Shell Thickness is changing my model dimension

[jweaver 2]

This was discussed under another topic, so I created a new post.. Hope thats OK.

I have been printing with CURA for a month or so and have found settings that I like.. I am typically using 1.0mm shell but yesturday learned (remembered) that this should be a multiple of my nozzle size.

At the time I was printing these:

http://www.thingiverse.com/thing:394364

And at 1mm, and used "scale" to get a perfect size.. But when I changed to 0.8/1.2mm shell it all went wrong.. All of the dimensions changed and they didn't fit.

So today I did a few test and am not even sure what I the issue is...

I have printed a few layers of this model, just so that I have a hexagon.. After doing this with various shell thicknesses, the difference is clear.

The picture should show the differences between them.

When I measured them afterwards, I found the following

0.8mm - Wall: 1.65mm Outer Width: 16.25 Inner Width: 13.0mm

1.0mm - Wall: 1.5mm Outer Width: 16.15 Inner Width: 13.2mm

1.2mm - Wall: 1.75mm Outer Width: 16.4 Inner Width: 12.8mm

I then loaded the STL in Sketchup and measured the 'real' dimensions, at it was

Wall: 1.3mm Outer Width: 16.00mm Inner Width: 13.00

So my prints with a 1.0mm wall are the closest.. Why are 0.8/1.2mm so far out? Whats going on here??

Does anyone have any ideas? I never imagined that changing wall thickness would have such an effect.. I wonder if this is purely down to the fact that the STL in question has quite a thin wall... Perhaps I need to do a test on something a little larger?

I look forwards to any thoughts you have.

Jon

 

[Daid 306]

I think you might be over-extruding then, which will give different results on different parameters.

 

[svanbennekom 0]

Is this with a printerbot?

It really depends alot on your setup.

A good idea is to create a gcode that extrudes a certain amount of material, say 100 mm, and then measure what your machine actually extrudes.

Multiply that by the average diameter of your filament, and you've got the total volume extruded. Divide by the diameter you've set in cura times a hundred and you've got the flow rate you should set, unless I'm mistaken or made a math error.

 

[jweaver 2]

Yep... A Printrbot.. When i got it, i did some calibration which was to feed a set amount using the GUI amd thrn make adjustments in the firmware.

Since then i have been able to print stuff with remarkable accuracy.. For example a 110mm calibration "L" came out at 110.1 each way.

I have started to get more confident with Sketchup and have been drawing basic shapes (washers, spacers, etc) and everything prints at approx +-1% accuracy..

Bit now i have trsted with exact multiples of my nozzle size its all gone wrong...

Can you clarify your calibration steps? Say i print 100mm, where do i measure.. Going into tge extruder or coming out? And are you suggesting i tweek the "flow rate" in CURA rather than setting thing in the Printers firmware.

[Daid 306]

What could be happening. And this is a guess, as the data I have is not from a printrbot.

What we see on Ultimakers, is that the material amount you are getting depends on the material flow rate. For example, if you are asking for 10mm^3/second, we're seeing for example 10% less extrusion then expected. While if we extrude without a hotend attached (so just the feeder) we see exactly the amount we're requesting. When we request 15mm^3/s, we're seeing 20% less material.

The faster we feed, the less material we are actually getting. And this all depends on the feeder, hotend and material used.

This isn't really new information (as some of these measurements where done quite a while back) but because expectations of machines and software is going up, this is starting to become an issue.

So, I'm suspecting you calibrated everything properly for a certain flow (in mm^3/s), and by adjusting the parameters, the flow changes, and thus the actual flow changes. You could easy check this by doing the same print twice, but only change the print speed.

 

[fluxline 0]

Thanks for reminding me of this, shows me something I need to do. I have my own extruder with a different gear ratio and enter my own eSteps. When Illuminati posted his back pressure and flow piece, I tested the calibration at higher and slower feed rate and got no difference. Now I see with no loading it would be the same. I'll have to test that again to come up with a good value at both high and low flow.

[jweaver 2]

I did another test last night.. I drew a 'washer' in Sketchup and printed it with different shell thicknesses.. The output was SLIGHTLY different between prints but all within acceptable tolerances..

I do think that the "thin wall" of the Nut Cap I was testing with might not have helped.. But I wonder if in normal operation, just sticking with the same shell size, will be fine.

To be honest, I am not seeing any benifit of going with 0.8/1.2 over 1.0.. So I might go back to 1.0, just because its what I am used too...