Process Capability study of CNC Machining Process

A

alongtain

#1
Hypothetical Question: Let's say I'm CNC machining a 1000 piece order for a new part and it has 3 key dimensions that require "inspection". How would you go about setting up and conducting a process capability study on that job?

I guess the main thing I need help with is determining how many parts to check when performing a Cpk study and at what frequency to check them?

My current method is listed below:

1) First, we dial in the CNC program, setup, speeds and feeds until we're able to produce a good part. The program is then commented with setup, speed & feed info and saved in the DMR (Device Master Record) for the part/part family! It is also referenced in the appropriate operation on our routing for that part (which is created in our shop management ERP system).

2) Second, we develop a detailed inspection report and we inspect the initial order per Z1.4, level II, AQL = 1.0. If we get a repeat order and we know we're in tight control and would like to reduce our inspection frequency, we'll take our Z1.4 results (from the first job) and analyze them to determine if our Cpk is acceptable.

I'm looking for some assistance in devising a simple procedure for conducting process capability studies on the parts we machine and could use some help because I don't have a firm grasp on how to execute a process capability study from a practical standpoint.
 
Elsmar Forum Sponsor

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#2
If you are doing precision machining with your CNC process, I recommend reading the following:

SPC for precision machining

If you have the correct sawtooth curve and it falls within your control limits - which means your process is in control, you are capable. If you have a normal distribution, you have incorrect sampling, or you are out of control. Details are in the link.
 
G

George Holz

#3
It seems that you are doing batch production. The best you can do is Ppk. You are not including all of the variables in your samples over time, which would include set-up variation. Your samples do not include enough of the long term variation to calculate a valid Cpk.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#4
It seems that you are doing batch production. The best you can do is Ppk. You are not including all of the variables in your samples over time, which would include set-up variation. Your samples do not include enough of the long term variation to calculate a valid Cpk.
Actually, if he is doing precision machining with the CNC operation, neither is applicable. If the process is in control, the results should be non-normal, so those capability indices are not applicable. Also, the use of AQL inspection is not good for production. The sampling rate should be a function of the tool wear slope and its intersection to the control limits. All of this is discussed in the link above.
 
