Harnessing High-Mix, Low-Volume
As manufacturers shift high-volume wire processing to countries with low-cost labor, domestic wiring harness assemblers are struggling to survive on the remaining work, which consists primarily of low-volume, high-mix jobs. Because this type of production requires more frequent changeovers, reducing setup time has become more important than ever.
Two main factors determine the productivity of a wire processing operation. One is the production rate of the wire processor. The other is the average changeover time.
The production rate is always an important consideration when selecting a wire processor. How important depends on the typical job size and how frequently the machine is stopped to produce a different job.
Changeover time becomes very critical when lot sizes get smaller. For a typical wire processor, the total time required for changeover is the sum of mechanical setup time and programming time.
Mechanical setup time depends on the design of the wire processor. Modern machines require fewer tooling changes to run different jobs. Some machines do not require hand tools to complete a changeover, so the operator can save time by not hunting for tools.
A fair amount of production time can be lost unloading the previous wire and replacing it with a different type, size or color of wire. Productivity can be increased with better job planning to minimize the frequency of wire changeovers.
All wire processing machines use blades of one type or another to cut the wire and strip the insulation from the conductor. The most common types are V blades, radius V blades, die blades and rotary blades.
V blades are universal for cutting and stripping operations. The blades overlap to form a square- or diamond-shaped opening, and the machine can be programmed to close to any diameter within a specific range. This makes the V blade very versatile, covering a wide range of wire sizes. Using V blades whenever possible will help shorten changeover times.
Radius V blades are used to strip thin insulation from large wire sizes. These blades have some adjustability and can cover a small range of wire diameters. Radius V blades are usually changed when switching to a different wire size.
Die blades are wire-specific and are typically used for Mil-Spec work. The blades close until they butt against each other, and they cannot be adjusted. These blades are changed every time a different wire type or size is run.
Rotary blades are universal. They can be programmed to close to any diameter within a specific range. Rotary blades can also be used to process coaxial or other multilayer cables. While rotary blades usually need a little more time to process wires, this is offset by the time saved by not changing blades between jobs.
Guide tubes direct the wire through the transport system and center it between the blades during cutting and stripping. Guide tubes are available in different diameters, and the operator usually installs one that is slightly larger than the wire to be processed.
Some wire processing systems can process flat cable, zip cord, multiconductor cable, power cords, and other odd-form wires. Typical operations include axial slitting, window stripping and combing the inner conductors for further processing. Most of these specialized applications require custom blades and tooling that must be installed for each job.
Programming time is the total time required to recall or program all parameters required to run the next job. Typical program parameters include wire length, strip length, blade depth, wire transport speed and acceleration, and stripping speed and acceleration. Optional program parameters include marking text and positions, window stripping, multistep stripping, and slitting.
Most wire processing machines have a user interface consisting of a keyboard and a display. When the operator receives a work order, he typically calls up a program from the machine's memory that has some basic parameters for the particular wire to be processed. Because the memory of a typical wire processor is limited to a few hundred jobs, it's impossible to store and recall parameters for all jobs, so templates are used. The operator then enters the parameters (overall length, strip length, quantity and batch size) from the work order. A few wires are run and checked to verify conformance with specifications before running the rest of the job.
The total time for entering the parameters and completing quality checks is operator-dependent, but it usually takes 2 to 3 minutes. Direct machine programming can be time-consuming, and data entry errors are possible. When machines are programmed directly, jobs are typically run as work orders are received throughout the day or week. Operators have no tools to help sort jobs by wire size and type, so they cannot plan their work to minimize the number of changeovers.
Basic wire processing software lets manufacturers connect a PC to a wire processor. This allows wire processing jobs to be stored on the PC, overcoming the limited memory of the wire processing machine. With a PC, memory is limited only by the capacity of the hard drive. Another benefit of basic wire processing software is the ability to program jobs off-line and the ability to upload or download jobs to multiple wire processing machines.
While these capabilities are desirable, they fall short of what's required to achieve much greater savings in production time and costs. Here's why. For any given wire, total cycle time is the sum of production time and changeover time, divided by the lot size.
Now let's look at two examples. In Example 1, a wire processor can produce a 4-inch long wire with an 1/8 inch strip on each end in 1 second. The changeover time between jobs averages 180 seconds. Total cycle time is 181 seconds for a lot size of one wire and 1.81 seconds for a lot size of 100 wires.
In Example 2, a wire processor can produce a 4-inch long wire with an 1/8 inch strip on each end in 1 second. But, the changeover time between jobs averages 60 seconds, instead of 180 seconds. Total cycle time is 61 seconds for a lot size of one wire and 0.61 second for a lot size of 100 wires.
