Even the simplest medical cable assembly projects contain numerous engineering considerations. And among the most complex applications, the quantity of requirements can skyrocket. Factors such as diameter, construction, material, coating, length, and tolerance are only a few of the often-mountainous specifications medical device engineers must consider. Regardless of the complexities of a medical cable assembly, the decision as to how to cut the cable is built in to virtually any medical device cable project, big or small, intricate or straightforward.
There are two fundamental ways to cut cable for medical applications, each possessing its own set of distinctive benefits and appropriate applications — mechanical cutting and electrocutting. It is precisely because each approach serves several specialized purposes, however, that it is vital to be well acquainted with the method that will meet the unique cutting needs of the application.
If a speedy, economical turnaround is required, for example, then mechanical cutting likely provides the quickest production run. And although electrocutting also can be utilized for high-volume production, in many instances, electrocutting may be slower and less economical than its mechanical counterpart. If, however, there is concern about fraying of the cable, especially when the cable is flexed, then electrocutting the cable may be the best solution, because a fused end prevents wires from separating under flexing conditions.
While these are merely two of the many variables that determine the right cutting decision, the bottom line is that the list of cutting considerations goes on and on. Getting the advice of a qualified endoscopic cable manufacturer will ensure that the intricate science of cutting medical cable, particularly for endoscopic purposes, is done right the first time.
Mechanical Cut Endoscopic Cable
Mechanically cutting cable is a process of mechanically shearing cable to length. With mechanically cut endoscopic cable, the cutting process does not bond the cable ends together, leaving the wires loose at the cut end. If an application does not require that the cable ends be bonded or swaged together, as in the case of an electrocut cable, then mechanical cutting may be well suited to the application. Among the most common applications for mechanically cut cable are those involving solid core wires regularly used in push-pull assemblies.
Lastly, one of the greatest advantages to mechanically cut cable is that it is often inherently faster, because the equipment required is not tasked with welding the wires together. This saves valuable production time, making mechanical cutting less expensive. The absence of the fusing process to form a single bonded surface edge allows for a speedier and more efficient production run, making mechanical cutting both easier to execute and control.
As mentioned, when cable is cut mechanically the wires are not bonded together. If preforming, or in the case of small cables, stress relieving is done properly, mechanical cutting should produce an acceptable cut. The cable can subsequently be used in further process steps, including swaging fittings to the cut end of the medical cable.
Mechanical Cutting Challenges
With mechanically cut endoscopic cable, the cutting blades are prone to wear over time and therefore must be carefully maintained and monitored to assure sharpness. If the blades get dull, the clean sheer required of the production run begins to suffer, and it is likely that burrs will appear over the end of the medical cable. Additionally, the cable spooling must be perfect to prevent the cable from wrapping on top of itself. When the cable is improperly spooled, the spool itself will periodically jerk ever so slightly; this disruption oftentimes is enough to create a bend in the finished product.
Electrocutting Endoscopic Cable
As the endoscopic cable assembly industry evolves, and medical devices become increasingly miniaturized, the assembly requirements become more difficult, challenging, and complex. As the industry “shrinks,” the quality of the cable ends used for these sophisticated devices are, now more than ever before, of critical importance in perfecting an endoscopic assembly. Today, there is little clearance between cable diameters and fitting internal diameters, so a perfectly fused cable end is ideally suited to the endoscopic, and more broadly, medical devices manufacturing industry.
Before cable is even being prepared for cutting, the cable is run through a set of rollers, ensuring that the wires do not come apart over time. This process is known as preforming. Because endoscopic cable is typically miniature in size, it can be difficult to preform. Thus, when the cable is cut, the ends have a tendency to fray or unravel. While there are methods to prevent fraying from occurring, electrocutting the medical cable ends eliminates the potential for fraying because it fuses the wires together. Once the ends are fused together perfectly, the decision to electrocut the endoscopic cable ends makes all subsequent manufacturing steps easier and more efficient throughout the balance of the manufacturing process.
