Capability Data for Paint Thickness on Painted Parts

heiss

Starting to get Involved
#1
Hi all,

Hope everyone is doing great!

I have some questions regarding capability analysis for painted parts. A customer is requesting Cpk and Ppk for paint thickness. This is done by taking multiple measurements of the same part at different locations. We have tried in the past to get capability data for paint thickness, though our process always meets the minimum requirement, we can't meet the Cpk/Ppk requirement. This is because of large variation between parts due to where the parts are at the time of painting (e.g. parts on top of paint rack will have a lower paint thickness than parts on the bottom of the rack). I was thinking of grouping parts based on rack location.

Does anyone have prior experience dealing with paint thickness capability? How did you do the subgrouping?
Any help on this is greatly appreciated

Thank you
 
Elsmar Forum Sponsor

Miner

Forum Moderator
Staff member
Admin
#2
I have some experience with measuring thickness of various coatings, but fortunately have not had to demonstrate capability of thickness. You have several issues with which to deal with that are causing problems. As you correctly pointed out, location on the rack has a consistently significant impact on thickness. This is a variation of mixed process streams (different mold cavities, different stamping dies, etc.). Next, you have within part variation. You mentioned that you take multiple measurements at different locations. What do you do with the measurements? Are you treating them as individual data points or averaging them? You will probably have to work out an agreement with your customer. Since the top of the rack has the thinnest coating, I would recommend sampling from that layer only. If that layer is capable, you can demonstrate and argue that if the top layer is capable, all other layers must be capable since they are thicker. Be prepared to demonstrate that graphically and statistically.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#3
What you are probably experiencing is the effect of the systemic (fixed) differences between the locations being included mathematically in the calculation of the standard deviation. This will automatically and falsely increase the standard deviation.

If you can plot the data by location (within piece and within rack) we can help with the correct math to use...
 

heiss

Starting to get Involved
#4
I have some experience with measuring thickness of various coatings, but fortunately have not had to demonstrate capability of thickness. You have several issues with which to deal with that are causing problems. As you correctly pointed out, location on the rack has a consistently significant impact on thickness. This is a variation of mixed process streams (different mold cavities, different stamping dies, etc.). Next, you have within part variation. You mentioned that you take multiple measurements at different locations. What do you do with the measurements? Are you treating them as individual data points or averaging them? You will probably have to work out an agreement with your customer. Since the top of the rack has the thinnest coating, I would recommend sampling from that layer only. If that layer is capable, you can demonstrate and argue that if the top layer is capable, all other layers must be capable since they are thicker. Be prepared to demonstrate that graphically and statistically.
Thank you Miner,

For some customers we average the multiple measurements and report as one individual point, and for other we would have to provide data for each measurement point.

Some customers are insisting we provide Cpk/Ppk indices, I tried explaining that with a paint process it would be difficult to obtain the minimum AIAG requirements.



Are there other statistical tools I can use to demonstrate process capability? How did you get away from providing capability analysis?
 

Miner

Forum Moderator
Staff member
Admin
#6
Fortunately, I was never requested to provide such as they were not considered to be CTQs by our customer. I agree with Bev. If you provide some data, we can provide better advice on how to proceed. I do have some thoughts on an empirical approach, but it should be considered a last resort if more traditional approached don't work out.
 

heiss

Starting to get Involved
#7
What you are probably experiencing is the effect of the systemic (fixed) differences between the locations being included mathematically in the calculation of the standard deviation. This will automatically and falsely increase the standard deviation.

If you can plot the data by location (within piece and within rack) we can help with the correct math to use...
Please post your data this will help us.

Hi Bev,

I don't have process data to share at the moment. Please see data that was randomly generated, this data closely resembles what we would normally get. The data is grouped by rack location (Top, middle, or bottom of rack). The individual measurements in the Data column are assumed to be the average of multiple points on the same part.

Thanks a lot.
 

Attachments

Miner

Forum Moderator
Staff member
Admin
#8
It would be helpful to see the data that would allow us to quantify the following sources of variation:
  • Within part variation
  • Within rack variation at a single level (side to side)
  • Within rack variation across levels (top to bottom)
  • Rack to rack variation over time (at a single level)
The data you provided addresses the 2nd and 3rd bullets, but you stated that it was randomly generated, so it isn't really helpful.
 

heiss

Starting to get Involved
#9
It would be helpful to see the data that would allow us to quantify the following sources of variation:
  • Within part variation
  • Within rack variation at a single level (side to side)
  • Within rack variation across levels (top to bottom)
  • Rack to rack variation over time (at a single level)
The data you provided addresses the 2nd and 3rd bullets, but you stated that it was randomly generated, so it isn't really helpful.

I was able to get some process data. The data in the attached sheet is for 12 parts, 5 measurements from each part.

There is no upper spec, the lower spec is 0.78. Please let me know what you think
 

Attachments

Miner

Forum Moderator
Staff member
Admin
#10
This is very helpful. As you indicated, there is definitely a difference between top and bottom of the rack. These should be evaluated as different process streams and the data not mixed together.
1585660571598.png

There is also a difference over time indicating that the process is not stable. This should be brought under control before assessing capability.
1585660655294.png

There is also an interaction between the location on the part and the parts over time. Notice that the first 6 parts have much greater variation within a part than seen in the last 6 parts. You need to determine the cause of this and correct it before assessing capability.
1585660866725.png
 
