Need help / feedback on Xbar/R charts and Cpk/Ppk applicability - Assemblies

X
#1
I am conducting SPC studies (using Xbar/R charts) on a grinding process, using variable data from three characteristics:

(1) assembly thickness after grind
(2) parallelism of surface to backing plate after grind
(3) flatness of surface after grind

To make charting easier, I am coding the data...in other words 0.001" will be 1, 0.010" will be 10, 0.007" will be 7, etc...

I am also setting the nominal as zero (0), and then will chart the difference from actual target. For example, if a part thickness target is 0.650", and the measurement taken is 0.652", then I record "2"...if a measurement is 0.647", then I record "3". What this does is it allows me to use one standard set of charts, regardless of the actual part thickness.

That's just a little background on what I am doing...now on to the question.

Ok, for THICKNESS, the specification is bi-lateral, however, for PARALLELISM and FLATNESS, the specifications are uni-lateral. Thickness is typically 0.650" +/- 0.005" (ie: in my case, 0 +/- 5). Parallelism and flatness are typically 0 + 0.010" max (ie: in my case, 0 + 10 max).

Ok, for calculating Cpk and Ppk, the formulae are as follows:
CpkPpk.gif

(I know that there is no PPU or PPL, I just used those terms for the sake of this question...)

That being said, I realize that these formulae work fine for my thickness studies, since they are bi-lateral, but what about for the uni-lateral tolerances (parallelism and flatness)??

For parallelism and flatness couldn't I just say that Cpk = CPU and Ppk = PPU (since there is no lower specification)?

Does anyone have any ideas on a better way to monitor these two uni-lateral characteristics (are Xbar/R charts even appropriate for these)???

:bonk: (I need a vacation!) :truce:
 
Elsmar Forum Sponsor
C

Craig H.

#2
Good morning, Xman.

Yes, this is what you should do.

The use of xbar should take care of any "normalcy" issues, and the use of the USL is entirely appropriate - the farther you are from the upper spec, (i.e. closer to zero) the better, right?

Just curious, but how did you determine your subgroup (n) size?
 
X
#3
the farther you are from the upper spec, (i.e. closer to zero) the better, right?
Yes, that's correct. The ground surface, ideally, would be perfectly flat/parallel (ie: 0), however since we all know that's virtually impossible, I have a tolerance of a maximum of +0.010" deviation from 0. So, the closer to 0, the better.

Just curious, but how did you determine your subgroup (n) size?[
I am using charts with 25 subgroups of 4 (for a total of 100 readings.) I chose 4 because it is the best "fit" for 100 readings, as well as the fact that our grinder runs 4 jigs/fixtures, so I can also track the variation from jig to jig by watching the charts as well.

Thanks for your reply! :)
 
R

Ravi Khare

#4
As Craig rightly pointed out, X-bar should take care of Normalcy issues. X-bar R Charts would give you a good indication of the stability of the process.

Your evaluation of Cpk = Cpk(U) and Ppk = Ppk(U) is also would give you the right picture of the Process Capability. Cp and Pp are undefined in such cases.

Since you have parameters of parallelism and flatness that have one sided specification, the approach you need to take for an out of control situation will difer slightly from that for parameters with two sided specs.

If you find that you have an out of control situation that takes the X-bar below the lower control limit, your process has drifted lower, but actually improved. You should then investigate what you have done right ( assignable cause of the right kind) to get this out of control. If you are able to replicate the assignable cause setting, and maintain the out of control below LCL, you could consider reapplying the Control Limits based on the new improved process study data. You could then maintain the process stable with reference to the new limits.

-Ravi
 
X
#5
With the parallelism and flatness characteristics, since there is no lower spec, wouldn't it make sense to say that there should also be no lower control limit (in other words the LCL should be 0), like the R chart (since range can only be =/< 0 as well)?

For example, one set of data I analyzed for flatness gave me a LCL of -0.12. However, since I have no negative (-) readings, and can have no readings less than 0, shouldn't the LCL be 0?
 
C

Craig H.

#6
Well said Ravi! A much more complete answer than mine. However, one warning.

I got in hot water with a VP recently for pointing out that a chart drifted "out of control" when the process improved. He waited until we were alone in his office to point out to me that we were in better control now than we were a month ago. He was more than a little unhappy.

The lesson: be careful when talking to "non-practioners"!!!
 
R

Ravi Khare

#7
The lower control limit is only meant to draw the line beyond which you know that the drift is significant. That way you will be able to differentiate random variations from assignable cause ones. The lower control limit is going to tell you when there is going to be a 'significant' (real) process drift towards improvement.

As for Craig's comments, I wholeheartedly agree with you Craig... We all have gone through this sometime or the other I guess. I have spent hours trying to argue with smug people who look at Control Charts and be happy that they are producing quality; when the Cpk is all the time flashing the disaster warning!
 

Darius

Quite Involved in Discussions
#8
If there is only one spec, I feel that you should try Cpmk or its equivalent using ds_total, as othe other posts said, you could get better Cpmk if the process is farther from the target and the variation is lower, than a process that is closer to the target and with higher variation, with the definition of quality improvement as "reduce variation centered on the target" that doen't make any sense.:confused:

look at http://www.scausa.com/capaqa.pdf


:truce: Be careful of wich control chart to use, I feel that there could be autocorrelation if this is a continuos process, you may need an X/MR chart.

