WireProcess Specialties

This is the final installment of Crimp Quality Process Validation and Monitoring.  In part one, we discussed pre-process validation. In part two we outlined real time monitoring process during production.  In this segment, we will discuss the calibration and machine capability of crimping presses as an integral part of an overall quality strategy.

Process Capability
Process Capability is the total variation in a production process and the ability for that process to be reproduce-able over time and within stated production specifications.  Measurements during the production process are taken then grouped together to form a histogram (bell curve). The distribution of measurement results provide an indication of present conformance of the item being produced.  Process Capability is a valuable tool for making changes to improve the production process. The capability of a production process is based on multiple factors which are common among almost all production processes The major factors are People, Machine, Methods and Materials. Each factor has its own process variation and contributes to the overall process capability.

In crimping a wire to a terminal, these factors above come into play and can be controlled at the plant level in different degrees.  For example:

Considering “People” as a factor, the manual locating of a terminal in a bench top crimp press is one level of variation. Training operators improves the process and is eliminated when an automatic processing machine is deployed.  Methods can range widely but could include maintenance schedules, operator and set up personnel training, set up and operating procedures.  Effects the Machine has on the process can include the machine age (wear and tear), maintenance and overall accuracy and repeatability of the machine itself.  Machines can include the crimp press, terminal applicator and in the case of an automated machine the robotic assembly to deliver a stripped wire to the crimp press.  The factor which the producer has less control is materials which include wire, terminals and weather seals.  In the case of materials, specifications are generally established by the material manufacturer within their own production process.

Crimp Press Capability

The machine component which can cause excessive variation in the process and which is the last area companies look to for process improvement is the crimp press itself.  Even with stable materials, methods, people and crimp applicators additional process variation can occur with the crimp press.  Presses have a long production life cycle and in that time bearing, crankshafts and ram assemblies can get worn. Which will show up as piece by piece variation.

The illustration at the left shows a chart of a crimp press which was calibrated and the press was cycled to determine peak crimp force per piece.  The top chart shows the shut height measurement and the bottom chart shows reference force.  Excessive variation can be found in the force chart on a piece by piece basis.  The shut height measurement was in control where the force was not in control.

The illustration at left was the same press after maintenance was performed to tighten up the press ram assembly.  The force measurements were brought into statistical control.  The shut height measurement also improved statistically. The result of this improvement will be a process that is in better statistical control and a product which will perform consistently better piece by piece.

The tool used is the C&S PAL3001 which can calibrate a press to the industry standard shut height and take force measurement readings to determine machine capability.

Wire Process Specialties can supply your crimp validation equipment and in-process measurement and data collection for ongoing process improvement.  Connect You Way to WPS find out more.

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.

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 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.
 

Validating and monitoring the process of crimpimg a wire to a terminal is a very critical in today’s wire harness production environment. So critical that some industries have mandated the use of advanced validating and monitoring crimp technology to reduce exposure (and liability) to financial loss if a defective harness is introduced into a a product. in this three part series, we will explore some of the techniques and technology being used in wire harness facilities worldwide. In part one, we will cover pre-production crimp validation.

Crimp Height

Crimp Height is an initial pre-production test to determine if the crimp compression meets the manufacturer’s specifications.  Crimp Height micrometers are used as they have a point on the spindle which rests on the underside of the crimp.  The anvil side has a flat surface which rests on the upper side of the crimp.  Using standard micrometers or verniers can provide false reading as they may pick up the points of the bottom anvil mark and not the center bottom of the crimp.  Using digital micrometers allow the crimp height reading to be downloaded to data collection/analysis software.

Pull Testing

Pull Testing is a destructive test to determine if the wire is properly secured into the wire crimp of the terminal.  Pull test procedures require the insulation support to be peeled back so the pull test reading is based on the wire to terminal crimp only.  A motorized pull tester pulls the wire from the terminal at a steady rate which improves the accuracy and consistency of the crimp.  And a digital output provides the ability to collect and analyze the pull test data.

The model PT100 shown is used as a standalone pull tester, interfaced with a computer for data collection/analysis/capability studies or fully integrated into a plant wide validation and real time crimp monitoring system.

Crimp Cross Section

Once you have established your crimp height and pull test, do you assume everything you see outside is the same inside the crimp?  Manufacturers of terminals work hard to ensure the wire to crimp size match properly and there is a science to establishing the proper shape and size of the crimp to fit a specified wire range.  But outside of the manufacturer’s work to determine the proper crimp size to wire, other factors come into play which can affect the crimp:

• wire OD which can vary within a specified wire range.
• tool wear.
• wrong tooling used or wire size used which does not meet the terminal specifications.

The effect of these changes can cause the wire to be unevenly distributed inside the crimp.  Something which cannot be determined without doing a cross sectional analysis.  Here is an example:

In this case, the wire crimp looks normal but the wire distribution is not even.

Did you know, this effect is one factor which can cause false readings on in-process monitoring of crimping using crimp force monitoring tools? False readings can be rejecting good parts or accepting bad parts.  The right wire gauge for the terminal, consistent wire distribution within a crimp and the proper crimp forming tools can provide consistent crimp quality.

Crimp cross sections are created by cutting the wire from the terminal, grinding/polishing the terminal down to the approximate mid point of the wire crimp and using etching chemicals to prepare the end of the wire.  The crimp is loaded to a microscope and a digial image is produced.  Further analysis include a calculation of the crimp area. A cross section is a pre-production analysis tool used in some critical applications but can be costly on a regular basis.  Having the documentation on a crimp cross section on the introduction of a new wire termination is a good practice (mandated in some industries) as it provides an understanding of what to expect from the crimp monitoring process. Wire Process Specialties can assist in the equipment required for processing cross sections in house or through our Cross Sectioning Service.

Data Collection

Data Collection is a critical part of pre-production and production to ensure your crimp process is meeting your specifications.  Data is used for validation prior to production, capability analysis and to document crimping results for review by your quality and production personnel, customers and external quality auditors to verify compliance with national and international standards.  Data collection can be as simple as using the tools above to manually or automatically recording results, within a local inspection station where all tools are combined to provide a bigger picture of the crimp quality.  For larger operations, client/server systems provide crimp specifications to the inspector, measurements are made and validated before production can commence.  Then in process measurements are taken and stored electronically across all processing machines plant wide.   In part two, I will outline typical tools for in process quality monitoring of wire to terminal crimping.

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.