Machine short term capability study with Cmk index in automotive industry

V

vavrochj

#1
Hi everybody,

my question refers to a machine capability study methodology (also known as short-term capability study with Cmk index). I was looking for the answer in all the threads but without success. I have been working in automotive industry for a quite long time but this is still not clear for me.

Quality standards and also our customers ask us to carry out the machine capability studies prior to the start of serial production. But the problem is in 2 different approaches/understandings of the machine capability study:

1) Some procedures speak about machine capability study in case of purchasing a new machine to be sure that the new machine is able to produce in the way our customers are requiring. In this case the process parameteres are recommended to be measured during the study.

2) The second approach is usually taken by the car makers. They often push us to make this study (Cmk) for each new project before we are able to present them Ppk or Cpk evaluation. In this case the product parameters have to be measured.

What is your opinion on these two approaches? Which one is the correct one? Is it the right way to do the machine capability study only for new machines and try to explain to our customers that Ppk and Cpk studies are enough for the product/process validation during development phase. Or do you see another solution?

In what way do you carry out the machine capability study in your practise?

Thank you very much for any kind of help, comments or opinions. :thanks:
 
Elsmar Forum Sponsor
W

wmeister

#2
Ahh.. Machine Capability!

Well, My background is at both sides of this issue: I have been with machine suppliers previously, and now the consumer of machines in which I specify Machine Capability. Machine Specifications are normally created by a specific test that the machine can repeat with a test that is uniform across different applications which is representative of the application which it will be used
.
Sometimes manufacturers will specify multiple machine capabilities, but that is rare as it confuses customers and is bad for marketing and sales.

(Short answer is Machine Capability is definitely dependent upon Machine Parameters! [Settings, Material, Applications, etc.]) You should buy/make the machine based upon the Machine Capability of your application!

An example of a simple machine tool: an increased feed rate will usually decrease the quality and range of quality in terms of dimensions, roughness, etc -> Slower is usually more consistant = better Capability, but poorer productivity.

------------------------------------------------

It really all comes down to variability partitioning. Variation is additive and by
the general rule:

Variation(Process) = Variation(Measurement System) + Variation(Machine*)

so:
Variation(Machine*) = Variation(Process) - Variation(Measurement System

------------------------------------

Then You can break it down further IF NEEDED!!!!! [IF Variation(Process) is poor]:

Variation(Machine*) = Variation(Parameter1) + Variation(Parameter2..etc + Variation(Material) + Variation(Tools) + Variation(Tooling) + Variation(Moon).

And of Course the old:
Variation(Measurement System) = Variation(Repeatability) + Variation(Reproducibility)

You find Variation Components by doing:
A Gage R&R to determine if the Variation(Measurement System).

and IF NEEDED a DOE/ANOVA for the Variation(Machine*) to see where the variation comes from.
++++++++++++++++++++++++++++++++++++++++++++++++++++

Brain Teaser for all you readers out there: If you start a process (variable-continuous) and determine the process is capable. Then do a process capability study with which includes the production gage which shows an acceptable Cp & Cpk (lets say 2.0 - 3.0+).

Pick One:

A. Now I do a gage r&r study.
B I did a gage r&r before I did the process capabilty study?
C. Don't run a gage r&r study?
E. What the heck is a gage r&r?

--> Tell me why!

---------------------------------------------
Outstanding Book on this Subject:
http://www.amazon.com/Measuring-Process-Capability-Calculations-Manufacturing/dp/0070066523
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#3
Here is my recommendation for precision machining capability - and it applies to new process or new machine (what's the difference?)

First, and foremost, when doing precision machining on a CNC turning center, the resulting distribution is the uniform or rectangular distribution - so any statistical notions concerning the normal curve do not apply whatsoever.

Second, the concept of a "machine capability" can be considered a misnomer. Unless you are comparing machines, there is no way to determine the machine capability. What you can determine is if the machining "system" is capable. The reason why this distinction is important is because the part - and its dimensions - that come out of the machine are the result of the machine, the tooling and the material. The most significant part to part variable should be tool wear, unless the bearings are about to fall out, or the chucking is loose, etc.

