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Solved Part size accuracy (like holes, e.g.)

Discussion in 'Troubleshooting' started by OutsourcedGuru, Sep 6, 2017.

  1. OutsourcedGuru

    OutsourcedGuru Active Member

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    Oh, the fun of designing parts which interact with non-plastic (screws).

    Currently using Autodesk Fusion 360 + Cura 2.3.1. The external dimensions of parts and even the internal dimensions are usually pretty close to expectations. What I'm seeing here as a problem—Cura, I'm probably looking at you—is the finished sizing of holes.

    1. Accurately measure the required shaft for a screw
    2. Design that into the part
    3. Print it out
    4. Screw really won't fit
    5. Re-design hole to be 50% larger in diameter
    6. etc

    For example, the holes to accept the filament in the run-out switch assembly. The filament is 1.75mm in diameter. As measured from the stock part, it was like 1.85mm or something like that so this was designed into the part. It wouldn't accept the filament even after reaming out the hole. Next iteration was a 2.7mm design which resulted in way too much. The last iteration was 2.0mm which won't accept the filament again.

    It would be nice to have a back-of-the-napkin calculation for hole diameter below a certain threshold (if you know the print resolution).

    So I guess there are some factors involved: typical shrinkage for PLA, width of a layer of filament in the ultra-fine setting in Cura, diameter of hole expected.

    Having measured the "tell" before, I've seen the width as much as 1.0mm and typically a little less like 0.8mm. Assuming a 0.3% shrinkage and a 1mm line width and a wanted 2.0mm diameter hole...

    ...it all depends upon whether or not Cura isn't compensating for line width. It repeatedly seems to be ignoring the line width on holes, if I'm guessing correctly. If I then imagine that 0.8mm line forming the circle and assuming that it's centered exactly on the circle, then that's a slop of 0.4mm on both sides. If I add 0.8mm to that 2.0mm design then I'm back to the > 2.7mm that I tried before (which resulted in a hole that was twice the width of the filament and too big).
     
    #1 OutsourcedGuru, Sep 6, 2017
    Last edited: Sep 6, 2017
  2. WheresWaldo

    WheresWaldo Volunteer ( ͠° ͟ʖ ͡°)
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    There is generally a compensation factor that can be applied, I know that Prusa Slic3r and S3D both have it. It is specifically there to adjust hole sizes. Also there might be a setting that allows you to print from the outside in instead of from the inside out with regard to perimeters and that can help maintain proper dimensions.

    Usually for filament pathway 2 mm is suggested. You will always have this issue with FDM printers you simply cannot control the flow of plastic that precisely. Best thing is just get is as close as possible and ream out the hole. Or print it at 4 mm, which should be easier and then use a piece of PTFE Tubing to get it back down to 2 mm.
     
  3. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    This is true to some extent for all slicers and in some (small) respects is inherent in the materials. They all swell or shrink a bit when extruded.

    There are cases where some slicers do a poor job of, well, slicing, at least in part because circles are "hard" to do with polygons :)
    Try other slicers is usually my suggestion for step one or model around it as best you can.
     
  4. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    Yes. This is our approach (although we usually just get them to self-tap)
     
  5. WheresWaldo

    WheresWaldo Volunteer ( ͠° ͟ʖ ͡°)
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    Just a tip, under-extruding is your friend when you are trying to deal with accurate sizing.
     
  6. OutsourcedGuru

    OutsourcedGuru Active Member

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    I'm so close to finishing this double-filament run-out box for the back of the C2. And since Mark's probably the only other one who might actually need this puppy, I may just ream it and call it "done" at this point.

    I'll eventually create a fit jig, I guess, and design a variety of holes to accept the classic metric bolts to see what actually will work (gradually incrementing the diameter each column).
     
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  7. OutsourcedGuru

    OutsourcedGuru Active Member

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    I guess another approach would be to print the 4mm hole all the way through, cut a 6mm long PTFE and glue that into the bottom half of the sensor box (with a notch for the microswitch).
     
