Logic to make the hole smaller.

[zBeeble 2]

I've attached my new bath knob.  Permissive commons license and already on thingiverse.  I was watching the print (3D printers can be so mesmerizing on a lazy Saturday)... and I think I detected one place where Cura can do better (although I don't know if I know where to twiddle... is this possible with a plugin?  I'm happy to roll up my sleaves if I'm not wasting my time.

 

I figure this is pretty common.  The hole narrows in the middle.  It's wider on both sizes and there are good reasons not to fillet (basically... the screw supplied with the original part wouldn't fit properly for one).  Your first reaction is that supports would make it print 100% successfully.  I'd say supports are hard to remove in there... but also, the current behavior is so close ... it just seems cura can do a tad better...

 

If you print the knob with the outside surface down (seems the sensible way), the hole starts at 12mm and narrows to 5.5mm (I know 'cause I made it).  On the first layer of bridging, it starts fine ... going across from the edge of the circule out until it is printing the part of the smaller circle that isn't a contiguous bridge... which continues until it is again.  This dumps that non-contiguous material down the hole and leaves two side bits almost-ok.  On the next pass, we almost get it right where the 2nd bridge layer is 90 degrees to the first ... and you get the square part of the circle-square being almost right... but still, on this pass, more material down the chute.

 

If you were amazingly smart about this, I'd imagine the best way would be to print this as a series of tangents to the inside circle that began and ended on the solid outside circle.  If you were less smart about it, printing the two 90 degree edge bits on the first pass, you might not be dumping the circular inner walls down the hole.

 

Thoughts?

 

BathKnob v6.stl

[burtoogle 513]

OK, what I always do in this situation is to include a skin in the model at the height where the hole goes from large to small diameter. The skin is 1 layer thick. I then put a drill through the hole when the print is finished. Here's an example where the hole is above air:

 

[burtoogle 513]

Oops, I realise that in this example the hole is actually above support but you get the idea.