Sampling Plan for Inspection of Metal Parts after CNC Machining

G

GRISHAG

#1
I need some help for the similar problem (3 questions):
1. What is the difference between lot and batch?
2. What is the difference between minimum protection level and probability of conformance?
3. I have to prepare sampling plan for inspection of metal parts after CNC machining according to SAE ARP9013/2 requirements.
The requirements are:
3.1 C=0
3.2 Minimum protection level (probability of conformance) is 97%
3.3 AQL (is not mentioned in customer’s order), I assume AQL=0.65
The sampling plan shall identify a parameter used to control customer risk, and identify the value of that parameter.
The value of quality parameters shown in the ANSI/ASQC Z1.4 Tables V.A, V.B, VI.Q, VI.6. Each sampling plan may identify both the AOQL and LQ values used to control customer risk.
How to proceed?

Thank you in advance,
Greg
 
Elsmar Forum Sponsor

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#2
Re: Sampling Plan for Inspection of Metal Parts after CNC Machinin

A quick "Bump" - My Thanks in advance to anyone who can help with the other questions.

<snip> 1. What is the difference between lot and batch? <snip>
A Batch is all the material being put through a process run. A lot will be a part of a batch, or a lot can be the batch size if a company doesn't want to sub-divide a batch.
 

Wes Bucey

Quite Involved in Discussions
#3
<snip> 1. What is the difference between lot and batch?
2. What is the difference between minimum protection level and probability of conformance?
3. I have to prepare sampling plan for inspection of metal parts after CNC machining according to SAE ARP9013/2 requirements.
The requirements are:
3.1 C=0
3.2 Minimum protection level (probability of conformance) is 97%
3.3 AQL (is not mentioned in customer’s order), I assume AQL=0.65
The sampling plan shall identify a parameter used to control customer risk, and identify the value of that parameter.
The value of quality parameters shown in the ANSI/ASQC Z1.4 Tables V.A, V.B, VI.Q, VI.6. Each sampling plan may identify both the AOQL and LQ values used to control customer risk.
How to proceed? <snip>
Sorry, I missed this the first time around.

Lots and batches can be interchangeable terms. The point is the organization should choose a word, define that word (in writing) and use ONLY that word thereafter.

Let's talk about sampling.
Are you making the machined parts in house? If yes, it is best to use a Statistical Process Control system - an example for a machine that produces 100 parts per hour might be:

  1. Each hour, set aside parts 96, 97, 98, 99, 100 in sequence and measure them for "critical characteristics" (if you are unfamiliar with that term, say so, and we'll elaborate.)
  2. Chart those measurements (measurements, NOT go/no go) - comparing hour to hour charts will tell you if the process is in control and allow you to perform all sorts of statistical magic to determine when to change tooling, etc.

If not in-house, try to get the supplier to provide SPC charts similar to that described above.

Alternately, if ALL the parts are delivered en masse, there are charts for how many samples (as a percentage of the whole) you will have to pull and measure to achieve an Acceptable Quality Level. Trust me, this is a P.I.A. (see 3rd definition at http://www.acronymfinder.com/PIA.html) best avoided, but, if necessary, we will steer you to such charts. Some of our Cove members are very savvy about SPC charting (bobdoering has written some good stuff on this topic)
 
Last edited:

Michael_M

Trusted Information Resource
#5
1. What is the difference between lot and batch?
The two terms (as Wes stated) are interchangeable outside a company. We have defined the two terms as:

Batch: one traveler/work order; usually consisting of the quantity we actually want to run and (usually) consisting of one heat lot of raw material.

Lot: each set of parts that go for outside processing. Sometimes we have to run 3,000+ pieces but break the batch up into smaller lots due to time/customer requirements.
 
G

GRISHAG

#7
Thank you Wes,
please find below the answers to your questions:
we are you producing complicated metal parts using cnc machines (in house). I know that it is best to use a SPC system -but we are not going to manage SPC. It takes about 30 minutes for each machining operation. Please see below our inspection process:
a. Set-up inspected and approved by QC personal, using CMM.
b. Sampling inspection of critical dimensions performed by CNC operator (we have collection of results). Visual 100%.
c. Random on-line inspection performed as collaboration between production and QC dept.
d. Final 100% visual inspection performed by QC inspectors.
e. Final inspection report based on set-up and process inspection results issued by QC.
Requirements of SAE ARP9013:
a. c=0
b. Minimum protection level (probability of conformance) is 97%.
(AQL is not mensioned).
d. The sampling plan shall identify a parameter used to control customer risk, and identify the value of that parameter.
My process is stable and we have minimum customer complaints.
Your suggestions please: how can I combine my inspection process with customer requirements (c=0 and SAE ARP9013 requirements)?

