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rich-k

Dimensional accuracy (older vs. newer versions)?

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Hi, all! New to this forum, and fairly new to 3-d printing as well. I have a Mendel90 ("Sturdy" version which uses MDF panels instead of acrylic or Dibond), which I just built late last year and am constantly improving and upgrading (latest improvement - better "Z" motors; next improvement: change hotend from a J-head Mk. V-BV to an E3D V6).

I have tried several slicers: KISSlicer, Slic3r, Skeinforge, and Cura. Of the four, I like Cura and Slic3r best, as they are the most user-friendly (Skeinforge is very user UN-friendly, IMO, and I haven't managed to use KISSlicer successfully either), but I tend to have problems with holes and nut-traps coming out the correct size - they are generally too small. From what I have read, this is a common complaint for both of these slicers. Nophead, a member of the RepRap team and developer of the Mendel90, uses Skeinforge for that reason - apparently there is something about Skeinforge that allows better dimensional accuracy.

I found out that earlier versions of Cura incorporated Skeinforge, unless I mis-read something. Is anyone here happier with those earlier versions of Cura, as far as dimensions and features go? If so, what is the best of those earlier versions, in terms of features, accuracy, and user-friendliness?

Thanks in advance!

 

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http://software.ultimaker.com/old/

cura 13.04 was the final version with the older slicer. Fortunately you can have as many cura versions installed as you want as the installer tells windows that they are all completely different programs.

I would be very surprised if skeinforge is any better. The problem is only for *vertical* nut traps and holes. It has to do with the material properties of liquid PLA. Does your friend print with ABS or some other material?

Anyway there are 3 reasons holes are too small but the biggest reason is the nature of liquid PLA where it acts like snot or a liquid rubber band as it is being laid down and stretches towards the nozzle which is inward for holes. It wouldn't happen if the layer below supported it better but that layer also stretched inwards. There is no good fix for this and different types of material and color additives and other additives can change this property although ABS hardens much sooner so doesn't have as much distance to pull on as you "draw the circle" (lay out a round trace) so it isn't pulling inward quite as much.

Now of course some slicer could compensate for all this. Outer corners also. I'm not sure if the error is consistent enough between printers, printing speeds, fan speeds and so on. Most people who use CAD will just design all their holes and such a bit bigger. I like to add .4mm to my vertical holes so 3.4mm holes for M3 screws. Smaller if you want M3 screws to "self tap".

More information here and great ideas for designing parts:

http://support.3dverkstan.se/article/38-designing-for-3d-printing

 

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"Nophead" (Chris Palmer) uses ABS in all the printed parts kits he sells for his Mendel90 machines (which he designed). My original parts set was printed in PLA by someone else, and I am in the process of printing replacement parts in ABS for the sake of temperature resistance (I plan to put my machine in a heated chamber eventually). I tend to make more mechanical parts than models anyway, so I mainly use ABS myself (yes, I know PLA is harder, but I prefer the overall qualities of ABS).

 

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I doubt that it is a dimensional inaccuracy in the slicer.

Here are a few things:

- Thermoplastics shrink as the cool so if you model a 3mm hole, the printer will print a 3mm hole but you get a little less than that as the part cools.

- Holes and round surfaces in general are (always?) approximated with small straight lines. This tends to cause the holes to print with a little less of the actual modeled dimension. Most slicers (all?) don't support true curves.

- If the hole or trap is through the first layer, there is usually some "squish" of extra material in both directions so that the infill of the first layer is solid but it also "squishes" into what should be an empty space. So you get a little lip that messes up tolerances. This can be worse if the leveling isn't perfect.

- If the hole or trap is close to the shell, the slicer might alter the dimensions to better "fit" a nozzle's width of material and this might mess up tolerances.

In general, I've found it to be somewhat trial and error. This will be more so if you switch material types. Something that might print perfectly in PLA will be different of printed in XT or ABS due to different shrinkage and flow.

 

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