Cura connects supports that need to be separated by gaps

[Riki3D 0]

I am struggling, so far unsuccessfully, to get Cura to "partition" support for my model in the necessary way.  The model ("TheBasicModel.png") has two long horizontal tunnels (which require internal support) but which terminate in space below an overhang at each end, which also requires support.  The naive support structure automatically generated by Cura would therefore have fat ends and a thin middle, making it impossible to pull out.  The situation is further complicated by the need for another support for the "arch" underneath; see the attached images or .3mf file.

 

What I have tried to do is make a custom support structure which has narrow breaks, 1 or 2mm wide, at one end or the other of the tunnels, dividing the overall assembly into two pieces which could be pulled out in opposite directions; see "SupportOnlyAsModel-Layer32" (for explanatory purposes, this was intentionally not sliced in "Print as Support" mode).  Similarly, the support underneath ends short of the tunnel fillers, so should be able to be detached downwards.

 

Unfortunately, Cura always connects my disjoint pieces, defeating the whole scheme.  See the other two images, which show that the support runs continuously from one end of the model to the other, through the tunnels.  And (not so clearly) is joined to the semi-cylindrical arch support in the middle.

 

I previously tried using support blockers to divide both auto-generated and custom supports into pieces, but again, below a certain size (?), they seem to be ignored.

 

The screenshots and project file included are actually a simplified version of the actual part I am trying to fabricate, which has some fairly complicated stuff on the upper surface.  (Had it been flat as shown here, I would have flipped it over to eliminate needing to support the arch on the underside.)

 

On a side issue and assuming I am ultimately able to get custom supports to behave as I would like, I also need to ask if my supports need to be explicitly sized and positioned to account for the desired Support Z (and X/Y) Distances.  That is, if I want to construct a tunnel 4mm wide by 2mm high to be printed at 0.2mm resolution, should I make the support, e.g., 1.4mm high and 3mm (or less) wide, and center it in the tunnel?

 

Hilderic Browne

Rimouski QC

Canada

 

custom-support-test-project.3mf

[Riki3D 0]

I should have mentioned that I am currently running Cura 4.11.0.