 

Wes Bucey

Quite Involved in Discussions
#8
I agree a slow production rate (almost a one-off) does not readily lend itself to SPC.

so that I am not guilty of assuming:

  1. 30 minutes is for one of several operations on a workpiece OR
  2. is it the time to finish ALL operations on one piece?
(i.e. only a few pieces per shift or two pieces per hour?)

At this point, it is necessary to revisit the concept of sampling at all.
Beginning way back when Military Standards dictated a lot of quality decisions, it was understood that certain conditions required a decision NOT to sample.

Following is an excerpt I had hanging around from an old document (sorry, no longer have the original or title details, but I suspect it came from an aerospace customer):
1.0 Sampling Decision
1.1
Prior to the creation of an acceptance sampling plan, it should be determined whether sampling is appropriate or not. This determination should be made by Quality Department personnel, with input from other organizations as appropriate. Answer the following questions to determine the viability of acceptance sampling as an inspection option:
a. Is there a need to perform acceptance sampling?
b. Does the drawing or related specifications prohibit sampling?
c. Is the rejection rate low?
d. Is 100% inspection disruptive?
e. Are the costs of escape low?
f. Are the costs of sampling low?
g. Has training been conducted?
h. Are the parts produced in high volume?
i. Will sampling provide a positive return on investment?
1.2
Answering “no” to any of the above questions may indicate that sampling may not be appropriate for the given task. However, corrective actions taken can change this situation to once again make sampling a viable option. If sampling is determined to be a
viable option, a sampling plan must be generated.
Look particularly at Item H and discuss with your customer whether sampling is appropriate for the low volume you describe.

Note that "c=0" means the entire lot fails if one dimension on one piece from the sample is nonconforming. Is it really smart to fail an entire lot because a chip may have stuck in a bore or threaded hole? Typically, failing a lot meant (back in the day) subjecting the entire lot to a 100% inspection, which most quality experts now agree cannot assure 100% conformance of parts which pass inspection.

Before we moved away from massive pieces on multi-axis machines with small quantities of finished pieces per shift, we did our operation pretty much as you describe. Our one overriding concern was that ALL dimensions from one machine setup or tool were conforming BEFORE moving to the next setup or tool so that we NEVER continued work on a piece already out of conformance. When the production rate is low, it makes sense to have an operator check all necessary dimensions on a finished workpiece while the next piece is running.

Elsewhere here in the Cove (see quote box below), I described our method of making operators the quality inspectors and using quality folks as instructors and arbiters between customer and supplier (either direction in the supply chain.)

So if we had two setups on one workpiece, we simply took the time to confirm the first work was in conformance (or met the interim measurements required at that stage of completion) before clamping into the next setup and expending more work on a piece.

We gradually segued into a 90% turning operation and ceased working on any part larger than a human fist with much higher production rates. Just as we were transitioning, we became aware of, but did not install non-contact optical and laser measuring instruments which could check dimensions without removing the piece part from a holding fixture.

We did install such devices on some of our turning machines, which allowed us to run about one shift a day completely "lights out" with the non contact (as well as some contact) probes rigged to stop a machine and alert a standby tech to investigate.
And, about my own operation

Operation:
Operators were all on MRB (material review board) in addition to Quality Manager, Finance/Purchasing, Marketing. MRB meetings were held in their [the operators'] conference rooms. If customers or suppliers were invited to MRB, they met there, too.

All training (in-house, machine tool suppliers, outside experts, cutting tool suppliers, heat treaters, platers, etc.) could be conducted on-site. Customers were encouraged to come and meet with operators running their jobs.

We had no quality inspectors (we did have quality trainers and guys who acted as "court of last resort" when a question would arise.) Operators did own first article inspections, based on control plan/inspection plan agreed with customer as part of contract review. Another operator would perform a redundant first article inspection with different inspection instruments. Marked sample with BOTH inspection reports was sent to customer for confirmation before production began.

In-process inspection, SPC, etc. was performed by operator in real time. If nonconformance was discovered, production would halt - all operators would collaborate on finding and curing cause, only calling in outside help if solution eluded them. Inspection records, charts, etc. went right to computer where they were available in real time to in-house folk and customers.

Operators had autonomy to bring in experts from our suppliers of material, capital equipment, and expendable tooling to stay up to date on industry innovation. Sometimes, we shut the whole shop down and chartered a bus to take us to the International Machine Tool Show to spend the day.

If an operator wanted to see a customer's operation and how his product was used, we made it happen. Similarly for a supplier's operation.

