How to Measure 3D Filament Average Spool Diameter and Tolerance

Measuring filament diameter incorrectly can sometimes give the impression that the filament spool is out of the specified diameter. Knowing the average diameter of a specific spool can also be helpful in fine tuning your print settings. Here's a quick guide that will help you more accurately asses the average spool diameter and the tolerance accuracy of your 3D printing filament.

Important: When measuring your filament, please first and foremost ensure that you are using a high quality caliper with a reading error of 0.01mm or better.

You will need to measure several different points along the spool, so we recommend unrolling at least a 6 meter section of filament for this purpose. (The longer section and more points you measure the more accurate your final result will be, but 6 meters will give you a pretty good idea of the average diameter and tolerance for your spool)

Measure your filament every 2 meters. For a 6 meter section, you will measure at 4 different points within equal distance along the section: 0m, 2m, 4m, 6m

At each of the 4 points, you will need to take 2 measurements at a 90° angle from each other as illustrated below (Measure using the tip of your caliper):

how to measure 3d printing filament diameter

Record your results. You should end up with a set of measurements, see example table below:

 Measuring Point Measurement #1 Measurement #2
0 meters 1.73 mm 1.69 mm
2 meters 1.70 mm 1.74 mm
4 meters 1.75 mm 1.79 mm
6 meters 1.77 mm 1.75 mm

Next, you will need to calculate averages for your measurements as shown in the example table below:

(Measurement #1 + Measurement #2) / 2 = Average point diameter

(Average point 1 + Average point 2 + Average point 3 + Average point 4) / 4 = Average spool diameter

 Measuring Point Measurement #1 Measurement #2 Average point diameter:
0 meters 1.73 mm 1.69 mm 1.71 mm
2 meters 1.70 mm 1.74 mm 1.72 mm
4 meters 1.75 mm 1.79 mm 1.77 mm
6 meters 1.77 mm 1.75 mm 1.76 mm
Average spool diameter: 1.74 mm

Finally, now that you know the average diameter of your spool, you can easily find out the tolerance accuracy of the spool as well:

1.75 mm - Spool average diameter = tolerance accuracy

In the case of the example above, this would be 1.75 mm - 1.74 mm = 0.01 mm tolerance accuracy.

Please keep in mind that although the results you get can certainly be helpful, they are still only based on sampling and of course, cannot be 100% accurate. Most state-of-the-art filament extrusion lines actually measure the filament along the entire length of the spool, which is the only way to produce a true representation of the accuracy and tolerance of the filament spool.

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