Capability for CMM Scanned Profiles on a feature with a Bilateral Profile Tolerance

R

Rand T

#1
I have been struggling with this for awhile and never really resolved it. :frust:

I have parts that get scanned via CMM with hundreds of points. The PCDMIS CMM output is the min and the max along with a value called "measured" which is the absolute difference between the two if different signs and the greater magnitude of the two if the same sign.

example; a) min = 0.05 mm & max 0.10 mm. "measured" = 0.10 mm
b) min = -0.1 mm & max = 0.05 mm. "measured" is 0.15 mm

I am trying to calculate capability on a feature with a bilateral profile tolerance of 0.25 mm. I have thought about a couple different methods, but both seem to have pros and cons. Of course, I want to determine the "worst case" condition and apply it against the tolerance.

1) Use the "measured" value. The downside is that this "measured" value gives very different values depending upon if the signs of the min and max are opposite or the same which may end up overstating the amount of variation from nominal. In exp. b above, my largest variation from nominal is still only -0.1 mm, but the value I have is 0.15 mm.

2) Select either the min or max depending on which one is larger. This option was the one that made the most sense, but has one big drawback. The min and the max can be very close in magnitude, but with opposite signs. When I select the larger of the two, I often end up with values the bounce back & forth between + & -, again now overstating the variation.

exp; part #1 is min.= -.032 & max = .034 use .034 since it is larger
part #2 is min= -.033 & max = .031 use - .033 since it is larger

Now it looks like I have a range of .067 when in actuality it is only .003


Anybody have any suggestions?

I could do min and max capability seperately, but that seems like a lot more work and I'm not sure there is value there either.
 
Last edited by a moderator:
Elsmar Forum Sponsor

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#2
Re: Capability for scanned profiles

What is the process that generates this feature?

Typically, trying to use a single number (capability index) to describe a relationship of many points within a part to a single tolerance is a knotted up mess of a concept - more so than just using one measurement. Not sure if you are going to end up with a meaningful analysis.

But, that depends on the process.
 
R

Rand T

#3
Re: Capability for scanned profiles

Machining of titanium. Customer is requiring 100% CMM inspection until we can show capability.

Unfortunately, don't have a good method of outputting the individual point data on the entire cloud of scan points. Logically, it makes intuitive sense to calculate based upon the single worst case point, however there are pitfalls as I noted above.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#4
Re: Capability for scanned profiles

Well, this may help. Is this profile made with one tool? If so, how many parts are made from a tool before an offset is required? How many before a tool change?

If not, then the analysis gets complicated further...

As I read through the algorithm you describe - if it is correct - I do not see how you can correctly calculate capability. You have apples (positive/positive) and oranges (positive/negative) - and capability is not a fruit salad.

Does PCDMIS allow any other analysis technique of the same data set? How many parts are you using as your sample set? Can you get the min and max for each part, and plot those values in a time order throughout the run? There might be hope there, if you can....
 
Last edited:
R

Rand T

#5
Re: Capability for scanned profiles

Not the machining expert, but I believe it is one tool. The runs are fairly short, (60 pc) so few changes once it is set up and running.

I do have the min and max for each part and could plot in a time series. Right now, I am using approx. 30-40 pcs as a sample set.

The 100% CMM is killing me from a time standpoint, and I have a customer who is requiring capability before relaxing that requirement. As far as PCDMIS is concerned, I have to rely on what my programmer is telling me, so I'm not sure if there is anything else I can get. So far he hasn't indicated there are other options, but I haven't given up yet.
 
Last edited by a moderator:
T

True Position

#6
Re: Capability for scanned profiles

Are you applying any type of automated filtering to the data? Scanning or individual points?
 
R

Rand T

#7
Re: Capability for scanned profiles

No All I have is the min and max from the cloud of scanned points. CMM measures 100+ scanned points and reports two values.

These mins and maxes are likely not even the same exact point from part to part, but whichever of the entire scan cloud were the min and max on each piece.

Ultimately, I am thinking I may need to do capability on both the min and max seperately and just ignore the one that has the highest indice.
 
T

True Position

#8
Re: Capability for scanned profiles

No All I have is the min and max from the cloud of scanned points. CMM measures 100+ scanned points and reports two values.

These mins and maxes are likely not even the same exact point from part to part, but whichever of the entire scan cloud were the min and max on each piece.
Is there outlier filtering or no? Do the points tend to move?
 
R

Rand T

#9
Re: Capability for scanned profiles

No filtering as far as I know. Not sure what you mean by "Do the points tend to move?"

