In wire harness manufacturing no single process is more important than the terminal crimping.  But how do you know you have a good crimp? Well there are several ways we’ll show you but in the end there are two end goals; Low electrical resistance in the crimp joint and strong mechanical strength of the joint.

Electrical Test

To assess the electrical properties of the crimp the milli-ohm meter should be connected to the terminal close to the crimp and on the other end on the wire as close to the crimp as possible (cut the wire back and strip it close to the crimp, we want to minimize the resistance of the wire itself).  Because of this requirement this test is generally considered destructive in nature and not useful for in process checking of connections, but rather used during setup and spot checks. A four wire kelvin connection should be used here to eliminate the effective resistance of the test leads.

Pull Force Test

The pull force test is one of the most basic tests you can perform on a crimp joint, it involves securing the terminal to some sort of fixture and then clamping the wire and pulling it with constant speed to measure the maximum force applied before the wire breaks or the crimp fails.  This result is then compared to industry standards such as UL486 (an example would be an 18AWG wire should be able to withstand a minimum of 20lbs of pull force on the conductor crimp). This test is performed normally on the conductor crimp only, if you have a insulator crimp it should be disabled to get accurate results.  This test is great except for one thing, it is destructive, you are testing until failure, even if you stop at the minimum specs you have put strain into the assembly and it would be very time consuming to do this for every crimp anyways so it is really used more often as a setup test and a spot check during a production run.

Crimp Height Measurement

Another test that can be performed is a crimp height measurement.  Once a good crimp is established and proven out with other methods you can measure the total crimp height for the specific terminal, wire, and tooling combination to use as a reference value.  Then subsequent crimps can also be measured and compared to the initial value to determine if the crimp was properly set. The crimp height measurement will identify tooling wear, setup mistakes, or press inconsistency, it is also non destructive, which can be great for spot testing a production run without scrapping materials, however it is time consuming and not easily automated without expensive camera based systems.  Crimp height measurements also require some amount of operator skill to get consistent results so some training is required to get the most out of it.

Cross Section Analysis

Cross section analysis is most often used as part of a final setup verification or to diagnose problems with a setup.  This is where the crimp is cut in half to show a cross section of the crimp. This area is then polished up and put under a microscope to check for wire strand compression, look for voids in the crimp, and verify the crimp flags aren’t contacting the floor of the terminal.  Cross section analysis is a destructive and time consuming test, often times only used to validate a completely new setup or to understand what might have happened to an old setup that is no longer meeting specifications.

Crimp Force Monitoring

Crimp force monitoring is now widely used in industry as it allows for fast, automatic testing of every single crimp in a production run.  Crimp force monitoring (CFM) basically graphs the force vs time profile during a crimp cycle and compares that to a known standard. CFM can detect all sorts of faults such as a broken wire strand, a missing wire strand from the crimp area, an improperly stripped insulator, a missing weather seal, worn tooling, improper setup, incorrect wire material, and incorrect terminals loaded.  And because it happens during the crimp without any intervention from the operator it does not require any additional time at all to perform and can automatically alarm out the operator, or trigger a scrap call in an automated machine so that bad parts are never used in production. The CFM system does however rely on the operator already having a known good setup that was validated using other methods.  The system just compares it’s known good sample data to the current crimp data, it can not aid in initial setup.

Putting it all together, the Terminal Crimp Quality Control Process

Given all the tools at our disposal, this is how we typically setup and then monitor our crimp process.  

1. Loosely crimp some terminals to our desired wire material and gradually increase the crimp force and decrease the crimp height as we go until we reach acceptable pull test results (meet UL 486 minimums)
2. Switch over to measuring electrical resistance on the next batch of samples.  Electrical resistance should decrease as we compress our crimp up to a certain point, however once we start to over crimp our terminal we’ll see the resistance actually start rising again.  Using this data we can determine our range of crimp heights that are acceptable.
3. Another pull test is performed to verify we are above UL486 standards.
4. We then will go do a cross sectional analysis of a terminal with this setup and verify there are no issues or additional changes we need to make (such as tooling shape/size).
5. Now we will set the CFM system to learn this new setup and crimp 10 or so crimps so the system can learn what a good crimp looks like.  We will also measure the crimp height on these samples so we have a reference value for a good crimp with this setup.
6. That’s it, we can now run our production cycle with the CFM system monitoring all the parts produced.  We can spot check as needed by performing additional pull tests or crimp height measurements but it shouldn’t be required if the CFM system is setup properly.

