Typically, rescue cutters have been marketed by maximum cutting force. The standard has been – the more, the better. Although cutting force and operator technique are very important, cutter blade design, which includes blade geometry and composition in conjunction with the manufacturing, forging and heat treating processes, is a key factor when determining a tool’s ultimate cutting capability.
Often overlooked is the cutter blade’s ability to draw material to the rear of the blade, which is a key component in cutting capability. An effective cutter draws the metal to the rear of the blade where the cut force is greatest.
Cutting Force at the Right Point
Today’s modern vehicles use highly advanced types of steel in their construction. These highly reinforced structures do not compress into a tight bundle like the A-posts, roof rails, and B-posts of passenger vehicles built in the 80s and 90s. Instead, when the cutter blades make contact with the high-strength outer layer of steel in today’s significantly larger posts, they are immediately up against the ultra-high-strength press-hardened Boron sheet metal and Martensite Boron Steel which lines the inside diameter of the structure. These advanced types of steel do not cut. And, compression of these structures is limited, which results in fracturing at greater than half their original diameter.
All of this means that when cutting large diameter posts on today’s vehicles, maximum cutting energy is required at the tips of the blades at near full open position.
Maximum Cutting Force
Many the hydraulic cutters achieve their advertised maximum cutting force as the blades’ leading edges cross past each other nearest to the blades’ pivot point. This means the cutter’s weakest point is when the blades are in the full open position.
Cutting Force Calculations
There is some confusion in the marketplace regarding how cutting forces are calculated and why advertised cutting forces may vary from National Fire Protection Agency (NFPA) guidelines.
In the Hurst Jaws of Life testing lab, engineers calculate force measurements based on known facts, and then conduct real-world tests with production-grade tools to verify the data. The company then tests and retests the products until it is certain they can handle even the toughest emergency rescues. This data is then published internally and shared with customers.
The theoretical maximum cutting force for many cutters applies at a material diameter close to zero, which is an irrelevant figure in real world applications and means the cutter’s weakest point is when the blades are in the full open position. Additionally, maximum forces for traditional cutters apply at small material diameters where the force which is required to cut the material is very low.
There is also confusion regarding the meaning of NFPA-compliant. To be clear, the NFPA does not provide certifications. The organization publishes a list of standards and independent labs, such as TUV and UL, certify that a product meets those standards.
The NFPA publishes a respected process for testing and making claims based on a cutter’s ability to cut different sized and shaped metals. These ratings are published as A7, B6, and so on. These letters and numbers represent different types and sizes of raw metals—specifically solid round bar, flat bar, square tube, round pipe, and angle iron—that must consistently be cut 12 times in a row with no damage to the cutter blade or the tool. All compliant cutters will feature a label stating they are compliant to NFPA 1936, Standard on Powered Rescue Tools.
This standard provides the industry with a valuable starting point, but it is important to remember that rescuers cut vehicles and other implements to free trapped and injured people in environments that are far from the pristine nature of a lab. Rescuers do not cut perfectly round bar stock at 1¾ inches, as would be the case with a tool listed as an A9 in NFPA tests.
It is also imperative to note that the NFPA has not issued a standard for cutting Boron-type materials.
With many hydraulic cutters, the most successful tool position for cutting through a highly reinforced B-Pillar post is parallel with the vehicle’s body ring. In this position the blades are able to close more before contacting the metal, which generates more force than if the tool was fully open and perpendicular to the vehicle. This means the tool is positioned in the window, which could potentially block access to the injured occupant for medical care or worse, the rescue cutter could rotate inward making contact with the patient.
This article is courtesy of Hurst Jaws of Life. For more information on, visit www.jawsoflife.com. Click here for information on the company’s S 700 blade design.