The values are fairly consistent, (doesn't seem to really be any outliers) but tolerance on a couple is a profile total of .08 mm, so not a lot of tolerance to begin with.
 
T

True Position

#10
Re: Capability for scanned profiles

No filtering as far as I know. Not sure what you mean by "Do the points tend to move?"

The values are fairly consistent, (doesn't seem to really be any outliers)
Are the min and max points always at the same place on the scanned parts? (Point #1 vs #60)
 
Thread starter Similar threads Forum Replies Date
L Capability of CMM's - The minimum tolerance in microns a CMM can inspect Capability, Accuracy and Stability - Processes, Machines, etc. 4
B Cpk for 3D data - Capability study from CMM data- Need Help Capability, Accuracy and Stability - Processes, Machines, etc. 1
B Capability Maturity Model (CMM) for Manufacturing Misc. Quality Assurance and Business Systems Related Topics 12
K CMM (Capability Maturity Model) Audits vs. ISO 9001 Internal Audits Software Quality Assurance 16
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
H Capability Data for Paint Thickness on Painted Parts Statistical Analysis Tools, Techniques and SPC 10
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
G Capability Study for Tapped Hole Reliability Analysis - Predictions, Testing and Standards 3
E Sampling for capability studies for variables and attributes Capability, Accuracy and Stability - Processes, Machines, etc. 4
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
T Final process capability results - What I am supposed to present? Cp and CpK? APQP and PPAP 11
G Plotting capability study data - I have a bimodular data distribution Capability, Accuracy and Stability - Processes, Machines, etc. 4
A Statistic Tools to Know Capability of Sealing Machine for Seal Width Capability, Accuracy and Stability - Processes, Machines, etc. 3
Q How to perform Process Capability for true position Statistical Analysis Tools, Techniques and SPC 11
V Generic requirements regarding capability study in automotive Capability, Accuracy and Stability - Processes, Machines, etc. 1
A Capability Analysis for Packaging Seal Strength with spec. >0.1 Kgf using Minitab Using Minitab Software 6
J Initial capability for injection molding part Capability, Accuracy and Stability - Processes, Machines, etc. 6
Ron Rompen Ford Method - Position Capability with MMC Modifier Capability, Accuracy and Stability - Processes, Machines, etc. 14
O Capability or Gage R&R Study for Leak Tester Reliability Analysis - Predictions, Testing and Standards 11
S Understanding control chart and measurement capability Statistical Analysis Tools, Techniques and SPC 2
L IATF 16949 Cl. 8.3.3.2 Manufacturing process design input - Process capability IATF 16949 - Automotive Quality Systems Standard 1
B Short term vs Long term Capability - We ship millions of parts in a year Capability, Accuracy and Stability - Processes, Machines, etc. 7
M IATF 16949 - Capability study on non critical dimensions? Statistical Analysis Tools, Techniques and SPC 4
J Are Capability Requirement(s) Valid for All Characteristics? Capability, Accuracy and Stability - Processes, Machines, etc. 6
Y Process Capability for Non-Normal Data - Philosophical Questions Capability, Accuracy and Stability - Processes, Machines, etc. 6
M Process Capability in a High Precision Environment Capability, Accuracy and Stability - Processes, Machines, etc. 7
M CAAC-145 Manuals - Looking for examples of MOM's, MMM's Capability Lists, etc. Federal Aviation Administration (FAA) Standards and Requirements 13
J Powdercoating Capability Employing a Robotic Arm Manufacturing and Related Processes 1
L Excel Template for Cgk Study wanted (Capability Gauges Study) Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 12
V Product Development - When to start calculating Process Capability Capability, Accuracy and Stability - Processes, Machines, etc. 3
V Capability for Correlated Charteristics Capability, Accuracy and Stability - Processes, Machines, etc. 2
S Proving Process Capability of a Packing Machine Capability, Accuracy and Stability - Processes, Machines, etc. 6
R Visual Attribute Inspector Capability and KAPPA Inspection, Prints (Drawings), Testing, Sampling and Related Topics 3
N Process Capability Study - Aggregate vs. Individual Processes Statistical Analysis Tools, Techniques and SPC 24
KyloDen Capability Study and Qualifying a Supplier Capability, Accuracy and Stability - Processes, Machines, etc. 5
V Process Capability for parameters with non-normal distribution. Capability, Accuracy and Stability - Processes, Machines, etc. 16

Similar threads

Top Bottom