Summary

While the process may seem involved it’s important to remember just how dependent your product is on the wire harness performing correctly under a variety of variables; vibration, chemicals, strain and stress, flex, and other non-ideal conditions.  The end product is only as good as the wiring connecting it all and for reliable performance these quality standards are absolutely necessary. Modern equipment is covered in electronic systems and wire harness quality standards are more important than ever.  As always, thanks for spending a minute with us today, and let us know if you have any crimp quality questions we can help with!

So your wire harnesses are built, but how then are they tested and validated?  Here at Rim Rock Wire and Assembly we use an automated electrical tester that measures between each point on the harness to every other point on the harness to check for opens, shorts, resistors, diodes, etc.  A map of the harness can be generated within a second or two and compared to the original schematic to ensure that every harness manufactured matches the design or a sample harness exactly. This is far superior to a manual electrical test because it is far faster and takes human error out of the equation.  The tester will never forget a test point! And an electrical test is obviously far better than any visual inspection process because abnormalities can be detected electrically that might visually appear fine.

I get it, your harness is complicated, it’s not an off the shelf cable, but that’s okay, our test fixture has a base setup of 156 points that can be measured on the harness, but is expandable up to 2048 test points in a single scan.  These points can all be broken out into a special mating harness or board designed to exactly fit your harness with mating connectors, flying leads, etc so that every aspect of your completed harness can be verified.

So what exactly are we checking for once a wiring harness is completed?  Well, actually a number of things. First, every single point of the wiring harness is tested electrically against every other point on the harness.  This will map out the connections, and also rule out any accidental connections such as short circuits. Not only that, but all of those electrical checks are highly accurate resistance measurements that are capable of identifying connection points that have higher than expected electrical resistance that might indicate a problem with the wire or termination.  Additionally embedded components such as resistors or diodes can be identified, measured, and compared against the standard to ensure they are in the correct place, of the proper value, and pointed in the correct direction. And if that wasn’t enough macros can be added to test in multiple stages, useful if the harness is attached to a switch assembly, the full assembly can be tested in each switch position possible.  This data is all saved and can be recalled later so every batch of your harness will be tested in the exact same manner. Additionally an unknown harness can be identified using this system, so if you have a lot of similar harnesses and aren’t sure which one was returned to you, we could run it through and match it with an existing model for you (testing the returned harness for abnormalities in the process).

If an exception is found that part can be easily flagged and an exception report can be printed showing exactly what the problem was and what needs to be done to fix it.  This makes identifying problems fast and a decision can be rendered immediately about if the harness should be reworked or scrapped. Time saved means lower costs for us and you.  Additionally this 100% testing process can be integrated into a serial number labeling system so every harness could have a serial number assigned at time of testing and that label could be permanently affixed to the harness to allow traceability in the field.  This data is also logged and could be sent showing the test results for all your traceable cables, useful if your industry requires 100% traceability of your assemblies, or to comply with quality standards and metrics.

When you order your wiring harnesses through us, you can be confident that everything you receive will be perfect.  And we can prove it! Thanks for following, have a great week and please let us know if there is anything we can do for you or your business.

The term “wire processing” typically refers to preparing wire from the spool for it’s end use, and includes things such as measuring and cutting to length, stripping, twisting, and solder tinning.  Processing wire by hand is very time consuming and difficult to do accurately, which is why even for low volumes this work is typically done by machine. A machine can do simple wire processing (cutting and stripping) very quickly and prepare thousands of pieces per hour to incredibly tight tolerances.  

