SPC Help - Probability of SL=CL

L

Spc

Hi all
I have a question, how about the probability of SL=CL in SPC study?
SL=specific limit CL=cernter limit

Any inputs appreciated.

ksanders

Involved In Discussions
Do you mean specification limit? If so, is you question whether or not this can happen?

SteelMaiden

Super Moderator
Super Moderator
In simplest terms (don't want to fire up any big debates) yes, your process can be running and centered on your specification limit. It's not a good thing, but it could happen. If your process is centered on the spec limit, it means that (around) half of what you are producing is out of spec.

ksanders

Involved In Discussions
With SPC, almost anything is possible. If the process average is shifted away from nominal to a point where it becomes equal to one of the specification limits, the result is a Cpk of zero (0), and 50% of the product is being produced outside of the specification limit. Take a look at the Cp value. If the Cp value is within normally acceptable limits (1.00 or larger), then there is hope. The process average can be shifted to the print nominal, and your process will again be producing most of it's product within specification. Further refinement of the process will result in larger values for both Cp and Cpk, increading your capability.

Basic rules for Capability Ratio's Cp and Cpk:

1. Cp tell the process potential (if it were centered). This is the best your process can do at it's current state.
2. Cpk tell you how your process is currently running against it's potential.
3. Cpk can never exceed Cp. If Cpk and Cp are equal, your process is perfectly centered within the specification. Any differences between the two, is an indication of how well your process is centered within specification.
4. A Cpk of 1.33 indicates that ±4 sigma are within the specification limits.
5. A Cpk of 1.00 indicates that ±3 sigma are within the specification limits.
6. A Cpk of zero (0) indicates that nominal is equal to one of the specification limits. (At least 50% nonconforming)
7. A negative Cpk indicates that nominal is outside the specification limits. (>50% nonconforming)

I hope this will help.

L

Originally posted by SteelMaiden
In simplest terms (don't want to fire up any big debates) yes, your process can be running and centered on your specification limit. It's not a good thing, but it could happen. If your process is centered on the spec limit, it means that (around) half of what you are producing is out of spec.
In this situation, how many Sigma does it imply?

ksanders

Involved In Discussions
Sigma is based on the variation present in the samples collected and is used to calculate the capability indecies. Capability doesn't imply sigma, it is a result of sigma as it relates to the process tolerance. The larger the tolerance relative to sigma, the better the capability. The larger the sigma relative to the tolerance, the worse the capability.

L Help with concept of PPM in SPC Statistical Analysis Tools, Techniques and SPC 6
S Can you help with a simple SPC sheet to judge if inspection is Good / No Good ? Capability, Accuracy and Stability - Processes, Machines, etc. 8
Q Errors in the AIAG's SPC manual? Need help reviewing charts. Statistical Analysis Tools, Techniques and SPC 8
C Help !! Problem of SPC on continuous variable Statistical Analysis Tools, Techniques and SPC 6
SPC for machining processes Capability, Accuracy and Stability - Processes, Machines, etc. 1
What is the best way to control special characteristics in Control plan? Is it Mandatory to have SPC for IATF 16949? IATF 16949 - Automotive Quality Systems Standard 7
SPC - Stamping parts - Small batch size Statistical Analysis Tools, Techniques and SPC 6
Attributes SPC study - Attributive control (Go gage) Statistical Analysis Tools, Techniques and SPC 5
Anyone Familiar with "SPC for Excel" add-in? Lean in Manufacturing and Service Industries 4
Assignable cause/corrective action list for SPC Software Statistical Analysis Tools, Techniques and SPC 3
SPC for special characteristics - CSR - Control of SC and CC Statistical Analysis Tools, Techniques and SPC 1
Determining SPC tolerance Statistical Analysis Tools, Techniques and SPC 21
SPC - Methods to collect data IATF 16949 - Automotive Quality Systems Standard 7
Steve Prevette's Statistical Process Control (SPC) "Library" Statistical Analysis Tools, Techniques and SPC 0
SPC for Eddy Current Testing Statistical Analysis Tools, Techniques and SPC 28
SPC (Statistical Process Control) for Unilateral Tolerance - Questions Statistical Analysis Tools, Techniques and SPC 6
Informational Use of Classical SPC in Precision Machining Statistical Analysis Tools, Techniques and SPC 9
Recommendations on Inspection/SPC Software Statistical Analysis Tools, Techniques and SPC 4
Real Time Electronic SPC/Control Charting Statistical Analysis Tools, Techniques and SPC 4
Other types of Control Charts described in Chapter II of SPC Manual Statistical Analysis Tools, Techniques and SPC 6
Electric terminal crimping machines - Is SPC appropriate for this process? Manufacturing and Related Processes 4
Properly understanding SPC - Newbie SPC questions Statistical Analysis Tools, Techniques and SPC 34
SPC Charting frequency calculator for X hi/lo chart Imported Legacy Blogs 0
Handy training tip for SPC - the Check Engine Light Analogy Statistical Analysis Tools, Techniques and SPC 0
SPC Software for MAC computers Statistical Analysis Tools, Techniques and SPC 2
SPC based Dashboards - By Steve Prevette (November 2017) Statistical Analysis Tools, Techniques and SPC 1
SPC Work Instructions (WI) Statistical Analysis Tools, Techniques and SPC 2
Doubts about SPC taken in Machining - Part has +-0.01 Tolerance Statistical Analysis Tools, Techniques and SPC 5
Short Term vs. Long Term SPC Study - Where is Cp and Cpk Defined Statistical Analysis Tools, Techniques and SPC 8
SPC for Dummies - The correct meaning of all the graphs Statistical Analysis Tools, Techniques and SPC 3
Can/should SPC be applied to Computer Assembly and Software Imaging? Statistical Analysis Tools, Techniques and SPC 8
Recommendations on SPC Software or Excel SPC add-in(s) Quality Assurance and Compliance Software Tools and Solutions 6
Not all characteristics follow a Normal Distribution - How do you do SPC Chart Capability, Accuracy and Stability - Processes, Machines, etc. 5
SPC in Insurance Back Office Operations Service Industry Specific Topics 4
C SPC for Particulate Alert/Action Levels Statistical Analysis Tools, Techniques and SPC 3
SPC Production - Getting Cpk and Ppk Statistical Analysis Tools, Techniques and SPC 8
M SPC with many data points from one dimension Statistical Analysis Tools, Techniques and SPC 16
Mixed data collection (SPC) for different mold cavities Statistical Analysis Tools, Techniques and SPC 2
M How to Start SPC from the ground up! Statistical Analysis Tools, Techniques and SPC 8
Available SPC and Metrology Consultants Paid Consulting, Training and Services 0
I SPC on Cyclic Data Statistical Analysis Tools, Techniques and SPC 10
SPC Card for One Sample Size - Customer requires for all critical dimensions Statistical Analysis Tools, Techniques and SPC 8
R Quality Management & SPC Manufacturing and Related Processes 1
Is SPC data For Special Characteristics always required? TS 16949 clause 8.1 ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 2
How to Start SPC in a Shaft Grinding Section Statistical Analysis Tools, Techniques and SPC 1
I-MR SPC used for Inspection Statistical Analysis Tools, Techniques and SPC 7
E SPC data from multiple Excel files Statistical Analysis Tools, Techniques and SPC 5
S Where to Get SPC Training to Measure our Processes? Training - Internal, External, Online and Distance Learning 12
SPC Short Run Advice of a Stamping Process Statistical Analysis Tools, Techniques and SPC 3
SPC in Heat Treatment Workshop Statistical Analysis Tools, Techniques and SPC 2