Process capability study

Submitted to- Submitted by-

Prof-JM Pant Sachin Khare

Sachin Sharma

Himanshu Shekahar

Submitted to- Submitted by-

Prof-JM Pant Sachin Khare

Sachin Sharma

Himanshu Shekahar

Introduction-

Process capability is the range within which a process is normally able to operate given inherent variation due to:

• Design• Materials• Equipment• People• Process steps• Others

•Also defined as the six standard deviation spread in the process

Contd…

•Process capability analysis involves measuring the natural

variation in a process

• Process capability is a function of the variation in a process

• Knowing the capability of a process means knowing if a

particular specification can be heldif the process is in control

Reasons for process capability

• Assess whether a quality characteristic or process is able to meet requirements

• Monitors the continuous reduction in variation

• Monitors the continuous improvement in a process

• Measures the uniformity of the output of a process

• Identifies processes or quality characteristics that need improvement

• Ensures that key customer requirements are met or exceeded

Capability indexes

• Capability indexes are values that measure the capability of

a process

• Capability indexes are used to predict the performance of a

process by comparing the width of process variation to the

with of the specified tolerance

• Process capability uses two indexes, Cp and Cpk,to compare

process variation and engineering requirements

Contd…

• Cp• Called Process potential • Width of the engineering tolerance divided by the capability, or spread, in the output of the process

• The Cp ratio does not take into account the relationship between the process average and the target specification

Cpk

• Called Process performance

•Cpk does not measure the process

performance in terms of Nominal or target value so

we use Cpk.

• Ratio of the distance between the process average and

the closest specification limit, to the three standard

deviations of the process variation

• Takes into account the location of the data mean

within the tolerance

• Called Process performance

•Cpk does not measure the process

performance in terms of Nominal or target value so

we use Cpk.

• Ratio of the distance between the process average and

the closest specification limit, to the three standard

deviations of the process variation

• Takes into account the location of the data mean

within the tolerance

Formulas for Capability indexes

Cp = 6

Engineering tolerance

Cpk = Min of USL - X X - LSL,

3 3

USL= Upper specification limitLSL = Lower specification limitEngineering tolerance = USL – LSLX = Average of the individual measurements = True population standard deviation

Assumptions for calculating Cp and Cpk

• The process must be in “statistical control”

• Process mean is stable and predictable

• Process variation is stable and predictable

• Specification limits are independent of the process

• Targets being used are goals for the process

Cpk and number of measurements relationship

• The more data used in the calculation, the more accurate will be the estimate of Cpk.

• Having only a few measurements can result in misleading Cpk results.

• Some customers required a minimum Cpk value for certain characteristics

• Larger Cpk values than the minimum requirement provide confidence in the process• Acceptable confidence level is 90%.

Comments-

•The Cp value does not change as the process centre changes.•Cp = Cpk when the process is centered.•Cpk is always equal to or less than Cp.•Cpk value greater than 1 indicates process confirms to specifications.•Cpk value less than 1 indicates that the process does not confirm to specifications.•Cp value less than 1 indicates the process is not capable.•Cp value greater than 1 indicates the process is capable.•Cpk value of zero indicates that the average is equal to 1 of specification limits.•A negative Cpk value indicates that the average is outside the specification limits.

•The Cp value does not change as the process centre changes.•Cp = Cpk when the process is centered.•Cpk is always equal to or less than Cp.•Cpk value greater than 1 indicates process confirms to specifications.•Cpk value less than 1 indicates that the process does not confirm to specifications.•Cp value less than 1 indicates the process is not capable.•Cp value greater than 1 indicates the process is capable.•Cpk value of zero indicates that the average is equal to 1 of specification limits.•A negative Cpk value indicates that the average is outside the specification limits.

Thank You