Materials.  Once of the most important factors in wire processing is the raw materials themselves.  The wire used should be of high quality, and be delivered on as large of spool core as possible or be pulled from a wire barrel.  Wire that is too tightly wound will be difficult to straighten before being fed into the machine. Another important factor in choosing wire is that the wire conductor is centered, and round inside of the insulation material.  Wire with strands outside the expected conductor area is easily nicked or cut by automated equipment and can make removing the insulation around strips more difficult. If the conductor is an oval shape the stripping blades can only cut as deep as the shallowest point to the conductor, meaning it will not cut all the way through the insulation and make removing the insulation slug more difficult and possibly cause some strands of conductor to pull out with it.

 

Pre-feeding.  After a proper material is chosen you must feed that wire into the machine properly, meaning at the correct tension, and after straightening the wire as much as possible.  A pre-feeder is often used to dereel the wire from the spool and take up any slack so that the wire processing machine gets wire at a constant tension even though it will often alternate between rapidly feeding wire and completely stopping the wire feed to perform operations on it.  The pre-feeder acts as a buffer that can speed up the wire pull from the spool and brake the spool as well as having a supply of wire already available for the machine to pull at any given moment.

Straightening.  Wire that isn’t straight will not get processed very well, the machines do everything possible to minimize the distances between a feeder and a blade or wheel, but if the wire has a lot of curl to it, it may not get placed in exactly the right spot for whatever operation is taking place.  Most wire processing machine include a set of wire straighteners on them, but if your wire is particularly difficult to work with you may need to add additional straighteners to get acceptable results.

Cutting and stripping.  The heart of any wire processor is cutting and stripping.  While typically done with a single set of blades for both operations, there are special circumstances where multiple blade sets are used, this can extend the life of a set of

 blades, or allow for stripping operations on difficult material such as PTFE where a more concentric cut may be required than standard V-blades can produce.  While most machines can be programmed to cut and stip both ends of a piece of wire, there are also machines that can strip out a “window” in the middle of a piece of wire, but for cost purposes it is usually better to spec your harness without these types of strips and instead do a splicing operation to keep costs down. Stripping can be done in two ways, either a full strip where an entire lug of insulation is pulled off, or a partial strip, where the insulation is cut and pulled back part way but left on the end of the conductor.  Partial strips are great for when the wire won’t be immediately terminated and will prevent the strands from fraying out during transport or packaging.

Other operations.  Today’s wire processing machines can do some amazing things but some of the more common secondary operations include twisting pairs of wires, stripping multi-conductor wire and cable, tinning stripped wire with solder, and even being integrated into other operations through automation.  While these additional features can cost quite a bit up front in equipment, they can save amazing amounts of time on higher volume jobs and should be considered if you are doing more than 10,000pcs.

Crimping is probably the most important process in wire harness manufacturing, it is the typically the sole interface between the wire carrying the signal and the where the harness connects to the next component.  A single poor crimp can ruin an entire harness and a typical wiring harness could have dozens or even hundreds of crimps, any one of which could be the weak link causing a failure. This is why having perfect crimps, every single time, is the most important aspect to manufacturing a wiring harness and why you should use a profession for your wiring harnesses if at all possible.

What is a crimp joint?  A crimp typically is how the connection between the wire and the terminal inside of a connector is formed.  The metal terminal will have some sort of flags or crushable barrel that gets pressed onto the wire conductor to form a mechanical and electrical connection between the two.  Then these terminals will typically be inserted into a plastic connector shell to complete the wiring harness assembly.

Types of Terminals.  There are three popular terminal types, open barrel, closed barrel, and insulated closed barrel terminals.  

