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A high-RPM machine is a machine that operates at a high rotational speed. For example, a bench grinder features a grinder wheel that spins super fast. While these are touted for removing scratches and sharpening knives, they aren't really made for kitchen knives. That's because the rapid speed heats up the steel in the blade, making it brittle and more prone to shattering while sharpening. Karl Frentz explained that another problem with high-RPM machines is that they grind away more material than necessary, drastically reducing your lifespan.

"You can get a sharp knife with this method, but the problem lies with the high RPM," Frentz said. "This creates sparks, which is, of course, dangerous, but you also run into other problems that ruin the integrity of the edge. You will end up softening the steel, so the edge will be sharp, but, at the same time, it will go dull much faster. It is also extremely difficult to get a precise angle since you have to do this free hand and keep it stable while fighting against the high RPMs."

When it comes to sharpening kitchen knives, rotary grinders have similar issues as bench grinders. The grinding wheels are smaller, but they still heat up the blade, remove too much steel, and produce force that you have to fight against to maintain a precise angle for the blade. That's because these handheld tools are developed to run at high RPMs, whether they're corded or cordless.

Although rotary tools with variable speed options are made for sharpening, polishing, and cleaning all kinds of materials, they're designed more for carpentry, plumbing, and DIY tasks than for maintaining kitchen utensils at home. "Rotary grinders work but are best wielded by a professional who knows the proper way to grind the edge without ruining the temper of the steel," Eric Rowse commented.

Angle sharpeners are designed to provide consistent results at specific angles. They all feature a jaw-type clamp to hold the knife in place while you use a stone clamped to a guide rod to sharpen the blade. While they work in respect to sharpening kitchen knives, one of the biggest problems is that knives have various bevel angles. Karl Frentz elaborated, "For example, if you want to sharpen a butcher knife, these are typically sharpened at 18-22 degrees. If you are using a set angle sharpener and the only option is 15 degrees, you are now changing the knife's angle, and it won't work as it was intended."

Even if you have an angle sharpener that allows you to adjust the angle, the clamps may not be strong enough to hold your knives so securely in place that they don't move at all. If the clamp can't be tightened enough, the blade will move inside of the clamp while you're sharpening it, messing with the angle of the bevel.

On top of that, your own safety could be at risk because of the direction in which the clamps hold knives — with the sharp sides facing outward. Frentz noted that, typically, angle sharpeners leave "the knife exposed while sharpening and involve movements that put you at risk of cutting yourself." He went on to mention another reason why these sharpeners are risky. "A lot of these sharpeners have the knife set in place, and you have to manually roll/move the diamond plate along the edge with your hand," Frentz continued. "This can be unsafe as one wrong move could lead to an injury."

Invented in 1993 by Otmar Horl, the roller sharpener has been manufactured by numerous companies and become prominent on social media. It consists of a magnetic block to hold your knife in place — usually at various angles on each side — and a cylinder with abrasive stone attached to each end that you roll against the blade. Despite the angled magnets, though, it's difficult to achieve a consistent sharpness on the blade because the angle can change easily. The biggest problem is that the magnets aren't always strong enough to prevent your knife from moving out of place.

Also, according to Eric Rowse, roller sharpeners "are just glorified diamond steels that add micro serrations to the blade (just much nicer and cleaner than a rock), which does work to an extent, but without a polishing stone or wheel, the edge will dull fairly quickly." He said that these roller devices don't work to sharpen a serrated knife or bread knife either. On top of that, the diamond coating on the stone ends doesn't last forever, and the cost of replacing it can be expensive in the long run.

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Among all the knife-sharpening tools available, you've probably seen pull-through sharpeners more often because they've been around for quite a while. Most of them have multiple slots, each with a different angle for the various bevels of knives. You use them by simply putting your knives into the appropriate slots and pulling them back and forth with even pressure for consistent sharpness.

However, Eric Rowse said that the slot angles aren't always sufficient, explaining, "These can be problematic for any knife that doesn't have a twenty-degree Western-style bevel or edge angle. If you had a knife that came from the factory with a 15-17-degree bevel, the pull-through sharpener would ruin the edge unless you just keep running the knife through it until it changes the edge geometry to 20 degrees." He added that another concern when using pull-through sharpeners is that tiny bits of steel can be left in the tool and on the blade, potentially ending up in your food.

Karl Frentz doesn't believe pull-through sharpeners produce the best results either, saying, "It is extremely difficult to apply even pressure as you are pulling through the sharpener and because of that, you aren't sharpening the entire bevel of the knife. This will cause inconsistencies along the blade and will leave you with an underwhelming edge. If not used correctly, pull-through sharpeners can take chunks out of the blade as you pull through." When you see small bite-like marks along the edge of the steel, he continued, the only way to fix it is to grind the material past the indentions in an attempt to restructure the bevel.

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So, if you've used the kitchen knife hack of slicing tomatoes or a piece of paper to test knife sharpness and found that the blade is dull, what should you use to bring the steel back to life? Eric Rowse prefers to use whetstones to sharpen his knives unless he has to grind out chips or sharpen multiple knives at once. For that, he outsources to a professional. He also noted that some electric home sharpeners might be worth a try as long as you can adjust the angle.

Since using a whetstone — like the Sharp Pebble Premium Whetstone Knife Sharpening Stone — takes skill, time, and effort, Karl Frentz recommends a sharpening machine with a low RPM setting so that it doesn't heat up the steel blade too much. The Tormek T-1 Kitchen Knife Sharpener, for example, has an adjustable bevel guide between 8 and 22 degrees and a slow speed of just 150 RPMs. By comparison, most bench grinders range between 3,450 and 3,600 RPMs, while rotary tools generally have a speed range of 5,000 to 35,000 RPMs.

"[The slow speed] is crucial for not only getting a sharpened knife in the moment, but also maintaining the longevity of the knife," Frentz explained. "You also want to be able to sharpen the knife at the angle it was intended to be sharpened at. You shouldn't sharpen sushi knives at the same angle as your butcher knives since they serve different purposes."