@Maker_s_Muse - Question about modeling for 3D. I just finished a night of frustrating work trying to migrate to Fusion 360. I am very adept at Autocad (standard software) and am pretty good at Sketchup. So after beating my model into submission (how I got it to come out like I wanted I still don’t know) I am printing it on the Monoprice Select Mini. The thing that caught my eye and was a “light bulb” moment was that some holes that I made in the model were spot on (in cyberspace) but when they made their way into reality I see that they are not the right size! Please confirm that I need to take into account, especially with respect to holes but the whole model in general, the nozzle width. My holes look a bit filled in (smaller inside diameter) than I thought. yeah, kinda small holes in the part I replicated but even smaller in the actual 3D print.
Now I have a stock 0.4 mm nozzle in the Select Mini and for that nozzle I would have to oversize the holes in the model by that same 0.4 mm when it comes to diameters (or I.D. for irregular holes), correct? And the inverse kinda goes for the exterior of a model, correct? I would have to bring in the edges 1/2 of the nozzle diameter to put the outside edges at the exact distance? Now this assumes no shrinkage at all and perfect placement of the filament but ignoring that reality, what are your thoughts on the essence of this theory? I ask because I’m a stickler for accuracy and while this model will not drive me nuts because high levels of accuracy are not necessary or wanted, I do like knowing how to “do it right” for the times when I do need to be spot on.
I appreciate any feedback you could provide or if you could address it in one of your 3D Printing 101 videos.
Dimensionality is, unfortunately, much more complicated. The nozzle diameter determines the minimum feature size, but the resolution of movement is determined by the mechanics of the printer. A 0.4mm nozzle can draw circles with 0.1mm or better resolution, so long as the wall is a minimum of 0.4mm.
There are many factors that will impact a print, but the there are two that stand out the most, in my experience.
The first, is the mechanical calibration, tuning and measuring the various mechanical and extruding parts of the printer so that you can determine what its actual, as opposed to nominal, behavior is.
The second is that all plastic shrinks. This shrinkage varies by design of part and by material. All holes, for instance, will shrink, although the amount will vary depending on the part’s shape and material used to print. This is a very complex area, but is also very similar to design concerns for injection molded parts. I suggest you research articles about design for plastic injection molding. ProtoMold also has some YouTube videos on the topic that are illustrative.
In addition to those possible sources of error, there’s also the possibility of overextrusion. If the printer is outputting more plastic than the software expects it to, dimensions will be off.
That effect is most noticeable in holes, or nesting pieces.
Every slicer will take the nozzle size into account, but if the nozzle draws a small circle you will get a slight overextrusion on the inner side of said circle even if everything is perfectly calibrated. I don’t know a slicer which can compensate for this, I usually do it manually, smaller curve radius and bigger nozzle needs more clearance. But if you do it often enough you can get it right mostly on the first try and if you parametricized correctly in F360 then you can change it in a minute.
Yes, a couple things fight you. “Polyholes” and melted plastic drawing a bit inwards, I recall the explanation was surface tension but that might not be the whole story. I just give extra clearance, like .5mm for a 3mm hole. Worst case, clean up the hole with a drill bit, or make a test part and use the results for reference.
What I meant is this: Assume these two lines are the same length and printed with a single extrusion movement each (ignoring start and end should be round) Then in the curved case the inner border is much shorter than the outer one which means the filament has less space to expand and therefore you’ll get overextrusion to the inside of the circle (and underextrusion on the outside, depending how the slicer calculates the length). missing/deleted image from Google+
if you want small dimensionally accurate holes, you need to not make them circles. hexagon, square, triangle depending on your particular printer. Run the test for your printer and find the sweet spot for each size hole.
@ChPech - I’m just learning F360 and the work flows and processes are not easy right now. I made the model pretty much the way I wanted it with respectable accuracy considering it’s my first actual complex model with multiple features.
Once I get a better handle on the processes to make the models then I think I will be able to edit them easier.
@Chapman_Baetzel - actually the “holes” were “slots”. Circular ends with straight sides. Unfortunately they are small though, 2mm x 1mm. missing/deleted image from Google+
I don’t know of any slicer that will handle features that size very well. Unfortunately, I think that you are stuck manually compensating for size. Print one, measure, adjust the model and print again - repeat as necessary… For features that size, you may also need a very small nozzle, print really slow, and use as much cooling as possible.
@Alan_Thomason - I agree with your assessment completely. Unfortunately I only have the 0.4 mm nozzle. The print came out really well and the holes are only ornamental. I am making an Amazon Echo Dot replica to use as a stand in to test some accessory model prints for an Echo Dot and there is of course the challenge of recreating the Dot itself.
My beginner 2c.
My acrylic Prusa knock off always prints holes by 0.125 to small ( I’ve been making M2 and M3 clearance holes and that is a trial and error conclusion on the error size)
In F360 you can setup preset dimensions in each model if you go to
“Modify” then “Change parameters”.
I make a " correct" preset for the hole so the model is theoretically correct. Then when it comes to exporting the STL I go to the sketches and change them to a modified preset I made that has the extra size added. A pain but quick to do on simpler designs.
Hope that makes sense
I’ve also found holes on the horizontal and vertical planes need different offset values.