Process Control Yields Cost Effective Weld Integrity

Orbitform 1600 Executive Dr. Jackson, MI 49203

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By Greg Reed, Contributing Editor, Design News

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Resistance Welding TechnologyFundamentally, resistance welding combines a set of processes that collectively produce a coalescence of separate and distinct work pieces as a result of heat generated by the resistance of work piece materials to the welding current (100 to 100,000 A) plus mechanical pressure (up to 30,000 psi). As the current passes through the work pieces, a molten fusion takes place at the connecting surfaces which is the point of high-est resistance. A stable fusion is accomplished by applying enough current to create a molten state while applying pressure at this weld zone while cooling takes place as the current is withdrawn. The combination of current and pressure must be sufficient to create a solid weld, but not so much as to create an expulsion of molten metal from the weld zone.

During the welding process, many factors impact the welding temperature which ultimately influences the stability of the joined work pieces.

Controlling such factors as proportions and material composition of the work pieces, elec-trode materials, electrode geometry, electrode force, weld current, and welding time is vital to achieving structural integrity of the fused work pieces. Figure 1 offers a look at the variables associated with resistance welding and a typical welding cell where the activity takes place.

Within the realm of resistance welding tech-nology, several distinct methods can be identi-fied, each with its unique set of attributes that govern which applications are appropriate for a particular technology. Spot welding, seam weld-ing, projection welding, flash welding and upset welding are common methods used in automo-tive, industrial, aerospace and defense, medical, and many other applications across a broad spec-trum of industries.

Dan Shirkey, Lead Engineer (Orbitform Group, Jackson, MI) says, “A superior welding process control system must have inherent flexibility that allows users to match their unique process-ing methods with a standard process monitoring system. First, one must determine what process is optimal, and then one must determine how to optimize that process.

Welding Control ToolsAs seen above, resistance welding technology assumes many forms. Engineering teams con-tinually assess operations and then match the correct welding solution to each situational challenge. When the welding processes are performed correctly and consistently, the man-ufacturer’s efficiency is maximized. However,

Process Control Yields Cost Effective Weld Integrity

Today, resistance welding takes many forms across numerous applications in diverse market seg-ments. While each company must decide the most efficient welding technology for its various operations, the parameters that govern such decisions often change, sometimes rather quickly over short time intervals. This means decision makers must constantly monitor welding processes to ensure the right technology is applied to each application and done so in a cost effective manner.

By Greg Reed, Contributing Editor

A White Paper / 2008

Figure 1Resistance welding process variables include heat, cur-rent, resistance of work pieces, time, and heat loss. A work cell offers localized resistance welding operation.

an out of control process may result in wasted raw materials, energy overruns, excessive worker downtime, and idle equipment. As a result, a manufacturer could face substantial losses or severe fines when engaged in third-party con-tracts. In a worst case scenario, safety, accident, or injury situations could arise. Any of these may threaten the economic vitality of the manu-facturer, up to and including closing the plant doors. Clearly, a reasonable expenditure on the right system for monitoring with closed loop feedback provides a critically important tool that enables manufacturing success and early return on investment.

According to Jeff Losey, Senior Welding Engineer (Orbitform Group, Jackson, MI), “The key for any process control system is to record and review process variables; and then know when to take action that prevents process failure while also understanding why you are making changes to process parameters.”

Besides assessing welding operations in real time, process control tools should record data which can be reviewed or analyzed by process managers, track positive and negative trends in machine condition, allow remote access for view-ing fault history and trending graphs, and offer modular or expandable components to accommo-date lean cell or line operation environments.

Elements of Process ControlProcess control systems begin by gathering and recording accurate information via sensors. Once input into a system, the data is analyzed and then actionable outputs allow process man-agers to alter the operation to keep performance within specified parameters. However, break-downs involving misinterpretations between monitoring systems and process managers are not uncommon. The most common root cause of such breakdowns is monitoring systems’ fail-ure to convert data into actionable information.A perfect example of this “failure to communi-cate” is illustrated in Edward Tufte’s recounting of the NASA Challenger disaster in his book, Visual Explanations: Images and Quantities, Evidence and Narrative. In the book, he demon-strates how the NASA Challenger disaster may have been avoided if the Morton Thiokol engi-neers had displayed their temperature vs. o-ring

failure data in a meaningful way. The engineers performed admirably in collecting the data; however, a failure to translate the data into use-ful and actionable information was the source of their downfall.

RPAS for Resistance WeldingFor manufacturers that rely heavily on weld-ing operations, several vendors offer reliable process control systems. One advanced system from Orbitform Group is the Resistance Process Analysis System (RPAS), which is briefly sum-marized in Figure 2. It is a proven welding process control system that incorporates all key elements with advanced visualization to facilitate easy process management (Figure 3). The system acquires data from a wide variety of sensors (e.g. current, voltage, force/pressure, distance, etc.) featuring advanced flexibility (monitoring +/-10Vdc signal from any type of sensor) and scal-ability (configuration from 8 to 64 sensors). It identifies undesired process variations, converts

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Figure 2RPAS offers a complete welding process analysis setup.