Reducing changeover time from 180 seconds to 60 seconds reduces the cycle time by 66 percent for one wire and 43 percent for 100 wires. However, the cycle time savings drop to 18 percent for 500 wires and only 10 percent for 1,000 wires. In general, the smaller the lot size, the more impact changeover time has on total cycle time. For batch sizes of one to 500 wires, the largest productivity gains can be realized by any means that can reduce the changeover time between jobs.
Better Processing Through Software
Wire list management software (WLMS) has all of the features of basic wire processing software. It also has additional features to help reduce changeover time.
For example, WLMS can import wire lists. Wiring harness manufacturers use production planning software, CAD software, Excel and other programs to keep track of wire lists. WLMS can import wire lists from these programs, eliminating the need to manually re-enter data. This saves a lot of time and prevents data entry errors. Many companies have a goal to move toward a "paperless factory." Using "open" software platforms can help assemblers achieve this goal by making it easy to transfer data from one system to another.
WLMS can also maintain libraries of raw materials. Many parameters must be programmed into a wire processor for it to optimally process a particular wire. Many of these parameters are directly related to the wire size and type. They include the outside diameters of the conductors and wire; the optimum handling speed and acceleration for cutting and stripping; and the ideal pressure and gap settings for the transport system.
WLMS can store the optimum machine settings for each raw material in a library organized by name, wire size and part number. When operators recall a specific wire from the raw material library by name, all of the associated processing parameters for that wire are used to automatically set up the wire processor. If a parameter change is made for a wire in the raw material library, any job that uses that wire will automatically use the new values the next time the job is run.
Not only can WLMS store wire lists, but it can sort them, too. A wire list may consist of all the wires necessary to produce a wiring harness, or all the wires that a particular wire processing station must produce in a given day or week. Using WLMS to sort wire lists by wire size, type and color will reduce the frequency of changeovers, maximizing productivity.
The WLMS can be set up to stop the machine after every wire or batch or only when a different wire must be loaded. It can be set to produce all of the wires necessary for one wiring harness, or it can be set to produce each wire on the list in the desired quantity before proceeding to the next wire.
Control Wire Marking
WLMS can control more than just cutting and stripping equipment. It can control wire markers, too. Wire harness customers often require that some wires be marked with terminal identification information, part numbers, serial numbers, date codes, bar codes and other information. The marking positions (from right and left ends and along the wire), as well as the marking text, can be easily managed by the WLMS, eliminating the need for manual entry.
The most common wire markers are hot-stamp and ink-jet systems. Laser systems are available, too, but their use is generally limited to Mil-Spec work.
Hot-stamp markers use heated character wheels and marking foil to mark directly on the wire insulation. The wire must stop during the marking cycle. Typically, the operator must manually change the character wheels to change the marking text.
Ink-jet markers print directly on the wire using a jet of ink. Many fonts and sizes can be marked on the wire. Because the wire is moving while being marked, ink-jet markers are faster than hot-stamp markers. A major benefit of ink-jet markers is the ability to produce a wire harness entirely from white-colored wire, instead of many different colors. Terminal identification information can be marked on each end of the wire and additional information, such as wire part number, logos and serial numbers can be printed along the length of the wire.
This approach can produce tremendous savings, because the number of colors for a given type and size of wire can be reduced from 10 to one. The frequency of wire changeovers is also reduced by that same factor. Additionally, this approach reduces purchasing, storage and handling costs.
WLMS can control both an ink-jet marker and a wire processing machine. It coordinates what alphanumeric text gets placed where on each wire, taking into consideration the offset distance of the ink-jet marker relative to the cutting blades on the wire processing machine. Such a system can efficiently produce one of each of the wires necessary to build a single wiring harness, making it ideal for just-in-time production. All the wires can have different overall lengths, strip lengths and marking text. The entire group can be produced without any operator intervention at an average speed of 2 seconds per wire, depending on wire length.
By contrast, if the operator had to manually program the inkjet marker and the wire processing machine for each wire, it would take an additional 1 to 2 minutes per wire, instead of an average of 2 seconds per wire.
Instead of marking the wire insulation, some customers require that labels be wrapped around the wire. Like hot-stamp systems, the wire must stop while the labeling machine applies the label. But, the position of the labels on the wire can be managed by the WLMS.
Some WLMS packages can display a scaled graphic depiction of the wire to be processed, including color, stripping values, marking text and window strips. This capability is valuable, because it allows the operator to quickly verify that the machine is correctly programmed for a given job before producing any bad pieces.
Like basic wire processing software, WLMS allows off-line programming, so future jobs can be prepared and reviewed while running current jobs. This is an excellent way to reduce changeover times. Wire lists can be easily sent over wired or wireless networks, eliminating the need for hard copies of work orders.