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.
Construction and Concepts
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.
Standard Setups
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 crimpafterconfiguring 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.
A Complete System
For a CFM system to be truly effective, the process, or system, must be a stable one. In this case, the “system” includes all those factors that come into play when creating the crimp: not just the wire, terminal and crimp, but the head room of the application, the applicator, the press, the operator in case of benchtop applications, the machine and the tolerance parameters for the CFM.Each of these variables can affect the resulting crimp curve, and all play a part in the resulting forces that the CFM will register via its sensors and software. Unfortunately, a CFM cannot isolate a specific variable, or variables, but monitors the process as a whole. Therefore, the entire system must yield consistent forces for the CFM to work correctly.
Terminals and Wire: Not all materials are created equal. Frequently, with less cost comes lower quality. There is often a point where paying less for materials may cost you more in the long run.
With respect to terminals, several factors contribute to terminal quality. For instance, variations in material stock thickness can cause force-curve variations. Rarely is this the main culprit in a problem application. Nonetheless, it is easy to imagine how these variations, if extreme, will adversely affect the ability of the CFM to do its job correctly.
Terminal material can also play a role in how much variation the CFM sees. Gold contacts, for example, typically show more variation than those made from other materials, because gold is a softer metal, and softer materials tend to vary more. For this same reason, CFMs cannot be used on most applications involving pre-insulated terminals. The plastic insulation is too soft and exhibits too much variation.
Using oil on contacts can add another variable. Although oil doesn’t necessarily cause problems when a machine is running at a normal pace, operators might see some errors immediately after returning from a break, because the oil on the terminals between the anvil and the oiler has dried slightly.
Finally, poor care of terminals on spools can be a source of problems, because the way the terminals are stored on the spool will affect the way they enter the applicator. For example, they might start to enter the system at odd angles, affecting crimp quality.
With respect to wire, nonconcentric wire will often exhibit stripping issues, while some insulation materials can adhere to the wire strands. The former is a case in which a CFM can be especially beneficial, because it will detect variations in the force curve when strands have been nicked or cut-errors that are often difficult to detect visually once a crimp has been executed.
Wire, Terminal Combination: We live in an imperfect world. All too often, manufacturers will specify a terminal that is either too small or too large for a given wire. Note surprisingly, this can cause problems in terms of quality and productivity.
It will be more difficult, for example, to monitor a 24 AWG wire crimped into a terminal that is rated for 24 AWG to 20 AWG, than it is to monitor the same wire crimped into a similar terminal rated for 24 AWG to 28 AWG.
Oversized terminals can also cause problems as a result of variations in wire placement. Specifically, the strands from the wire can end up in different areas of the crimped terminal, depending on their orientation prior to crimping. The result can be dramatically different crimping forces, even in a pair of terminals that look identical.
Head Room: Head room refers to the difference between the crimping forces when the wire is present and when it is not. This difference can be used to estimate whether a CFM will be able to identify those crimps in which a strand has been left out.
Let’s say, for example, the difference in forces with and without the wire for a particular application is approximately 47 percent of the total crimp force. Roughly speaking then, with a seven-strand wire, each strand is contributing about 6.7 percent of that total force differential. With a 19-strand wire, on the other hand, each strand is contributing roughly 2.5 percent.
When using a ±4 percent tolerance parameter, you should therefore be able to pick up one missed strand on a seven-strand wire, but not on a 19 strand wire, because the latter’s force differences are so small.
If, on the other hand, the peak force of the curve drops only 26 percent when the wire is omitted, the affect of a single wire out will be a 3.7 percent reduction of the total force with a seven-strand wire and a 1.4 percent reduction with a 19-strand wire. As a result, using the same tolerance of ±4 percent, the CFM will probably not recognize a crimp with one strand out as being defective with either wire type.
Applicators: Applicator quality plays a critical role in CFM effectiveness, because an applicator in bad condition will inevitably be a source of variation. The crimp may look fine from the outside, but it will not pass muster with the CFM.
The biggest factors at work in this kind of situation are applicator age and lack of maintenance. Variation on the crimp curve can also result from worn tooling, inconsistencies in feed or bell-mouth position, or a ram that is not sliding smoothly enough.
On occasion, an applicator might need to “settle” after an adjustment or after new tooling has been installed. Specifically, force values will continue to drop until the applicator has become correctly seated. Once it settles in, force measurements will be consistent again.
Note that because of their sensitivity, CFMs serve as an excellent kind of protection against applicator damage in the event there is a processing problem. It doesn’t take many missed crimps to crack a die or anvil, and with an automatic machine, one missed crimp can quickly become five or six. With a CFM, the significant force variations that result from this kind of defect should stop the machine before the tooling gets damaged.
Presses and Operators: If a CFM is to be effective, the press must be rigid and consistent in terms of speed and shut height. This can be a problem for those manufacturers using older presses that have lost some rigidity. Presses manufactured within the past five to 10 years are typically fine, provided they have been well maintained. However, anything older may not be rigid enough.
During actual production, it is critical that wires be place consistently to avoid dramatic variations in the force curve. This is especially true with respect to the depth of insertion, something that can often be problematic for novice operators or poorly maintained machines.
For more on crimp force monitoring,click here.
