Process Capability

The main purpose of a process capability evaluation is to determine the ability of a process to be reproduced over time and stay within required production specifications. The evaluation also determines the total variation in a production process.

To begin the evaluation, a sufficient sample size, typically 30 or more pieces, is collected. Next, measurements are made on each piece. In the case of wire termination, the measurements are crimp height and pull-off force. Crimp height determines if the crimp compression meets specifications supplied by the terminal manufacturer. A wire-pull tester is a manual or electric device that applies horizontal or vertical force to a crimped terminal to determine its pull-off force.

All of these measurements are plotted manually on graph paper or with statistical analysis software to form a histogram having a bell-shaped curve. The manufacturer uses this curve to determine whether or not the process is currently within tolerances and will remain there in the future. Typically a process in tolerance has a value of 1.67 Cpk (process capability index) or higher. 

Process capability is based on five factors common among all production processes: people, machines, methods, materials and the environment. Each factor contributes to overall process capability, but may also increase variation.

The people factor is the amount of manual tasks performed in the production process. More manual work increases the likelihood of variation. For example, manually locating a terminal in a benchtop crimp press will likely cause more variation than deploying a fully automatic processing machine. Training of operators also lowers the people factor variation and improves the process.

Machines used for wire crimping include a crimp press, a terminal applicator and, in fully automatic systems, a robotic assembly that delivers a stripped wire to the press. Machine performance and variation are affected by wear and tear, maintenance and overall accuracy and repeatability. 

A crimp press is particularly susceptible to wear and tear even though it has a long life cycle. Over time its moving parts (bearings, crankshafts and ram assemblies) become worn, leading to variation in press force.

Regular calibration of the crimp press can prevent variation, but it can also give manufacturers a false sense of security in crimp press performance. Case in point: A calibrated press’s shut height can be within tolerance, yet its press forces can vary widely and be inconsistent.

What’s needed is a crimp press capability study, done in the same manner as the general process capability study. How often it should be performed will vary from company to company and depend on process cycles and environmental conditions.

Typically, a study should be performed every six to 12 months. Results from the latest study should be compared with those of previous studies to determine if press capability has degraded between calibration cycles.

A handy tool for assessing the performance of crimping presses is the PAL3001 press analyzer from C&S Technologies. This device takes force measurement readings to determine press capability. It also calibrates a press to the industry-standard shut height.

Wire crimping materials include wire, terminals and weather seals. The harness manufacturer has little control over materials because specifications are established by the material manufacturers within their own production processes.

The methods factor usually encompasses maintenance schedules, setup and operating procedures, and training of operators and setup personnel.

Environmental factors also need to be considered where material performance may be affected by variation in temperature and humidity. These environmental factors can also affect an operator’s performance.

Preproduction Validation

Before initiating production on a wire harness or lead, a manufacturer should test crimp height and pull-off force, and perform a crimp cross-section analysis. Crimp height specifications are supplied by terminal manufacturers and calculated using compression ratios to provide the optimum electrical performance of the crimped wire assembly.

Standard micrometers or calipers should not be used to determine crimp height. This is because the measurement surfaces may rest on the points of the bottom anvil impression rather than the bottom center of the crimp, resulting in false readings.

Specially designed micrometers should be used. Their spindle point rests on the underside of the crimp. The micrometer’s anvil side has a flat surface that rests on the upper side of the crimp. Digital micrometers allow crimp-height data to be downloaded and analyzed.

Standard pull-off test procedures require the insulation support to be peeled from the wire so the pull test reading is based on the wire-to-terminal crimp only. Sources of pull-test specifications are numerous and include terminal manufacturers and electrical approval organizations such as UL.

For accuracy and consistency, motorized pull testers are better than manual ones. A motorized tester pulls the wire from the terminal at a steady rate. One example is the PT100 from C&S Technologies. It can be interfaced with a computer for real-time data collection, analysis and capability studies, or it can be fully integrated into a plantwide system.

