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  FMEA and Control Plans
  Special Process Characteristics

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Author Topic:   Special Process Characteristics
MSAFAI
Forum Contributor

Posts: 24
From:Tehran - IRAN
Registered: May 2001

posted 07 May 2001 04:03 AM     Click Here to See the Profile for MSAFAI   Click Here to Email MSAFAI     Edit/Delete Message   Reply w/Quote
Hello friends,

Any one can help me out with following:

How do you identify SPECIAL PROCESS CHARACTERISTICS using PFMEA's ?

Thanks
MSAFAI

[This message has been edited by MSAFAI (edited 07 May 2001).]

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D.Scott
Forum Contributor

Posts: 37
From:Wellington, OH USA
Registered: Feb 2001

posted 07 May 2001 08:29 AM     Click Here to See the Profile for D.Scott   Click Here to Email D.Scott     Edit/Delete Message   Reply w/Quote
Not sure what you are looking for here MSAFAI but I will offer the following:
Depending on the customer, there is sometimes a symbol or mark on the print showing SPCs. You would use the same symbol or mark (or a suitable mark of your own) to identify them on the PFMEA. - See appendix C of QS-9000.

If you mean how do you know which they are, the defination given by AIAG (PFMEA Reference Manual) is "A special process characteristic (e.g., critical, key, major, significant) is a process characteristic for which variation must be controlled to some target value to ensure that variation in a special product characteristic is maintained to its target value during manufacturing and assembly."

IMO, this would include anything shown with a dimension requirement on the print and anything identified by the customer as special.

Hope this helps.

Dave

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MSAFAI
Forum Contributor

Posts: 24
From:Tehran - IRAN
Registered: May 2001

posted 08 May 2001 07:45 AM     Click Here to See the Profile for MSAFAI   Click Here to Email MSAFAI     Edit/Delete Message   Reply w/Quote
THANK YOU VERY MUCH SCOTT.
PLEASE FIND MY REMARKS IN CAPITAL LETTERS BELOW:

quote:
Originally posted by D.Scott:
Not sure what you are looking for here MSAFAI but I will offer the following:
Depending on the customer, there is sometimes a symbol or mark on the print showing SPCs. You would use the same symbol or mark (or a suitable mark of your own) to identify them on the PFMEA. - See appendix C of QS-9000.

I DID NOT MEAN HOW TO IDENTIFY SPECIAL PROCESS CHARACTERISTICS ON THE FMEA'S. SUPPOSE YOUR CUSTOMER HASN'T DESIGNATED ANY SPC'S, HOW DO YOU IDENTIFY THEM USING YOUR FMEA'S?

If you mean how do you know which they are, the defination given by AIAG (PFMEA Reference Manual) is "A special process characteristic (e.g., critical, key, major, significant) is a process characteristic for which variation must be controlled to some target value to ensure that variation in a special product characteristic is maintained to its target value during manufacturing and assembly."

THIS IS OF COURSE TRUE. BUT STILL IT DOESN'T SAY HOW TO FIND OUT WHICH PROCESS CHARACTERISITICS ARE SPECIAL USING THE PFMEA'S.

IMO, this would include anything shown with a dimension requirement on the print and anything identified by the customer as special.

ANYTHING SHOWN ON THE BLUEPRINT WOULD BE A SPECIAL PRODUCT CHARACTERISITIC, WOULDN'T IT?

Hope this helps.

Dave



NICE DISCUSSING WITH YOU. HOPE TO HEAR AGAIN FROM YOU.

THANKS
MSAFAI

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Marc Smith
Cheech Wizard

Posts: 4119
From:West Chester, OH, USA
Registered:

posted 08 May 2001 08:03 AM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
-> SUPPOSE YOUR CUSTOMER HASN'T DESIGNATED ANY SPC'S, HOW DO
-> YOU IDENTIFY THEM USING YOUR FMEA'S?

If your customer does not specify any CCs (on which to do SPC), then you only have to worry about yours.

-> HOW TO FIND OUT WHICH PROCESS CHARACTERISITICS ARE
-> SPECIAL USING THE PFMEA'S.

There is no black-and-white which says something on the PFMEA has to be controlled. But - the PFMEA is meant to predict possible failure modes. If you know how to make an FMEA (have you had trqaining?) you will understand how you use the FMEA to decide which items you should or must control as well as the extent of control. FMEAs are supposed to be team efforts so the team would decide these issues.

-> ANYTHING SHOWN ON THE BLUEPRINT WOULD BE A SPECIAL
-> PRODUCT CHARACTERISITIC, WOULDN'T IT?

Not necessarily. Prints typically have many dimensions and often other callouts in notes and such. I often see prints with no critical or special characteristics defined but there are lots of dimensions, etc. On a complex part you may have 100+ dimensions with only 1 or 2 designated as critical or special.

Take a read through: /APQP/ and /FMEA/

Your questions lead me to believe you need FMEA training. You might want to purchase the FMEA (and possibly the APQP) guide files at: /courses.html

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MSAFAI
Forum Contributor

Posts: 24
From:Tehran - IRAN
Registered: May 2001

posted 08 May 2001 09:09 AM     Click Here to See the Profile for MSAFAI   Click Here to Email MSAFAI     Edit/Delete Message   Reply w/Quote
Dear Marc,

Thank you for your attention.
I think because of the language barrier, I could not make myself understood properly.

