Dimensional accuracy

Discussion in 'E3D-v6 and Lite6' started by JustinSB, Mar 14, 2018.

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  1. JustinSB

    JustinSB Member

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    Hi All, I've just got my E3D Lite6 & Titan set up & working, & as I can print again, I'm now back to designing stuff. However, I'm having problems with dimensional accuracy. I'm designing & printing some new bearings for my Prusa i3 clone, but when I design a simple 8 mm hole in a 10 mm circle, it prints the hole out at around 7.5 mm. My external diameter came out at 9.99, which was lovely. I have my Extrusion Multiplier set correctly, as I've measured the thickness of the top couple of layers of a double thickness wall & compared that with what it should be - I got 1.10 which seems fairly reasonable. My Titan is extruding exactly 100mm when I tell it to too.

    So what do I need to do to get it to print an accurate hole? Thanks.
     
  2. Antoine

    Antoine Well-Known Member
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    Getting internal geometries to be perfectly accurate is a constant struggle with 3D printed parts. Getting accurate-ish extrusion is not sufficient, as any small changes in extrusion width will accumulate into a larger dimensional inaccuracy later on. One method you can try is to start printing your outer perimeters first, to prevent this from happening.
    You can also work with the horizontal offsets of your slicer, as once your extrusion is perfectly calibrated it will become a slicer issue.
     
  3. jfb

    jfb Well-Known Member

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    Not really a solution to the problem in general (depends on your use case) but an interesting look at some of the causes of the problem, which might help you find solutions - for some parts, at least. http://hydraraptor.blogspot.co.uk/2011/02/polyholes.html

    Personally, I tend to either print slightly larger holes (using a 'fudge factor' that mostly works and is largely guesswork) or ream the hole out with a drill later. Sometimes I print a many sided polygon instead of a round hole. That said, it's never been really critical for me to get very accurate holes, so I've not put a lot of time into it.
     
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  4. JustinSB

    JustinSB Member

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    Thanks @jfb for the HydraRaptor link. Alas, it's one that I've already discovered, & I've been using his hole formula with successful results to fudge my own designs. I've been trying to print my own LM8UU bearings, as my stock Geeetech ones are a tragedy, hence the desire to solve my accurate holes issues. Unfortunately, thus far, my home made (downloaded from thingiverse.com) bearings are even worse than my Geeetech ones (stickiness & friction), so I may try my hand at designing my own, & see if I can do any better.
     
  5. dc42

    dc42 Well-Known Member

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    Two tips for getting small holes closer the size you asked for:

    1. In your slicer select "External perimeters first" or "Outside in" or equivalent.

    2. Print the perimeters of internal holes very slowly. The speed setting in slic3r for "Small perimeters" is very helpful here. Sadly no other slicer appears to provide this setting yet.
     
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  6. Paul Winter

    Paul Winter Well-Known Member

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    Hi, not really an answer to your issue of correctly sizing holes (which btw is also dependant on temperature, filament viscosity, heated bed temp etc all causing the extrusion to spread sideways), but after spending some time trying downloaded LM8UU bearings, and designing my own, all unsuccessfully, I bit the bullet and ordered a full set of Igus Drylin bearings. Not terribly expensive, and remarkably effective. I am currently using them with stock 8mm stainless steel rods.
    Search for 8mm Igus Drylin RJ4JP-01-08 on Amazon
     
  7. JustinSB

    JustinSB Member

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    @Paul Winter , I've been looking at Drylin bearings, but I've just managed to get a full compliment of fully working LM8UU's - so my plan is to use them for a while, whilst I work on ironing out my other glitches, then possibly upgrade to Drylin bearings in a few months. Of course, I may weaken, & just buy a set from Amazon if/when my current batch of LM8UU's start to fail again.
     

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