J. Ruben Lozano 2017-11-07 05:04:51
by: J. Ruben Lozano, B.I.E. Vice President of Sales-Americas Lakes Precision Inc. 1900 Epler Court Three Lakes, WI 54562 USA www.lakesprecision.com Wire Processing Essentials The conundrum of comparing vs. contrasting: Oftentimes you see yourself selecting, picking, choosing or electing between two or more choices available to you, right? Most likely, you are going to make your selection by means of comparing. Why comparing? Well, in my opinion, as a society, we are trained from childhood on, to compare between products, things or persons. Just look at any message or advertisement on commercial or political media, the advertising part usually boasts some major benefits “as compared” to what other adversarial part is offering. Very few (if any) messages offer a “contrasting” point of view, why? Because in my opinion, “To contrast” is the most sober, mature and refined sibling of “To compare”. Comparing is more emotional in nature while contrasting is more cerebral. Ever heard the expression of trying to “compare apples to oranges”? Of course, we all have, and it recalls a quick asseveration of something that doesn’t make much sense when comparing two dissimilar or unrelated things (except in politics were comparing apples and oranges is elevated to an art form). So either we quickly dismiss it as nonsense or challenge the notion that the two things have something in common. But have you ever heard the expression of “contrasting apples to oranges”? No? neither have I. The above thoughts can be applied to the two major modern technologies for processing wire available to us. That is, measuring, cutting to length, exposing conductive ends and applying interfacing materials to those exposed extremities: either by metal blade-based wire processing or laser beam-based wire processing. Which option do you think is “the best”? Well, in my opinion, there is really no comparison between the two technologies, it would not even be “comparing apples to oranges”. Most likely, it is like comparing “apples to light bulbs” where the only commonality perhaps is that apples and light bulbs have a spherical shape, but they exist in two different realms. However, there are contrasting elements in the way each technology approaches the same wire processing task. To understand these contrasting elements, let’s look at the basics of each: Blades: Essentially, blades are a combination of two simple machines: the wedge and the guide. The wedge element of the blade is the feature that performs the job of cutting the insulation material while the guide element performs the job of centering the wire end to achieve a circular and perpendicular cut around the conductor core relative to the longitudinal axis of the wire. In Figure 1, we see three examples of the wedge simple machine, at 45°, at 30° and at 15° inclination with their corresponding mechanical advantage coefficient (MA). The performance of a wedge is measured by its mechanical advantage.That is, a wedge with the highest mechanical advantage requires less effort to penetrate a given material, although the caveat is the density and coefficient of friction of the material being sliced, for example chopping wood with an ax vs. slicing tissue with a scalpel. In Figure 2, we see the guiding angles most commonly used in wire processing blades; 90° slope, 60° slope and a combo angle of 90° and 60°slopes (although these angles are an open choice for some blade designers and are not constrained to these three common ones, though the function is basically the same). When these two simple machine features are combined, a wire processing blade is born. In one tool you combine three functions, which interact when the blade pair closes in onto the wire being processed. Wire is coaxed into a centered position, and the insulation is cleanly sliced by the mechanical advantage of the blade’s wedge. And the combined complex geometry of the wedge, guiding angles, radiuses and predetermined shut heights, form a precisely located envelope around the conductor core, which avoids contact and damage to the copper filaments. See Figure 3 and Figure 4. Laser: Laser technology per se is not a recent development, not even recent as it pertains to wire processing. It was originally conceptualized by NASA in the mid 70s when the need to avoid any physical damage to the conductor cores of critical wiring in aerospace and spacecraft such as the space shuttles and others was a concern. And since then, it has been developed and improved to this date by several private global laser wire processing equipment OEMs. In contrast to metal blades, in which mechanical principles for the removal of plastic insulation materials are used, laser-based wire processing applies concentrated Laser radiant energy to vaporize the insulation materials and thus expose the interior conductor cores. To understand how the laser energy is used for this process, let’s explore the fundamentals of this application without diving too deep into the complex science of laser energy generation. 1.The source laser beam must have sufficient raw power density to deliver enough energy to be able to physically vaporize the wire’s insulating material. Power density is defined as the amount of power in watts over a cross-section perpendicular to the laser beam’s longitudinal axis and it’s measured in units of watts/cm². For example, a 75-watt laser head emitting an 8 mm (0.8 cm) diameter laser beam has a raw power density of approximately 149 watts/cm² (using the formula PD=Watts/πr2). 2.Perhaps the raw laser beam could have enough energy to cut through the material, however, the beam diameter is too wide to do a precise cut. Thus, laser cutting systems incorporate optical lenses to concentrate the beam into a much narrower focus (like the way most children use looking glass lenses to concentrate the sun’s heat to burn something, although laser focusing lenses and other optical elements must be very precise and thus are expensive to make). 3.When you focus the beam, it takes an “hourglass shape” (the physics of this phenomenon are beyond the scope or the intention of this article). The point at which the beam is at its highest focus and smallest cross section is where the highest power density is found. This point is called the “Focal length” (see Figure 5 for reference). 4.Although the insulating material enveloping the conductor core might be relatively thin by itself (when laid flat), it forms a cylindrical shape around the core and the focused laser beam must have enough power to vaporize material at any given point across the cylindrical cross section. This is where the “Depth of field” physics comes into play. In lay terms, the depth of field is the section of the focused beam away from the focal length where there is enough power density to vaporize the material. See Figure 6. 