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Original Date: 07/25/2005
Revision Date: 09/12/2006
Best Practice : Qualified Manufacturing Lines
Northrop Grumman Electronic Systems implemented a Qualified Manufacturing Line process that has allowed the company to realize significant savings in material (reduced rejection rates), in man-hours, and in reduced chemical usage and associated costs. A Qualified Manufacturing Line is a statistical process control method used to manufacture and accept a product that has statistically stable and capable machine processes.
Northrop Grumman Electronic Systems (NGES) implemented a Qualified Manufacturing Line (QML) process with significant success. This is a control method used to manufacture and accept a product that has statistically stable and capable machine processes. This documented top-level policy provides for:
Process characterization implementation
Determination of line qualification status
Interim acceptance prior to qualification
Initial process qualification
Maintenance of a qualified process
Performance of process inspections
A process failure-and-findings system
Action plan for findings
Dequalification of the process
Below the top-level policy, each production area develops a process characterization map that becomes the foundation of the QML. Each of the production areas has its process characterized along with the elements that are under control limits. A Control Team is assigned to each production process consisting of a manufacturing engineer and both quality assurance and operator personnel who use the QML to manage the production process.
The QML is designed to eliminate breaks in the process flow. Monitoring the controls and process inspections versus stop-gate sorting is part of this system. This has led to improved process understanding by the operators and decreased lead-times for parts produced in that section of the facility. NGES’ QML has four levels of qualification that are clearly defined in policy and are constantly displayed on the plant floor by a “light stick.” This “light stick” is a stoplight-type fixture that indicates one of four conditions as defined by the plant’s QML policy: “No Light” means an unqualified process; “Red Light” means a major finding is identified and the production line is stopped until corrective action is implemented; “Yellow Light” indicates that a minor finding is identified, production resumes if previously stopped, and the process is monitored at greater frequency; and “Green Light” indicates the process is operating in a normal, acceptable manner and normal monitoring is in process.
Other advantages of the QML include:
The plating line, where an automated tool was developed to monitor and adjust vat concentrations that were under Statistical Process Control (SPC) cycle times was optimized for tank temperatures and concentrations. The rejection rate went from two to three parts per week to one part in three years. Chemical usage was reduced 50% over a 5-year period, and more than 1,600 man-hours per year were saved.
Sheet metal punching became statistically controlled, with both upper and lower control limits placed on stamping tolerances. A standard coupon was developed, with three random job audits performed daily. This process found that machines required more frequent adjustment than manufacturer-recommended to stay in tolerances. The rejection rate went from two to three parts per week to two to three parts per year. Labor hours saved using the surveillance audit versus traditional inspections exceeded 1,200 hours per year.
A new laser welding process was developed to weld pins to thin sheet aluminum. This process met design requirements for conductivity and mechanical attachment of pins to .016-inch-thick aluminum. An audit inspection of hundreds of pins per sheet using sampling, machine parameters, and coupon destructive test was used. Without this process, a major radar program would not have been effective.
A Kitamura machining process was previously controlled by incremental post-process inspection, with final part 100% inspection. There were hardware backlogs and process variations. After the process was put under QML control, machine behavior was statistically evaluated over time for both thermal and tool wear. Variations and corrections were found to be at the limits of the machine’s capability. Special in-process gauging was developed to control key machining characteristics. The rejection rate went from 15% to 20% to 1%, and man-hour savings of 360 hours per year were realized.
NGES has realized significant savings in material, cycle time, and inventory with the implementation of its QML, ensuring a continued competitive advantage for many of its products.
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