Introduction to knife sharpeners
Knife sharpeners remove material from dull knives to create new bevels and new cutting edges. When they're designed and used with the best of them they create razor sharp results, the first time.
As soon as you start using your knives they start to dull. It's kinda like the tires on your car. The harder you drive the balder they get. So also with your kitchen knives. The harder you use them, the duller the get.
Your best knife likely has the best edge on it. A knife sharpener put it there by removing material and crafting a sharp knife edge from a dull knife blade.
The best part of a pull through knife sharpener (as an electric sharpener or otherwise) is the first sharpening. After that the results diminish and the shape of your cutting edge becomes more and more difficult to maintain.
Intro to pull through sharpeners
While there are stones, belts, wheels and diamond rods to use to sharpen knives there are also scrapers sharpeners that can assist the sharpening process. A scraper knife sharpener is typically crafted of two carbide edges (or stone/ceramic wheels) set at an angle. The carbide is harder than your knife material and thus can coarsely remove material and "sharpen."
These pull through sharpeners, however, rarely return a knife to its original sharpness. Infact, they frequently ruin edges over time. Why?
Your pull through knife sharpener becomes rounded out
That makes sense. The sharp edge of the manual pull through sharpener wears under use, like a knife does. As the angle of the knife blades pass by the sharpening metal, small steel bits scrape off. That's good. However, in the same motion smaller bits of of the sharpener scrape off too.
Over time your sharpener becomes a "rounder" tool. The groove at the very tip of the sharpener wears and rounds out. This creates a rounded edge on your knives as well. In the worst cases these sharpeners actually dull knives vs sharpening them.
The apex of your cutting edge gets shaved off
Because of the way pull through knife sharpeners are constructed, both electric knife sharpeners and those that are unpowered (i.e. manual), the very tip of your cutting edges run into the sharpeners in a blunt way. This is different than other sharpening methods where only one bevel or edge is worked on at a time.
With two simultaneous pieces of material in contact with the dull knife at the same time the burr, something essential to sharpening, can only get so big. Thus even the best electric knife sharpener, capable of putting more pressure into a sharpening, will make a semi sharpened edge and leave a coarse sharpness of lower quality than other methods.
All of these will also commonly grinding off the very point of your cutting edge, the apex.
There is no way to thin the knife behind the edge
Professional knife sharpeners have invested time and skill into the craft. They also have a wide set of tools to assist with crafting straight durable edges to knives. Part of that process is to thin the blade behind the cutting edge.
Scraper sharpeners, used to slide a dull knife through them, are not equipped to maintain the performance of a knife over time by "thinning the knife." This lack of performance will come because a knife's cutting edge will move toward the spine with each sharpening and the blade is thicker toward the spine. Scrapers are only equipped to work the edge of the blade, not the sides.
You'll likely build a dip at the heel and maybe at the tip too
The profile, from the side, of your knife will likely change by sharpening with a scraper style manual sharpener. At the tip of your blades you're bound to see a flattening. At the heel of your blades you're bound to see space created under the blade in a kind of arch.
This will happen with both hand powered or electric sharpeners of the scraper style.
Four reasons scraper sharpeners are tough on knives
In review, while these sharpeners can be purchased for just a few dollars they also create lackluster results. Knife sharpeners of the manual or electric knife sharpener styles are 1 not equipped to maintain a uniform thickness of your blades.
2. They are typically gouge your edges at the heel or the tip adding a dips or bows or the like to your knives over time.
3. They have a hard time building a substantial burr on the knife and therefore struggle to make them razor sharp.
And finally, 4. over time think twice about putting your best knife in sharpeners of this style. These are bound to wear out and actually dull your knives by rounding them vs sharpening them.
Note: Interested in extending your sharpness? Read up on how to use a hone. Honing every time you use your knifes could seem like overkill. However, the practice of pushing the slicing edge back straight before use can save a lot of time and deliver a a much more superior and enjoyable cutting experience.