I have a 3D printer, which wasn't used for a longer time. There is a nozzle, but don't know what diameter is it. I used several widths - from 0.2 to 0.8, switched them depending on my needs, but don't remember which one was used lately. The nozzle is a little bit worn down, so the diameter on the side is not visible.
How to get the nozzle diameter, without taking it off the printer?
This is more of a theoretical question, because I can simply swap the nozzle, but still - eager to know.
The best method is to have a scale on your microscope that looks like a ruler drawn in the optical path. However I usually visually compare an unknown nozzle with known nozzles under a microscope. If you don't have a microscope, you can get USB otoscope cameras (15mm focal length for looking in ears) for under $20.
You can get a fairly good idea of the size by telling the printer to extrude in air then measure the extrusion diameter with calipers. Of course, it's better if you can compare with a known nozzle. If you push the filament through by hand, the extrusion will be too thick. With the stepper motor extruding, I measure the extrusion diameters between 0.3 and 0.5 mm on a 0.4 mm nozzle. Low cost calipers are less than $20. The main different with the low cost calipers is the slides are not smooth, reducing the accuracy in the 0.01 mm place.
If your nozzle is worn enough that you can't read the size stamped on one of the hex flats, you need to replace it anyway.
That said, the best ways I know to measure the aperture are with either a pin gage (known diameter high precision wire, find the largest that will enter the nozzle aperture), or optically -- take off the nozzle or hot end and put it on a scanner bed; use the scanner to image the nozzle at highest available resolution, then count the pixels across the opening in the image and multiply by the pixel size (for instance, if 6400 ppi, you'd multiply your pixel count by 1/6400 in/px * 25.4 mm/in).
I've used the optical method to measure photographic pinholes (drilled in delicate, .001" thick brass shim stock) that were smaller than 0.5 mm, so I know it works in this size range.
Measuring the extruded filament may not give an accurate diameter, for two reasons -- the hot, freshly extruded material may either swell or stretch after extrusion (and may well do both in a short length of extrusion, giving varying diameter), and it's difficult to accurately measure small round samples (especially of plastic) with either micrometer or dial/digital caliper -- it's too easy to compress the sample and obtain an artificially undersize result.
I got interested in the question and did some research. I found PCB drill bits from 0.1-1 mm in 0.1 mm steps for around 7€ and believe they are an interesting option. They should help clean the nozzle (or quickly break in the process?) and I think one could experiment with revitalizing worn nozzles by drilling them up a bit (as they are carbide, they should work well with brass nozzle, hardened steel ones might be too much). Having them (and a Dremel-style tool and proper clamping) could open up a world of nozzle hacking, see e.g. CNC Kitchen on YouTube for what happens if you solder a copper wire into the nozzle.
I would find some wires (piano wire, CAT5 copper, paperclip, nozzle unclogger, etc. ) and choose one that should fit the nozzle (eg. a 0.5 mm copper wire will fit a 0.8 mm nozzle) and push them into the nozzle. Heat them up a bit to soften the filament in the nozzle and brush with a metal wire brush to easily clean it up.
Extrude some filament, let it hit the desk or whatever surface the printer is resting on; but not the bed as it may be too close. The initial filament will form a coil. After extrusion stops measure the straight part of the extruded filament as close to the coil as possible. This will give you the true current nozzle diameter. (Edit: I mean measure with a micrometer)
Just as ancillary info, the nozzle diameter increases as the filament is passed through it. This is called nozzle wear, and since the diameter is changing the quality of your prints is going to drift. If you have a brass nozzle then after every spool of filament, you should reverify your nozzle diameter. Steel or other hardened nozzles would be much more resilient.
Majority of nozzles have a .4 or a .3 on it, that indicates how big it is in millimeters. If yours doesn't, take one of these digital rulers (the ones that have the prongs that stick out on the side and slides, I don't know the name off of the top of my head), extrude some filament and measure the filament, make sure that you find a piece that was extruded after the bottom hit the ground and make sure that you find one that isn't bending that way its not stretched or compressed. (Your measurements won't be exact due to things cooling off and shrinking)
