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Understanding Process Variations in Manufacturing Process to Control Product Quality

Variations exist everywhere and in manufacturing process where consistently good quality is desired in output of mass production goods,  we would need to manage the source of variations.  

The best finished goods should be  consistently fullfill the specification of quality characteristics such as dimension of a product (figure b).  If finished products have too much variations, it means there could be parts which does not meet lower and upper specification (LSL and USL) and consider as reject per illustration (figure a).


In process,  input is required to make an output,  for manufacturing process the inputs are materials,  man, methods, machine, measure and environment known as 5Ms and 1E.  



Each of the process input has variation and will be reflected in the finished products.  If process inputs has too much variation then final product will have inconsistent quality with a lot variation too. Therefore we will need to control process input variations to enable us to get good quality output overtime with very little variation.  This type variation is known as a common or natural cause variation where we can predict the product's variation.  If the process inputs  has too much variation and unpredictability  then we will have unstable and unpredictable quality product.

A simple example would be a paper cutting process  where we need to cut a big piece of paper into paper strips of  3 cm width and 10 cm length. The table show what are the process input , process and output.

Inputs 5Ms 1 E
Process
Output
Man - operator who cut the paper






Paper cutting






Strips of paper 10 X 3 cm
Materials – big piece of paper
Machine – scissors, pen,  fixture
Methods -  work instruction
Measure – inspection process to ensure finish part is according to specs
Environment – ambient temperature/ humidity

If all process input are controlled well per below,  then the resulting cut paper with will be predictable  with control  common cause variation in paper width and length measurement :-

  1. Man - operator are well trained with proper training plan
  2. Material -  paper from reliable supplier with consistent  thickness and hardness
  3. Machine -  scissors are sharp, fixture well maintained
  4. Method -  there is step by step work instruction for trained operator to follow
  5. Measure -  measurement process that does not impact product
  6. Environment -  control temperature and humidity

You can predict the outcome only if the process is stable

Common cause variation in cut paper  quality feature Width/length  -  preferred process.




 Cannot  predict the outcome only if there is too much process variation
Special  cause variation  with unpredictable product Width/length variation  -  unwanted process


Table below compare the differences between 2 types of variations in  manufacturing process - common cause and special cause :-


Common cause
Special cause
Exist all the time
Exist when one process input is out of control or change
Contributed  by all process input variations
Could  be contributed  by one or two sources of process input  
Cannot be eliminate but can be reduced
Must be eliminated to get good quality  product

Easy to detect its presence
Do not impact product quality
Adverse effect on product quality
Popular approach use to reduce common cause  could be six sigma
Approach use to eliminate special cause  is corrective action preventive action methodology such as 8D or 5C problem solving, error proof, kaizen

Major source of  variation in manufacturing comes from process input 5Ms and 1E.   In order to increase the opportunity to  produce a predictable and consistent good  quality products we must be able to detect and eliminate special cause variations and control common cause variation.  Before we can control source of variations we would need to understand and able to detect where the source of variations come from. 

Besides manufacturing process variations, there are other source of variations throughout the supply chain  which also contributed to product quality.  For this article we shall only focus on understanding major sources of variations from manufacturing process.  I will post an article on detection of variations in manufacturing process in the future.

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