Figure 3Key features to an advanced welding process control system keep equipment within established welding parameters.

A White Paper / 2008

the data into useful and actionable information, and communicates to the cell controller (e.g. PLC) over standard protocols. Recent process signature data and all historical summary data is available via any type of plant Ethernet (LAN/WLAN/VPN/WAN). The system alerts process managers with user-defined trend indicators, screen captures that detail out-of-sync process parameters, and alarms that prompt process adjustments before quality issues arise.

Converting Data into Actionable At the heart of the RPAS system, is a well-developed visual presentation architecture that graphically presents the system data in an easy to understand format. The advanced visualiza-tion delivers previously unavailable or difficult to understand information in real time so that

process managers can easily interpret problems and then take corrective action in time to restore welding operations back within process param-eters. Figure 4 displays a chart which contains a snapshot of sample data accumulated with a multi-sensor system installed on an automated robotic resistance welding cell used for the assembly of automotive components.

Jeff Losey points out, “The RPAS system con-verts process data into summary graphs that are easy to interpret and act on. Using RPAS accel-erates operators and process managers’ progres-sion down the learning curve. This acquired pro-cess intelligence pays earlier and more frequent dividends over the life of the process.

InformationAt the heart of the RPAS system, is a well-developed visual presentation architecture that graphically presents the system data in an easy to understand format. The advanced visualiza-tion delivers previously unavailable or difficult to understand information in real time so that process managers can easily interpret problems and then take corrective action in time to restore

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RPAS Specs Resistance Process Analysis System Product Specifications Processes

• Spot welding

• Projection welding

• Hot upset Sample Rate

• Up to 6 kHz Process Data Presentation

• Process signal (signatures)

• Summary data (trends) Debug, Benchmarking, Documentation

• Managing process limits, tear-down frequencies, and maintenance intervals ensures a good start to the life of the project Automatic Notification of Faults and Alerts

• Network communication to a PLC

• Email Continuous Improvement

• Documentation of the change of the improvements

• Narrowing limits to reflect established process improvements

• Predictive maintenance

• Reduce tear-down frequency Flexibility

• Up to 64 sensors (with additional input cards and junction boxes)

• +/- 10vdc sensors Traceability

• Historical data can be tracked to the second

• Improvements and potential defects are docu-mented and can be tracked easily Accessibility

• Web browser (view fault history and trending graphs)

• Live viewer (fault history, trending graphs, and signatures)

Figure 4In this chart, the vertical line shows the range of a linear transducer that is parallel to the weld cylinder on a transgun. The horizontal line displays time over a two-hour period. Data collected: 1) This gap shows an initial variation of .008 in. with electrodes com-bined (from electrode dressing). 2) Electrode re-dress creates break. 3) Dress time �8 seconds. 4) Missing component for .0�� in. 5) Downtime for electrode change 7 min. �� seconds. 6) .0�9 in. difference between new / old electrodes. 7) Process running con-sistently. 8) Duration of example—two hours.

A White Paper / 2008

welding operations back within process param-eters. Figure 4 displays a chart which contains a snapshot of sample data accumulated with a multi-sensor system installed on an automated robotic resistance welding cell used for the assembly of automotive components.

Jeff Losey points out, “The RPAS system con-verts process data into summary graphs that are easy to interpret and act on. Using RPAS accel-erates operators and process managers’ progres-sion down the learning curve. This acquired pro-cess intelligence pays earlier and more frequent dividends over the life of the process.

ConclusionSeveral challenges inherent to resistance weld-ing processes continually crop up across a broad spectrum of manufacturing situations. Rebounding back and forth between “hot” and “cold” welds, knowing when to replace cables, performing too many destructive tests, and trac-ing welds to specific shifts and times are com-mon issues. A welding process control system should always allow users to define process lobes, enable easy equipment benchmarks, alert process managers to abnormal data trends, and provide traceability to the millisecond.

To resolve these issues, a highly accurate data acquisition system must gather reliable inputs from numerous properly placed sensors. Moreover, the display methodology must render

the data in a manner that allows operators and managers to both interpret process anomalies and then take corrective actions. If the process control system fulfills these needs, productivity will be maximized, return on investment will be realized quickly, and profitability will be ensured for welding processes.

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Resistance Welding Selection Criteria As described in the main article, several varia-tions of resistance welding such as spot welding, seam welding, projection welding, and upset welding are possible, each appropriate for spe-cific applications.

In assessing the value of resistance welding, ben-efits include: high speed, easy automation, high production rates, and cost effective operations.

Some compromises may include: investment in equipment, lower tensile and fatigue strength, and added weight and material.

An optimized welding process will eliminate the following problems: cracks, electrode deposit on work pieces, porosity or cavities, pin holes, deep electrode indentation, improper weld pen-etration, surface appearance, weld size, and irregular shaped welds.

The RPAS system documented herein offers one effective method to optimize resistance welding processes so that benefits may be fully real-ized while common problems can be eliminated or minimized to the extent they remain within specified process parameters.

A White Paper / 2008