  8. OutsourcedGuru

    OutsourcedGuru Active Member

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    Version 5 of the run-out box has printed with success. Huzzah. I have come to the conclusion that the biggest problem here is when printing holes which are aligned in the Z axis. The printer chokes on this, to be honest. I re-aligned the part so that the filament holes were in the X axis (and bumped their diameter by another 0.2mm to 2.2mm) and they now seem perfect without reaming.

    And I'm designing the bolt gauge part amid interruptions from the gf. On that note, there's a cool feature within Autodesk Fusion 360. Insert -> M-C component -> find the part -> drill into the P/N link -> drill into the Product Detail link -> scroll down and select "3D STEP" from the pulldown and click the Save button.

    Move/rotate the bolt added
    Modify -> Combine -> select part -> select bolt -> Operation = Cut

    Insert.jpg
     
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  9. WheresWaldo

    WheresWaldo Volunteer ( ͠° ͟ʖ ͡°)
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    That feature has been there since the beginning. Have you looked at the stuff in the Add-Ins, there is more interesting things there. There are also a lot of free apps in the Fusion 360 App store.

    Fusion 360 is written in Python, and so are all the add-ins, just in case you ever wanted to dabble in that playground.

    https://apps.autodesk.com/FUSION/en/List/Search?facet=__pricetype::FREE
     
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  10. OutsourcedGuru

    OutsourcedGuru Active Member

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    Thanks a lot. This will help with the quad-copter project. Otherwise, I'd be stuck in a quagmire of rest inertia.
     
  11. OutsourcedGuru

    OutsourcedGuru Active Member

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    I designed/printed a bolt gauge/jig and as expected, the z-axis holes are laughably small compared to what Autodesk Fusion 360 thinks it's importing from the M-C components. For example, an M3 aluminum bolt reasonably fits into an M4-threaded hole. I'll need to verify this but I would guess that merely re-orienting the gauge will result in a good fit.

    I'll do a lot more investigation into this but I'm thinking that I'll need to create a library of adjusted bolts to import into projects to accommodate z-aligned bolt holes.
     
  12. WheresWaldo

    WheresWaldo Volunteer ( ͠° ͟ʖ ͡°)
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    Robo's settings are strange for the number of steps in the axes. They have them all at 80.0395 rather than 80 which I think is the correct number mathematically Z is set to 800.24 instead of 800 as dictated by the hardware. You could temporarily change them to 80 and then run a test print to see if it is closer.

    You can do this with M92
    Code:
    M92 X80 Y80 Z800
     
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  13. OutsourcedGuru

    OutsourcedGuru Active Member

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    Well, that's worth remembering... but I think Z isn't the problem (even though these are z-oriented holes). This is more X/Y if you think about it. This could be a one-layer-high part and the vertically-aligned hole would be small by a ratio of 3:4.

    And of course, this could be Cura. It's probably not compensating correctly for the extrusion width in the X/Y plane. I'd need to buy S3D to confirm that theory, though.
     
  14. OutsourcedGuru

    OutsourcedGuru Active Member

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    I printed the bolt gauge and measured it against a collection of standard-sized bolts. In many cases, the wanted-versus-printed ratio for Z-oriented threads was around 2:3, 3:4, 4:5 in the metric-sized bolts. I didn't see a linear progression but I'd need to run more tests.

    I just designed/printed a working part; however, that takes all this into consideration with success. I have a variety of #6-32 aluminum bolts for this project and there are two Y-oriented and four Z-oriented cases of those. For the Y-oriented threads, I used the standard M-C part's STEP file for the #6-32 and it worked out great (with only a little pressure required for it to fit). For the Z-oriented threads, I used the #8-32 and it seems to compensate for the hole inaccuracy as seen from Cura. Other parts for this project are also nicely accommodating four bearings and and their axles, similarly compensated for this Z inaccuracy.

    Rather than changing slicers, I think I'll go with this as a solution.
     

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