Open barrel terminals are the most popular type and are usually just made from pressed metal that is formed into the shape of the terminal pin, plus the retention feature, and then the two sets of “flags” that are wrapped around the wire when crimped.  These flags wrap around the wire to form an ‘M’ shape. The first set around just the bare metal wire conductor, and the second set around the wire insulation, this prevents the conductor from sliding out of the insulation if somebody pulls on the wire improperly.  The shape of these crimps is vitally important and you must use a crimper designed for open barrel terminals, and preferably the exact tool or die made for the exact terminal you are crimping. This is where it gets hard for the DIY person to produce a quality crimp, because without the exact right tool, the crimp will never be as strong as it should be, and may have much higher electrical resistance as well.  This high resistance can lead to heat, sparking, and arcing which can melt the connector and ruin not only the wiring harness but also whatever it was plugged into. The main advantages of open barrel terminals is that they are cheaper to produce, the crimping process can be highly automated with machines, and they are easy to inspect after the crimp to determine the crimp quality.

The next type of terminal is a closed barrel, this type of terminal has a round “tube” that the wire is inserted into , then this tube is crushed around the wire to form the crimp.  Much like the open barrel terminal it is important to use the correct crimping tool to form this crimp, there are many different styles available but it’s best to use the tool recommended by the terminal manufacture.  Different terminal types have different shapes they must be crimped into to hold the wire and maintain their integrity. The downside to the closed barrel style terminal is that because the wire must be inserted into the tube, it is harder to automate and high volume wiring harnesses will typically choose not to use closed barrel terminals if it can be avoided.  There are some industries that require closed barreled terminals to be used however so check your specific requirements.

The final terminal type, insulated closed barrel, is typically not used with plastic connectors, but rather on single wire connectors such as spade, ring, or bullet style connections.  These are exactly like the other closed barrel terminals except that they will have a crushable plastic around the barrel as well. When this type of terminal is crimped it is usually with a color coded crimp tool that matches the color of the terminal.  The colors (red, blue, yellow) will correspond to the wire size that should be used as well. These types of terminals are far more universal than other terminals as far as the tools that can be used to crimp them, but they do suffer when it comes to strength and repeatability.  

 

Quality.  Crimp quality comes down to two main characteristics that can be measured.  First is mechanical strength, is the wire secured to the terminal sufficiently that it can be expected that it will never loosen up or slip out?  This is most often measured with a destructive “Pull test” that measures the amount of force required to break the connection between the wire and the terminal.  There are a number of ratings based upon the size of the wire, the application, etc to benchmark your pull test results to. The second consideration is electrical connection.  You want the electrical connection between the terminal and the wire to be as low of resistance as possible to ensure a minimum of heat transfer or signal loss between them. Milli-ohm meters can be used to measure this resistance to ensure a quality connection in a non-destructive way.  Other methods of verifying a crimp quality is to cut the crimp open with a grinder and view the cross section under a microscope, called a cross section analysis. Another is to measure the crimp height and compared to a known good crimp. And finally camera systems can be used to perform visual inspections that can look for any number of faults, and measure all final dimensions of the crimp joint.

Possibly the most important decision to make when building a wiring harness is the wire itself.  Choosing a wire involves knowing the environment the wire will be used in, how much current it is expected to carry, how long of run that current needs to travel, what types of load or signal is going to pass through it, how the person installing or serving the product in the future will identify the wires for certain components and much more.  

Material.  The first decision that is likely to be made is to choose a wire material or type.  There are many choices of wire and you can start to narrow down what you need by starting with if you need a single conductor wire, or if you need some sort of specialty wire/cable such as a twisted pair (for differential signals such as CAN Bus), or maybe a standard impedance cable such as a coax cable, etc.  While typically cables will not be included inside a wiring harness assembly, it is possible to do so. To simplify things we’ll assume you’re picking individual conductor wire for your project, and if any specialty cables are to be used, you’ll understand the requirements those must meet.

For automotive wiring harnesses PVC (Polyvinyl Chloride) wire is typically used for general purpose wiring.  PVC will typically be good for -40degC to +85degC and have pretty good abrasion resistance as long as it is properly secured, and any places where there is movement (door jams, etc) thick rubber strain reliefs and protectors are used.  PVC wire is also typically available in two insulation thicknesses, standard (GPT), or thin (TWP). Thin insulation can be used in tight spaces or in connectors without a lot of extra room for insulation. GPT is most common insulation type.