Thread starter Similar threads Forum Replies Date
L How to evaluate the process capability of a data set that is non-normal (cannot be transformed and does not fit any known distribution)? Capability, Accuracy and Stability - Processes, Machines, etc. 12
G Plotting capability study data - I have a bimodular data distribution Capability, Accuracy and Stability - Processes, Machines, etc. 4
Y Process Capability for Non-Normal Data - Philosophical Questions Capability, Accuracy and Stability - Processes, Machines, etc. 6
L Cpk after Data Transformation - How to Transform LCL /UCL to get Capability Report Capability, Accuracy and Stability - Processes, Machines, etc. 8
E What Data do we need for Welding and Repair Process Capability? Six Sigma 1
K Calculating Capability of a process when data is skewed in the upper range Capability, Accuracy and Stability - Processes, Machines, etc. 4
R Capability Analysis (using Histogram) for Subgrouped Data Statistical Analysis Tools, Techniques and SPC 16
E Binomial Process Capability Sample Size - Variables Data Capability, Accuracy and Stability - Processes, Machines, etc. 4
E Predicting Machined Components Capability based on Historical Data of Similar Parts Capability, Accuracy and Stability - Processes, Machines, etc. 1
B How to calculate Capability of Nonnormal Data Distribution in Minitab Using Minitab Software 6
C Process Capability - Attribute Data Analysis (Template) Document Control Systems, Procedures, Forms and Templates 0
C Process Capability - Variables Data Analysis (Template) Document Control Systems, Procedures, Forms and Templates 0
P Data Analysis - Johnson Transformation/Weibull Distribution/Capability Analysis Capability, Accuracy and Stability - Processes, Machines, etc. 15
P Calculating Process Capability from Data of attached spreadsheet Statistical Analysis Tools, Techniques and SPC 4
S Auto-Correlated Data and Capability Index Quality Tools, Improvement and Analysis 5
S How to conduct Capability Analysis for this data? Statistical Analysis Tools, Techniques and SPC 12
J Attribute Data and Capability Study of Holes in Parts Capability, Accuracy and Stability - Processes, Machines, etc. 11
O Capability Studies: Can I combine Attribute Data and Variables Data Capability, Accuracy and Stability - Processes, Machines, etc. 2
T Advice on PPAP Attribute Data Capability - Go/No-Go APQP and PPAP 3
A Process Capability Study using Variables Data (weight of the product) Statistical Analysis Tools, Techniques and SPC 9
S Capability Analysis of Non-Normal Data Statistical Analysis Tools, Techniques and SPC 23
kedarg6500 Process Capability for Attribute Data - Excel .xls spreadsheet wanted Capability, Accuracy and Stability - Processes, Machines, etc. 5
M Heat Treat Data - Best method for determining the capability of a carburizing process Statistical Analysis Tools, Techniques and SPC 3
T Capability with Non-Normal Data - Component Strength Statistical Analysis Tools, Techniques and SPC 11
B Visual Inspection - Capability Study on Attribute Data Capability, Accuracy and Stability - Processes, Machines, etc. 13
S Asked by customer to supply Retrospective Capability Data IATF 16949 - Automotive Quality Systems Standard 12
J Fitting an Equation to a set of data? Does minitab have the capability? Using Minitab Software 4
T Process Capability Data (TS16949 8.2.3.1) - How often should it be reviewed? Capability, Accuracy and Stability - Processes, Machines, etc. 9
A Capability Analysis - Dealing with non-normal data in Minitab Using Minitab Software 8
B Cpk for 3D data - Capability study from CMM data- Need Help Capability, Accuracy and Stability - Processes, Machines, etc. 1
C Is X-Ray Data Variable or Attribute? How to do a capability study? Statistical Analysis Tools, Techniques and SPC 3
R Bootstraping and Resampling Statistics - Capability for non-normal variable data Inspection, Prints (Drawings), Testing, Sampling and Related Topics 6
S Capability Study for Leak Tester Capability, Accuracy and Stability - Processes, Machines, etc. 8
C Capability in sewing processes CP,CPk,PP,PPk Capability, Accuracy and Stability - Processes, Machines, etc. 10
K Machine Capability Capability, Accuracy and Stability - Processes, Machines, etc. 2
C How to Establish the Calibration & Measurement Capability (CMC)? ISO 17025 related Discussions 1
lanley liao How to keep the manufacturing capability under the API monogram Oil and Gas Industry Standards and Regulations 6
M Minitab Capability of the Population (no sampling) Using Minitab Software 11
A Capability Study - in the beginning of your career what should you have known about the tool Quality Tools, Improvement and Analysis 11
B Two excellent examples of process capability analysis from Quality Magazine Capability, Accuracy and Stability - Processes, Machines, etc. 5
Q Capability study with a minimum spec Statistical Analysis Tools, Techniques and SPC 8
Q Capability - CPk comparison values Capability, Accuracy and Stability - Processes, Machines, etc. 12
R Capability analysis - What is going on this chart? Manufacturing and Related Processes 15
N Is capability applicable for a destructive test? Capability, Accuracy and Stability - Processes, Machines, etc. 9
S Capability or Gage R&R Study for Leak Tester? Reliability Analysis - Predictions, Testing and Standards 15
D Pre-Production Capability Assesment Reliability Analysis - Predictions, Testing and Standards 5
T Iec 60601 Impedance and current-carrying capability IEC 60601 - Medical Electrical Equipment Safety Standards Series 1
S Machine Setup for a CNC Machine Capability Study Capability, Accuracy and Stability - Processes, Machines, etc. 1
D O Ring capability and measurement - What is the automotive 'norm' for capability studies on O Rings? General Measurement Device and Calibration Topics 3
M Measuring Capability of Process with Multiple Specifications Capability, Accuracy and Stability - Processes, Machines, etc. 6

Similar threads

Top Bottom