The practice of putting sharp edges on cutting tools dates back more than two million years. That’s when we think genus Homo started using hammerstones to break off parts of a rock to form sharp cutting edges. Things have changed since then. Based on numerous YouTube videos devoted to the topic, knife sharpening has evolved into high art. Now, it’s reached the point where robots can do a respectable job of sharpening knives. You’ll likely find one of these robots at your local hardware store.

Back in my Cub Scout days, I discovered that the fastest way for me to dull an official Cub Scout pocket knife was for me to try to sharpen it with a whetstone. Neither of my parents knew how to sharpen cutlery, so I had no role models and no instructions. These days, I have an inexpensive 1×30-inch Harbor Freight belt sander with an assortment of sanding belts from coarse to fine grit. This $50 tool is a darling of the knife-sharpening YouTubers and allows me to ruin knives far more quickly that I can by hand. There’s a real art to knife sharpening and the rule of thumb is that you need to ruin several knives by attempting to sharpen them before getting the knack.

Imagine my surprise when a new knife-sharpening robot appeared in small town Utah at my local Ace Hardware. For $6.99 per knife, the Resharp machine puts a new “factory” edge on a wide variety of knives in a claimed 90 seconds. The Resharp robot is one of the most interesting embedded systems I’ve seen in a while. It combines electronics with mechanical, optical, and air-handling technologies in a truly unique way. Think of it as a smart, specialized, compact multi-axis CNC machine with vision-based (or optical) sensing that allows the tool to follow the arbitrary curves of any knife’s edge and a dust-collection system to sweep away the metal shavings created during the sharpening processes.

The automated Resharp robot is ready to automatically sharpen knives at a hardware store near you. Image credit: Resharp

The Resharp’s design reflects an elegance of thought in its seemingly simplistic design. An automated vise grips the knife. A “trained associate” (we’ll come back to this term later) opens a door in the front of the machine and places the knife on an open vise where it’s held temporarily by a magnet. The operator lines the beginning of the knife’s heel (the rearmost part of the knife’s sharpened edge) with a vertical green laser beam and then presses a button on the machine’s touchscreen, causing the vise jaws to close on the knife to hold it firmly in position. The Resharp machine needs to know where the knife’s heel ends so that it knows where to start sharpening.

At this point, the Resharp machine raises the vise that’s gripping the knife and then scans the knife edge’s profile using a multipurpose rotary head that rides along a linear axis running along the length of the knife to be sharpened. This rotating head contains two stations that bring scanning equipment, the green laser for heel alignment, helical grinders, and spiral elastomer wheels impregnated with polishing abrasive to bear on the workpiece. Rotating the head brings the correct station to bear on the knife. The head traverses the length of the knife edge and tilts to accommodate curves in the knife’s working edge.

A robotic vise keeps a firm grip on the knife during sharpening and moves the knife up and down to accommodate the curves of the knife edge during sharpening. The green laser serves as an alignment guide for initial knife placement. Image credit: Resharp

However, the Resharp machine’s rotating head does not move up and down to accommodate vertical changes in the knife edge’s location. Instead, the knife height is dynamically positioned by the vertically moving vise as the scanning head passes over the knife from heel to tip.

Once it profiles the knife’s edge along its entire length, the moving head repositions itself back to the blade’s heel and moves the helical grinding wheels into position. Two passes of the grinding wheels create an edge profile with a 24° bevel and a 36° secondary or micro bevel. The double bevel produces a sharp edge from the micro bevel with extra durability due to the wider main bevel. The Resharp machine then rescans the ground edge and regrinds it if the first pass doesn’t prove satisfactory. Because of the way the Resharp machine sharpens cutlery, with opposed grinding wheels, it cannot sharpen knives with serrated or scalloped edges although it will sharpen any smooth-edge blade with a two-sided bevel, if it fits within the prescribed overall dimensions for the knife.

Two helical grinding wheels put a bevel and a secondary or micro bevel on the knife during sharpening. Image credit: Resharp

The patent for the Resharp machine is intentionally non-specific about the technology used for scanning the edge. It suggests two methods: inspection by visual imaging and laser diffraction. Knife-edge diffraction and the resulting interference patterns turn out to be a whopping big topic in electromagnetic theory, so feel free to investigate it if it’s of interest. The obvious presence of specular red laser light around the Resharp machine’s scanning head during knife inspection suggests that the machine is using laser diffraction for profiling the knife edge and measuring edge sharpness.

Once the Resharp machine has ground the knife edge to its satisfaction, it polishes the ground edge with a pair of spiral wheels made from an abrasive-impregnated elastomer. I’ve bought similar polishing wheels for my Dremel rotary tool, so it was easy to recognize what’s being used in the Resharp machine. Once the machine is satisfied with the resulting edge, it parks the moving rotary head and signals the operator to remove the sharpened knife. The entire process, from pressing the “start” button to extracting the sharpened knife, truly takes only a couple of minutes per knife.

I took three knives and a cleaver into my local Buck’s Ace Hardware store in Santa Clara, Utah one recent Saturday. It’s a very friendly store with plenty of helpful people working in the store. It’s just what I want in a hardware store. I told the first person I met that I wanted to sharpen some knives. We went to the Resharp machine, which is in the front of the store near the entrance. The store employee first picked up my cleaver and measured it using the rulers pasted to a pull-out chart on the machine. The cleaver met the machine’s size limitations, so he opened the machine’s front door and placed the cleaver in the vise using his left hand.

