Engineers need to understand how crimp force monitors work to take full advantage of their capabilities.
Crimp force monitors (CFMs) are useful for ensuring quality when terminating wire. They can save considerable time and money by reducing scrap and preventing damage to applicator tooling. They also help keep customers happy by ensuring quality crimps.
Although CFMs are more accurate and easier to use than ever before, engineers still need to understand how they work to take full advantage of their capabilities. Failure to do so can lead to frustration. In some cases, manufacturers have been known to actually shut off their CFMs entirely because the devices weren’t performing as expected.
Many engineers believe that implementing a CFM will mean an end to their quality problems or that they can put a CFM on any press. However, this is not the case. Some will ask the unqualified question, “Can the CFM detect one strand outside of a crimp?” In fact, this question can only be answered after a series of questions about the application.
The following points apply primarily to those applications in which a fully automated system is being used to cut, strip and terminate each wire assembly. CFMs are most commonly implemented on these kinds of machines, as opposed to benchtop presses, because the speed with which an automated system terminates wire makes it difficult to perform an effective inspection any other way. However, the same basic principles apply to benchtop applications.
Crimp force monitors create a force-time or force-angle curve that allows engineers to observe exactly how the crimp is being performed. Note the bump at the left side of this curve that resulted when the tooling first came in contact with the terminal’s wings.
Force Sensors: All CFMs are similar in that they rely on a load cell to measure the crimp force; some type of triggering device to tell the CFM when to start reading; and a control unit that performs the analysis.
Force sensors can be placed on the press frame, in the press ram or in the base plate. Frame sensors are the most common, because they are less expensive and easiest to install. However, for those applications requiring greater sensitivity, it is better to use a ring sensor placed in either the ram or the base plate, where it will be in a direct line with the pressing force.
On the downside, ring sensors cost more because the sensors themselves are more expensive, and they require custom parts if they are to be correctly installed. Typically, the only time an assembler needs to use a ring sensor is when working with very small wires and terminals.
Triggering Devices, Electronics: A triggering device is required to tell the CFM when to start analyzing the force signal during each crimp. There are many types of triggering devices, including proximity switches, light barriers mounted on the body of the press, and encoders connected to the press shaft. Encoders are typically the most reliable and accurate of the three. However, they are also the most expensive.
As a press executes a crimp, the signals from the sensors and triggers are fed into an electronic control unit, which generates a force-angle or a force-time curve consisting of a few hundred data points on an X-Y axis-with the X axis designating time or angle, and the Y axis designating force.
Complex algorithms are then used to analyze the curve for shape and amplitude at each point and compare these characteristics to a known “good” reference curve. Different manufacturers use different algorithms, but all require the user to input the parameters that define a good or bad assembly. Some manufactures take a more user-friendly approach that is not as flexible. Others want to be more flexible, but these tend to be more complicated. Contrary to what some may think, not all applications are the same, and sometimes finding the correct parameters can be tricky.
Crimp force monitors are most often used with automated systems, but they can also be implemented on a smaller, slower benchtop press.
The way to implement a CFM is basically the same, no matter what type of system is being used.
As a first step, the crimp needs to be verified for all specifications-including crimp height, a pull test and a visual inspection-to ensure a good baseline. This may seem obvious, but a surprising number of manufacturers will make additional changes to a crimp after configuring their CFM, and then try to run production. Not surprisingly, this often results in a lot of scrap, because the CFM sees a different crimp curve and assumes the crimps are all bad.
Once an assembler has established the correct parameters, the second step is to “teach” the CFM to recognize a good crimp. This usually takes one to six crimps, regardless of the specific system being used.
During the programming process, it is very important for the operator to verify that all the parts are, in fact, good assemblies. If the operator runs the teaching process with a crimp height that is too high and then verifies that these values are correct, the CFM will not know otherwise. It will look for and accept parts with high crimp heights, at the same time possibly rejecting crimps that have been performed correctly.
Once a curve has been created, the next step is to create a set of tolerance parameters. It is important that these not be too stringent. Otherwise, good parts may be identified as bad crimps, and the machine will stop too often. This not only frustrates operators, but wastes time and materials.
At the same time, tolerances need to be tight enough to ensure that no bad crimps will pass as good, which will anger customers-an even worse result than undue scrap.
Force curves can be analyzed in several ways. The most common approach is to look at both the area under the curve and the shape. It’s a good idea that the two be monitored simultaneously, because it is possible for one parameter to be within tolerance at the same time the other is out. Typical tolerances are around ±4 percent of the total force being applied.
Many engineers assume that one set of parameters can be used for all applications. However, this is not always the case. If it becomes necessary to increase tolerances to more then ±7 percent, there is too much variation, and the system needs to be checked to see if the applicator is functioning correctly or if there is some other problem.
Note that only a part of the curve is usually analyzed. The “noise” at the beginning of most curves, in particular, has little bearing on the quality of the crimp. This noise-in the form of a small bump in the force reading-occurs as the tooling makes contact with the terminal and begins bending the crimp wings. Because the actual crimping process has yet to occur-the wings have not yet contacted the wire-these readings have no bearing on ultimate quality.
Similarly, it is sometimes necessary to analyze a particular zone, or zones, of the curve to catch especially fine defects. To use a zone approach, the rest of the process has to be very stable. Each zone will then have its own tolerance parameters.
This cross section illustrates what can happen when a manufacturer specs a terminal that is too big for the wire. In this instance, failure to correctly position the wire has resulted in a void to the left.