issue: February 2009 APPLIANCE Magazine
The Reality of Crimp Force Monitoring
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Rob Boyd, crimping product manager, Schleuniger Inc.
Understanding how to use crimp force monitors (CFMs) for crimp quality verification can save OEMs considerable amounts of time and money.
In the last 15 years, CFMs have come a long way. They are much more accurate and easier to use than in the past. Some CFM manufacturers focus on user-friendliness, but this tends to make the device less flexible. In contrast, when manufacturers want to give the user a larger degree of control for a greater range of applications, the devices tend to be more complicated to work with. Overall, in spite of improvements, there are still considerable frustrations over using these devices. This is why we still see machines and presses with CFMs that have been turned off.
When used correctly, CFMs can save appliance manufacturers and their suppliers considerable amounts of time and money in terms of applicator tooling and material scrap. This article aims to address the common challenges and misconceptions about what can be a very useful production machine.
The Complete System
The Complete System
Before any CFM system can be used, the process or system has to be stable. In this case, “the system” refers to all of the factors that come into play when using CFMs.
When there is a challenging situation involving a CFM, most will only consider the wire, the terminal, and the resulting crimp. The crimp might appear to be fine, but the CFM has identified it as defective. Many other quality metrics (e.g., crimp height, crimp width, brush length, etc.) involve only one parameter. However, there are many more factors to consider with CFMs. The user must consider the terminals, wire, head room of the application, applicator, and press. Each of these variables can affect the resulting crimp curve, and all play a part in the resulting forces that the CFM inevitably “sees.” Unfortunately, the CFM cannot isolate specific variable(s) to analyze. In other words,
it cannot pay attention to some and ignore others. It sees them all as a whole. Therefore, the entire system must yield consistent forces in order for the CFM to work properly.
Materials: Terminals and Wire
Materials: Terminals and Wire
Not all materials are created equal, and typically, with less cost comes lower quality. Material quality must be consistent.
Terminals. Variations in material stock thickness will cause variation that CFMs might detect. Variations are to be expected to a degree and are usually not the main culprit. But it is easy to imagine how these variations, if extreme, will adversely affect the ability for the CFM to do its job correctly.
Terminal material may also play a role in how much variation the CFM sees. Gold contacts typically show more variation than the same contact in another material. Gold is a softer metal, and softer materials will exhibit greater variation in forces. This is also the reason why CFMs cannot be used on many typical appliance applications involving preinsulated terminals. The plastic insulation is too soft and exhibits too much variation.
The way in which the terminals are stored on the spool will affect the way in which the terminals are presented to the applicator. If terminals enter the applicator at odd angles, the crimp forces can be affected. Figure 1 shows terminals that have not been well cared for. The different angle will cause variations in forces. Terminals entering the applicator properly can improve positioning over the anvil and terminal feed.
Wire. Nonconcentric wire, as many of us know, will lead to stripping issues. Also, some insulation materials will adhere to the strands and cause stripping problems. If the insulation concentricity or adhesion is not consistent, a problem may be even harder to isolate. However, the CFM can detect variations in the force curve when strands have been nicked or cut better than can be seen with the naked eye. These errors frequently can’t be seen after the crimp has occurred.
The number of strands in a wire also points to a question that many appliance production engineers ask regarding whether the CFM can detect one strand out or not. One strand in a seven-strand wire will have a much larger impact on the force of a crimp than one strand of a 41-strand wire. So, if the CFM can see one strand out of a seven-strand wire, two or three strands may need to be out for a 41-strand wire.
Wire and Terminal Combination. Sometimes OEMs may use a terminal that is slightly too large for the wire. It will be more difficult 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.
When the wire is small in relation to the terminal, wire placement can be a critical issue. The operator may only see that the terminal is crimped on the end of the wire, but the CFM may be seeing significantly different forces. Figure 2 shows cross-sectional pictures of two consecutive crimps in which the wire is undersized for the terminal. The strands of the wire end up in different areas of the crimped terminal, which may result in different forces. This is a case where it might be difficult to use a CFM.
