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Negative Space problem

Posted by muntahunta 
Negative Space problem
November 03, 2016 05:54AM
For a long time now i've had trouble printing anything that is supposed to fit together or be printed together.
I've never figured out why until I heard about negative space tolerance.

Now my printers x, y and z axis are all calibrated correctly, as is the extruder, so how do I change negative space?

I've tried many slicers, Cura being my main slicer for over the past year, but I have skeinforge, Slic3r and Simplify3d.
I got simplify3D yesterday and so far really love the software, but I did some modifications to the default .fff file (i'll post my profile below if it helps)
I think, somewhere, I have the wrong settings and that is what is messing up my negative space (although it does seem to happen on every slicer I try)

The nozzle i'm using is 0.3, i'm using 1.75 PLA on a prusa i3.

Any help would be very much appreciated.
open | download - prusa1.fff (9.3 KB)
Re: Negative Space problem
November 03, 2016 08:24AM
Just printed a calibration cube and stopped half way, the cube walls are supposed to be 1mm thick, mine are 1.8mm thick.
Re: Negative Space problem
November 07, 2016 07:11AM
This has had my head battered.
I got the walls down to the perfect amount, but it required me to adjust the extrusion multiplier to 0.36.
I have calibrated my extruder so that when i click 100mm extrusion, it extrudes 100mm so why do I need to set the extrusion multiplier so low so that I can get the correct size walls?
Re: Negative Space problem
November 07, 2016 07:14AM
I think that you need to re-calibrate the extruder - it's not 100mm of filament coming OUT of the nozzle, it's 100mm of new filament going IN to the top of the extruder.

Look here: RepRap

Edited 1 time(s). Last edit at 11/07/2016 07:15AM by David J.
Re: Negative Space problem
November 07, 2016 09:36AM
yeah, thats how i've calibrated it.
1.75mm, mark 100mm from the top of the motor, extrude 100mm then measure how much the mark is off.
Its calibrated.
But in order for me to get the correct wall thickness (1mm for the 20mm square calibration cube (or 0.4mm for the thin wall calibration)) I have to set my extrusion multiplier at 0.36
Re: Negative Space problem
November 10, 2016 10:49AM
Managed to get it to print by reducing the amount of filament extruded to 36% but this is making the parts very fragile... Is there another way to fix the walls being too thick?
Re: Negative Space problem
November 10, 2016 06:59PM
I think your designing parts wrong. (most things on thingiverse are broken)

A X mm tab will not fit in a X mm slot. The real world requires some play

There is also the old polyholes issue where by cylinder cuts outs are smaller than expected as they are approximations generated by polygons.
Re: Negative Space problem
November 11, 2016 06:10PM
Is your nozzle size set correctly in your slicer?
Re: Negative Space problem
November 12, 2016 07:27AM
As an example I was printing a 20mm cube with a 1mm wall all the way around.
The model was fine and its fairly hard to mess up such a simple model.
I also tested on a 0.4mm thing wall in a square.

I know that Xmm part wont into Xmm slot, but the problem I am having is that Xmm part isnt printing at the same wall thickness as it should.

If I print a 1mm wall thickness, its coming out at 1.8mm unless I change the extrusion multiplier to 0.36, which then gives me a 1mm wall.

The nozzle is set to 0.3mm on the slicer which is correct, The X, Y and Z axis' are all calibrated to give me the correct size and the extruder is calibrated so that when I send in 100mm from the top, it is within a 5% error margin (95mm to 105mm).

I have ordered a 0.2mm nozzle which will arrive wednesday so maybe that will help, but there has to be something else that is wrong.
Re: Negative Space problem
November 12, 2016 11:13AM
What shell thickness do you have set up in the slicer?

I had problems with Cura recently when I was trying to print a 1.2 mm thick wall with a 1.2 mm shell thickness set. The result came out 1.6 mm thick. Reducing the design wall thickness to 1 mm gave me the 1.2 mm thickness that I wanted. There are some strange interactions between design wall thickness and the shell thickness set in the slicer when you print fairly thin wall. I use a 0.4 mm nozzle by the way.
Re: Negative Space problem
November 12, 2016 05:26PM
my shell thickness is set to 1mm and the top/bottom is 1mm, It should be working fine by my calculations. but maybe my calculations are wrong haha.
Re: Negative Space problem
November 12, 2016 05:53PM
My understanding from the short manual with Cura is that the shell thickness should be an exact multiple of the nozzle diameter. Hence my choice of 1.2 mm with a 0.4 mm nozzle.
Re: Negative Space problem
November 13, 2016 08:23AM
but you said that once you changed down to 1mm, it came out at 1.2 which you were after, but at 1.2 it was coming out at 1.6. If that were true, then having the shell thickness at 1.2 would have worked fine.
Re: Negative Space problem
November 14, 2016 12:53PM
No, I reduced the design thickness of the wall in the MODEL to 1 mm which then printed at 1.2 mm thick. There is some strange interaction in the slicer when printing walls that are thinner than two shell thicknesses and I haven't got to the bottom of this yet.
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