An accurate value for your filament diameter (as a setting in your slicing software) can have a large impact on your overall print quality. Most filament brands will list their dimensional accuracy but I think it's important to have a real value. Here is my process for finding filament diameter.
I do this for every new roll of filament. I keep a record of filament diameters in a notebook designated for 3d printing. I also like to print a sample chip for each type and color of filament I have. With a direct-drive extruder, I can often use the length I measure to print a sample chip.
Time needed: 20 minutes.
Getting an average filament diameter.
Cut off a section of filament about three feet long.
Make sure to restrain the end of the filament at all times so it doesn't get tangled.
Make a mark on the filament every inch or so.
You want 20+ marks. More is better but I usually do 25-30. I use a black sharpie for light filament and a silver one for darker filaments.
Measure at each mark using your digital calipers. Note down the measurement.
Be sure to use the wider, flat area of the caliper jaws so it doesn't cut into the filament and give a false measurement. Also, you should rotate the calipers around the filament to make sure it isn't oval.
Average your measurements.
Simply add all of them together then divide by the number of measurements.
- Enter the result in your slicing software.
You can get a decent set of digital calipers on Amazon for around $20 USD. I see a lot of machinists recommend Igaging so they might be worth checking out. If you want to pay a little more to get a higher quality tool, you really can't go wrong with Mitutoyo. They make excellent quality measuring tools.