What is the difference between Process Capability and Machine Capabilty

S

Sivakumar CS

#1
What is the difference between process capability and machine capabilty by definition ?

How to calculate ?
 
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S

Surehawk

#2
From what I learned the difference is how you take your samples.

Machine capability, wich is to take into account just the machine and not the process, you take out all your samples in a row (from 30 and up is what I learned) and you do the calculations.

Process-capability, wich wants to take in the whole process (including how the operators manage the machine)m you take for example 5 products every half hour (depending on the speed of your process) and do this over a couple of hours til you have the amount of samples you want, and then do the same calculations.

Don't know if it is Swedish standard, but we usally let the machine capability give the value Cm and Cmk, while the Processcapability gives the more comon Cp and Cpk.

As for the formulas:

Cp = (UTL-LTL)/6s

UTL = Upper tolerance limit
LTL = Lower tolerance Limit
s = Sigma or standard deviation

Cpk = Minimum of [(UTL-3s)/3s;(3s-LTL)/3s)]
So in other words, the one of the 2 calculations that gives the lowest value. Hope this made sence
 
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S

Sivakumar CS

#3
Thanks a lot friend

but still the real difference between Cp, Cm and Pp are not very clear

where to use the appropriate metric in real time ?
 
S

Surehawk

#4
Well, Cm and Cmk is usally to test if a machine is capable of producing what it needs. I think a Cmk level of (at least) 1.33 f?r old machines and 1.66 for a new machine is a sort of standard.

While Cp and Cpk is for when you test your process, I suppose it can be intresting to know if you are looking at sampling frequency for example.

pp I am not that good with, but there are others here in the cove who is.

A little ps here... I have often seen that companies write Cp or Cpk when they have actually calculated what I call Cm or Cmk. So not everyone makes this seperation.
 
T

t.PoN

#5
The Machine capability is usually short term measurement. while the process capability is measured for a long term.

for example:
You take one staff (usually the best) and make him produce the same product using the same material in a short period of time (lets say a day or 2), the variation is your Cm.

for process capability, you take more than one staff (operator repeatbility and re reproducibility will take effect) and you make them produce the same product using different material for a long term (lets say a month). this way environment will take effect, material variation has its effect, maintenance and setup variation will make difference.

The above example if we assume that your production depends only in one machine.

but if your production depends on more than one machine, the process capability will consider all the Cm in accounts.

for example: You could have two stages with two different machines to make your final product.
or you could have two similar machines that are scheduled to produce the same product in different shifts.
the Cp will consider the variation in the Cm as well
 
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T

t.PoN

#7
Cp is used when the process is in control where sigma = R'/d2
Pp is used when the process is NOT in control. Pp = USL - LSL / 6s, where s=sqrt (Sum ((xi-x')^2)/n-1)

when the process is stable, normally distributed and in control Pp = Cp

but if you used Pp in your report, it will only be understood that you are in the process of determining stability of your process.

For example:
You'll be using Pp when you are using a new machine, or you are testing new material
 
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