What 3d printing is
A 3d printer is a device that builds up 3-dimensional structures layer by layer. This way, you can design something 3-dimensional on the computer, print it and hold it in your hands as an object in real life afterwards.
What 3d printing is not
Prints from a 3d printer often are not usable as-is. They are not as sturdy as injection molded pieces, they have problems with heat (depending on the material), they need post-processing depending on what was printed and the surfaces usually are rough and should be sanded and painted.
Still, they are sturdy enough for many uses. So if you don't mind the "printed" look with the visible layers and your project has sane requirements for strength, then yes, 3d printed parts can be used as they come from the printer.
Using an FDM printer
FDM is a type of printer that uses plastic filament and extrudes it though a hot nozzle. Popular examples are the Creality Ender3 and the Prusa i3. While there are other printer types (SLA for example), I don't have experience with those, so I won't talk about how to use them.
Calibration: Motor Current
3D printers usually work with stepper motors, and they have a certain current they are supposed to be driven with. If it is too low, they will skip steps and ruin the print. If it is too high, they will overheat and degrade.
Look at the print on the motor, into its datasheet or the service manual of your printer to find the right value, and configure that either with the small potentiometer on the individual stepper drivers or if your printer supports that, in firmware. Google will help you find the correct way to set a certain motor current for your specific printer.
Your computer tells the printer to move distances in mm, but the stepper motors only know steps. So you have to tell the printer how many steps make up a mm.
For the movement axis, mark the position of the nozzle or bed with a piece of tape, tell the printer to move 100mm and the measure the distance. If it is not exactly 100mm, the steps/mm need to be changed. There are online calculators for that, and instructions for your specific printer on how to change the values permanently.
For the extruder axis, mark the filament 150mm from where it is entering the machine with a felt tip marker or something similar, then extrude 100mm and measure the distance again. If it is not exactly 50mm (i.e. 100mm were pulled into the printer), your steps/mm have to be changed for the "E" axis. Make sure you go SLOW for this test, so the nozzle has it easy to melt the filament.
Some people may tell you to tweak the steps/mm for the extruder if the print does not look right. But the steps/mm is the bare translation from "mm" to "steps" and has nothing to do with extrusion. I would never change steps/mm from that hard translation factor. More on that later.
Calibration: Bed Leveling
Bed Leveling is done to ensure the print bed is parallel to the X and Y axis and the distance in Z is exactly what the slicer software is telling the printer to use. So "Z = 0.0 mm"" means the nozzle is touching the print bed. And "Z = 0.2 mm" means the distance between the print bed and the nozzle is exactly 0.2 mm.
Always level your print bed at working temperature. The print bed has to be pre-heated before leveling as it is large and material expansion can have an impact. The nozzle does not have to be pre-heated for leveling, it just has to be clean at the bottom.
To level a print bed, you need a gauge. This can be a fancy feeler gauge or something as simple as a sheet of paper. First, determine the exact thickness of your gauge. In case of standard office paper like for an invoice, this will be 0.1 mm. Then, home your printer using the homing command. Then, move your Y-axis up by the thickness of the gauge. In our example X = 0.1 mm.
Without changing the Z, move the print head over all four adjustment screws that are under your print bed. For each one, put the gauge between the nozzle and the print bed and move it a little. Adjust the adjustment screws so the gauge does scratch but does not stick. Then, your nozzle is exactly the thickness of the gauge away from the print bed.
Do it again. The first pass over all four adjustment screws is the coarse leveling, the second pass is the fine adjustment.
If your print bed is level (i.e. not bent), you are done with leveling.
For every combination of printer, material, print speed, layer height, room temperature and air current (open windows for example), you need to dial in the right nozzle temperature. The values printed on the spools are a starting value, and not a value that always works.
Find a heat tower / temp tower on Thingiverse, follow the instructions on how to change the printer temperature after each segment, and the segment that looks best is the temperature you should use.
Most heat towers have a pointy feature (that should look pointy), an overhang feature (that should not droop) and a bridge feature (that should be very horizontal).
You might want to re-print the temp tower after finishing calibration to fine-tune it.
Calibration: Flow Rate
The flow rate determines how much material is used, like a fine tuning after the extruder-steps/mm.
Print a small cube (20mm for example) with 100% rectangular infill. Ignore the first 5 layers to mitigate any errors from an imperfect print bed. Then, look at the individual strings of plastic that are extruded next to each other. Only in the middle of the cube, far from its edges. If there are gaps between them, increase the flow rate during the print. If there is material squished upwards between them, decrease the flow rate during the print. You will quickly find the perfect flow rate this way. To make it permanent, configure that flow rate in the slicer and set the printer back to 100%.
Filament is springy. If you push on it, it will compress a little. How much depends on the type of filament. This springiness - together with a few other things - leads to unwanted material extrusion at the end of an extruded line. Retraction is a way to avoid that problem.
A good starting point for retraction is 1mm for extruders with the drive close to the nozzle and 2mm for extruders that are connected to the nozzle with a long bowden tube.
Print two small items (for example 10mm cubes) that are a good distance away from each other. Like 100mm for example. Watch closely. If there is a "blob" of material before the nozzle moves away from one of the cubes and/or there is some material collecting at the outside border where the nozzle travels to one of the cubes, then your retraction is too low. This is a lot of trial and error, get the retraction distance as low as possible without creating blobs and outside material deposits on the model.
Printing slow means printing pretty. The faster you get, the more vibrations will make your prints ugly. Finding the perfect speed is what this chapter is all about.
Print a larger cube (50mm for example) with "vase mode". This will create a single, continuous perimeter and hides other potential issues to interfere with our calibration.
Increase the print speed until your printer gets an offset, does not print in the right place anymore. Re-start the calibration with 80% of that speed and look at the outside walls. If they are flat and smooth, you are done. If there are ripples after each corner the nozzle takes, decrease print speed until the walls are smooth.
Calibration: Acceleration and Jerk
Print a larger ring (50-100mm outer diameter, 10mm thickness) with a single perimeter and 100% rectangular infill at the calibrated maximum speed from before. Increase acceleration until the printer is getting an offset and does not print in the right place anymore. Use 80% of that value. Now increase the jerk value in small increments (like 2-5mm/s) until the printer loses its position again. 80% is a good value here, too.
Do the speed calibration (descibed above) again. If the ripples don't go away, reduce jerk in small steps until the walls are smooth.
Usage: Filament Tolerances
Filaments usually are 1.75mm in diameter, with a tolerance of +/-0.05mm, 0.03mm or even 0.02mm, depending on the manufacturer. This is a big deal and you should really care about getting filament with tight tolerances. There are filaments out there with diameters between 1.55 and 2.1mm within one spool. This translates to a tolerance of +0.35/-0.20, which not only is "less pretty" but completely useless. Let me explain.
We will need some math here. The area of a circle is pi*r^2. For 1.75mm, it is 3.14*0.875^2 = 2.40mm². So if the extruder for example pushes out 10mm of filament, it will extrude 10*2.4 = 24mm³ of filament. This is the amount your slicer uses in its calculation. This is what the extruder thinks will be put onto the print bed. If your diameter is off however, the amount of filament will be different.
Off by how much? Well, see for yourself. The percentage is the actual "flow rate" you would get with the
given actual filament diameter.