The chief rule of a sound electrocut cable is that the cutting process should not allow any loose wires or hangers in the finished cable assembly. Each individual wire will, therefore, be retained within the welded end itself. So, when the application calls for the cable to achieve its maximum strength, lifespan, and durability, an electrocut, or fused end, is recommended.
Before electrocutting existed, the wires were typically soldered together to achieve the bonded cut end seen in an electrocut alternative. However, this now obsolete approach introduced the unwanted side effect of solder, and its lead contaminants, being directly applied to the product. Additionally, the soldering process proved too labor intensive, time-consuming, and expensive. Soldering medical cable ends ultimately increased material and manufacturing costs, and produced, by comparison to today’s electrocutting approach, an inferior product.
Today, electrocutting means that the cable is fuse-cut by way of machine, which welds the cable ends together to form a continuously bonded material at the cable’s cut end. This cutting process essentially makes the cut end a single point, where all the wires merge into one, ensuring the cable’s integrity is uncompromised by the cutting process. Once the cut is fused to perfection, enough material is deposited at the cut end of the cable, so it subsequently may be formed into various profiles, such as a taper, ball, sphere, and so on. With an electrocut, the cable ends can be fashioned into the ideal geometry for the application.
When the cable is ultimately meant to be inserted into tight, narrow spaces, such as a lumen or tube, a fused end, with no risk of fraying, makes electrocutting the cable the logical cutting method as well. Lastly, if the cable’s appearance is an important a design consideration, the electrocut process can be modified to cut under inert gases, so that when the cable ends are fused, inert gas floods the work area and yields a clean, color-free cable end.
These processes, coupled with the strand geometry, combine to form the rigidness and features needed for medical devices and endoscopic equipment, such as endoscopes, implants, orthodonture, and many others.
For all its benefits, electrocutting endoscopic cable isn’t without its nuisances, however. For instance, in endoscopic cable sizes <0.0020 in., occasionally the cut will produce weld splatter, which may introduce a burr to the cut end of the weld. Burring may also increase variability with cut lengths, where perfect lengths are critical to the application. While a secondary operation, such as sanding, swaging, or buffing may eliminate the burred end, it is recommended that the production team devote more time to the setup process. Making sure that electrocutter blades are machined properly, that gaps are setup correctly, and that heat and time delays are set to process routing requirements, all help to mitigate the risks of burring. What’s more, the potential for burring to occur on endoscopic cable ends can commonly equate to an inevitable increase in production time and, subsequently, costs to produce a burr-free electrocutting cable end could increase as well.
Another challenge inherent in electrocut cables is that at such small medical device cable sizes, the individual strands are often as thin as human hair. Endoscopic cables at sizes so miniature can make electrocutting especially difficult to ensure that all the wires are contained in the weld. Therefore, the cutting operation may result in small wires breaking free of the weld. If the cut end of the cable is to be inserted into a fitting, for example, fraying may be anticipated in the assembly’s design. However, because the very nature of electrocutting promises a perfectly fused cut end, it is highly unlikely that an electrocut cable is meant to be obscured by a fitting or terminated in some other way that would render fraying unimportant.
There’s no single solution for cutting cable that applies universally to all endoscopic projects. And because both mechanical cutting and electrocutting ensure full tensile strength, choosing either won’t risk comprising the cable’s mechanical properties or load-bearing qualifications. So, given that each cable cutting approach possess strong empirical arguments for their usefulness, discussing cable cutting requirements with a contract manufacturer’s engineering team is vital.
If an application does not demand a fused end for instance, or if a fitting is not being applied to the cut end, mechanical cutting the endoscopic cable will produce a perfect product. If, however, the endoscopic cable application requires a fused end, where there can be no risk of the material fraying from the cut end, or strand breaking away from the product, an electrocut end is essential.
For endoscopic applications, it is important to note that the cable cutting technique applied influences the time spent making the device and its production complexities, as much as it does the medical device’s ultimate function.
This article was written by Scott Dailey, Vice President of Sales and Marketing, and Greg Soja, Vice President of Engineering, Carl Stahl Sava Industries, Riverdale, NJ. For more information, visit here.