Then, he needed to close the door of the machine and press the “start” button, which is located on the far left of the machine’s touchscreen, on the left side of the front panel. This positioning forced the employee to cross his right hand under his left hand to press the button while his left hand held the knife in place. Once he pressed the button, the vise jaws closed and gripped the knife. This sort of handedness problem crops up frequently in user interface (UI) design and, as a left-hander, I run into these kinds of problems frequently. The Resharp machine was clearly designed for right-handed people. Once the machine had a good grip on the cleaver, it passed the scanning head over the edge of the cleaver and rejected it (more on that later), so we moved on to the knives.

The Resharp machine made quick work of the three knives, and I studied its operation while it sharpened them. When we’d finished sharpening the three knives, I realized that we’d not properly aligned the cleaver in our first attempt, so I asked the store employee to try again. I’d noticed that when he inserted the cleaver during our first try, the handle was sticking so far back that it prevented the machine’s door from closing, and he’d nudged the knife forward and reclamped it in the vise. I advised the employee to move the cleaver further forward and to line up the back of the cleaver’s edge with the green alignment laser. The cleaver’s new positioning didn’t interfere with the front door of the machine and the machine accepted the cleaver after scanning it. Two minutes later, we pulled a sharpened cleaver from the machine.

I wrote earlier that we’d get back to Resharp’s claim of the “trained associate.” A store like Ace Hardware contains tens of thousands of stocked items, and no employee can be trained on the particulars of every item and every service on offer. In addition, my local store periodically adds new employees, and I don’t assume they all get training on the Resharp machine as soon as they start. The solution here, I believe, is a better user interface that provides better instructions.

My online research on the Resharp machine took me to the Website of Jeff Kastenbaum, who joined Resharp in early 2018 and served as the company’s Director of Engineering through 2021. Kastenbaum, who is a mechanical engineer, was the first non-founder to join Resharp. He took the lead in developing an impressively compact design for the overall machine. The retail version that I saw was the machine’s third generation. The first two generations had limited placements in the San Francisco Bay area. 

I spoke with Kastenbaum via Zoom, who agreed with me that the Resharp machine is indeed basically a 3-axis CNC machine with some auxiliary axes, including the adjustable knife vise and the positioners for the helical grinding wheels. Kastenbaum told me that a Raspberry Pi controls the overall operation of the Resharp machine. It also provides WiFi connectivity, to report each successful sharpening (the machines are placed in stores with a revenue sharing arrangement) and for remote diagnostics and debugging. The Raspberry Pi also operates the machine’s user interface and manages a microcontroller that provides real-time control of the stepper motors (augmented with optical encoders for positioning feedback) that position the machine’s multi-station head and the knife vise. 

Similar small CNC systems don’t need a lot of electronics support in terms of computing power. For example, my 3-axis (4-axis-capable) Two Trees TTC450 CNC machine incorporates an Espressif ESP32 microcontroller module, which manages the CNC’s stepper motors, the spindle motor, limit switches, and the touchscreen UI. 

According to Kastenbaum’s site, more than 900 Ace Hardware stores now have Resharp machines, and I see from a YouTube video that my old independent hardware store haunt, McGuckin Hardware in Boulder Colorado, also has one. The Hillman Group in Cincinnati purchased Resharp in August 2019 and integrated it into the company’s Robotics and Digital Solutions segment. The Hillman name may be familiar to you because they furnish a lot of the nuts, bolts, screws, brackets, and other metal bits and bobs found in most of the major US hardware chains including Ace Hardware, Do It Best, Home Depot, Lowe’s, Tractor Supply, and Walmart. The company’s Website says it has 114,000 active products, which includes personal protective equipment such as work gloves, kneepads, and safety glasses. The Resharp machine fits into Hillman’s kiosk product group, which currently offers commercial robotic kiosks for key duplication (Minute Key) and pet-tag engraving (Quick Tag) in addition to the Resharp knife-sharpening kiosk.

I took my freshly sharpened knives home from Ace Hardware and tried the chef’s knife on a handy tomato sitting on the counter next to the wooden knife block. The chef’s knife sliced through the tomato cleanly and effortlessly. I then noticed a spotty black sludge on the newly sharpened knife edges, obviously left there by the Resharp machine. It took some vigorous scrubbing to get the residue off the newly sharpened cutlery. You need to be careful when you clean these knives. They are sharp! To be fair, the safety orange cardboard protective sleeves slipped onto the sharpened knives at the hardware store advise you to wash the knives before using them. However, that warning is in a tiny, unbolded font on the back of the sleeve, just below the warning “caution: seriously sharp knives” printed in boldface and just above the barcode. Meanwhile, the front of the protective sleeve proclaims “Factory sharp without the factory” in a gigantic bold font.

Does Resharp’s knife sharpening meet with the approval of professional and experienced knife sharpeners? Absolutely not, if you believe the Internet traffic. Hand sharpening with professional tools takes a long time, much longer than the 90 seconds taken by the Resharp machine. If you look at the edge finish of a knife sharpened by the Resharp machine with a microscope, it will not look as refined as a professional hand-sharpened edge made with a whetstone, diamond stone, or a belt sander with multiple sanding and conditioning belts. However, some non-purists will admit that the Resharp machine creates knife edges that are 90% as good as a hand-finished edge, which is much better than I can do with my Cub Scout training. Score another one for the robots.

References

Jeff Kastenbaum’s Web site

Hillman Solutions Corp Prospectus, August 27, 2021

Gary Zinn, Micro Bevels are a Knife Sharpener’s Best Friend