In general, traditional CFMs are most effective for appliance applications of 24 AWG and larger. Smaller applications can be difficult. Many of the factors discussed in the coming sections play a part, but the primary reason is that the forces related to just crimping the terminal onto the wire are too low compared with the other forces involved. In some cases, it is possible to detect 26 AWG applications, but the smaller the application, the more important it is to have good head room and an applicator that is in good condition.
The head room of a crimp is the difference in crimping forces when the wire is present and when the wire is not present. This concept plays a large part in answering the question, “Can the CFM detect one strand out?”
Figure 3 shows an example of a 16 AWG wire application. The difference in force with and without the wire is approximately 47%. Therefore, each strand of a seven-strand wire will contribute approximately 6.7% of the force. If it were a 19-strand wire, each strand would contribute roughly 2.5%. This is not exact, but it is a good approximation. However, if an engineer is using tolerance parameters of ±4%, he should pick up one strand out on a seven-strand wire, but not on a 19-strand wire.
In the example shown in Figure 4, the peak force of the curve drops by only 26%, so the effect of the wire on the overall force of the crimp is not nearly as much. In this case, each strand of a seven-strand wire will affect the force by roughly 3.7%—only 1.4% for a 19-strand wire. It is easy to see that if the same tolerances of ±4% are used, the CFM will probably not see a crimp with one strand out as a defect.
Some applications have a head room of 8 or 10%. These applications will be very difficult to work with because the majority of the force is just to crimp the terminal.
Applicator quality plays a very big role in CFM effectiveness. An applicator that is in bad condition can introduce variation that the CFM will see.
Two different applicators with the same wire and terminal were tested on the same automatic machine. The wire was a 16 AWG bare copper, and the terminal was a rear-feed, brass quick-disconnect; crimp heights and widths were identical. You can see in Figures 5 and 6 that the resulting crimps from the two applicators are very similar. However, the older applicator yielded a Cpk value of 0.65, and the newer applicator yielded a Cpk of 1.20. Although both values are not good, there is clearly a difference considering the same wire and terminals were used.
Figure 5 shows the crimp results from the new applicator that yielded a Cpk of 1.20. Figure 6 shows the crimp results from the older applicator that yielded a Cpk of 0.65.
Although the crimp may look fine from the outside, the CFM can see defects because the forces are varying. One of the biggest contributors to this problem in the appliance industry is applicator age and lack of proper maintenance. Over time, applicators will wear out. Noise on the crimp curve can be introduced by any of the following: a ram that does not slide smoothly, worn tooling, inconsistent feed, or inconsistent bell-mouth position. These issues might not be detectable by the human eye, but the CFM will see variation.
The best solution for this is a regular maintenance plan for the applicators. It is strongly recommended that anyone considering implementing CFMs should consider the age and quality of their tooling. This is especially true if purchasing a new piece of automatic equipment, regardless of the brand. Putting an old, worn-out applicator on a new machine is like putting old tires on a new Corvette. Operators simply won’t be able to get the optimal performance from the machine.
In order to use a CFM, the press has to have consistency in speed and shut height, as well as be very rigid. The primary concern is with the older presses that are not rigid enough for use with a CFM. However, presses manufactured in the last 5 to 10 years are typically fine, provided they are in good condition.
When used properly, CFMs can be a tremendous asset on any production floor. They can save considerable amounts of money in tooling and scrap costs and ensure that only the best connections are made within motor and front panel assemblies. Quality monitoring is always seen as a positive.
However, all factors need to be considered. For new users, there can be a long learning curve. It is important that at least one key person is involved in the process and that this person is thoroughly trained. This individual must be willing to take the time to really understand the best way to utilize the CFMs.
Users should also understand that there are many components to the system and that the CFM cannot analyze certain variables and ignore others. CFMs will look at variation of the entire process, which includes the wire, terminals, applicators, operators, and machines. Because of this, not all applications are considered equal. Make sure that your equipment is well maintained and that you are getting consistent quality from your materials.