Capability - Process vs Product - Look at process characteristics to improve product?


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For process capability studies one measures product characterisitcs to determine if process is capable then improve the process. Is there an opposite where you look at process characteristics to improve product?

I am on a team, because I am the ISO Coordinator, looking into Capability studies. I have limited experience in this area, just enough to be dangerious. Thanks in advance!


Feedback Loop

Can't say I ever heard of formally doing this, but after some discussion here we decided that yes, in some cases process studies can improve product. It mainly boils down to communication.

Normally, engineering decides what the product needs to be then production manufactures to those requirements. However, if process capabilities are shown to be well within tolerances, and the company has a good PDCA cycle so that engineering gets good feedback, then the engineers may be able to use tighter tolerances to improve product features such as weight, ease of use, reliability etc.

Hope this helps.


Atul Khandekar

I believe, since it is the process that produces a product, it should be possible to measure and monitor process characteristics to improve the product. Especially, in cases where it is not possible to directly measure the product characteristics we should be able to measure process parameters, identify the cause/effect relationships and correlate them with product quality. I think this technique is used in some chemical processes and software development processes. In toolroom or short/small run production batches where we may not obtain enogh samples/readings in one run, it would pay to concentrate on the process rather than the product. I remember having read about it in one of Thomas Pyzdek's books. Unfortunately I do not have the reference right here (posting from home), but I will certainly look it up tomorrow and post if there is any relevant information.

- Atul

KenK - 2009

Of course it will work. This is the very foundation of what Six Sigma (and the pile of methods that were around before it for those no so thrilled with the SS label) is all about.

This website gives some of the fundamentals of what I call process characterization - a somewhat standard approach to process improvement via process measurement and optimization:

Don't forget to also do measurement system analysis before you start measuring stuff.

Here is another site too:

(broken link removed)

This approach has been modernized and generalized into today's DMAIC process.

Atul Khandekar


The book I referred to is:

"The Six Sigma Handbook" - Thomas Pyzdek (McGraw-Hill, ISBN 0-07-137233-4)

He has discussed this approach as applicable to Short Run, Small Run and even Single Piece Processes.

The key seems to be:

- Planning: Prepare a Process Control Plan (PCP) before going into production, by thinking about all those process elements (sources of variation) that may have any bearing on the output characteristics.

- By preparing a PCP the inputs to the process are controlled, thus assuring that the outputs from the process will be consistently acceptable.

This essentially is the same as what has been said in this thread already.

There are also some examples of a CNC machining process, Wave soldering process, and software development process in the book.... Well,.... impossible to quote the whole chapter here!:)

- Atul.

Al Dyer

It is very doable and should be done. As opposed to part dimensions being the outcome of the process, their are areas of the process that can be monitored using basic SPC techniques and hopefully related up to the end product.

If you are drilling a hole in a piece of steel you want the end product to be the correct diameter. One way to prove and improve would be to do a process study on the drilling function of the machine. Basically called Mean Time Between Failure.

If you are running product, it has already been approved for shipment and you need to continuously improve the process performance, which is usually the machine used to manufacture the product. In doing MTFB to improve the end product you would also be covering any preventive maintenance requirements in effect.

Small projects can lead to large improvements!:bigwave:

venkat - 2011

I would like to know the methodology of drawing the process capability charts with UCL and LCL values.

Can any of the viewers help me with an example and explain step by step (or) any website references please


You can get the basic information from the AIAG SPC manual. Once you have collected your data you can plug it into an excel spreadsheet and chart the results. Or you can purchase a canned SPC package.


The continuous process improvement via process measurement and optimization is a way to make the output of our process more consistent, and that the objective of the capability analisys, not only to make the caracteristics of the products get into the specifications, but to reduce the variation, and that's SPC.

For charting capability charts, some people like to chart a normal distribution alone with the Lower and Upper Specification Limit, others to chart the frecuency histogram with the normal distribution and limits, and others don´t like capability index and preffer the use of box-plot. The box-plot could be usefull for visualization acrosss the time, I charted the index in an individuals and moving range control chart and the box-plot as a companion chart and I tink is very ilustrative.

I hope this help;)
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