Defects or nonconformities are often a result of multiple aspects of the manufacturing process not working as intended. There is however, a “root” cause which is primarily responsible for the defect/nonconformity.
In this post we will discuss the two main techniques that are commonly deployed for identifying the cause for a defect/nonconformity. These techniques work in conjunction with one another, though they can be employed separately.
The first thing we’re going to focus on is ‘Flowcharts‘ which are a symbolic/graphical representation of the actual process that takes place on the factory floor. Next, we are going to discuss ‘Cause-and-effect Diagrams‘ which are used to enumerate and categorize the possible causes for problems, hence making it easier to arrive at the root cause of a problem.
- Initiates discussion between personnel from all departments of the plant. Also, Establishing consensus about what actually happens not what’s supposed to ideally happen.
- Can reveal non-value-added activities such as redundant inspection and steps reworks, bottlenecks. With enough detail flowcharts can also reveal unnecessary movement and processing loops.
- Promotes clarity with a graphical representation of the steps in creating the product, while elucidating key details about the processes that personnel may be unware of.
- Makes everyone question all the steps involved in the process, leading to optimization.
- Allows for analysis at multiple levels, via macro, intermediate, and micro flowcharts.
- Highlights the quality characteristics that should or need to be monitored.
* From an SPC point-of-view,
Flowcharts help reveal the stages in the process where data may be collected.
- Cause-and-effect analysis helps identify main causes for problems and the steps we can take to solve them, once the pervious steps give us an indication as to what the problem is and where it might be occurring in the process.
- The best part is often the process not the results, as with any team effort. With representatives from all departments, each member can share their expertise and experience with the problem, they assists clarification of potential causes, and building consensus for causes.
- Circumvents the trap of “Pet Theories”, where people assert that they already know the cause of a problem without taking part in a collaborative analysis. If the person is wrong then it leads to a waste of time and resources, even if the theory is correct, it may be only partially correct, i.e., it might address a symptom or secondary cause rather than the root cause.
- Cause-and-effect diagram promote sense of ownership over the process and motivates everyone to implement and maintain the solution. Without a team effort, personnel may feel their input to problems is neither needed nor valued.
Flowcharts are invaluable in initiating discussions during Cause-and-effect analysis. The team effort helps get everyone on the same page, while revealing key details.
Ensure that everyone understands and agrees on the actual processes that really take place on the factory floor, rather than what is supposed to happen.
They also help to find opportunities to optimize the process at various locations and more data collection, while highlighting non-value-added activities.
Steps involved in creating a flowchart
* For future reference note the names of team members, the date, and the purpose for creating the chart should be included.
Step 1: Determine the start and stop points that the chart will cover. The level of detail should be maintained at a consistent level of detail, so it’s imperative to decide on it from the beginning. Macro-level flowcharts show key action steps but no decision boxes, intermediate-level flowcharts show action and decision points, while micro-level flowcharts also include intricate details.
Step 2: Brainstorm amongst all the team members, either allowing anyone to chime in with an idea or going round-about giving everyone the opportunity to contribute. Ask everyone to list all the steps involved between start and stop points of the flowchart, finally, put the steps in order.
Step 3: Draw the flowchart using the standard symbols to signify Inputs/Outputs, Processing, Decision, Storage, Delay, Data Entry, Movement, and Inspection. Each step needs to be labeled. Arrows should be used to indicate the flow of steps. Label branches of decision boxes, e.g., “Yes”, “No”, etc.
Step 4: Test the accuracy and completeness of the resultant flowchart, by making sure,
- All symbols are used correctly
- Steps are identified correctly and clearly
- Process loops are closed (i.e., every path flows to a logical end)
- If process box has more than one output arrow, consider a decision diamond
- Get external review done
Step 5: Look for opportunities to improve the process by comparing the actual process to the ideal process, in order reduce non-value-added activities and eliminate,
- Unnecessary redundancies including bottlenecks, multiple inspection points for the same quality characteristic, etc.
- Many Movements, for example to a staging area, to storage, to holding area, to production, etc.
Suggestions for eliminating non-value-added activities using flowcharts
* Macro flowcharts lack the level of detail required for optimizing the process.
- Rearrange sequence of steps
- Change work methods
- Redesign forms and documents
- Improve supervision
- Eliminate unnecessary steps
- Rearrange physical location of the operator in the system
- Change type of equipment used
- Improve operator training
- Consolidate process steps
Once flowcharts, or through other team activities, have identified points in the process where the problems might occur, as well as where measurements were currently being taken.
To be able to address this problem,
i. identify the root cause
ii. test potential solutions
Types of Cause–and–Effect Diagrams
There are two common formats used for Cause-and-Effect diagrams,
Focuses more on structuring the diagram according to the major possible causes for nonconformities such as machines, methods, materials, operators, and environments.
This format focusses on enumerating the steps involved during the production process such as incoming inspection, ripping, sanding, moulding, etc.
Steps involved in creating a cause-and-effect diagram
Step 1: The problem/nonconformity/effect needs to be clearly stated and understood by everyone. There needs to only be one effect that’s being examined at a time.
Step 2: Select one of the two available formats as your format of choice, either dispersion analysis or process classification.
Step 3: Before brainstorming for the possible causes, based on the format. First, draw a blank cause-and-effect diagram, with only the major categories filled in.
Step 4: The team will now brainstorm potential causes that may fall under each category, defined in the previous step. Though, it’s typically done in a rapid-fire manner without regard for which category each cause falls under. Asking questions about the manufacturing process can lead to a healthier discussion.
Step 5: Once all the various potential causes for the effect being studied have been listed, go over the now filled in cause-and-effect diagram in search for the root cause. This will involve a lot of discussion and a healthy debate amongst the team members. At some point everyone will arrive at a natural conclusion, either by exhausting all possibilities or reaching a consensus.