Machine Capability Studies: How many parts needed?

D
#1
I'm looking but not finding a general statement anywhere here stating exactly how many parts should be checked as a general guideline for a machine capability study. Any body?
 
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A

AJLenarz

#2
Hello Dawn,

The rule of thumb as stated in both the SPC manual is “25 or more subgroups containing about 100 or more individual readings give a good test for stability and, if stable, good estimates of the process location and spread”.

This can be found in SPC manual pg 31.

The PPAP manual states: “… at least 100 individual samples…”

This can be found in PPAP manual pg 6.
 
A

Al Dyer

#3
Dawn,

I'm not sure if you will find a definitive standard. The guidelines in the AIAG manual are at the best "adequate"

What I like to consider is the product being produced, the tools used, and cycle time. At a minimum I would consistantly graph 5 samples per subgroup during and 8 hour continuous production cycle.

This is not to mean inspect every part, just measure and plot one group and when that is done get the next 5 pieces, production continues while you are doing the study.

This is probably labor intensive but by doing it through 1 or 2 production shifts you will be able to get a handle on both random and assignable causes of variation.

ASD...
 
K

Ken K.

#4
I agree that the SPC Reference Manual and the PPAP Manual quantities are pretty standard.

Here is a method based on an article called "Recent Developments in Process Capability Analysis" by Robert N. Rodriguez (JQT, October, 1992)

I'll show it in Excel format:

XX=DELTA-NORMSINV(A/100)*SQRT((1/(9*N))+(CPK^2/(2*(N-1))))

where
A is the Type I confidence level expressed as a percentage, typically 90 or 95, {0<B<A<100}. n increases as A increases.

CPK is the upper bound estimate of the Cpk, n increases as the CPK estimate increases.

DELTA is the desired resolution. Half-width of the desired one-sided lower confidence interval. n increases as DELTA decreases toward zero.

In all the above inputs, if unsure, use the more conservative value - the one that would provide the larger sample size n.

N is a proposed sample size that will be evaluated using the XX value.

Start with N = N' shown below:

N'=CEILING(NORMSINV(A/100)^2*(1/9+CPK^2/2)/DELTA^2,1)

Increase the value of N by integers until the smallest positive value for XX is obtained. The resulting N is the recommended sample size, n. Of course this assumed normality.

Typically the final number n is simply N'+1.
 
D

DICKIE

#5
We typically use a minimum of 25 subgroups. Most of the customers for special machines we build ask for 25 subgroups of 5.
 
D
#6
Thanks!
If you are running a study for 8 hours, 20 hours, 24 hours, etc., is this without ANY adjustments?
 
A

AJLenarz

#8
Capability indices can be divided into two categories: short-term and long term. Short-term capability studies are based on measurements taken from one operating run (the time frame could vary depending your particular production process). The data are analyzed with a control chart for evidence that the process is operating in a state of statistical control. If no special causes are found (a machine adjustment could be considered as a special cause), a short-term capability index can be calculated. This type of study is often used to validate the initial parts produced from a process for customer submission. Another use is for machine capability studies.

Long-term capability studies consist of measurement consist of measurements that are collected over a longer period of time. The data should be collected for long enough, and in such a way, as to include all expected sources of variation.
 
A

Al Dyer

#9
Originally posted by Dawn:
So are you telling me there is no difference between a process capability study and a machine capability study? I understand the aforementioned to be a process capability study.
Maybe it's just my line of business but I can't for the life of me see any discernable difference between machine capability and process/product capability.

During an initial study, as I noted above, we are monitoring the machine by defining offsets, tool changes etc... and determining a baseline or MTBF. As I noted there may be situations that warrant longer or shorter study frequencies. This is also giving us a view of our inherant variation as a tool change/offset is expected and not a special source of variation.

The tools used by the machine directly affect the variation in the end product which lead us to a process/product capabilty study.

This is not to say that there are studies that can and need to be performed on aspects of the machine that do not directly affect the end product. I choose to do these types of studies as part of our preventive/predictive maintainence system.

This is a real time system that works for us and I would like to here some additional discussion on machine vs product/process studies, I might be going down the wrong road.

ASD...
 

SteelMaiden

Super Moderator
Super Moderator
#10
Al, be careful what you ask for, you might get it!

My SPC knowledge is rather limited, but I can think of a couple reasons to look at machine capabilities vs. process. If your process is slowly trending (or quickly, who knows?) towards one limit, it could be that your equipment is not as capable as it was due to age, wear and tear, some undiscovered malfunction, or whatever.

Another thing that some of my past coworkers have used is machine capabilities to plan for new or "changed" product. i.e., if the machine has a capability of X.0 and the revised customer specification is X.7, you will probably need to invest some money into upgrading or replacing the existing equipment, not?

Just my thoughts in regards to your statement about not seeing a discernable difference. You are probably right, statistically speaking ;-), most of the time a process capability study is probably sufficient for most of us. Maybe that is why we aren't all capable of reciting the correct methods and requirements of a machine capability study off the top of out heads.
 
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