In my own practice, "suspects" were referred to a Material Review Board. The MRB had power and authority to make unilateral decision on suspects detected in-house. If a suspect arose at a customer, MRB worked jointly with customer to determine true status and devise a remedy acceptable to all parties. Often, a customer would be called in to affirm a "use as is" determination, regardless of where the suspicion arose.

From 1980 on, my MRB (Material Review Board) was always cross-functional, so it had the experience, knowledge, and power to make decisions on the spot (high efficiency - no delays in making decisions about N/C on incoming or outgoing material.)

I did essentially the same thing with the groups which made decisions about Contract Review and plans for new capital expenditures.

It just seemed like good sense to me. I'd be willing to bet lots of other executives independently came to the same conclusion as I.
The point we need to keep in mind is:
For a number of reasons - operator variation, measuring instrument variation, personal interpretation or estimate of an instrument reading, etc. - folks inspecting the same part may come to different conclusions regarding conformance to specifications. It is good company practice to have a process in place to routinely resolve the issue when such instances arise. In my contract machining business, such instances arose frequently enough that we codified the resolution under our Material Review Board, regardless of whether the issue arose in-house or not. This is definitely NOT a matter for discipline or punishment, but for simple, methodical resolution, with NO FINGER POINTING!

ADDED IN EDIT: the simplest agenda follows these steps:

  1. do we have the expertise here on the MRB to decide if material is conforming or not?
  2. do we need outside help?
  3. if we need outsiders, do we have authority to get and pay (if necessary) such experts?
  4. does our customer have to be involved in the decision?
  5. once we decide whether material is conforming, do we have the power and authority to ship it/rework it/scrap it?
I've had several people ask me why I spent so much money to provide high quality inspection tools at each machine. My partner and I looked at it this way:
If we spent from $250,000 to $500,000 on a CNC turning center which could turn out a complete, intricate piece from raw stock without changing a setup, why wouldn't we spend a few more thousand to have all necessary inspection equipment at hand to assure ourselves AND the customers that we were operating at the high cpk levels which assured and justified our profitability? (To do anything else would have defied my mom's advice when I was five: "don't be penny wise and pound foolish.")

Further, if an operator could run an expensive CNC, why couldn't he also inspect his work? If you can trust him to do one, you can trust him to do the other (confirmed by spot checks and observations.)
 
G

GRISHAG

#9
Thank you, Wes
I'm studying your detail answer.
The first question is what are your CNC operators doing when CMM inspection is needed? Do they place each part on CMM machine?
What is proportion between CNC operators and QC personal in your plant?
How did you convince CNC operators to work simultaneously as quality inspectors? I understand your position regarding c=0, but this is requirement of our customer.
 

Wes Bucey

Quite Involved in Discussions
#10
Thank you, Wes
I'm studying your detail answer.
The first question is what are your CNC operators doing when CMM inspection is needed?
Everyone was trained and judged competent on CMM, kept in climate controlled quality lab. For first Article, if CMM was required, a redundant inspection by another operator was performed.
Do they place each part on CMM machine?
YEP! but not EVERY part unless it was short run and required by contract and control plan. Once we turned to primarily turning operations, we rarely used the CMM.
What is proportion between CNC operators and QC personal in your plant?
Once we made the transition, we had THREE specialists who trained, worked with suppliers and customers AND operators to devise Control Plan and Inspection Plan (order of checking dimensions, instruments to use) They also acted as arbiters or "court of last resort" before bringing in outside specialists (metallurgists, X-ray inspection, micrographs, etc.)
How did you convince CNC operators to work simultaneously as quality inspectors?
It was never a problem. Operators were eager to learn and (I suppose) free themselves from the tyranny of Kwality Kops they had encountered in previous employment.

I understand your position regarding c=0, but this is requirement of our customer.
It might be worth a discussion with customer, especially if quantity is low and cost of product is high.
I hope my answers in bold face red answer your questions sufficiently.
 