The first problem people run into when running a machine capability is they measure one of the resulting part diameters. The emphasis is on 'one'. How many diameters are there in a circle? There are an infinite number. So, how can you describe or predict an infinite number of diameters with one measurement? You can not. So, to resolve that problem, you need to pick a specific diameter - such as 24.000 mm from the end of the part (so that taper does not affect your data). You need to measure around the diameter, and determine the largest and smallest diameter measurements. Then, plot both of those dimensions on a X hi/lo -R chart, with UCL and LCL at 75% of the tolerance for X and 30% of the tolerance for R. The range is the difference - or the roundness. Continue that for about 100 pcs. (Make sure your gage R&R is less than 10%! Try to use the same material lot, if possible, operator, etc.) Is that enough pieces? Usually. If the process is 'in control' you should see the diameter increase as the tool wears (for an OD, opposite for an ID). You might see some fluctuation at the beginning as the machine warms up. That is a special cause that you cannot remove, but need to consider. If the tool wears until the X hi data is up to the upper control limit, then adjust until the X lo reaches the lower control limit. (Adjust during a capability study?? Yes.) If you get two cycles, you will have a real good idea of how the 'machine system' is going to perform. If the machine runs 100 pieces with no need for adjustment, but the data is gradually increasing at a steady rate, then you can likely extrapolate when the adjustment would need to be made. If you have the luxury to find out how long you can run before adjustment with more pieces, then all the better. There are occasions where the tool will wear to the point of breaking or poor finish prior to need for adjustment - and that information is good to know too. But, as long as you can keep the process between 75% of the tolerance, and you are capable to at least 1.33 [capability=(USL-LSL)/(UCL-LCL)].

The next thing to review from the run is roundness (R chart). If the roundness is less than 10% of the tolerance, you have no worries. If it is greater than 30%, then you will have to watch that process like a hawk with frequent SPC checks. I would vote to resolve the roundness issue (which is most likely a machine or machine set-up issue, particularly in chucking or bearings). If it can not be improved, I suggest passing on that process and finding one that can maintain the roundness better.

By the way, this also works in milling , just substitute height or length for diameter and parallelism for roundness!

Now you have some real data on whether that 'machining process' will work for you! This is the short and sweet lesson, but I hope it corrects some of the misconceptions on the topic.
 
Last edited:

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#4
Brain Teaser for all you readers out there: If you start a process (variable-continuous) and determine the process is capable. Then do a process capability study with which includes the production gage which shows an acceptable Cp & Cpk (lets say 2.0 - 3.0+).

Pick One:

A. Now I do a gage r&r study.
B I did a gage r&r before I did the process capabilty study?
C. Don't run a gage r&r study?
E. What the heck is a gage r&r?

--> Tell me why!
ALWAYS run a gage R&R prior to any process evaluation. Its purpose is to determine if the gage is the correct gage for the measurement (its variation is statistically insignificant to the decision you are making). You can not justify your "decision" if you can not convince anyone that the gage you used to make the decision is any good.

And...by the way...all gages are bad, but they may be good enough. And Gage R&R is how you find out!
 
