The 800 mm radius myth started a number of years ago. Larry Brooks, a well respected retired shear sharpener, wrote a book on how to sharpen beauty scissors. To my knowledge this is still the only book on how to sharpen beauty shears. I bought it and read it and found it had a lot of good information Larry noticed that shears from Japan have a curvature to the edge of the blade, a belly so to speak. He wanted to know what the curvature was and took several of the shears he had at that time and laid them on his barn floor and drew a circle from the shape of the edge of the blade. Lo and behold, all the shears he had an 800 mm radius. This piece of information was interesting but accurate for only the shears he picked up and measured. It is not accurate for all shears.
The best radius for a shear for the least push and the best cut is 800 mm. HOWEVER, just like there are specific paint brushes for specific purposes for painting there are different edges of shears that make them specifically better for slide cutting, point cutting, blunt cutting, texturizing and more. These will all have a different radius to make them more suitable for a particular purpose.
When sharpening, the sharpener should when possible maintain or reestablish the original radius. Sometimes due to damage to shear from previous sharpening, deep nicks or other issues it is not in the best interest of the longevity of the edge to restore the original radius. All of this means that the sharpener and his or her equipment must be able to follow the edge of the blade and can create any radius that is on the market and be prepared for the shears of the future.
For those who want to know more from the manufacturer’s point of view this is a video clip from a sharpening convention in which a USA scissor manufacturer answers to question of a sharpening concerning the 800 mm radius. https://youtu.be/bmvMe6SfKcoThe Bonika Firefly Shear has a different radius on each of the blades. The top blade has a radius of approximately 1200 mm (almost straight) and the lower blade has a low radius of around 600 mm (curved). In addition, the edge angle of the two blades are different as well. This shear is one example why a 800 mm radius would be inappropriate for this shear. Other shears like the Bonika Fishbone has a low radius to slide cut and point cut better while most German type shears like the Kretzer have a high radius for straight line precise cutting.
If you are looking for sharpening equipment please be sure you purchase equipment like the Scimech Scissor Flathone and others that can address all the different radii found on today’s shears from a 500mm to 1500mm as well as the 800mm. If you have equipment that will only sharpen a 800 mm radius my suggestion is to purchase another clamping system so that you have the flexibility to sharpen any type of shear or scissor on the market today and will appear on the market tomorrow.