:bigwave:
 
X
#9
Thanks for the input (and the link Darius)! :bigwave:

I think I will look further into this and re-evaluate how I am monitoring the uni-lateral toleranced characteristics.
 
#10
The formula loks acceptable tome and as Darius indicated there are other methods. However, I am curious about the tolerance. +/-0.005 on thickness, +/- 0.010 on flatness & perp.
Typically the Flatness & Perp should be less than the size tolerance. preferably 1/2 the size tolerance.
Just a thought.
 
Thread starter Similar threads Forum Replies Date
SocalSurfer AS9100 new certificate, but need QMS software, help Quality Assurance and Compliance Software Tools and Solutions 2
S Need help with analysing a survey on minitab Using Minitab Software 1
K A proposal for the model Quality Management - I need help for the project ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 3
R Probability - Need a help to solve the below question Statistical Analysis Tools, Techniques and SPC 5
8 Need Help - Runout - Function Gage Inspection, Prints (Drawings), Testing, Sampling and Related Topics 7
S New to FAIR, need help in filling it out AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 8
R Pls help --- Need expert advice on Video Measurement Measurement Uncertainty (MU) 0
Ajit Basrur Need help to understand HIPAA requirements ISO 13485:2016 - Medical Device Quality Management Systems 17
Z I need help with getting a CFS in China Manufacturing and Related Processes 2
S Need help wrapping my head around confidence vs beta error Statistical Analysis Tools, Techniques and SPC 2
P Received a minor for not having good measureables/goals. Need help with KPIs. IATF 16949 - Automotive Quality Systems Standard 52
A Failed the AS9120B Exam Twice.....Need Help AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 7
Ajit Basrur Need help in Conditional Formatting for the entire row Excel .xls Spreadsheet Templates and Tools 3
H Need help on Quality Improvement Plan (QIP) Quality Tools, Improvement and Analysis 5
S Help: Need Control Self Assessment Questionnaire for HR Departmental Functions Process Audits and Layered Process Audits 0
M Need help in Calibration of Directional Coupler General Measurement Device and Calibration Topics 2
L Need HELP with Internal Audit Program ISO 13485.2003 Quality Management System (QMS) Manuals 3
J Supporting Processes - Internal Audits - Need help settling a debate IATF 16949 - Automotive Quality Systems Standard 4
A Cpk Formula seems off, need help!? Capability, Accuracy and Stability - Processes, Machines, etc. 11
A Need Help With AEA Auditor Related Situational Questions AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 1
E Transformer Overload Test - Need help IEC 60601 - Medical Electrical Equipment Safety Standards Series 3
C I Need Help with Drafting Conventions!!! Design and Development of Products and Processes 4
M Need help on QMS Development in an AS9100C certified organization AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 10
R Need Help on Analysis: How to know potential causality model from historical data Problem Solving, Root Cause Fault and Failure Analysis 5
B Need help tracking training for my business Quality Assurance and Compliance Software Tools and Solutions 3
B Need help with a Control Plan for Encapsulation Mold / Trimming / Forming Tooling FMEA and Control Plans 9
S I need help from all of you bros Oil and Gas Industry Standards and Regulations 1
LRE67 Need help identifying this "thing" Coffee Break and Water Cooler Discussions 9
G Need help on Device Description in the Acceptance Checklist for Traditional 510(k) Other US Medical Device Regulations 2
K Need help finding a book on Documentation Book, Video, Blog and Web Site Reviews and Recommendations 8
J Need help/advice on video editing software etc..... Coffee Break and Water Cooler Discussions 8
L Need Help with Case Study Questions Manufacturing and Related Processes 16
W Help explaining the need for the Process Approach AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 4
Ron Rompen Unusual problem with Excel - Need some help Excel .xls Spreadsheet Templates and Tools 7
S Need help on new TS 16949 Rules IATF 16949 - Automotive Quality Systems Standard 5
M Need help with Network Analyzers and which DUT can be Calibrated with it Calibration and Metrology Software and Hardware 2
R Need help on defining scope for Design Verification File for Class III IVD 21 CFR Part 820 - US FDA Quality System Regulations (QSR) 8
J Help I need your opinion on a News Letter Misc. Quality Assurance and Business Systems Related Topics 15
N Need Help Preparing for Medical Device Quality Engineer Interview Career and Occupation Discussions 2
B Need help on an application we are working on Coffee Break and Water Cooler Discussions 4
S Need help with Project Charter for my Six Sigma Green Belt Project Six Sigma 1
Q Need help on MSA study on a torque analyzer Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 3
B Need help in adding second location to FDA Registration Other US Medical Device Regulations 4
T A survey on problems during ISMS implementation - need help IEC 27001 - Information Security Management Systems (ISMS) 2
Z My first Gage R&R - Paper/Poly/Foil Material Bond Test, need help Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 15
R Need help on cost analysis for copper tube manufacturing Manufacturing and Related Processes 1
T SPSS for Analysis of Medical Data - Need help Statistical Analysis Tools, Techniques and SPC 12
T Metal Stamping Defect--Need Help with Corrective Action Manufacturing and Related Processes 5
E New to Quality & Need Help Career and Occupation Discussions 6
T Need help developing an ECO (Engineering Change order) procedure Document Control Systems, Procedures, Forms and Templates 3

Similar threads

Top Bottom