Thread starter Similar threads Forum Replies Date
P Lot Acceptance Plan that specifies BS 6001 Double Sampling Plans, general inspection Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
V Sampling Plan for Final Inspection and Process Verification Inspection, Prints (Drawings), Testing, Sampling and Related Topics 21
M Sampling Inspection at QA - Inputs on the following sampling plan Statistical Analysis Tools, Techniques and SPC 3
J Dodges 1943 paper on A Sampling Inspection Plan for Continuous Production Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
R Sampling Plan for Wafer Inspection (Small Lot) Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
Q SMT Final Inspection Sampling Plan for boards off an SMT line Inspection, Prints (Drawings), Testing, Sampling and Related Topics 3
N AQL - Sampling plan (variable, inspection level ii, normal inspection S-method) AQL - Acceptable Quality Level 2
A Combining Lots under ANSI/ASQC Z1.4 Inspection Sampling Plan Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
V Can give me the sampling plan for Receiving and Final inspection - AS9100? Inspection, Prints (Drawings), Testing, Sampling and Related Topics 3
M Acceptance sampling inspection plan compliant to Boeing X31764 Inspection, Prints (Drawings), Testing, Sampling and Related Topics 2
S Sampling plan for Inspection by Variables (high volume production) Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
H Incoming Inspection Sampling Plan ISO 2859-1/ ISO 2859-2 Inspection, Prints (Drawings), Testing, Sampling and Related Topics 7
T Acceptable AS9100 Receiving Inspection Sampling Plan - ANSI/ASQCZ1.4 vs. Mil-STD-105E Inspection, Prints (Drawings), Testing, Sampling and Related Topics 10
C Sampling Inspection Plan - Cell phone distributor offering programming services Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
J How are the inspection processes and sampling plan determined? Inspection, Prints (Drawings), Testing, Sampling and Related Topics 5
S Which Sampling Plan(s) Should I Use? Inspection, Prints (Drawings), Testing, Sampling and Related Topics 7
L Sampling Plan Risks AQL - Acceptable Quality Level 6
A Sampling plan for in-process QC (medical devices) Inspection, Prints (Drawings), Testing, Sampling and Related Topics 13
W LTPD, AQL, Ppk and Cpk validation sampling plan table Inspection, Prints (Drawings), Testing, Sampling and Related Topics 0
E Change in control plan - Do I have to do sampling? IATF 16949 - Automotive Quality Systems Standard 1
S Is using ANSI/ASQ Z1.4-2008 the correct sampling plan to determine Pass/Fail of Apparel measurements? AQL - Acceptable Quality Level 4
N Sampling Plan for Internal Audits - ISO 2859 or 3951 - Or Neither? Internal Auditing 6
R Reduced sampling plan for sterial products APQP and PPAP 0
T Defining sampling plan for different AQL AQL - Acceptable Quality Level 3
B AS9100D Sampling Plan AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 2
Nicole Desouza Sampling plan for a customer who wants AQL 1.0 (per ANSI Z1.4) AQL - Acceptable Quality Level 5
M Sampling Plan for Alumin High Pressure Die Castings Manufacturing and Related Processes 0
E Sampling plan for orthopedic implant - Process Validation Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
S How to make a Sampling Plan - Conveyor belts Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
S Help develop sampling plan for prototype samples (medical devices) Statistical Analysis Tools, Techniques and SPC 7
M Zero Defect Sampling Plan Standards Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
L ISO 2859-2 LQL sampling plan linked to %confidence/%reliability Inspection, Prints (Drawings), Testing, Sampling and Related Topics 2
K Sampling Plan for transparent tubular plastic parts from 32 different cavities Inspection, Prints (Drawings), Testing, Sampling and Related Topics 3
A AQL Sampling Plan SOP / WI - Medical Device Industry Inspection, Prints (Drawings), Testing, Sampling and Related Topics 14
J Sampling Plan Questions from the CQE Practice Exam Inspection, Prints (Drawings), Testing, Sampling and Related Topics 2
M Sampling Plan for for Chemical Analysis Testing of Medical Devices EU Medical Device Regulations 3
V Sampling Plan for Destructive Testing Inspection, Prints (Drawings), Testing, Sampling and Related Topics 2
J Can ANSI Z1.4 sampling plan be used for sampling alone without the Acc/Rej criteria? Inspection, Prints (Drawings), Testing, Sampling and Related Topics 5
C Sampling plan for destructive tests on small lots ISO 13485:2016 - Medical Device Quality Management Systems 20
B Designing a Sampling Plan - AQL AQL - Acceptable Quality Level 19
N Use of arrows in ISO 2859-1 Table 1 and 2-A sampling plan Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
S AS9100 Sampling Plan Requirements - 8.2.4 AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 5
S Military Sampling Plan 105E vs. ISO 2859-1 AQL - Acceptable Quality Level 2
kedarg6500 Acceptance Sampling Plan for Variables - Product Performance Testing Inspection, Prints (Drawings), Testing, Sampling and Related Topics 5
M Manufacturing Process Validation Destructive Testing Sampling Plan Advice Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
S Where could I get reference to an OQA Sampling Plan of 1 per 1000 ? Quality Manager and Management Related Issues 4
R What to Report when Using C=0 Sampling Plan Supplier Quality Assurance and other Supplier Issues 15
R Results: Monte Carlo Simulation of Acceptance Sampling Plan AQL - Acceptable Quality Level 3
G Sampling Plan according to SAE ARP9013 C=0 Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
S Class III Sampling Plan - Explain AQL vs. 95%C & 90%R Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1

Similar threads

Top Bottom