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Introducing an absolute exclusive: the SHARP-METR sharpness meter.
We have long been interested in the question: How to quantitatively measure the sharpness of a sharpening? Quantitatively, not just qualitatively, with tests on paper, hair or cloth, because the aforementioned tests, like many other such tests, are very subjective and depend on many specific conditions. The goal was to get a quantitative assessment of sharpness.
You can see what the result was in the video.
First we had to choose a working body for the measurements. It seemed to us most likely to take as a basis a fishing line (monofilament) with a diameter of 0.2-0.3 mm. This diameter was chosen empirically, based on testing different diameters. This choice allowed us to obtain reliable readings over a wide range of measurements. Peak readings remain on the display until the next reset to zero. One of the most important conditions for stable readings is to use the dynamometer not only to determine the cutting force, but also to set the thread tension. It is the thread tension that determines the cutting force reading. In this case, a thread tension of 200g (2n) was chosen. The stronger the thread tension, the less cutting force required and the lower the absolute reading. In addition, the monofilament is prone to stretching, its tensile force is about 10 kg. The so-called "braid" - a thread woven from many fibers - is much smaller in this respect (almost zero elongation coefficient), but it has a specific property when cutting - there is no clear breaking moment.
When creating the device we tried to observe several conditions: ease of use, work without additional tools. All manipulations with the measures are very simple, the thread itself is on a special dispenser for threads, the stock of thread on the spool for about 500 measures (about 50 meters). The thread can be bought in any fishing store. Through a special window in the dispenser you can always see the amount of thread on the spool. The spool of the dispenser is equipped with a friction brake, which prevents the thread from tangling, so the thread is pulled out without much effort. The thread locking knots are made to make it easy to wind and hold the thread. There are even pins to keep the clamps from turning on their own. The dynamometer itself has a "PEAK" mode that records the peak cutting force. The reading is stored on the display until the next reset. The dynamometer is powered by 2 AAA batteries. The batteries can be replaced without removing the dynamometer from the base.
This design utilizes dynamometers from different manufacturers, but are exactly the same in functionality, size, and even price. The only matter of choice is stock availability and delivery time. There are no differences other than appearance, they even come in identical cases.