Manual pull testers are a cost-effective alternative to motorized models. However, piece-to-piece variation may occur due to human operation. A digital output enables manufacturers to collect and analyze data.

Having validated crimp height and the wire’s pull-off force, a manufacturer might assume everything is OK. This is the wrong assumption because what is seen externally may not match what is happening inside the crimp.

Crimp cross-section analysis is needed to view the inside of the crimp to ensure proper wire distribution. One or more factors can cause the wire to be unevenly distributed inside the crimp, even though the crimp has a normal shape. The most common causes are wire-diameter variation, tool wear and using tooling or wire that does not meet terminal specifications.

The cross section must be properly prepared prior to analysis. After the terminal is cut in half, the exposed end is polished and dipped in etching solution. The cross section is then mounted under a microscope. Software measures the crimp, calculates crimp compression and verifies whether or not wires are properly distributed.

Gathering cross-section analysis data is a good practice for a harness manufacturer to follow when introducing a new wire-termination process. Some OEMs require this documentation from their approved suppliers.

Collecting data from all three tests is critical. Collection may involve downloading data from instruments (micrometers, pull testers) or cross-section software, or recording results manually.

In some cases, an inspection station may be set up as a central location for all tools that collect and process data. At large assembly facilities, these tools may also be located near production equipment.

The crimp inspector on staff must validate the data before harness production can begin. He does this by making sure the collected data for a test meets the corresponding specification. The inspector can use a client-server network, such as the True Soltec Co.’s BBMX, to access each specification.

Once validated, the data can be shared internally with quality and production personnel, and externally with customers. It can also be provided to third-party quality auditors, who use the data to verify that the manufacturer’s procedures meet the requirements of a specific standard, such as ISO.

Crimp Force Monitoring

Manufacturers use various methods to maintain crimp quality during production, but not all of them are equally reliable. Crimp height, pull testing and manual visual inspection are time-consuming and do not provide a graphical representation (force over time) of the crimp. In addition, these methods do not enable a manufacturer to see what is happening inside the crimp.

To obtain a graphical representation of each crimp and ensure it is free of defects, many harness manufacturers use a crimp force monitoring system. In fact, crimp force monitoring has been a standard practice for more than 15 years and is mandated as a minimum quality measurement tool for automotive and appliance harnesses.

The system uses a load cell to measure the crimp force of each termination in real time throughout the crimp cycle. The load cell is mounted in the base plate under the applicator, in the ram adaptor above the applicator or on the press frame.

Besides a load cell, some crimp force monitoring systems use a triggering device that initiates reading of force measurement. However, all systems use a control unit to analyze the data. The system is used with benchtop crimp presses or automated crimping machines.

System setup begins by entering teach mode in the control unit. After an initial sample, software in the unit automatically calculates and converts the readings into a curve (force over time) that represents an adaptive (reference) crimp. Crimp analysis occurs during the production crimp cycle and immediately determines whether or not a crimp is good or defective.

When a crimping defect is detected, a signal is sent to the control unit to stop or initiate a reject cut off sequence. Common defects include high or low insulation, wrong wire gauge and cut strands. Another defect is a strand or strands that are partially encapsulated and laminated to the top of the crimp.

Uneven wire distribution, a problem easily detected with cross-section analysis, can cause a false reading during crimp force monitoring. This is because wire distribution may shift from one side to another during crimping, resulting in different crimp forces on each side of the terminal. False readings are a concern because they can lead to the rejection of good parts or acceptance of bad ones.

Monitoring systems can be used with a benchtop press or integrated with multistation automated wire processing machines. The MX system from C&S Technologies can be connected with the BBMX network to provide cross-platform data logging and crimp force monitoring throughout a plant.

 To complement crimp force monitoring, some manufacturers are now using vision systems with high-speed cameras and monitors. A crimped wire passes under the camera, and the crimp is measured by a control unit with special software. When defects are detected, the wire processing machine is stopped or initiates a reject cut off of the faulty crimp.