Six Sigma and Beyond: Design for Six Sigma, Volume VI

The Process Failure Mode and Effects Analysis (process FMEA) is a method for identifying potential or known processing failure modes and providing problem follow-up and corrective actions.

OBJECTIVE

The process FMEA is a disciplined analysis of the manufacturing process with the intent to identify and correct any known or potential failure modes before the first production run occurs. Once these failure modes are identified and the cause and effects are determined, each failure mode is then systematically ranked so that the most severe failure modes receive priority attention. The completion of the process FMEA is the responsibility of the individual product process engineer. This individual process engineer is the most knowledgeable about the process structure and can best anticipate the failure modes and their effects and address the corrective actions.

TIMING

The process FMEA is initiated during the early planning stages of the process before machines, tooling, facilities, etc., are purchased. The process FMEA is continually updated as the process becomes more clearly defined. The process FMEA must be totally completed prior to the first production run.

REQUIREMENTS

The requirements for a process FMEA are as follows :

1. Form team

2. Complete the process FMEA form

3. FMEA risk ranking guidelines

DISCUSSION

The effectiveness of an FMEA on a process is dependent on certain key steps in the analysis, including the following:

Forming the Team

A typical team for the process/assembly FMEA is the following:

• Design engineer

• Manufacturing or process engineer

• Quality engineer

• Reliability engineer

• Tooling engineer

• Responsible operators from all shifts

• Supplier

• Customer

A design engineer, a manufacturing engineer, and representative operators are required to be team members . Others may participate as needed or as the project calls for their knowledge or experience. The leader for the process FMEA is typically the process or manufacturing engineer.

Describing the Process Function

The team must identify the process or machine and describe its function. The team members should ask of themselves , "What is the purpose of this operation?" State concisely what should be accomplished as a result of the process being performed. Typically, there are three areas of concern. They are:

1. Creating/constructing functions: These are the functions that add value to the product. Examples include cutting, forming, painting, drying, etc.

2. Improving functions: These are the functions that are needed in order to improve the results of the creating function. Examples include deburring, sanding, cleaning, etc.

3. Measurement functions: These are functions that measure the success of the other functions. Examples include SPC, gauging, inspections, etc.

Manufacturing Process Functions

Just as products have functions, manufacturing processes also have functions. The goal is to concisely list the function(s) for each process operation. The first step in improving any process is to make the current process visible by developing a process flow diagram (a sequential flow of operations by people and/or equipment). This helps the team understand, agree, and define the scope. Three important questions exist for any existing process:

1. What do you think is happening?

2. What is actually happening?

3. What should be happening?

Special reminder for manufacturing process functions: Remember, if the process flow diagram is too extensive for a " timely " FMEA, a risk assessment may be done on each process operation to narrow the scope.

The PFMEA Function Questions

Each manufacturing step typically has one or more functions. Determine what functions are associated with each manufacturing process step and then ask:

1. What does the process step do to the part?

2. What are you doing to the part/assembly?

3. What is the goal, purpose, or objective of this process step?

For example, consider the pen assembly process (see Figure 6.22), which involves the following steps:

1. Inject ink into ink tube (0.835 cc)

2. Insert ink tube into tip assembly housing (12 mm)

3. Insert tip assembly into tip assembly housing (full depth until stop)

4. Insert tip assembly housing into barrel (full depth until stop)

5. Insert end cap into barrel (full depth until stop)

6. Insert barrel into cap (full depth until stop)

7. Move to dock (to dock within 8 seconds)

8. Package and ship (12 pens per box)

Figure 6.22: Pen assembly process.

Remember that another way to reduce the complexity or scope of the FMEA is to prioritize the list of functions and then take only the ones that the team collectively agrees are the biggest concerns.

Describing the Failure Mode Anticipated

The team must pose the question to itself, "How could this process fail to complete its intended function? Could the resulting workpiece be oversize, undersize, rough, eccentric, misassembled, deformed, cracked, open , shorted, leaking, porous, damaged, omitted, misaligned , out of balance, etc.?" The team members are trying to anticipate how the workpiece might fail to meet engineering requirements; at this point in their analysis they should stress how it could fail and not whether it will fail.

