Ultrasonic Chef's Knife

Well, you have a knife, and it vibrates, so your claims to this extent are factually correct. However, in the video linked by the o.p., there are several exaggerated or misleading claims or images. For instance, at 1:41 of that video, there is a pair of SEM images showing blade edges that are severely rolled (left image) and blunted (right image). Since there is no scale on the SEM we can’t see what the extent of the deformation is but the narration indicates that these are typical conditions for a kitchen knife in normal use. In fact, the left image is clearly not a kitchen knife (apparent from the profile of the blade behind the edge) but likely a fine scalpel abused in a way that such an instrument should not every be used. The right image has the correct profile for a heavy kitchen or utility knife but the blunted shape of the edge is not from normal use; that has clearly been pushed edge down onto a harder substrate such as glass. In normal, non-abusive use, the edge of a well-honed kitchen knife used for slicing and chopping will first roll at the very edge and then wear away, leaving a more rounded profile which is perceived as gradual dulling.

Second, in the discussion of how the piezo elements cause the blade to vibrate, there is an animation (starting at 2:33) showing the response of the blade. The narration and the note at the bottom right corner does indicate that the animation is exaggerated but doesn’t offer a scale factor, and the motion of the animation shows the blade flexing and waving around with the tip deflecting more than 1/2 inch up and down, giving the impression to an unsophisticated viewer that this is the mechanism by which the blade is effecting cutting. The reality is that the deflection is scaled by at least a factor of 100000X or more, and the actual benefit that comes from vibration is preventing food from sticking to the face of the blade.

The video showing the cutting of the tomato (starting at 3:33) compares the cutely named “J. Robot Choppenheimer” cutting a tomato with a ‘dumb knife’ versus the Ultrasonic Chef’s Knife by just pushing the edge down into the fruit. This is a very misleading comparison as this isn’t the way any reasonably experienced cook would try to slice a soft-bodied fruit like tomato; they would push the knife forward or (or draw it back) to facilitate cutting through the comparatively tougher skin. It is also much easier to do this with a fillet knife with a narrow bevel rather than a broad-faced chef’s blade which is actually intended primarily for chopping and mincing; in fact, a sharp fillet knife will cut its way through a ripe tomato just on the weight of the knife alone and produce slices that no thicker than cardstock.

As I noted above, the vibration may indeed help to prevent food and residue from sticking to the blade but that is a ‘problem’ that most cooks learn to cope with procedural steps, i.e. by using their non-cutting hand to remove and organize cut material. The notion that one should use the vibratory motion vice learning the simple task of quick honing and occasional full-edge resharpening with a flat stone is basically trading learning a basic skill (which, it should be noted, is still necessary for all of the other knives needed for applications where this chef’s knife doesn’t serve) for a convenience of an at-best questionable practical utility. This is a $400 ‘solution’ to a problem that doesn’t really exist for the home cook who is more into gadgets than learning actual craft of cooking. And frankly, as someone who has spent many, many hours prepping hundreds of pounds of produce and meat for cooking at mass scale, I would far rather have a ‘dumb’ knife with good ergonomics and balance that I spend a minute or two honing for every several hours of use than a big, blocky-handled cutting gadget that is likely to run out of power or just stop working when it is most inconvenient.

I’ve spent several years as a cook working in commercial kitchens using all kinds of kitchen cutlery, and going on three decades as an engineer doing mechanical design, structural and dynamic analysis and testing, and mechanism synthesis, including the design, analysis, and qualification testing of cutting, severing, and penetrating devices for aerospace, military, and emergency rescue/extraction applications. I’ve owned a lot of knives for cooking and other applications (including some approaching the price of this gadget, albeit for the steel, balance, and quality of handle materials) and to me this product seems like a gimmick with little practical benefit for the cost and promoted with misleading comparisons.

Stranger

Those pull-through sharpeners are garbage, even the ones from well-known manufacturers which should know better than to promote them. With care a ceramic or diamond-coated honing steel can help touch up the edge between uses but depending on usage (but at least every few months) they should be retuned on a fine grit sharpening stone. You can get inexpensive diamond-coated bench stones like this that tuck away into a drawer and will retune the edge to a kitchen knife (even a cheap one, which you can use to practice and improve technique) with just a few light swipes on each side as long as you don’t abuse the knife by blunting or taking chunks out of the edge. (But don’t tell your friends and family because once they learn that you know how get knives to hair-popping sharpness they’ll be constantly nagging at your to sharpen their knives.) If you have a knife that needs more work (because, purely as a hypothetical example, someone took a $200 santoku out and used it to prune a plumb tree, and the edge needs to be completely reprofiled), then you’ll probably want to use something like this hand belt sharpener, which can also serve to sharpen scissors, axes, lawnmower blades, and other cutting implements that don’t sharpen easily on a bench stone, but you do have to be careful to not inadvertently profile off the tip.