Your comments are enlighting and very useful. But I think I have to explain some points:

1- By SPC's, I meant "special process char" not statistical process control.
2- you are right. Our customer hasn't specified any special process characteristics, therefore, we are trying to find out on our own.
3- I've had a bit of modest training on FMEA and I fully understand the basics.
4- May I explain what I actually meant?
a- High severity rankings in PFMEA's can indicate presence of special PRODUCT characteristics. Do you agree?
b- many of the causes can be considered as PROCESS characterisitics, like conditions of the die, temperature set, speed etc. Do you agree?
c- Now my question was can we decide among so many causes which are "SPECIAL PROCESS CHARACTERISTICS"?
Or maybe this premise that PFMEA helps in identifying special process characteristics is wrong?

Can you suggest a method to identify special proces characterisics?

5- You are absolutely right. Not every dimension on the blue print is a special product characterisitc. The point I was trying to make was that: what comes on the blueprint CAN only be a special PRODUCT charaterisitic, not a process characteristic?

I hope this clarifies my point.
Thanks again for your kind attention.

MSAFAI

[This message has been edited by Marc Smith (edited 08 May 2001).]

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Al Dyer
Forum Wizard

Posts: 622
From:Lapeer, MI USA
Registered: Oct 2000

posted 08 May 2001 05:56 PM     Click Here to See the Profile for Al Dyer   Click Here to Email Al Dyer     Edit/Delete Message   Reply w/Quote
MSAFAI,

You posted:

"....The point I was trying to make was that: what comes on the blueprint CAN only be a special PRODUCT charaterisitic, not a process characteristic"?

Hypothetically we have a customer that notes on a blueprint that they want a certain product machined on an XYZ Machine only.

I believe that would be a process related item, not a product characteristic. I realize this example is a stretch and would like to hear more.

We usually keep it pretty simple (as we have no call-outs for special characteristics) and define our own special characteristics a give them the proper (our decision) notation on control plans and FMEA's.

A good many of the internal special characteristics we identify do not affect the final customer but the internal customer which for us is the next operation in the process.

ASD...

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Marc Smith
Cheech Wizard

Posts: 4119
From:West Chester, OH, USA
Registered:

posted 08 May 2001 06:16 PM     Click Here to See the Profile for Marc Smith   Click Here to Email Marc Smith     Edit/Delete Message   Reply w/Quote
-> a- High severity rankings in PFMEA's can indicate
-> presence of special PRODUCT characteristics. Do you
-> agree?

Yes - typically.

-> b- many of the causes can be considered as PROCESS
-> characterisitics, like conditions of the die, temperature
-> set, speed etc. Do you agree?

Yes. In injection molding, for example, critical characteristics *may* include injection time, hold time, hold pressure, mold temperature, hotrunner temperature (if you're using hotrunners), etc. Turn speed and feed speed are sometimes critical in cutting operations. But again, it depends upon the product. If you're injection molding small 2 inch toy dolls for cracker jack boxes you're not going to be real critical about some parameters. On the other hand, in airbag cover injection molding, particularly because of the tear strip characteristics (which are *very* critical), all I mentioned above and more are critical to the process and thus have to be monitored and controlled including calibrated thermocouple systems, etc.

-> c- Now my question was can we decide among so many causes
-> which are "SPECIAL PROCESS CHARACTERISTICS"? Or maybe
-> this premise that PFMEA helps in identifying special
-> process characteristics is wrong?

When determining what YOUR critical or special characteristics are you first look to the document which defines what, to your company, should be a critical or special characteristic. Then you look at everything and ask for each one "How important is this and why?" Is hold time critical? What are your tolerances.

Now look at your PFMEA and ensure what you think may be critical are on the pfmea. Remember that to do this, someone has to be on the PFMEA team who understands the process and knows what, at least in general, is critical. For example, if injection molding you shold have a process engineer (or similar) who understands injection molding and understands the product. For example, I'm not an injection molding expert, but I know (from experience) what is typically important and what is not with respect to a specific product (which I think I exampled herein with injection molding). Now the pfmea team looks at the items and 'goes to work'. This is a 2" doll. Hold time will probably not be critical. Mold temperature may be important but there's a wide tolerance on the temp so maybe we don't need calibrated instruments but do track temp on a 'reference' gage. There would be no reason to use a hotrunner for such a product. Injection pressure is probably like temperature. You're probably going to get a good fill-out (assuming a rather simple mold) with a wide range of pressures (we're looking at the 'tolerance' here). Flow is not an issue (flow is typically a critical issue in airbag covers because of the tear strip criticality - you can't have any flowlines at the strip). If you don't have an 'expert' your PFMEA team cannot work.

-> Can you suggest a method to identify special proces
-> characterisics?

Specifically what process and what is the product?

-> what comes on the blueprint CAN only be a special PRODUCT
-> charaterisitic, not a process characteristic?

Yes - there can be a mixture of product characteristics, materials requirements, processing requirements, packaging requirements and other 'requirements'. Lots of stuff *can* be put on a print.

[This message has been edited by Marc Smith (edited 08 May 2001).]

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MSAFAI
Forum Contributor

Posts: 24
From:Tehran - IRAN
Registered: May 2001

posted 12 May 2001 05:28 AM     Click Here to See the Profile for MSAFAI   Click Here to Email MSAFAI     Edit/Delete Message   Reply w/Quote
Thank you Al Dyer and Marc Smith for taking time to answer my questions.

I learned a lot and I think I got my answers.

Hope to discuss again.

Thanks
MSAFAI

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