The process of crimping a wire to a terminal is a time tested electrical connection method. But gone are the days of a crimp being processed using low grade wire strippers and crimp tools. A wire lead requires tools and processes which are highly accurate and repeatable. A quality improvement process identifies the processing inputs that are used in an assembly. A baseline quality level is created and the company works to improve the overall process over time.
The quality of a crimp is the sum total of the process capability of five elements in the crimp process: Wire, Terminal, Crimp Press, Crimp Tooling and Operator.
To improve the overall quality, each individual element must be studied and improved. This article will outline some of the aspects of each element which can cause a less than desirable capability which in turn can affect the overall crimp quality. Also we will cover some of the typical tools used to assess and monitor the whole crimp process. This article deals with the crimp process for terminals on a reel but can equally apply to loose piece terminals.
The Five Elements of the Terminal Crimp Process
The wire is one of the material elements of a terminal crimp. Typically process variation from the wire comes from a few external sources but mainly downstream processing steps leading up to the operator presenting the wire to the terminal for crimping.
Overall wire quality is an external source of process varitiation.
- Does the supplier have the necessary quality systems in place to reduce process variation? This includes:
- individual wire strands
- overall wire inside diameter
- insulation material (consistency and overall outside diameter)
Changing wire suppliers or improving material quality can reduce variation.
Downstream processes can impact process variation.
- Wire stripping and handling:
- Manual wire stripping or incorrectly adjusted wire processing machines. Correct strip length or piece by piece variation.
- Blade Condition: cut strands
- Wire ends frayed from handling. Semi-stripped wire reduces damage to the stripped end of a wire.
The other material element of the terminal crimp is the terminal itself. Like wire, supplier selection and supplier process controls are critical. External factors (supplier side) affecting process variation from the terminal and packaging include:
- crimp (wire and insulation) barrels. Straight or misaligned
- terminals feeding straight on the terminal strip.
- can affect loading position in crimp applicator
- packaging (reel and carton) protecting the terminals from damage from handling and de-reeling.
The selection of the proper wire range to match the terminal crimp sections is very critical. Terminal manufacturers design the geometry of a crimp to match the wire size. This includes the wire and insulation crimp sections. Mismatched terminal to the wire (gauge and insulation OD) is a major cause of process variiation.
The crimp tooling includes the crimp punch and anvils as well as the feeding mechanism (the applicator). Some of the major factors in process variation are:
- worn crimp tooling
- feed track not adjusted properly, causing terminals not to feed straight.
- wire stop too far forward or backward. Or excess play in the wire stop itself. Causing variation in target position of the wire to the terminal. Failures are typically high insulation (insulation crimped in the wire crimp), low insulation (insulation not fully captured in the insulation crimp). Or inconsistent piece to piece variation.
The crimp press is the one processing tool which typically receives the least attention. Even if all of the above (wire, terminal and crimp tooling) are in control, press wear can cause excess variation. Wear on the crank or casting of the press can cause the press shut height and crimp forces to vary. This can create piece to piece variation. In excessive cases of wear, crimp height and pull test can go out of specification which can cause failure of the electrical connection.
The operator is the final element of the five. The operator can cause variation due to improper loading of the wire to the applicator and not monitoring the inbound de-reeling of the terminals from the reel, through the applicator to the crimp tool. The operator can check the terminated parts and without controls in place has the power to accept rejected parts. Or not fully inspecting the batch of wires that have been processed. In the case of an automated machine where the wire placement is handled by a robotic arm, the wire placement is generally more consistent (assuming the machine is maintained and the servo motors or pneumatic actuation are providing repeatable wire position). Monitoring the terminal de-reeling and inspecting the crimped wires is still an operator based process.
Operator training and process monitoring can improve process variation.
Process Improvement Tools
Crimp Height Micrometers
A specially tooled micrometer with a pointed spindle and flat anvil. The spindle is positioned on the underside of the terminal to avoid false readings from the anvil crimp tool impression on the terminal. A pre-process and in-process inspection tool. A model with output capability can provide data for capability studies and for data archive.
A destructive test. Checking the secure-ness of the crimp. It should be noted that most pull test specifications are for the wire crimp only. The insulation crimp must be peeled back prior to performing the pull test. A pull tester is normally a pre-process inspection tool. Output as described above is a good feature.
Crimp Cross Section
Cutting the crimp in half and viewing inside is becoming a required quality validation process in a growing number of industries. A cross section analysis provides valuable information on strand distribution within the crimp and overall compression analysis. Designed as a pre-production analysis tool as well as validating the crimp after changes to the process (any of the five elements).
Crimp Force Monitor
Statistically speaking, 100% visual inspection is only 80% effective, leaving a high risk of 20% of production not fully inspected. A crimp monitor measures each termination in real time and compares the results with initial samples of terminations inspected and validated for production. All five elements described above are monitored as a whole. Crimp monitors take the decision making process away from the operator and provide a base line for the crimp process as a whole. As a process improvement tool, single improvements to the process will show as an improvement in the variation of the crimp curve. Providing an opportunity to reduce the process tolerances to detect smaller variations. The crimp monitor is a standard in process monitoring tool for most industries. Even without a customer mandate, a crimp monitor is a good tool for in process inspection and process improvement.
Calibrating a press to the required shut height only assures capability to the single calibration cycle. Ensuring the press meets a statistically controlled capability is an extension of the calibration process and assures the press can provide the repeatability required. Typically companies calibrate and run capability studies once per year, semi annually or quarterly depending on the volume being processed. Analyzing a press’ shut height and reference forces also allows maintenance personnel to prioritize maintenance schedules.
Improving the overall quality of a terminal crimp requires an evaluation of all five process inputs. Assuming what previously was acceptable is still acceptable or what is seen on the surface is acceptable under the surface is not a good quality strategy. Implications can be as basic as a circuit failure or as broad as product failure causing damage to property or injuries to the end user. There are real life examples of these conditions occurring which have caused damage to people, property and your company’s reputation. The risks in today’s business conditions are real but can be greatly reduced by adopting and maintaining a comprehensive quality validation, monitoring and improvement strategy.
More information on quality validation and monitoring of the terminal crimp can be found in our three part special: Part One, Part Two and Part Three.
Connect Your Way to WPS to hear how we can support your wire processing requirements.