filing.

Hope that helps.  :)

Nate

Thanks for the reply Nate!

So it's just finding the right average diameter of the filament and setting the right flow rate? Hmm??

Yes, holes can be sized using a drill, but what about bearings etc. Do i have to modify all kind of STL parts from github etc. ???
Do you think all slicer programs have the same result with inner hole sizes? It's kinda strange inner holes are not calculated right in slicer programs...

Thanks!

Br,
Webmug

Quote from Webmug on February 2nd, 2014, 08:03 AM

Thanks for the reply Nate!

So it's just finding the right average diameter of the filament and setting the right flow rate? Hmm??

Yes, holes can be sized using a drill, but what about bearings etc. Do i have to modify all kind of STL parts from github etc. ???
Do you think all slicer programs have the same result with inner hole sizes? It's kinda strange inner holes are not calculated right in slicer programs...

Thanks!

Br,
Webmug

I would start with tweaking settings first.  It took me a good month to do my first time.  Modifying STL files isn't really that fun, if you can get the original source file you would be better off.  You can try other slicing programs, they all have their own little ways of calculating tool paths.

Quote from firepinto on February 2nd, 2014, 08:24 AM

I would start with tweaking settings first.  It took me a good month to do my first time.  Modifying STL files isn't really that fun, if you can get the original source file you would be better off.  You can try other slicing programs, they all have their own little ways of calculating tool paths.

Yes, I do this step first, tweaking... Then see if it's changing the inner dimensions. Very frustrating!!! :-)

Br,
Webmug

Just for sharing...

Well this is just great, NOT!! Slic3r has removed its Arc compensation  :@
https://github.com/alexrj/Slic3r/issues/1613

A few links what creates smaller holes in the printing process:
http://reprap.org/wiki/ArcCompensation
https://dl.dropboxusercontent.com/u/3775100/Slic3r%20-%20Dimensional%20Errors.pdf
http://fabmetheus.crsndoo.com/wiki/index.php/Skeinforge_Stretch

Reprap uses Arc compensation, I try this if it will run with a K8200 3DPrinter.
http://reprap.org/wiki/Installing_RepRap_on_your_computer

What software are you using Nate?

Br,
Webmug

I use Slic3r, but used to use Sfact.

I'm not sure if the arc stuff really applies when using an STL file... There really isn't any true arc capability in the file, since its just a bunch of straight line segments to form what looks like an arc.  Matt Watts was talking about arcs capability on CNC routers one time, he might understand that better.  :)

Its good to see that they are(were?)working on calculating the issue though.   I think the reason people complained about the change making holes too big, is because those people already designed their model to compensate for the problem, which is what I do.