The Reality of Crimp Force Monitoring
When it comes to wire processing, many crimp-force monitors are used neffectively. The condition of the press, applicator, and terminal all affect tolerance settings.
Crimp-force monitoring systems used in the wire processing industry are useful tools for maintaining quality, if they are used correctly. However, users often find them difficult to work with. Many companies have inactive crimp monitors on machines because they could not use them effectively. Most users can benefit from taking the time to fully understand the complexities of operating crimp-force monitors.
Although there are numerous manufacturers, the basics of crimp-force monitors are the same. As the crimp occurs, a load cell picks up the forces and feeds them back to the electronics. The electronics converts these force readings into a time-force curve or crimp-force signature. Different monitors calculate the signatures differently, but the purpose is the same: to find where the force curves vary. The first crimps are used to "teach" the monitor the signature of a good crimp, deriving a reference crimp. Every subsequent crimp is then compared to the reference crimp. If any crimp-force signature falls outside the preset tolerances, then the crimp is flagged as "bad."
Different manufacturers prefer different types of load cells and their locations. Some use piezo-ceramic load cells while others use piezo quartz. Some place the load cell in the ram of the press, while some place it in the base plate beneath the applicator. In calculating and analyzing the crimp-force curve, some manufacturers use the area under the crimp-force curve, while others use actual points on the curve. There are pros and cons to each system, but the difficulties are the same regardless of manufacturer or method.
FIGURE 1: A typical crimp-force monitor tracks the force of the crimp over time to one-tenth of a Newton, flagging any deviation as a bad crimp.
Most users do not realize how sensitive crimp monitors truly are (see Fig. 1). Depending on the tolerance setting, crimp monitors can detect if a single strand is out. Some can even detect a strand outside the crimp that is compressed along the outside of the crimp barrel. Crimp monitors are sensitive enough to detect if the terminal feed is inconsistent. An applicator can have a negative effect on crimp force if it is not lubricated correctly or is worn and does not close correctly. Similarly, if the press is old and has too much flex, the crimp curve could vary widely. For this reason, crimp monitors are not effective on most pre-insulated terminals. The durometer of the insulation is inconsistent, making the force signatures inconsistent.
FIGURE 2: The experience of an operator can affect the tolerance parameters of a crimp-force monitor.
Terminals
Not all terminals are created equal. Tolerance settings may need to be adjusted depending on the terminals used (see Fig. 3). For instance, gold is a softer material than tin. A gold-plated terminal gives much more variation than a tin-plated terminal of the same size. Because of this, the optimal tolerance parameters are different for gold and tin. Similarly, thinner, more flimsy terminals require a larger window of acceptance. Heavier terminals typically have less variation. Finally, even the same terminal part number may vary in terminal stock thickness from one spool to the next. Users must commonly reference the crimp monitor when terminal spools are changed.
FIGURE 3: The optimal tolerance settings on a crimp-force monitor depend on many parameters, including the material of the terminal and a change in terminal stock from one spool to another.
As crimping presses get old, they become worn. The nice thing about most presses is that they last for many years. Over time, crimping presses may cycle properly, but the bearings may loosen or the press may have a little more flex. It is therefore not prudent to put crimp monitors on older presses. If a press is not consistent in cycle speed, the crimp monitor may see a different reference curve and flag the crimp as unnecessarily bad. Similarly, worn bearings or additional flex can cause a crimp monitor to give false bad signals. Tolerance settings may need to be adjusted depending on the press being used. Older presses that have not been well maintained need a larger tolerance window.
An applicator can affect crimp force in numerous ways. An applicator that is old or not well maintained might not close consistently, which may trip the crimp monitor. An old applicator also might feed inconsistently. A terminal placed in different positions over the anvil may roll differently in the tooling. Finally, if the tooling is worn, the crimp does not occur as smoothly as when it was new. Once again, even though all these factors may affect the crimp force, the crimp may appear to be good. Tolerance settings should be adjusted depending on the applicator being used.
The wire characteristics can vary but not as much as the other parts of the system. However, if insulation thickness varies too much, the crimp monitor will detect it. Sometimes different colors give different force readings. White insulation is typically softer than black insulation of the same type. However, this also depends on how tight the insulation crimp is on the wire.
Operator & Machine
Most crimp monitors are used on automatic processing machines. Usually wire placement is not a problem, but if the terminals misfeed or if the wire is not crimped, a terminal may get stuck in the tooling. Considering the production rates that today's machines can achieve, stuck terminals can quickly crack tooling. Therefore, crimp monitors can also save on tooling costs.
Crimp monitors can detect a wire placed improperly in the terminal on bench applications. But again, the consistency of the placement can cause differences in the crimp signature that the monitor sees. More experienced operators are more consistent than novice operators. Therefore, a larger tolerance window might be necessary for beginners.
Crimp-monitor "drift" is defined as minor changes in the reference curve caused by terminal stock thickness variation or slight changes in the wire. Drift accounts for the difference between the crimp signature that is read at the beginning of the day when the press and applicator are cold and the signature read at the end of the day when the components are warm and cycle more easily. Throughout the course of a day, the reference curve changes slightly to accommodate these minor fluctuations.
Some crimp-monitor systems have PC software to optimize each individual working parameter. For a repeating job, the optimal parameters can be saved and sent to the crimping device through a network to save setup time. Software is also useful for troubleshooting difficult applications because it gives you a better idea of what is happening and what the monitor is seeing.
Here are a few things to remember when using crimp-force monitors. First, you should be completely ready to run production before referencing the crimp monitor. Crimp heights and pull-test values should be verified and all machine settings confirmed. Second, keep in mind that different applications require different working parameters. Fortunately, some crimp-monitor systems help keep track of the parameters with PC software. Next, when troubleshooting is necessary, the crimp monitor evaluates the entire system, so you must, too. Finally, using crimp monitors is much simpler with well-maintained presses and applicators.
Crimp monitors are more complex than a simple "plug and play" system. However, crimp monitors are a useful tool in tracking crimp consistency and saving on tooling costs. Crimp monitors force users to maintain tooling, presses, and machines so the entire system is brought to a higher-quality level.
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