Diameter Calibration for Holes and Cylinders (2,0-3,9 _ 0,1Steps) (updated)

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This is a small plate with various cylinders and holes, ranging in outer diameters from 2.0mm to 3.9mm, increasing by 0.1mm increments. I had trouble with tolerances in the past, but now I know why - my printer is not accurate. I measured the print, applied some Excel magic, and the results are clear: the maximum differences are between +0.1mm and -0.44mm. The closer a line is to the blue one on the graph, the better it represents accuracy. Now that I know how much my printer is off, but not how to fix it, I'm looking for suggestions. One possible clue is that I need to scale up measurements of models, such as the 20mm calibration cube, by about 2% in both the x and y axes. Suggestions are welcome! ======================================== Update: March 30, 2017 Thanks for all the great suggestions! Here's what worked: 1. Check stepper-driver voltage (by Shooter40000): If motors don't get enough energy, magnets can't hold correct position, causing issues like layer-shifting. I already did this months ago, but it's still a good idea to check! 2. Material shrinkage (J741): As warm plastic cools down, it shrinks, especially with ABS. This is a common issue in 3D printing. 3. Check stepper motor's "steps per mm" in firmware (by Solarenemy and J741): The printer needs to know the distance of one step, which its stepper motors actually do. I didn't completely fix the problem, but it helped get more even results (-0.26mm) and I no longer need to increase model sizes by 102% in my slicer. For a HOW-TO on calibration, check out Kossel / Delta Dimensional Calibration with Instructions (by Coffea). My Make with instructions for a non-delta printer with Marlin firmware is also available. 4. Try slower printing speed (by Plust): To increase print details, try going slow. The printer is more accurate and plastic has time to cool down before the next layer is applied. The difference between normal and half-speed prints is interesting: inner diameter got worse, but it's worth a try! 5. It's the polygons' fault (by Chantling and Scabby): 3D models are made of many triangles, so every round shape differs from its original. This is why we need to consider tolerances up to 0.5mm when designing for FDM printing. The smaller it gets, the more difficult it becomes. Common issues in 3D printing! Print Settings: Printer: Geeetech Prusa I3 Pro C Rafts: No Supports: No Resolution: 0.2mm layer height Infill: 10% Notes: PLA Printing speed: 3600.0mm/min (60mm/sec) Extruder temp: 185°C (first layer: 190°C) Bed temp: 50°C (first layer: 60°C) Nozzle: 0.3mm Slicer: Simplify3D Top/bottom layers perimeters: 3/3/3 Part cooling fan: Yes