[This is a companion article to “Using Design FMEAs to Identify Special Product Characteristics.” These two articles should be read in sequence.]
Identification and application of special product and process characteristics help to focus assembly and manufacturing processes on the most important areas, in order to achieve the right outcomes.
“Science is the process that takes us from confusion to understanding” – Brian Greene
What is a process characteristic?
From chapter 5 of Effective FMEAs:
Process Characteristics are process variables and parameters that have a cause and effect relationship with the variation found in product characteristics. Examples include mold temperature, cycle time, pressure, flow rate, tool speed, etc.
Are some process characteristics more important than others?
Yes, certain process characteristics are more important than others, and have a disproportionate influence on product safety and performance. Identifying the most important characteristics is an essential step in controlling them, and achieving safe and reliable products. Although there is no universal standard defining process characteristics, many companies identify special process characteristics.
What are special process characteristics?
The Merriam-Webster dictionary defines “special” as “readily distinguishable from others of the same category; unique.”
Special process characteristics are unique process-related characteristics that can affect the ability of the manufacturing process to meet special product characteristics. They are input to a given manufacturing operation.
How are special process characteristics identified in PFMEAs?
One of the objectives of a Process FMEA is to identify special process characteristics. To understand how special process characteristics are identified in Process FMEAs, we have to begin with the definition of “cause” in a Process FMEA.
By definition, a cause in a Process FMEAs is the potential manufacturing or assembly deficiency (or source of variation) that results in the failure mode. Think about that for a minute. What is a process deficiency? Deficient means insufficient or inadequate. A process deficiency is something inadequate or insufficient about the manufacturing or assembly process. This can often (not always) be related to a process characteristic: process variables and parameters that have a cause and effect relationship with the variation found in product characteristics.
If the Process FMEA team properly identifies causes as process deficiencies, and if the risk associated with the process deficiency is sufficient, the associated process characteristic can be a candidate for being designated as a special process characteristic.
How are special process characteristics used in PFMEA?
In order to understand how special process characteristics are used, we need to take up the subject of Process Flow Diagrams and Process Control Plans. The following paragraphs are from chapter 5 of Effective FMEAs.
A Process Flow Diagram is a graphical representation of all of the process operations that result in the manufactured or assembled product, and are within the scope of the Process FMEA project. This is essentially the process hierarchy, including manufacturing and assembly operations, shipping, incoming parts, transporting of materials, storage, conveyors, tool maintenance, and labeling, and any other steps of the operations that are within the scope of the Process FMEA. Each of the process operations is represented by a symbol representing the type of operation, such as Fab, Move, Store, Get, Inspect, Rework, Scrap or Contain, and the symbols are connected in the precise sequence of the operations in the manufacturing or assembly process. Use of such symbols is easily tailored to company needs and policy.
A Process Control Plan (PCP) is a “summary description” of the methods used in the manufacturing environment to minimize variation and control product and process characteristics in order to ensure capability and stability of the manufacturing process. It is a structured approach for the design, selection and implementation of control methods, and reactions to problems with the manufacturing and assembly operations when they do occur.
Below are examples of the application of special process characteristics, showing how to organize and display the important information relating to special characteristics and map them to the Process FMEA and Process Control Plan.
Example of application of special product and process characteristics
In the companion article “Using Design FMEAs to Identify Special Product Characteristics” an example DFMEA is described on a shaft, which is part of rock grinding equipment. One of the concerns is shaft fracturing, resulting from low shaft hardness due to material heat treat incorrectly specified. The DFMEA team chooses to make “shaft hardness X” as a potential special characteristic, and follow up in the recommended actions to increase shaft hardness (by using more rigorous heat-treat standard #ABC) and deploy shaft hardness X as input to the Process FMEA and corresponding Process Control Plan.
The Process FMEA team fills out the Process Flow Diagram Worksheet, including special product and process characteristics. (If special process characteristics are not known at the time when Process Flow Diagram Worksheet is first developed, they can be updated later.)
Example of Process Flow Diagram for shaft hardening process:
They proceed to the Process FMEA for the operation: Induction harden shafts. One potential Failure Mode is “Shaft hardness less than BHN X,” with Cause “Induction machine electrical voltage/current settings incorrect for part number.” “Induction machine electrical voltage/current settings: V1/A2” becomes a candidate Special Process Characteristic.
Example of PFMEA for shaft hardening process step:
In this example, both the special product characteristic (shaft hardness X) and the corresponding special process characteristic (Induction machine electrical voltage/current settings: V1/A2) are entered in the Process Control Plan, along with other information that assures the manufacturing process is stable and capable.
Example of Process Control Plan for shaft hardening:
One of the important inputs to a Process FMEA is special product characteristics that are associated with the item being manufactured or assembled. The Process FMEA team should ensure that the Design FMEA team provides this information, as part of PFMEA preparation.
FMEAs are legal documents that support the demonstration of due care in product development. FMEA teams should ensure their worksheets are consistent with good legal practices for documents. The next article in the Inside FMEA series outlines legal guidelines for doing FMEAs.