The next most common automotive wire material is XLPE (Cross Linked Polyethylene).  XLPE wire is often times used in engine compartments, motorcycles, and other powersports and agricultural applications due to its higher temperature rating of -40degC to +125degC.  It is also more abrasion resistant than PVC and will hold up better in areas where chemicals or moisture is present. XLPE can resistant many types of oil, grease, gasoline, acids, and solvents.  This wire is also available in 3 thicknesses of insulation SXL, GXL, and TXL (Standard, thin, extra thin respectively).

If your application has special requirements there are many other types of wire available such as PTFE (Teflon), Silicone, Marine Wire (PVC), Fiberglass, etc.  Typically these wires would be used when you need very high temperatures, extreme flexibility, or special environmental considerations. PTFE for example can handle very high temperatures (-90degC to +260degC, is very abrasion resistant, can handle many chemicals, and is required for many military, aerospace, and commercial applications.  If you have any questions about what type of material you should be considering feel free to reach out to us for help.

Color and Marking.  Colors aren’t just about looking good when it comes to wiring harnesses, colors fill an important role in identifying individual circuits to make finding and fixing problems in the future far easier.  If a vehicle has a problem in the future and the circuit must be traced back the service technician can refer to a wiring schematic of the vehicle and see what color wires are involved in the circuit he or she is troubleshooting to make finding them much easier.  Usually these wires are buried in the vehicle and wrapped in tape or some other covering material making it impossible to track a connection by any other method. There are usually around 12-14 standard colors, but the wire vendor can often also stripe the wire in another color to produce dozens of combinations so that every circuit can be an individual color combo that is unique to it alone in the vehicle.  Additionally wires can also be marked with labels, printed on, or laser etched to further identify them as is required in some industries such as aerospace.

Size.  Wire can be produced in a huge variety of sizes and it is important to size your wire correctly to ensure safety, correct operation, and to keep costs down.  Wire is not sized by the outside dimension of the finished wire and cable, but rather by the cross sectional area of the conductor only (the metal part inside the insulation).  This is important to understand if you are choosing your wire to fit into an existing application or replace a portion of existing wiring. You must measure the actual conductor, not the outside of the wire.  As mentioned in the materials section, the size size wire may be available in 2 or 3 insulation thicknesses. Using a wire that is too large for a given application will cost more due to more copper (or other conductor) being used.  Sizing a wire too small will cause electrical friction (think about a pipe that is too small), this results in higher resistance, and thus result in a higher voltage drop across the wire, and a lot more heat. The electrical resistance of different wire sizes is easy to obtain and by simply using Ohms law you can calculate the voltage drop at different current levels as well as the power loss (Voltage drop * Current) that the wire will have to dissipate.  If for example you calculated that a particular size wire will cause a 60W power loss, think about grabbing an old fashioned 60 w incandescent light bulb after it has been on a while, you’ll get burned! The same thing can happen in a wire and it will self heat and if that temperature gets higher than the rating on the insulation the insulation will begin to melt off and pose a fire, electrocution, and burn danger. There are three common wire sizing standards, AWG, SAE, and Metric (cross sectional area).  Another size consideration would be the thickness of the insulation, which is usually dictated by the dielectric withstand it is rated for. Or put more simply, the voltage potential you can have on either side of the insulation without an arc burning through. Most automotive wire types are rated to withstand 300v or 600v, which is plenty for an automobile that typically has around 12v in most circuits. If dealing with AC power from a building, or a high voltage circuit like a spark plug wire, you will need to pay far more attention to the insulation thickness and type for a given application.

As you can see picking wire isn’t as easy as stopping by the local home depot and grabbing a spool of “blue”, but we’re here to help if you need help designing or manufacturing your wiring solution so don’t hesitate to contact us for help!