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Holes too small and outer diameter too big ? What should I do ?


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Posted · Holes too small and outer diameter too big ? What should I do ?

Hey guys, hoping to get your feedback on this subject 

 

I printed this setup yesterday in tough PLA and generic PLA

 

Capture.thumb.PNG.38011215cfa1fecea0add6ac7e56a4b8.PNG

 

The Inner Diameter should be of 4.000 inch and it resulted in 3.980 inch which is 0.508mm too small on the diameter. Outside diameter should be 4.25 and is 4.244 (0.15mm difference (good result)

 

Similar results were observed on the smaller cylinders. Inside diameter should be 0.500 and is 0.480 (.508 mm difference). Outside diameter should be 0.8750 and is 0.883 (this time it’s too big of 0.2mm)

 

Any idea what could have caused this shrinkage and expansion ?

 

Same thing happened to all the cylinders, the hole is too small and the outside diameter is a bit bigger but closer to the target dimension.

 

The four plates were almost on target though. Why would they be ok but not the cylinders ?

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    Posted (edited) · Holes too small and outer diameter too big ? What should I do ?

    Holes create a special problem.  The nozzle drags the plastic in a circle and the plastic wants to cut corners so it pulls towards the center making an ID smaller  You don't notice it as much on an OD because usually there is an inner wall to act as a dam to keep the plastic outboard.  How much it pulls to the middle is affected by a lot of things.  The phenomenon is known as the Gr5 Snot Factor after it's discoverer who had a runny nose at the time.

     

    The Shell | Hole Horizontal Expansion setting can be used to enlarge a hole.  Not all holes are round, it acts on any vertical holes in the part.  Horizontal features don't suffer from the snot factor.  In Nicko-Airbus example, the ID is .5mm too small.  A Horizontal Hole Expansion of +.25 would get the ID at least into the ballpark of being correct.  The hole horizontal expansion number works on the radius.  The downside is that the smaller holes will probably need a different HHE number to be correct.

    Edited by GregValiant
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    Posted · Holes too small and outer diameter too big ? What should I do ?
    On 1/9/2021 at 8:40 PM, GregValiant said:

    Holes create a special problem.  The nozzle drags the plastic in a circle and the plastic wants to cut corners so it pulls towards the center making an ID smaller  You don't notice it as much on an OD because usually there is an inner wall to act as a dam to keep the plastic outboard.  How much it pulls to the middle is affected by a lot of things.  The phenomenon is known as the Gr5 Snot Factor after it's discoverer who had a runny nose at the time.

     

    The Shell | Hole Horizontal Expansion setting can be used to enlarge a hole.  Not all holes are round, it acts on any vertical holes in the part.  Horizontal features don't suffer from the snot factor.  In Nicko-Airbus example, the ID is .5mm too small.  A Horizontal Hole Expansion of +.25 would get the ID at least into the ballpark of being correct.  The hole horizontal expansion number works on the radius.  The downside is that the smaller holes will probably need a different HHE number to be correct.

    Thanks a lot ! Really helps

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