5.Vaporizing a very small and precise drill hole on the insulation wouldn’t be of much use. Thus, to separate the insulation away from the core, you either need to move the laser beam across and/or length wise over the wire surface, or move the wire itself to achieve the vaporized cuts. Some bench operated machines move the wire and other machines move the beam head on the X-Y axis while maintaining the relative position of both the focal length and the depth of field, while still others use software-controlled optical mirror devices to redirect the focused beam leaving the laser head stationary. And all this while maintaining focus depth and depth of field under control. Another contrasting feature between laser energy and metal blade technologies is the difference in physicality and setup between the two. The foundation of the blade-based cutting and stripping operation is the cutterhead assembly. There are two blade holder setup techniques: 1.Stationary strip mode setup. The blades are mounted into the blade holder and are located away from the cutting edge of the central cut-off blade by means of specifically dimensioned inserts. This setup allows for the cutting and stripping blade insulation penetration to be performed simultaneously during the closing motion of the machine’s cutterhead drive. This by the way is very precisely actuated by micro-processor controlled servo motors or actuators, controlling the depth of penetration of an OEM-grade blade like Lakes Precision blades as to avoid conductor filament core damage over a geometrically stable core configuration and extrusion. 2.Dynamic strip mode setup. The blades are mounted in the blade holder and are located by a fixed-dimension, reference-only inserts. The strip dimensions on the trailing and leading ends are not dictated by the individual inserts as is the case in the stationary mode setup. Rather, the on-board equipment computer takes the reference of the fixed dimension inserts and calculates how far to displace the holding wire clamps to achieve the stripping dimensions programmed by the machine’s operator on each wire end before actuating the penetration mode of the cutter head. This mode takes away the simultaneous cutting and penetration mode achieved by the stationary setup, increasing the machine’s cycle time just a bit, but accomplishes savings in machine down time and extra inventory on the shelf. As you can tell from the illustrations in Figure 7 and Figure 8, the physicality of the blades allows the wire ends being processed in either stripping mode to be guided and supported by the blade geometry after the cut. This physical support and control of the limp wire ends by the blade body itself also is a major feature to achieve controlled and precise strips even though the insulation penetration is not all the way through the core itself. In contrast, a limp, curling wire end cannot physically be supported or guided by a laser beam, the physicality is just not there (Figure 9). Does this mean that the laser beam is disqualified because of lack of physicality? The answer is no. The lack of physicality is just a contrasting element between the two technologies. So it’s best if we understand the application limits of each one. Obviously, the answer to lack of physicality of the laser beam is to hold the wire taut while the laser action takes place, this adds another operation to the complexity of the equipment and extends the cycle time accordingly (Figure 10). Another physicality contrast between the two technologies is that the stripping blade edge in the blade-based cutter head is used to dislodge the insulation slug from the wire end just before the strip arm rotates towards the terminal crimp station (or other end process). In contrast, the laser requires three cuts, two cross cuts and one slit cut between the strip window. And at the end of this operation, the insulation slugs could probably remain lodged on the strip window. In this case, an ancillary dislodging method must be applied to the process—perhaps a compressed air blast or other means, although any type of oil contaminated pneumatic blasts or even vaporization gases inside the laser strip chamber could potentially contaminate the optical elements of the system and compromise the laser beam’s power density. At any rate, dislodging the laser cut insulation slug adds more complexity to the machine (Figure 11). Finally, another contrasting element between the blade-based and laser-based technologies is that in the former, the cutting operation (separating the wire section from the bulk) is performed just immediately prior to the stripping and positioning of the wire ends in front of the terminal presses or other final operation. And this is done within a relatively small footprint of the blade-based machine. On the laser-based system, the cutting operation must be performed after the window stripping and it is not done within the footprint of the laser chamber. The window stripped section of the bulk wire has to be moved to a subsequent cutter head where a physical metal blade performs the separation of the stripped section from the bulk wire. This would add yet another degree of complexity to the equipment (see Figure 12), because the laser-stripped window must be positioned exactly referenced to the cut blade position in order to split the wire into two stripped dimensions as programmed by the operator. Undoubtedly, the laser-based stripping technology is more precise in penetrating odd shaped insulation cables, up to a point. I might add, for example, consider the two examples in Figure 13. In these examples, the best candidate for laser-based stripping would be the one on the left because it contains a metal shield, which allows the laser to follow the complex contours of the insulation surface, while the right multi-conductor one can’t be processed with laser because there is no metal shield separating the inner conductor insulations from the outer jacket. The laser energy would vaporize all in one swoop (except for the inner conductor cores), surprisingly, as shown in Figure 14. The multiconductor on the right is a good candidate to be processed by a metal precision die-type blade, due to its quasi-cylindrical shape. So, what is the conclusion? In my opinion, both technologies can co-exist, as they have done for the last four decades or so. Laser-based technology is an amazingly efficient way to process irregular surfaced cables and wires and specialty high-precision insulation removal processes in the aerospace, electronics, medical and other wire processing industries as attested by case studies presented by Laser-based OEMs in this magazine and other mediums. Blade-based wire processing technology has been permanent, long-lived and very cost efficient as well as precise method of processing wires at high demand and high volume in cost-competitive industries such as the automotive, appliance, consumer goods and others of that nature. Thanks for reading these articles and this amazing magazine. See you next time. www.lakesprecision.com About Lakes Precision, Inc. From hand strippers to fully automated wire processing equipment, Lakes Precision Inc. has the tooling to meet your requirements. Lakes Precision offers the widest range of blade sizes providing the ideal blade to wire application. The firm’s extensive inventory allows same-day shipment on the most commonly used perishable tooling and accessories. Lakes Precision is your global source for wire processing and perishable tooling. www.lakesprecision.com
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