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Determining sharpness of sheet metal edges

Metal fabricators need to be proactive about safety in the design phase 

Determining the sharpness of sheet metal edges

Fabricators need to learn how to navigate standards and tests to ensure product safety when it comes to sharp sheet metal edges.  Gettty Images

Only a few modern standards exist to define sharpness, whether in products that are supposed to be sharp or those that aren’t. Manufacturers usually use these few standards or proprietary ones, but it’s hard to find a balanced, cost-effective way to deal with sharpness.  

When sharpness is not desirable for safety reasons, you should follow a hierarchy for making safe products, as defined in ISO 12100.2010: 

  1. Design out hazards. 

  2. If that can’t be done, protect against them. 

  3. If that isn’t possible, warn against them. 

Often manufacturers give up too easily on the first two and settle for warnings. These can either be placed in the manual or on product labels, and supposedly pass the risk on to the consumer. If there is an injury, you as the manufacturer may blame the consumer for not heeding warnings and misusing the product. While this may be true, if a product liability lawsuit ensues, you need to be able to prove it to the jury. 

This is best demonstrated if your design process largely follows ISO 12100.2010 and you tried to follow the hierarchy but found no reasonable way to achieve the first two methods. Be sure to include a reasonable, objective, and documentable safety review as part of your design documentation.  

While it would be great if consumers could be relied upon to read product manuals and safety warnings, this should be the last defense. Often, depending on the product, consumers have come to expect that common products are inherently safe. 

Standards and Testing Methods 

There are few published standards for defining sharpness. Some of these are intended for products that are supposed to be sharp, and some were written to prevent products from being sharp. 

In the latter case, UL Standard 1439 was an attempt to address unintended sharp edges on products. It was first issued in the early 1970s; since then several other, similar standards have been issued by other organizations.  

Determining the sharpness of sheet metal edges

Figure 1
A sharp edge used on a small contact area will result in higher stress than on a large contact area, even with the same force. 

UL 1439 was intended to provide a standardized method to test for sharpness, adding that “an edge for an enclosure opening, frame, guard, knob, handle, or the like shall be smooth and rounded so as not to cause a cut type injury when contacted during normal use or user maintenance.” It called for pressing and sliding product edges over special layers of adhesive tapes under a controlled contact force of 1.5 pounds. If the layers could be cut by the edge, then it was considered sharp. A Sharp Edge Tester device was developed to hold the tape and control the contact force, and versions of it are still sold today. 

Manufacturers of knives and razors test for sharpness with other tests, many of which are subjective. A frequently quoted standard is BS EN ISO 8442.5:2005, which is similar to the National Institute of Justice specification NIJ Standard-0115.00. These standards can be used to test the sharpness of objects that are supposed to be sharp. Test equipment is available that employs silicone rubbers and cards as the testing medium while measuring the contact force and penetration criteria.  

Some manufacturers of razor blades use optical equipment such as laser interferometers to measure the sharpness of their own blade edges. In addition to these controlled methods, other low-tech methods often are used, including cutting into man-made materials and looking for a reflective light glare along the cutting edge. Of course, these are of questionable use for identifying hazards to consumers.  

Another related National Institute of Justice standard is NIJ Standard-0101.05 for testing the effectiveness of body armor against stabbing penetration by sharp objects. The standard covers stabbing action only and mentions the need for “simulant flesh” materials, such as clays, but it gives little detail on how to determine or specify desirable material properties.  

Since the process of cutting involves bringing a material to its failure stress level, you have to consider how to control the stresses in the material before applying the test blade. A material that is stretched tight before a razor is applied to its surface will fail more easily than a material that is not. On looser material, the blade must produce all the stress and therefore must be applied with a larger contact force.  

Some tests are conducted simply to determine relative sharpness, such as testing two edges to see which one is sharper. For this type of testing, low-cost materials such as polymers, rubber sheeting, clays, foams, chamois cloth, and felts can be used.  

Stress and the Cutting Process 

Materials, including skin, fail when they are contacted by an object that exceeds a certain stress.  

Stress is a measure of a force when it is applied over an area. Stress is what matters in sharpness, because even a large force will produce a low stress if it is spread out over a large enough area, and vice versa. Depending on the contact area, you can have the same force and have a different stress.  

The difference between force and stress is seen in the use of snowshoes. When someone walks on snow, their weight (a force) presses against the snow, creating a stress. The bigger the snowshoe, the lower the stress, and the less sinking into the snow.  

This is the same effect that makes knives sharp. In Figure 1, the forces applied to two objects are distributed over an area, which reduces the contact stresses on the surface. 

Determining the sharpness of sheet metal edges

Figure 2
Manufactured products can have many kinds of sharp edges. Small changes to the edge, such as chamfering a square corner, can reduce the contact area and thus the sharpness.  

Mechanical cutting methods—slicing, sawing, chopping, shearing, abrasive cutting—typically employ a hard, sharp object to generate some combinations of axial and shear stresses to produce cutting action.  

Figure 2 shows several theoretical sharp edges that can be seen on manufactured products. On these, small changes to the edge, such as chamfering a square corner, can reduce the contact area, and thus the sharpness. But there are also nonintuitive sharpness hazards, such as the edges of sheared sheet metal. They can pose a greater danger than cutting blades because the danger is hidden.  

