On a dispute, I measured the diameter of the shaft without a barbell. Accuracy to tenths

Friends, I congratulate everyone on the New Year holidays.

I will share with you a trick that has helped me out more than once. Have you tried measuring diameters without a caliper? Shafts, pipes, pulleys? Do not use a ruler to do this... although who knows, maybe it will do... But in situations where there is no access to the end for measurement? Do you think this is possible? Of course yes. Accurate to the tenth of a millimeter! I even won an argument a couple of times with men with a hot temper. Now I'll tell you how.

In the photo there is an engine with a shaft diameter of 5 mm with a plus tolerance for fitting into an interference fit of a plastic coupling. Let's find out its exact diameter without resorting to measurements with a caliper.

We take sewing thread. Preferably thin and strong. We make 10 turns around the shaft, turn to turn perpendicular to the axis. But so that they do not overlap.

The number of turns affects the measurement accuracy. If you don't need to catch tenths of a millimeter, you can limit yourself to one turn.

Draw a line along the axis of the shaft with a gel pen. It is advisable to do this in one motion to keep the line thin.

Done. Now we remove the thread and measure the length of 10 turns with a ruler (between the first and last line).

The marks on the thread are not very visible, so you need to look very carefully. The main thing is not to miss the round. For reliability, you can recalculate the marks. There will be eleven of them.

In my case, it turned out 158 ​​mm. For greater scrupulousness, you can interpolate and take 157.8 mm. Mathematics will follow.

We calculate the length of one turn:

157.8 / 10 = 15.78 mm

From the school course, we remember that the length of a circular arc is the product of the diameter by the number Pi. Accordingly, to get the diameter, you need to divide the length of one turn by Pi. Again, for "super precision" let's take a number with five decimal places.

15.78 / 3.14159 = 5.023 mm

A caliper, by the way, showed a result of 5.01 mm on this shaft. The thread was only one hundredth wrong. But it was rather lucky. I would not promise accuracy to hundredths in a dispute over a case of beer)))

That's all for me. Thanks for reading. If the article was helpful, subscribe to the channel and of course put your thumbs up 👍. Once again, Happy New Year! All the best!