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Corexy print dimension not correct

Posted by Yoko 
Corexy print dimension not correct
March 09, 2019 02:07PM
Hi All,
I own a corexy printer with marlin software
when i print a 20x20 test cube I have the following measures
x=19.84
y=20.10
i tried to change steps per unit in marlin but it has no effect
any suggestion what to check?
Re: Corexy print dimension not correct
March 09, 2019 09:52PM
I know it's called a "calibration cube", but 20mm is too small to calibrate anything. At that small size, heat from successive layers of plastic being deposited on still hot plastic causes distortion in the print. In addition, every measurement you make is accompanied by some error. If you calibrate steps/mm based on measuring such a small object, when you print larger objects the absolute error will be multiplied. If you really want to calibrate, print a large part and measure it. It too will have error, but when you print a part that is smaller than the calibration print, the absolute error will be divided.

There are other reasons for parts not being exactly the intended size/shape. If the belts are not properly positioned parallel to the guide rails, and you print away from the center of the bed, the print geometry will be distorted. If the X and Y axes are not square, prints of square shapes will be rhomboid. Test for squareness by measuring diagonals of rectangular prints. If the diagonals match the guide rails are square. If they are out of square it is probably because you need to tweak the belt tension.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Corexy print dimension not correct
March 10, 2019 12:02AM
Quote
the_digital_dentist
I know it's called a "calibration cube", but 20mm is too small to calibrate anything...
Ok i will print a larger file and check again
Re: Corexy print dimension not correct
March 10, 2019 09:03AM
When you're trying to find/fix errors of 0.1 mm or so, you have to be sure the extruder is well calibrated. The dimensions of the prints are partially a function of the line width. The slicer sets the line width by calculating the the volume of plastic going into the extruder. It uses whatever diameter you set. If you just use the nominal 1.75 mm diameter and the actual filament is thicker or thinner than that, the plastic coming out of the nozzle won't match what the slicer thinks and the line width will be thinner or thicker than you specified. If the line width is off by 0.05 mm, you'll have a 0.1 mm error in the print size. It doesn't take much error in the filament diameter to lead to that much error in line width.

If you're going to try to calibrate the steps/mm in the axes, calibrate the extruder first. Measure the filament diameter in 20-30 places and calculate an average from that. Use that average diameter when you print an extruder calibration part. Print something that has 2 or 3 perimeters at a specific line width- I like to use the nozzle diameter for the line width- then measure the wall thickness of the print. If the line width was 0.4 mm and you used 3 walls, the wall should be 1.2 mm thick. Measure in several places, a few mm above the bed (heat from the bed can distort the first few mm of the print) and away from corners (which are othen thicker because of acceleration and jerk settings), and calculate the average wall thickness. Tweak the extruder steps/mm accordingly and print again. Once you're satisfied that that is working as expected, print your XY calibration part, using the same filament diameter you used for the extruder calibration. Note that if your Z axis has problems like Z wobble, the wall thicknesses you measure will include that error- the line width will appear to be larger than it is. You really should fix the Z axis problem before you try to accurately calibrate the extruder and the XYstage.

If you think about it, you'll quickly come to the conclusion that getting consistently accurate prints will require adjusting for the filament diameter every time you change spools. You can either measure the diameter in 20-30 places, calculate the average value, and use that each time you slice (or print if you use volumetric extrusion), or you can print a calibration part with each spool and set the extrusion multiplier when you slice or when you print to get the right line widths (and so the right print dimensions). Whichever way you go, mark the spool with the filament diameter or the extrusion multiplier so you only have to calibrate once for each spool.

In the last couple years, even cheap filament makers have been getting better at controlling the filament diameter, so when you buy 1.75 mm filament, it is often very close to that diameter with little variation. Look for filament ads that say something like "1.75 mm +/- 0.03 mm".

Edited 1 time(s). Last edit at 03/10/2019 09:29AM by the_digital_dentist.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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