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April, 2012


Moving from Quality Assurance to Quality Control


By Bernard Sutton, Product Marketing Manager, Valor Division, Mentor Graphics Corp., Wilsonville, OR Q


uality assurance (QA) has long been the recognized term and method for manufacturing qual-


ity processes. But we should question this, as additional benefit can be gained from focusing on Quality Control (QC). These benefits can move quality from a “non value- added cost” to actually supporting lean manufac- turing and cost reduction. It might seem odd that to sug-


gest differentiating Quality Assur - ance from Quality Control, when many people interchange the two terms to mean the same thing. But the presumption that the two are the same leads to organizations missing the cost reduction opportunity that quality, in particular, Quality Con - trol can bring.


How QA Evolved


Many of us have seen the evolution in action; there is an issue with product quality. Our first reaction is to add another quality check. Each new QA point gives us the warm feeling that we are doing something to remove defects and improving quality. In fact what we have done is add


costs, production delays and further opportunities for process defects. The additional costs are easy to under- stand along with the additional delays caused by adding another process step. But we have also added opportu- nities for process defects, as no inspec- tion or test is perfect. For all quality processes there


are four possible outcomes:


1. Bad products fail — a correct outcome. 2. Good products pass — a correct outcome. 3. Bad products pass — an incorrect outcome. 4. Good products fail — an incorrect outcome.


There can be several reasons


why we have the incorrect outcomes, but here we will examine the results of these outcomes, which can be to cause “good products” to be scrapped (outcome 4) and “bad products” to be labeled as good (outcome 3). Once we understand these possi-


ble outcomes, we can see that adding more “assurance tests” not only adds to the direct production costs but also to indirect costs related to the incor- rect outcomes. For anyone trying to implement “Lean Manufacturing” all this is bad news unless we also ques- tion the need for additional “Quality Assurance” operations as our re -


Complete monitoring of the line allows quick corrective actions and analysis of line performance.


facturing” route, where we strive to do only those activities that add value or cannot be avoided. This should exclude unnecessary Quality Assurance operations and their asso- ciated handling, data recording and later QA analysis. Somewhat like a golfer hitting a


good shot, if we keep our eye on the ball and focus on the process of hit- ting the ball, we can predict the out- come. Similarly, we can use Quality Control to predict the outcome of manufacturing and, at the same time, lower the overall cost of manu- facturing.


Defining Quality Control Unfortunately you will still see


definitions of Quality Control advocat- ing inspection as the primary feature of QC. See Wikipedia for an example. The more progressive approach


is to implement “control” methodolo- gies. Control should mean there is only one way a product can be processed through manufacturing and that is the prescribed route with the prescribed skills and resources. Poka Yoke, which we have learned from Japan, has long advocated the mistake-proofing of manufacturing, implemented in simple steps like placing the correct parts in a logical


nectivity we can now look beyond Poka Yoke in each work cell for the next advances in Quality Control. Therefore we can cite definition


for Quality Con trol as: “The monitor- ing and controlling of the manufac- turing execution to a specific opera- tions plan at specified oper- ations points with specified re sources of defined capa- bility.”


There are some funda-


mental building blocks of control. The first is to have a plan specifying the exact route that manufacturing products need to follow and the resources needed at each point to correctly process the product. While an obvious step, many peo- ple can miss the signifi- cance of having a plan that will be executed exactly and repeatedly. The plan needs to have a clearly defined flow through the engineer- ing and quality functions, usually starting in the ERP work order level and then on to association with the operations plan for that product type. The plan should also associate the operations and equipment sets with the required instructions and operator skills.


No


Loop count monitor


Repair


sponse to quality issues. From this simple premise we


can see the route to lowest cost man- ufacturing is also the “Lean Manu-


sequence of bins in each assembly cell. Poka Yoke has been very effective in reducing errors within a work cell. With advances in shop floor con-


Absolute Routing Control The most common form of prod-


uct routing control is the “paper trav- eller”. Routing control done by paper travellers is a simple and efficient solution for low volume, slow moving, manufacturing organizations where work typically progresses in small batches associated with a fixed and linear build plan and usually fixed batch quantities. This solution does,


however, have some major drawbacks: l


it contains. l


and only accessible on examining the paper traveller. This means only those who can put their hands on the product batch and its paper traveller can extract the knowledge


All actions are recorded manually


traveller completed. l


next goes unmonitored until the final operation is completed and the


pleting the paper traveller. l


Incorrect process routing is only noted by the diligence of those com-


for repair and retest can only be managed by holding back the whole batch or more commonly by raising new paper travellers splitting the batch — something that can take time and is prone to error. Absolute and auditable routing


Changes to the required routing


control requires automated monitor- ing and control of the actual process


Yes Assemble Inspect Test Pass? Next Operation Progress from one operation to the


Demonstrating a predetermined repair


and re-inspection point for that repair on failure at test, with the addition of “loop count monitoring”.


route in real time to the pre- deter- mined operations plan. The plan should also allow for conditional rout- ing on pre-determined events like a test failure at a specific test process.


Restricting Rework Loops Each item within the batch can


be allowed to follow the predeter- mined failure route while allowing the test passed items to proceed and Continued on page 58


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