To be fair, if you go far enough into the video the promoter does say that the knife can be resharpened with a honing steel or bench stone but the implication from the start of the video is that the benefit is that you really don’t have to sharpen it because the vibratory action will do all of the cutting. And realistically, typical users will probably tire of this gadget long before it actually gets so dull that the average home cook realizes that it does, in fact, need to be sharpened (which, in my experience, is not until you actually have to look at the bevel to figure out which side is the edge and which is the spine).

Stranger

I’m not sure if the word “garbage” is exactly right, but basically, yes, I agree. The pull-through sharpeners really don’t work very well at all. The sharpening steel is better. But to those who say it doesn’t really sharpen, it just straightens little micro-dents, I beg to differ. After about a dozen good strokes I can run a paper towel over the steel and pick up tiny dust-like metal filings. But I’ll have to consider a sharpening stone as you suggest.

Yes, just like any knife, the edge will wear over time. And, like any knife, it performs best when the cutting edge is sharp. However, there’s still a reduction in cutting force when the knife is on, even when the cutting edge is dull. That’s because a cutting is both about cut initiation (biting into a tomato skin) and cleaving (pushing through the middle of a potato, or a block of cheese). The former is all about the bevel edge, but the latter is about geometry and friction. And even if the ultrasonic knife is completely dull, you still gain the benefit of reduced friction on the blade face.

You can see these phases of cutting revealed in the data when you measure cutting forces directly. If you’re curious, I did a whole project around this; https://youtu.be/GUQy0Sdp8Hc

Dude, I cite all my sources in the video. Also, you’ve spent decades in engineering and it’s weird to you to see an FEA simulation with an exaggerated scale? That is so common that it’s the default. I then go on to show actual movement of the blade under a microscope and state the 10-20 micron amplitude. So the claim that I’m presenting misleading information just doesn’t hold up.

I’m not going to rebut your statements individually. You’ve clearly made up your mind that I have I’ll intent and are trying to find evidence to support your position.

I observed, as noted in my post, that no scale for the animation even though that is typically presented in the plot (which is actually required in our command media for presentations, and shows up by default in a displacement animation in HyperView, my preferred finite element post-processor). Without that, the uncritical viewer might really be impressed by what the “piezoceramic [sic] crystals” are doing to cause “… the metal of the blade itself stretches and contracts …” (your emphasis) apparently gyrating wildly for what is actually just very slight normal modal vibration, which if you animated a driving down the street at the same scale would make it appear to be in some process of constant explosion. In fact, the blade should have three or maybe four modes of vibration from base-end excitation (one lateral, one or two torsional, in addition to the up-and-down mode shown in the video) of which the presented one is almost certainly the smallest displacement but looks most impressive while describing the enhanced cutting action of the knife.

Sure, that’s your privilege. The o.p. solicited opinions about the product based upon the claims and price and I offered my view as both someone who has previously worked as a cook and an engineer who works with structural dynamics and cutting systems as well as someone who is just general skeptical about kitchen gadgets which ‘solve’ a problem that is (in my view) created to sell that gizmo. I certainly don’t have the last word in opinions, and for what it is worth I don’t think you have ‘ill intent’; I think you’ve invented a product and are really enthusiastic about it, but I don’t share your fervor or conviction of its value. There are other professional cooks and experienced home cooks here who may have different opinions to share but you have to realize that when you put out a product and make bold claims on the internet you’ve going to get critical questions and skepticism that require a thicker skin to persevere.

Stranger

I know that this is a brand new product, but have you loaned or given at least a few of these knives to disinterested 3rd parties or even friends and family? I would love to see any feedback that you’ve gotten so far.

It’s still pre-production, and I’ve just gotten the first few units that will be distributed to independent reviewers.

Hey, welcome to the straight dope message board, @seattlefoodgeek . We can be abrasive, but it’s a good group of people. Check out the other forums, too. I hope you enjoy your time here.