[4.2.1] Brim creation issue

[Dougal57]

Application version
version 4.2.1

Platform
Linux Mint 18.3 Sylvia 64-bit

Printer
Malyan M200

Reproduction steps
Using OpenSCAD, I created an .stl file for a wheel crossbrace as part of a 1/8 scale model DH Mosquito I am building. This involves six rods each terminating at their outside ends in very small (2mm, 3mm) flanges, joined together at their inside ends in the middle. In anticipation of printing, I rotated this model so that four of the flanges would contact the printing base plate. By eye, the correct rotation appeared to be 92 degrees.

I imported the .stl file into Cura, where I had selected Brim as build plate adhesion, layer height as 0.0875, and initial layer height as 0.2625

Actual results
Unlike every previous occasion in which I have aligned something 'by eye', two of the flanges appeared with no brim, as can be seen in the first screen capture attached below.

I calculated what the correct rotation should have been, and discovered that my 'by eye' estimate should have been 91.7357degrees. Close, but evdently not close enough. I calculate that this raises the offending flanges 0.146466mm above the build plate. Quite by chance, this figure is larger than the layer height, but smaller than the initial layer height.

I used the recalculated rotation in OpenScad, and re-imported the model into Cura. This time, all brims were generated as required, so problem solved. (I then also enlarged the brims so that they merged into one, as can be seen in the second screen capture below.)

I've done such alignment 'by eye' more than once in the past, and usually if there is a tiny gap, Cura generates a tiny support with its own brim. Is it possible that in this case the tests for needing support and generating a brim have conspired to omit the latter?

Expected results
See second screen capture below

Additional information

[Ghostkeeper]

Most likely this 1.46466mm is larger than half of the initial layer height. Is your initial layer height 0.27mm? Then it'll make the slice at 0.27mm/2 = 0.135mm. Only if you set the Slicing Tolerance to Inclusive will it take the first slice at 0mm.

I think the support was omitted because of the Support Z Distance.

[Dougal57]

On 24/09/2019 10:30, Ghostkeeper wrote: Most likely this 1.46466mm is larger than half of the initial layer height. Is your initial layer height 0.27mm? Then it'll make the slice at 0.27mm/2 = 0.135mm. Only if you set the Slicing Tolerance to Inclusive will it take the first slice at 0mm. I think the support was omitted because of the Support Z Distance. — You are receiving this because you modified the open/close state. Reply to this email directly, view it on GitHub <#6386?email_source=notifications&email_token=ANID3N75ULUOIPS4JSQG5FDQLHM4RA5CNFSM4IYX4VJKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOD7NXIPI#issuecomment-534475837>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ANID3N2FN2NWKF2V42QV4ODQLHM4RANCNFSM4IYX4VJA>.

Hi Ghostkeeper I note this says "Reply to this email directly", so I'm giving it a try. Thank you for taking the trouble to understand what must have been minimal information. This was my first attempt to report an issue, and I made a simple 'newbie' mistake. I attempted to follow the guidelines, but (for reasons obvious to me afterwards <!>) none of the lengthy explanation I wrote actually appeared in the final result. In deep embarrassment, I immediately closed the state. I think you are right about the Support Z Distance. My Malyan 200 is quite hard-worked, and has a PEI layer on the heated bed.  On this occasion I was using a layer height of 0.0875mm, an initial layer height of 0.2625mm as you guess, and a support z-distance of 0.1mm. When models are as delicate as this one is, I find I have to use a brim, sometimes a substantial brim, to support the printing. I use OpenScad for my designs, and on this occasion had rotated the final (somewhat delicate) model by 92 degrees to approximately align four of its 'feet' with the printing base plate. This was in a hasty attempt to finish a quick trial print before bedtime. This is something I have 'got away with' on other designs many times before: either a brim will be generated, or support and a brim will be generated: either is fine for a quick trial. I later calculated that my rotation should have been 91.7457 degrees: quite by chance two of the 'feet' were therefore 0.146466mm away from the print bed.  And 0.146466 is indeed larger than half the initial layer height. I classified this as a 'bug' because it seems to me that, if a brim is needed to support something resting on the print bed, and a brim is needed to support any support, a brim is surely also needed on those very rare occasions (like this one) when the print bed itself is 'considered to' provide the support? By the way, I note now that 0.146466 is also larger than the Support Z distance, so shouldn't support have been generated in any case? regards

[Ghostkeeper]

Thank you for taking the trouble to understand what must have been
minimal information. This was my first attempt to report an issue, and I
made a simple 'newbie' mistake. I attempted to follow the guidelines,
but (for reasons obvious to me afterwards <!>) none of the lengthy
explanation I wrote actually appeared in the final result. In deep
embarrassment, I immediately closed the state.

Yeah we have a template that has HTML comments in them, and you put your explanations inside the comments. I removed the comment markers so your explanations became visible. Happens more often.

I use OpenScad for my designs, and on this occasion had rotated the
final (somewhat delicate) model by 92 degrees to approximately align
four of its 'feet' with the printing base plate. This was in a hasty
attempt to finish a quick trial print before bedtime. This is something
I have 'got away with' on other designs many times before: either a brim
will be generated, or support and a brim will be generated: either is
fine for a quick trial.

I often find it easier to move the model down slightly underneath the build plate. All layers underneath the build plate are left out so it'll stick properly. Of course the total height of the print will be inaccurate then though.

By the way, I note now that 0.146466 is also larger than the Support Z
distance, so shouldn't support have been generated in any case?

Sadly, the support is still limited to be printed on a layer so it can't be printed halfway along the layer's thickness. So we're rounding it. I believe we're currently calculating the Z distance by dividing it by the layer height and then looking that number of layers higher if something needs to be supported. This would be incorrect for the initial layer. But it'll be difficult to make that work right because not the entire model is near the initial layer. We can look into that.

[GregValiant]

I'm cleaning house.
Is this still a problem with current Cura versions (5.8.0 and up)? Can I close this?