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Surface finish on bridges?

Martin
(@mprowe)
Eminent Member

This (left hand image in attached file) is a PCB mounting frame that I have drawn in FreeCAD (v0.19.x). To reduce the amount of support filament needed, I am printing it upside down. This, of course results in two bridges. Examining the surface of the bridges after printing I am unhappy about the finish?

Instead of the expected 2 layers of perimeter and 45 degree skin, I see this (right hand image)? However, if I print the other way up, it prints the skin as I would expect it to (see bottom image in composite image). What do I need to do to get correct (my wished for) layout on these bridges?

Regards, Martin

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Topic starter Posted : 30th April 2021 8:45 am
BHudson
(@bhudson)
Former USA Robox Support Moderator

@mprowe This is because you have a bridge. Bridges are calculated to run along the bridge to give the best chance of success. If you don't like the result you will have to use support material. If that still doesn't work for you, I suggest using dual materials with a solid support under the bridges to give you a good surface to build on. 

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Posted : 1st May 2021 4:48 am
Martin
(@mprowe)
Eminent Member

Yes, I know it is because I am printing a bridge. My apologies for forgetting to mention that I am printing with a support.

I'm not convinced that my issue is related to the support style though? Using alternative support (Grid or Lines) styles makes no difference. However, it does seem that the Slicer is a significant issue? If I compare the behaviour of the slicer between a "Bridge" and a "Pier", I can change the skin pattern. In the attached file, the top row shows two 3D models. On the left is a bridge (or will be when printed face down) and on the right is a pair of piers (excuse the pun!).

The next row down shows the print simulation of the last support layer (in this case, Grid but Lines make no difference) before the skin is printed.

The last row, is showing the print pattern for the first layer of the skin of a bridge (left) and a pier (right).

Why the difference? Why would a bridge with a small gap in it print in a significantly different (un-commanded) way?

Regards, Martin

ReplyQuote
Topic starter Posted : 2nd May 2021 9:11 am
BHudson
(@bhudson)
Former USA Robox Support Moderator

@mprowe Because it is a bridge. Bridges are a distinct feature in slicers because they need to be handled in a specific way. A bridge is a suspended part between two sides. The first layer of a bridge is always going to be made in long, straight lines across the bridge to create a platform for the next layers. This is so they can be printed without support if the user elects not to use support. If the bridge is printed without support and the slicer attempts to make the diagonal pattern it is much more likely to fail. Some slicers allow you to turn off the detection of bridges but others do not. Turning off bridge detection is not currently available in AutoMaker. 

The image you shows has a bridge and an overhang. Overhangs don't have a second side like bridges do, so they are calculated the same way normal surfaces are calcualted. An overhang is expected to be printed with support, though short overhangs can be printed without support. Because bridges and overhangs are different features to the slicer they are calculated differently. They need different styles to complete successfully. 

ReplyQuote
Posted : 3rd May 2021 4:02 am
Martin
(@mprowe)
Eminent Member

Because bridges and overhangs are different features to the slicer they are calculated differently. Turning off bridge detection is not currently available in AutoMaker. 

Understood. Thank you.

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Topic starter Posted : 3rd May 2021 6:38 am
Mike
 Mike
(@17bt)
Prominent Member

@mprowe You can always have a look at AutomakerOS and compile a version, as bridges is one area where you can make improvements over the current AutoMaker &  AutoMaker Pro build, but you'll have to add these Cura4.8+ feature sets

I haven't looked at your post for an actual print image, so it's hard to suggest ways to improve, but temperature, slower speeds, better extruded filament cooling will all tend to reduce sagging. Putting holes in a bridge can be problematic too. :)

Mike                                        

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Posted : 4th May 2021 8:30 am
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