This topic contains 31 replies, has 6 voices, and was last updated by jhr 1 year, 4 months ago.
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@dr-woo I have printed a cylindrical part 25mm diameter x 10mm high. The photo shows the 5layer repetitive effect. Due to high magnification the focus range was very limited hence blurred near area.
Next is to work out how even the layers are in height. The easiest is to work with the troughs of each layer but is going to be difficult, mainly due to optical distortion effects as the focal length is very short.
more to follow
John
@john Thanks for your post. Interestingly it looks on your printer more like a ripples-with-amplitude-every-5-layers compared the 5th-layer-rim I’m seeing on some of my printers. I’m pretty sure though that the cause is the same.
I have done a bit more on this issue. As I couldn’t measure the printed layer thickness accurately on the parts I decided to try a different approach so printed a bracket to mount a dial gauge on the horizontal guide rails down to the bed and used the manual Z-Axis control in the status window to raise the height in 0.1mm steps. The Graph attached clearly shows there is a 0.5mm repetitive height variation on the Z-Axis. I did it twice to verify the findings as the dial gauge has an accuracy of 10microns.
Additionally there appears that on this machine that there is 50micron backlash on the axis, depending on the head movement, when the up and down movement is unidirectional or bidirectionally. This clearance allows the machine to operate “normally” without the screw getting jammed.
@pete @chrisyt is this normal that there is a variation in the screw positional accuracy? It will limit the ultimate quality of the prints.
regards John
@john Very interesting post, although I don’t fully understand where the dial gauge was mounted and exactly how deviation looks and how it influences the print/how it is responsible for the repetitive wavy pattern. Can you post an image of your measuring setup?
Attached photo of dial gauge as used. I printed a plastic bracket to support the dial gauge and ensure that no damage was done to the rails. It’s based on the shipping bracket that came with the Robox.
The idea was to measure height variations from the height commanded.
If we assume the extruder outputs a constant volume per unit of time and the head movement is also constant then the area that is printed is also constant. Variations in height from layer to layer will affect the width printed i.e. the greater the height change the narrower the layer width and conversely the lesser height change the wider the layer is printed.
From the graph there is up to 0.03mm change in layer height, 30% of layer spacing when <fine profile> is selected, In the case where the layer height reduces by the 30% the extruded width on that layer will be greater (I haven’t done the maths so won’t put a figure to it), this means the inner peripheral layers will also be wider, forcing the outer peripheral layer to be squeezed out even further, it has only one direction it can go in.
hope that makes sense
John
Are we on something here? Or is this normal for FDM printers of this build-type?
Question is: What’s the cause? I don’t think that it is the threads, unless they have been irregularly cut/lathed, which I just doubt. I rather could imagine that it has to do with the stepper motors.
Rounding error? Makes no sense but if there was an issue with precision it would have explained it, although I do not expect it to be the case…
@dr-woo I have run the same test on my “spare” printer. The measured Z-axis deviations on this one is +- 10 microns (the dial gauge is used for centering parts on a lathe so repeatability versus accuracy was important). The result was the “spare” printer which has only done about 200hrs printing is within +-10 microns the more used used printer, which has more than 1500hrs, has a variation of +- 25 microns. I still have to do a comparative print to verify this, but will do.
@click this appears to be mechanical rather than software.
I will be investigating further
john
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