Thread starter Similar threads Forum Replies Date
ncwalker Process Capability Study - Aggregate vs. Individual Processes Statistical Analysis Tools, Techniques and SPC 24
Q When to use Cpk or Ppk for a Process Capability Study? Capability, Accuracy and Stability - Processes, Machines, etc. 14
D Process Capability Study which involves MMC (Maximum Material Condition) Capability, Accuracy and Stability - Processes, Machines, etc. 8
J Process Capability - Interview Case Study Capability, Accuracy and Stability - Processes, Machines, etc. 1
A Process Capability Study using Variables Data (weight of the product) Statistical Analysis Tools, Techniques and SPC 9
M Capability Study on a New Machining Process - Issues Capability, Accuracy and Stability - Processes, Machines, etc. 40
sridharafep Process Capability Study for Electrostatic Powder Coating Statistical Analysis Tools, Techniques and SPC 6
J Process Capability Sheet for a short term 30 piece study needed Statistical Analysis Tools, Techniques and SPC 1
B CNC test pieces for milling and turning - I need to run a process capability study General Measurement Device and Calibration Topics 2
E Minumim number of parts to conduct the Process Capability Study APQP and PPAP 18
D Where can I find Process & Machine Capability study software? Statistical Analysis Tools, Techniques and SPC 3
M Measuring Capability of Process with Multiple Specifications Capability, Accuracy and Stability - Processes, Machines, etc. 6
L How to evaluate the process capability of a data set that is non-normal (cannot be transformed and does not fit any known distribution)? Capability, Accuracy and Stability - Processes, Machines, etc. 12
T Final process capability results - What I am supposed to present? Cp and CpK? APQP and PPAP 11
Q How to perform Process Capability for true position Statistical Analysis Tools, Techniques and SPC 10
L IATF 16949 Cl. 8.3.3.2 Manufacturing process design input - Process capability IATF 16949 - Automotive Quality Systems Standard 1
Y Process Capability for Non-Normal Data - Philosophical Questions Capability, Accuracy and Stability - Processes, Machines, etc. 6
M Process Capability in a High Precision Environment Capability, Accuracy and Stability - Processes, Machines, etc. 7
V Product Development - When to start calculating Process Capability Capability, Accuracy and Stability - Processes, Machines, etc. 3
S Proving Process Capability of a Packing Machine Capability, Accuracy and Stability - Processes, Machines, etc. 6
V Process Capability for parameters with non-normal distribution. Capability, Accuracy and Stability - Processes, Machines, etc. 16
J Process Capability - How much percentage of tolerance? VDA Standards - Germany's Automotive Standards 3
S Regression Control Chart - Process Capability Statistical Analysis Tools, Techniques and SPC 18
M How to determine Process Capability using Minitab 17 Capability, Accuracy and Stability - Processes, Machines, etc. 6
B Process Capability - Changing Limits to Improve Cpk or find Root Causes? Capability, Accuracy and Stability - Processes, Machines, etc. 6
B Long Term Process Capability from miltiple Machines and Fixtures Capability, Accuracy and Stability - Processes, Machines, etc. 3
E What Data do we need for Welding and Repair Process Capability? Six Sigma 1
K Calculating Capability of a process when data is skewed in the upper range Capability, Accuracy and Stability - Processes, Machines, etc. 4
S What is the difference between Process Capability and Machine Capabilty Capability, Accuracy and Stability - Processes, Machines, etc. 13
V How to Assess Process Capability at the Development or Optimization Phase Capability, Accuracy and Stability - Processes, Machines, etc. 6
C Minitab Binomial Process Capability Analysis Chart Explanation Using Minitab Software 1
kedarg6500 Sample Size for Short Term & Long Term Process Capability Capability, Accuracy and Stability - Processes, Machines, etc. 2
J Is it the same process capability or performance in 8.2.3.1 and SPC? IATF 16949 - Automotive Quality Systems Standard 1
E Binomial Process Capability Sample Size - Variables Data Capability, Accuracy and Stability - Processes, Machines, etc. 4
M On Subgroups in Control Charts and Process Capability Statistical Analysis Tools, Techniques and SPC 3
N SPC and Process Capability Using a Romer Arm Statistical Analysis Tools, Techniques and SPC 6
J Interpreting Process Capability results and ratios Capability, Accuracy and Stability - Processes, Machines, etc. 1
J Go or No-Go Process Capability Quality Tools, Improvement and Analysis 8
S Process Capability with Negative Exponential Distribution Capability, Accuracy and Stability - Processes, Machines, etc. 4
C Process Capability - Attribute Data Analysis (Template) Document Control Systems, Procedures, Forms and Templates 0
C Process Capability - Variables Data Analysis (Template) Document Control Systems, Procedures, Forms and Templates 0
A Cnpk (Non-Parametric Capability Analysis) to assess whether the Process is Stable Capability, Accuracy and Stability - Processes, Machines, etc. 7
V Process Capability to reconfirm the Inspection Frequency Manufacturing and Related Processes 3
B Sample Size based on Process Capability Indices (Pp/Ppk)? Capability, Accuracy and Stability - Processes, Machines, etc. 20
P Calculating Process Capability from Data of attached spreadsheet Statistical Analysis Tools, Techniques and SPC 4
D How to measure Process Capability for Homogeneous Batch Manufacturing Capability, Accuracy and Stability - Processes, Machines, etc. 4
A Burrs affecting measurements and Process Capability Capability, Accuracy and Stability - Processes, Machines, etc. 7
G Assessing Process Capability on Variation (Hardware Adjustment Mean Shift) Capability, Accuracy and Stability - Processes, Machines, etc. 4
D Calculating Upper-CL/Lower-CL and Process Capability Statistical Analysis Tools, Techniques and SPC 8
V Defining Subgroups and Importance of Subgroups in SPC and Process Capability Statistical Analysis Tools, Techniques and SPC 6

Similar threads

Top Bottom