Thread starter Similar threads Forum Replies Date
G Uncertainty of staging a short line scale standard on longer measuring machine? Measurement Uncertainty (MU) 4
K Regulatory requirement of SaMD with machine learning component IEC 62304 - Medical Device Software Life Cycle Processes 1
P Need a programmer for QVI's VMS software for optical inspection machine Inspection, Prints (Drawings), Testing, Sampling and Related Topics 0
B IQ/OQ/PQ of CNC Milling Machine Qualification and Validation (including 21 CFR Part 11) 3
G Engineering retest in line - Machine stability issues IATF 16949 - Automotive Quality Systems Standard 8
R Machine directory (2006/42/EEC) is applicable or not Other Medical Device Related Standards 0
B Product Safety Responsibility - Job shop such as a machine shop AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 10
Marc The German Enigma Machine of World War II After Work and Weekend Discussion Topics 0
R Revalidation machine line and Stability Studies impact. Medical Device and FDA Regulations and Standards News 3
K Validation of new machine (second machine of the same type) Pharmaceuticals (21 CFR Part 210, 21 CFR Part 211 and related Regulations) 2
P How far an operator can reach into a machine before it becomes an ergonomic problem CE Marking (Conformité Européene) / CB Scheme 5
GreatNate Qualifying a CNC machine Manufacturing and Related Processes 3
C Machine Verification Plan Manufacturing and Related Processes 2
F AS9100D Process Matrix for a small cnc machine shop AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 6
B Using non CE parts in a machine CE Marking (Conformité Européene) / CB Scheme 1
Marco Bernardi Plane measurement with a 3D touch machine like a CMM. Calibration and Metrology Software and Hardware 3
Marco Bernardi Diameters measurement with 3D touch machine like a CMM Calibration and Metrology Software and Hardware 4
S Companies that maintain your machine should be in ASL? AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 2
S Machine Setup for a CNC Machine Capability Study Capability, Accuracy and Stability - Processes, Machines, etc. 1
GoSpeedRacer Veterinary anesthesia machine CE Marking (Conformité Européene) / CB Scheme 1
H Should I mention machine/Equipment password In SOP? Qualification and Validation (including 21 CFR Part 11) 4
R How to properly use Survival Analysis to predict time to machine failure Reliability Analysis - Predictions, Testing and Standards 0
K Mammography Machine Bench Testing - FDA 510(K) US Food and Drug Administration (FDA) 5
J Extent of Validation - New machine installed in manufacturing ISO 13485:2016 - Medical Device Quality Management Systems 8
P Average access height into an industrial machine (average base plate height) Human Factors and Ergonomics in Engineering 2
B Machine process sheet requirements for pill bottling operation Pharmaceuticals (21 CFR Part 210, 21 CFR Part 211 and related Regulations) 1
B Does EMC Directive 2014/30/EU cease to apply when the Machine Directive 2006/42/EC is applicable? CE Marking (Conformité Européene) / CB Scheme 4
Q Gage R&R for Instant Measurement Machine Manufacturing and Related Processes 6
jjmusicjr Implementing Machine Replacement Part Reverse engineering technology Manufacturing and Related Processes 3
T QMS Template Help - Small machine shop/job shop Manufacturing and Related Processes 1
D 4M (Man, machine, method, materials) Management Manufacturing and Related Processes 3
jjmusicjr Starting from scratch - a plethora of challenges for a new Lumber Mill Machine Shop Quality Management System (QMS) Manuals 11
P Calculating a weight for a machine for the CE label CE Marking (Conformité Européene) / CB Scheme 0
K Is Compliance Testing of IEC 60601-2-63 mandatory for Europe - Dental X-Ray machine CE Marking (Conformité Européene) / CB Scheme 3
K IEC 60601-1-9 Compliance mendatory or Not - CT Scan machine CE Marking (Conformité Européene) / CB Scheme 8
M (How To) Getting Data From Vision Machine To Reporting Software Misc. Quality Assurance and Business Systems Related Topics 0
G Assigning a calibration tolerance - An x-y coordinate machine - Uncertainty as my verification tolerance General Measurement Device and Calibration Topics 4
T EN 10204 3.1 Inspection Certificate - Machine Shop Various Other Specifications, Standards, and related Requirements 4
N IATF 16949 7.1.5.3.2 External Laboratory - Calibration of Nikon machine in factory IATF 16949 - Automotive Quality Systems Standard 24
M AS9100d (section 7.1.3) example of compliance in machine shop environment. AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 3
A Statistic Tools to Know Capability of Sealing Machine for Seal Width Capability, Accuracy and Stability - Processes, Machines, etc. 3
J Help with Arburg Allround 320s Injection Molding Machine Manufacturing and Related Processes 3
P Injection Molding Machine Validation Scope Capability, Accuracy and Stability - Processes, Machines, etc. 6
G Can Measurement Machine Bias be part of Uncertainty? General Measurement Device and Calibration Topics 1
G How to include machine error in uncertainty calculations? Measurement Uncertainty (MU) 5
B ISO 9001:2015 for a Small Machine Shop need Resources ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 5
H AS9100D Cert question - Small machine shop - Where can I buy a certificate? AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 6
A Machine Relocation within Facility - Is re-validation required? ISO 13485:2016 - Medical Device Quality Management Systems 8
H Validating Nondestructive leak testing machine Qualification and Validation (including 21 CFR Part 11) 3
WEAVER Can automatic machine testers be subject to GR&R? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 4

Similar threads

Top Bottom