The purpose of a process FMEA (PFMEA) is to analyze and evaluate a process on its ability to perform its functions. Therefore, the initial assumptions are:

1. The design intent meets all customer requirements.

2. Purchased materials and components comply with specifications.

Once failure modes are determined under these assumptions, then determine other failure modes due to:

1. Design flaws that cause or lead to process problems

2. Problems with purchased materials, components, or services

Describing the Effect(s) of the Failure

The team must describe the effect of the failure on the component or assembly. What will happen as a result of the failure mode described? Will the component or assembly be inoperative, intermittently operative , always on, noisy , inefficient, surging, not durable, inaccurate, etc.? After considering the failure mode, the engineer determines how this will manifest itself in terms of the component or assembly function. The open circuit causes an inoperative gage. The rough surface will cause excessive bushing wear. The scratched surface will cause noise. The porous casting will cause external leaks. The cold weld will cause reduced strength, etc. In some cases the process engineer (the leader) must interface with the product design engineer to correctly describe the effect(s) of a potential process failure on the component or total assembly.

Describing the Cause(s) of the Failure

The engineer anticipates the cause of the failure. The engineer is describing what conditions can bring about the failure mode. Locators are not flat and parallel. The handling system causes scratches on a shaft. Inadequate venting and gaging can cause misruns, porosity, and leaks. Inefficient die cooling causes die hot spots. Undersize condition can be caused by heat treat shrinkage , etc.

The purpose of a process FMEA (PFMEA) is to analyze or evaluate a process on its ability to perform its functions (part characteristics). Therefore, the initial assumptions in determining causes are:

• The design intent meets all customer requirements.

• Purchased materials, components, and services comply with specifications.

Then and only then, determine causes due to:

• Design flaws that cause or lead to process problems

• Problems with purchased materials, components, or services

Typical causes associated with process FMEA include:

Fatigue

Poor surface preparation

Improper installation

Low torque

Improper maintenance

Inadequate clamping

Misuse

High RPM

Abuse

Inadequate venting

Unclear instructions

Tool wear

Component interactions

Overheating

And so on

Estimating the Frequency of Occurrence of Failure

The team must estimate the probability that the given failure mode will occur. This team is assessing the likelihood of occurrence, based on their knowledge of the process, using an evaluation scale of 1 to 10. A 1 would indicate a low probability of occurrence, whereas a 10 would indicate a near certainty of occurrence.

Estimating the Severity of the Failure

In estimating the severity of the failure, the team is weighing the consequence (effect) of the failure. The team uses the same 1 to 10 evaluation scale. A 1 would indicate a minor nuisance, while a 10 would indicate a severe consequence such as "motor inoperative, horn does not blow, engine seizes, no drive, etc."

Identifying Manufacturing Process Controls

Manufacturing process controls consist of tests and analyses that detect causes or failure modes during process planning or production. Manufacturing process controls can occur at the specific operation in question or at a subsequent operation. There are three types of process controls, those that:

1. Prevent the cause from happening

2. Detect causes then lead to corrective actions

3. Detect failure modes then lead to corrective actions

Manufacturing process controls should be based on process dominance factors. Dominance factors are process elements that generate significant process variation.

Dominance factors are the predominant factors that contribute to problems in a process. Most processes have one or two dominant sources of variation. Depending on the source, there are tools that may be used to track these as well as monitor them. Table 6.8 gives a cross reference of the dominance factors and the tools that may be used for tracking them. The following list provides some very common dominance factors:

• Setup

• Machine

• Operator

• Component or material

• Tooling

• Preventive maintenance

• Fixture/pallet/work holding

• Environment

Statistical Process Control (SPC)

• X-bar/R control charts (variable data)

• Individual X-moving range charts (variable data)

• p; n; u; c charts (attribute data)

Non-statistical control

• Check sheets, checklists, setup procedures, operational definitions/instruction sheets

• Preventive maintenance

• Tool usage logs/change programs (PM)

• Mistake proofing/error proofing/Poka Yoke

• Training and experience

• Automated inspection

• Visual inspection

It is very important to recognize that inspection is not a very effective control because it is a reactive task.

