Process Capability Study | Cp Cpk Study with Excel Template

Process Capability Study | Cp Cpk Study with Excel Template

→ Process capability study (Cp Cpk Study) is the study to determine the output of a process is meeting customer's requirements, specifications, or not?
👉 Download the Excel Template for Process Capability Study or Cp, Cpk Study.
→ Process Capability Study | Cp Cpk Study is a part of Statistical Process Control.

Four steps for Process Capability Study (Cp, Cpk Study) with Excel Template

→ Perform below four steps for Process Capability Study | Cp Cpk Study
  1. Select the parameter and Fill basic details in Excel template of Cp, Cpk Study
  2. Select the Subgroup size and frequency for data collection
  3. Collect the data and put into SPC Excel template and find out Cpk.
  4. Interpret the graph pattern and Cpk value.

➤ Step 1: Select the parameter and Fill basic details in SPC Excel template

Define the parameter for study and mention Part Name, Part Number, Tolerance band, Machine name, Date, etc. in SPC Excel template.
We will write down all the details in the SPC Excel template.

Step 1 Process Capability Study

➤ Step 2: Select the Subgroup size and frequency for data collection

→ Define the sample size “n”.
→ We can take 2,3,4...,n samples in each subgroup.
→ Usually, four or five are common sample sizes.
→ We will take 5 samples in each subgroup.
→ We can define the frequency of sample checking like...5 samples in every 30 minutes or 5 samples in every 60 minutes, etc...
→ For, X-bar and R chart sample size should be 9 or less.
 Also, determine the frequency of sample measurement.
 We will collect 100 measurements in a subgroup size of 5 samples.
 So we have to take a reading 20 times.
→ Then we will calculate the average value and range for each of the 20 subgroups having 5 samples in each.
→ The range is the difference between the highest and lowest value in each set of 5 sample measurements.
→ In this case, we will take 5 samples every 60 minutes.
→ We will collect data as per our defined subgroup and frequency and put in excel template.

Step 2 Process Capability Study

➤ Step 3: Collect the data and put into Cp, Cpk Excel template, Cpk will be calculated

→ The Upper Control Limits (UCL) and Lower Control Limits (LCL) will be calculated.
→ X-bar and X-double bar (the average of the averages), which is represented on the X- bar chart by a solid center line are also calculated.
→ Find out the average of the sample ranges or “R” values it is called R-bar
→ The value of R-bar is the centerline of the Range chart.

➥ The formula of Cpk Calculation.

→ Cpk will be calculated by the below formula.
→ Standard Deviation (𝞼) = R bar/d2
→ Cp = (Upper limit - Lower limit)/6𝞼
→ CPU = (Upper limit - X double bar)/3𝞂
→ CPL = (X double bar - Lower limit)/3𝞂
→ Cpk = Min of (CPU & CPL)
→ The control limits are not defined in drawing or it is not set by the engineer.
→ It is derived from the data.
Step 3 Process Capability Study

➤ Step 4: Interpret the graph pattern and Cpk value

 In this step, we will interpret the Cpk value and graph pattern.
👉 Control Chart Rules, Patterns, and Interpretation
→ Check for the special causes, is it available in process or not?
If special causes are available then take necessary action and ensure the process is under control.
Step 4 Process Capability Study

Interpretation of Cpk (Process Capability Index) value:

→ Process Capability Study (Cpk Study) will give the outcome of Cpk Index.
→ It provides an indication of the ability of a process to consistently provide an output which is within required specifications.

→ There are four different cases are mentioned below:
→ (1) Cpk <1.00 or Cpk =1 then Process is poor and it is not capable.
→ (2) 1.00< Cpk <1.67 Process is fair and conditionally acceptable.
→ (3) Cpk >1.67 Process is Excellent and Capable)
→ (4) Cpk = 2 for a 6𝞼 process (i.e. a 6 sigma process)

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