Determining sharpness of sheet metal edges (2024)

Only a few modern standardsexist 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 desirablefor safety reasons,youshould follow ahierarchy for making safe products, as defined inISO 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.

Oftenmanufacturers 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 onto the consumer. If there is an injury,you as themanufacturermay blame the consumer for not heeding warningsand 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 ifyourdesign process largely follows ISO 12100.2010and you tried to follow the hierarchy but found no reasonable way toachievethe 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 thenseveral other,similar standards have been issued by other organizations.

UL 1439was intendedto 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 calledfor pressing andsliding product edges over special layers of adhesive tapes under a controlled contact force of 1.5pounds. 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 issimilar totheNational Institute ofJustice specificationNIJ Standard-0115.00.Thesestandards 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, theseare of questionable use for identifying hazards to consumers.

Another related National Institute of Justicestandardis NIJStandard-0101.05for testingthe effectiveness of body armor against stabbing penetration by sharp objects. The standard covers stabbing actiononlyand 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,youhavetoconsider how to control the stresses in the material beforeapplying the test blade.A material that is stretched tightbefore a razor is applied to its surface will fail more easily than a material that is not.On looser material, the blademustproduce 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 aspolymers, rubber sheeting, clays, foams, chamois cloth, and feltscan be used.

Stress and the Cutting Process

Materials, including skin, fail when they are contacted by an objectthatexceeds a certain stress.

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

The difference between force and stressis 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.InFigure 1,the forces applied to two objects are distributed over an area, which reduces the contact stresses on the surface.

Mechanical cutting methods—slicing,sawing, chopping, shearing, abrasive cutting—typicallyemployahard,sharp object to generate some combinations of axial and shear stresses toproduce cutting action.

Figure2shows 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 notbe 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 productedges 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 1910.132and 1910.138.The lattermandates that employers select and require employees to use“appropriate”hand protection whentheirhands are exposed to hazards, to protect them againstcuts and lacerations, punctures, and abrasions.In addition,1910.132(b) stipulates thatevenif 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 createsapass/fail criteriafor work gloves based on teststhatrun sharp blades across glove materials under a controlled force. The blades are replaced between each test to ensure consistent sharpness and results. If the glovesdo not get cut through, they can be rated for the weight that was applied.

Suppliers of safety glovesareexpected 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 productscancause debilitating injuries. Often these could have been avoided with a simple deburring operation. Obviously, the manufacturers did not intend for these injuriesto happen, and of course manufacturers cannot foresee extreme misuse of their products.

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

In one case, amanufacturermade an effortto 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—afact thatwas 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-handedassembly 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.

Many productsusesheared sheet metal, yet only onestandard,UL1439,addresses the issue of sheet metal sharpness directly. The other sharpness standards exist to define when productsthat are supposed to be sharp are sharp enough, or to rate products that areintended to protect users from sharp products.

Obviously, itwould be difficult to devise a single standard thatprotects consumers from the wide variety ofavailable sharpproducts.But this does not letmanufacturers off the hook.In the absence of a relevant standard, they must address product safety on a case-by-case basisbasedon their existing and extrapolated knowledge oftheir 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 theindustry 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, withoutspecifying a contact force. This makes such a testnonreproducible 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 agencyHealth and Safety Executive,the best way to prevent injuries from sharp edgesis 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.

AsFigure3shows, a sheared sheet metal edge has several attributesthatmake it sharp. The edges may appear flat, but shearingdiesleave burrs. While they may not be razor-sharp, they pose a significant threat, especially if the dies are worn or improperly setup.

The product inthephoto, 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 consumerlost the use of several fingers permanently.

Themanufacturer in this case took steps to prevent the sheet metal injuriesbut did not determine they were needed on thisparticular 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 toconflict with profits. Yetthe costs of deburring a part can be lower than the cost of settling an injury claim or suit.

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