The difficulty in determining if product edges are safe is not a license to manufacture an unsafe product. While the difficulties should not be underestimated, neither should the ramifications of avoiding the problem. Severe injuries have resulted when manufacturers underestimated a dangerous edge.  

PPE Testing 

Given the lack of appropriate standards for testing products for sharpness in high-force situations, engineers can consider novel approaches to testing product edges for safety. These tests also can be used to determine the effectiveness of personal protective equipment (PPE). 

Hand protection is outlined in the U.S Department of Labor’s OSHA Regulations 1910.132 and 1910.138. The latter mandates that employers select and require employees to use “appropriate” hand protection when their hands are exposed to hazards, to protect them against cuts and lacerations, punctures, and abrasions. In addition, 1910.132(b) stipulates that even if employees bring their own PPE equipment, the employer is responsible for its adequacy, maintenance, and sanitation. 

The most prevalent standard for testing and rating the effectiveness of work gloves against allowing lacerations is ANSI/ISEA 105-2016, which is a voluntary standard. It creates a pass/fail criteria for work gloves based on tests that run sharp blades across glove materials under a controlled force. The blades are replaced between each test to ensure consistent sharpness and results. If the gloves do not get cut through, they can be rated for the weight that was applied.  

Suppliers of safety gloves are expected to test and rate their products to these standards. These should be reviewed with care, as the ASTM test for this standard is for sharp edges, and there is a separate ASTM standard for abrasion resistance. 

Designing for Safety 

Seemingly innocuous products can cause debilitating injuries. Often these could have been avoided with a simple deburring operation. Obviously, the manufacturers did not intend for these injuries to happen, and of course manufacturers cannot foresee extreme misuse of their products.  

The solution is a comprehensive object safety review process as part of the design stage. While you cannot predict every future situation, you can develop a process that attempts to do so and that takes as much responsibility off the consumer as possible to prevent injuries. 

In one case, a manufacturer made an effort to make the product safe but left one sharp edge that was exposed during assembly. Rather than deburr the sharp edge, the manufacturer recommended that consumers use safety gloves. The problem was that the product could not be assembled with safety gloves on—a fact that was confirmed by one of the engineers in a deposition. The manufacturer knew there was a risk in not deburring the sharp edge, and it cost the plaintiff, a 30-year-old, right-handed assembly technician, the use of his right hand. The sharp, burred sheet metal edge cut through his palmer finger tendons, and the jagged edge splintered the tendon ends, preventing them from being reattached.  

Determining the sharpness of sheet metal edges

Figure 3
A sheared sheet metal edge has several attributes that make it sharp. The edges may appear flat, but shearing dies leave burrs.  

Many products use sheared sheet metal, yet only one standard, UL 1439, addresses the issue of sheet metal sharpness directly. The other sharpness standards exist to define when products that are supposed to be sharp are sharp enough, or to rate products that are intended to protect users from sharp products.  

Obviously, it would be difficult to devise a single standard that protects consumers from the wide variety of available sharp products. But this does not let manufacturers off the hook. In the absence of a relevant standard, they must address product safety on a case-by-case basis based on their existing and extrapolated knowledge of their product and their industry.  

Sheet metal manufacturers know about sheared-edge hazards, and special precautions for handling are well-publicized in the industry. But many of the  industry precautions are anecdotal. For example, a “sharpness test” recommended by one manufacturer’s group calls for running a “toothpick or other object” over the edge, without specifying a contact force. This makes such a test nonreproducible and subjective.  

According to “Preventing injuries from the manual handling of sharp edges in the engineering industry” (www.hse.gov.uk/pubns/eis16.pdf), a publication of the U.K. government agency Health and Safety Executive, the best way to prevent injuries from sharp edges is to design out, remove, or cover the sharp edges. According to the paper, “the use of gloves should be a last resort.” Websites for metal industry organizations also acknowledge the dangers of sheet metal and offer test suggestions.  

As Figure 3 shows, a sheared sheet metal edge has several attributes that make it sharp. The edges may appear flat, but shearing dies leave burrs. While they may not be razor-sharp, they pose a significant threat, especially if the dies are worn or improperly set up.  

The product in the photo, made of 0.016-inch-thick sheared sheet metal, was released to the public. The manufacturer felt the edge was not a hazard because it was protected during normal use; however, the edge was not protected during assembly, which is when the injury occurred. In this case, a consumer lost the use of several fingers permanently.  

The manufacturer in this case took steps to prevent the sheet metal injuries but did not determine they were needed on this particular edge. While the sheet metal was very thin, making most standard deburring methods unsuitable for use, the manufacturer could have made the product safe in other ways—methods it used on other parts of the same product. However, the manufacturer ignored the sharp edge because “it was only exposed during assembly.” 

Product design processes should include safety reviews that cover every conceivable product usage scenario to identify and quantify risks and design criteria. Engineers should drive this process while relying on ongoing feedback from sales, customers, and manufacturing. This is the best way to determine what design changes are needed, including for product safety.  

Safety at What Cost?  

Sometimes as a manufacturer, you face a trade-off between safety and cost. You need to define the risks, and then decide how to address them.  

The costs of product safety can seem to conflict with profits. Yet the costs of deburring a part can be lower than the cost of settling an injury claim or suit. 

Victor Alexander Popp, PE, is a principal of VPOPP Inc., vpoppinc.com.