In part one of this series, we covered the critical steps used to validate a wire to terminal crimp prior to initiating production on a wire lead or harness. In part two we will uncover some of the processes companies currently deploy to monitor the crimping process during production.
Although companies still perform an in production spot check of crimp quality using validation methods (crimp height, pull test), these methods show compliance to crimp specifications on a single piece at a time during production and do not demonstrate full statistical capability of the process. Visual inspection is another method. But as we have seen with the cross sections illustrated in part one, significant variation could be occurring inside the crimp that is invisible to the inspector. It has been shown that 100% visual (human) inspection is only 80% effective over time. Given 20 out of 100 parts may not be properly inspected, there is a potential for parts to be supplied that are not to specification.
In process monitoring methods are now used to ensure each wire termination meets crimp force specifications. Crimp force monitoring is the common in process method used which we will outline. New methods are emerging in the future, one which we will briefly cover.
Crimp Force Monitoring
Crimp Force Monitors (also known as Crimp Quality Monitors, CFM or CQM) measure crimp force of each terminations in real time. CFMs take force readings from the start to the end of the crimp cycle using a force sensor which is mounted in the base plate under the applicator, the ram adaptor above the applicator or on the press frame. The readings are received by the crimp monitor base unit which compiles and creates a crimp curve. The crimp curve is compared with a reference crimp curve, created by a teach in process at the beginning of production. Crimp errors such as high and low insulation, wrong wire size and cut strands are typical defects detected. Not so typical defects include strands partially encapsulated and laminated to the top of the crimp. When a defect is detected, a signal can be sent to the processing machine to stop or initiate a reject cut off sequence if installed.
You will find below an example of a normal curve from data points compiled from a single crimp. Also two more crimp curves showing common crimp defects.
The illustration above shows a normal crimp curve with no variation from the teach in reference sample. In the case of the C&S MX series crimp monitors, the crimp curve is split into three vertical sections which have tolerances applied on each side of the reference (teach in) curve. Each zone has a separate tolerance band which is progressively tighter as the press reaches bottom dead center and crimp force is at its peak. The chart below the curve is a point by point visual of the variation above or below the reference point from the teach in. In the case of the normal curve, very little variation is shown.
The illustration above shows the effect of strands cut and missing from the crimp. The measured result (in yellow) shows lower results and an error condition starting in the second and continuing through the third zones. The point by point variation also shows this condition.
In the illustration above, the insulation was embedded in the conductor crimp which is also known as “high insulation”. As the conductor crimp wings make contact with the insulation and see excess resistance, the forces spike quickly but then drop off as quickly in the first zone. Then the crimp force does not reach the required peak through the third zone and an under load error occurs.
The crimp force monitoring process is now a standard in most wire and cable harness facilities and is mandated as a minimum quality measurement tool for automotive harnesses. The C&S MX series crimp monitors operate as a standalone solution for a bench press or installed onto multi station automated wire processing machines. Connected with the BBMX client server network, the MX series provides a cross platform data logging and monitoring system for plant wide quality assurance.
New Crimp Monitoring Technologies
Crimp force has been the standard in process method of measuring crimp quality for over 15 years. In the coming years, vision systems will be an additional tool for monitoring crimp quality. The video below shows the C&S CVM-2 vision system with two cameras and monitors in simulation of a crimped wire passing through and being measured at high speed by the cameras.
Wire Process Specialties partners with C&S Technologies, a global leader in the supply of crimp process validation and monitoring systems. We can supply simple inspection tools or create a complete integrated system for plant wide control of the crimping process. Connect Your Way to Wire Process Specialties to get a personalized assessment of your requirements.
Part 3 of this series outlining crimp press calibration coming soon.