Surface Finish on machined brass (or other metals)- Attribute or Variable inspection?

ScottK

Not out of the crisis
Leader
Super Moderator
#1
This is directly realted to another thread I started about Surface Finish Capability http://elsmar.com/Forums/showthread.php?t=26288

I'm wonder what practice is out there.

In my particular application I don't see why it can't be an attribute.
As long as the surface finish is below, say, 64Ra the part will be fine. It will function the same at 63 as it will at 4.
So even though I'm "measuring" why shouldn't my device be a go/no-go?
You can do attribute gauge R&R and attribute capability studies.

Please poke holes in my logic and tell me why I might be wrong.

why do I ask? because there are people out there that are demanding that if I do not meet 1.33 Cpk based on variable capability studies (see previous thread I reference) I need to do 100% inspection.
And that's really getting up my nose because I know from history that once the machine is set, barring a crash that will mess up every other characteristic on the part , I will not exceed the maximum even if the range is such that I am below even 1.00 Cpk.
AND all of the finished parts that this component goes into are 100% tested as part of hte assembly process anway!!!
(ok - that last bit was a rant, but I'm really :frust: )
 
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Duke Okes

#2
Re: Surface Finish on machined brass (or other metals) - Attribute or Variable?

And that's really getting up my nose because I know from history that once the machine is set, barring a crash that will mess up every other characteristic on the part , I will not exceed the maximum even if the range is such that I am below even 1.00 Cpk.
By definition, and assuming a normal distribution, if the Cpk is 1.0 or less you will have some parts out of specification (estimate would be 100%-99.73%, or 27 out of 1000 for a Cpk of exactly 1.0).
 

ScottK

Not out of the crisis
Leader
Super Moderator
#3
Re: Surface Finish on machined brass (or other metals) - Attribute or Variable?

By definition, and assuming a normal distribution, if the Cpk is 1.0 or less you will have some parts out of specification (estimate would be 100%-99.73%, or 27 out of 1000 for a Cpk of exactly 1.0).
yes - I understand that.
All too well I understand that.
And, if you look at the other thread, I do not believe Cpk is warranted in my case.
 

Jen Kirley

Quality and Auditing Expert
Leader
Admin
#4
Re: Surface Finish on machined brass (or other metals) - Attribute or Variable?

In my particular application I don't see why it can't be an attribute.

As long as the surface finish is below, say, 64Ra the part will be fine. It will function the same at 63 as it will at 4.

So even though I'm "measuring" why shouldn't my device be a go/no-go?
You can do attribute gauge R&R and attribute capability studies.
A scratch is an attribute. Surface finish is a variable.

An rhr comparator can be used as a go/no go gage when inspection criteria calls out "only 64 rhr and finer is acceptable" but the finish is still variable to the extent of your process refinement.

A plug go/no go gage is the same way. The size of the hole is variable, but the inspection is defined as go/no go.

So I guess the only thing I can advise is to let go of the attribute vs. variable terms unless you are charting performance; in that case, a c, np or p chart would be used instead of a u chart. I do not see why you can't find a cpk with this data. Here is a thread that talks about cpk with attribute data: Cp, Cpk calculation for np chart - No USL or LSL - SMT Production
 

ScottK

Not out of the crisis
Leader
Super Moderator
#5
Re: Surface Finish on machined brass (or other metals) - Attribute or Variable?

A scratch is an attribute. Surface finish is a variable.

An rhr comparator can be used as a go/no go gage when inspection criteria calls out "only 64 rhr and finer is acceptable" but the finish is still variable to the extent of your process refinement.

A plug go/no go gage is the same way. The size of the hole is variable, but the inspection is defined as go/no go.

So I guess the only thing I can advise is to let go of the attribute vs. variable terms unless you are charting performance; in that case, a c, np or p chart would be used instead of a u chart. I do not see why you can't find a cpk with this data. Here is a thread that talks about cpk with attribute data: Cp, Cpk calculation for np chart - No USL or LSL - SMT Production
Yes - it's ultimately a variable as it can be measured. But I want to treat it as either good or bad.
That's excactly what our specification/inspection criteria calls out.
Max Ra for surface finish = X.
it doesn't say 0-X or Y+-Z.

Sure I can do a Cpk. It's just a calculation.
I don't believe it's a valid measure of the capability of the process in this application (not to mention I'm dealing with people who are not open to negotiation on that point :mad:).
That's based on history and the distribution, which is not normal.
 

Bev D

Heretical Statistician
Leader
Super Moderator
#6
Re: Surface Finish on machined brass (or other metals) - Attribute or Variable?

WARNING: RANT COMING

By definition, and assuming a normal distribution, if the Cpk is 1.0 or less you will have some parts out of specification (estimate would be 100%-99.73%, or 27 out of 1000 for a Cpk of exactly 1.0).
...and there in lies the fallacy of the Cp-Ppk abomination perpetuated by "check the box people" who don't understand statistics or physics...most processes are NOT Normal. even minor deviations from Normality in the tails can lead to terribly innaccurate "predictions" about defect rates.

I urge you to read Bert Gunter's series on this topic - it is available thru ASQ...

The tyranny of this urban myth is unbelievable.
 

Kales Veggie

People: The Vital Few
#7
Re: Surface Finish on machined brass (or other metals) - Attribute or Variable?

:2cents: (assuming you are measuring Ra in micro inch, not microns)

I am assuming that the part is machined and would be classified as smooth turned. The Ra value is determined by the speeds and feeds of the lathe and/or machine centre (basically how fast is the tool going across the surface). Ra < 32 is usually ground or polished.

Is this surface finish a special characteristic? How significant is the surface finish for the application?

If the answers are "no" and "not" (I believe that is what you indicated) I would accept attribute data from a supplier.

The other question is if attribute data is sufficient for process control at the supplier. I believe not. Variable data must be used.
 

Bev D

Heretical Statistician
Leader
Super Moderator
#8
Re: Surface Finish on machined brass (or other metals)- Attribute or Variable inspect

back to the original question: yes of course you can treat the surface finish as Attributes data and then calculate the Cpk value. The effectiveness of this approach depends on how the surface finish functions in the assembly and how the specification limit was derived (anatomical, historical or engineered)

When I have found myself in this position, I have gotten the data (control charts, multi-vari chart vs spec limit, etc.) that shows that I have a stable controlled process that meets spec (no defects historically). Then I have a face to face discusion with the supplier engineer to understand the assembly need and my process's performance. When actually confronted with the data and the physics, logic typically rules. be warned however, that there have been times when the supplier engineer has explained that the spec is wrong and they are trying to control the extreme values near the limit that actually cause failures, btu the politics keeps them from actually changing the specs...
 

Jim Wynne

Leader
Admin
#9
Re: Surface Finish on machined brass (or other metals)- Attribute or Variable inspect

This is directly realted to another thread I started about Surface Finish Capability http://elsmar.com/Forums/showthread.php?t=26288

I'm wonder what practice is out there.

In my particular application I don't see why it can't be an attribute.
As long as the surface finish is below, say, 64Ra the part will be fine. It will function the same at 63 as it will at 4.
So even though I'm "measuring" why shouldn't my device be a go/no-go?
You can do attribute gauge R&R and attribute capability studies.
A lot depends on how critical the surface finish is. You can buy a set of exemplars called "thumbnail" (or "tactile," or "touch") comparators that provide a visual and tactile way of comparing one finish with another. You can see an example here. It's an adequate method in some instances when all you really need to know can be ascertained by a simple comparison.
 
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