Estimating the Detection of the Failure

The detection is directly related to the controls available in the process. So the better the controls, the better the detection. The team in essence is estimating the probability that a potential failure will be detected before it reaches the customer. The team members use the 1 to 10 evaluation scale. A 1 would indicate a very high probability that a failure would be detected before reaching the customer. A 10 would indicate a very low probability that the failure would be detected , and therefore, be experienced by the customer. For instance, a casting with a large hole would be readily detected and would be assessed as a 1. A casting with a small hole causing leakage between two channels only after prolonged usage would be assigned a 10. The team is assessing the chances of finding a defect, given that the defect exists.

Calculating the Risk Priority Number

The product of the estimates of occurrence, severity, and detection forms a risk priority number (RPN). This RPN then provides a relative priority of the failure mode. The higher the number, the more serious is the mode of failure considered . From the risk priority numbers , a critical items summary can be developed to highlight the top priority areas where actions must be directed.

Recommending Corrective Action

The basic purpose of an FMEA is to highlight the potential failure modes so that the engineer can address them after this identification phase. It is imperative that the engineer provide sound corrective actions or provide impetus for others to take sound corrective actions. The follow-up aspect is critical to the success of this analytical tool. Responsible parties and timing for completion should be designated in all corrective actions.

Strategies for Lowering Risk: (Manufacturing) ” High Severity or Occurrence

Change the product or process design to:

• Eliminate the failure cause or decouple the cause and effect

• Eliminate or reduce the severity of the effect (recommend changes in design)

Some "tools" to consider:

• Benchmarking

• Brainstorming

• Mistake proofing

• TRIZ, etc.

Evaluate ideas using Pugh concept selection. Some specific examples:

• Developing a robust design (insensitive to manufacturing variations)

• Changing process parameters (time, temperature, etc.)

• Increasing redundancy, adding process steps

• Altering process inputs (materials, components, consumables )

• Using mistake proofing (Poka Yoke), reducing handling

Strategies for Lowering Risk: (Manufacturing) ” High Detection Rating

Change the process controls to:

• Make failure mode easier to perceive

• Detect causes prior to failure mode

Some "tools" to consider:

• Benchmarking

• Brainstorming, etc.

Evaluate ideas using Pugh concept selection. Some specific examples:

• Change testing and inspection procedures/equipment.

• Improve failure feedback or warning systems.

• Add sensors/feedback or feed forward systems.

• Increase sampling and/or redundancy in testing.

• Alter decision rules for better capture of causes and failures (i.e., more sophisticated tests).

At this stage, now you are ready to enter a brief description of the recommended actions, including the department and individual responsible for implementation, as well as both the target and finish dates, on the FMEA form. If the risk is low and no action is required write "no action needed."

For each entry that has a designated characteristic in the class[ification] column, review the issues that impact cause/occurrence, detection/control, or failure mode. Generate recommended actions to reduce risk. Special RPN patterns suggest that certain characteristics/root causes are important risk factors that need special attention.

Guidelines for process control system:

1. Select the process.

2. Conduct the FMEA on the process.

3. Conduct gage system analysis.

4. Conduct process potential study.

5. Develop control plan.

6. Train operators in control methods .

7. Implement control plan.

8. Determine long- term process capability.

9. Review the system for continual improvement.

10. Develop audit system.

11. Institute improvement actions.

After FMEA:

1. Review the FMEA.

2. Highlight the high-risk areas based on the RPN.

3. Identify the critical and major characteristics based on your classification criteria.

4. Ensure that a control plan exists and is being followed.

5. Conduct capability studies.

6. Work on processes that have C _pk of less or equal to 1.33.

7. Work on processes that have C _pk greater than 1.33 to reduce variation and reach a C _pk of greater or equal to 2.0.