Machine Shop Operator Inspection Frequency

[bryjarmoc]

Not sure if this is the right place to post this... but here goes.
I am a QE developing standards or (best practices) procedure for Machine Operators (Lathe, CNC Mill, Manual Mach, etc). One of the aspects is inspecting the parts that come off the of the machine throughout the run. Does anyone have a suggested sampling plan? Or even what they use so I can get some ideas on what we should be doing. We are looking at any orders under 10 you 100% inspect all dimensions created in the operation. 11 or more then the operator would be responsible for checking every 3rd part. In addition, there might be dimensions you would only have to verify on the 1st part only.

Thoughts????

 

[BillPSU]

It depends on some many issues and what you are making.

The operator can only correct certain dimensions on a part. How much does the part cost? The type of tolerance. The process making the dimension.

Say a part has a reamed hole 1.0000" +/- .0010. A reaming process can easily hold the tolerance in normal types of material (gray iron or soft steel part.) Now lets change the tolerance to 1.0000" +/- .0002. How often do you check it. Now lets change the cost of the part from $50 to a $2500 casting. Now how often do you check the part. Now lets change the material from gray iron to Inconel. The purpose for the hole is a high reliability feature on a rocket motor used in a nuclear missle. Tell me how to answer your question, it depends.

 

[bryjarmoc]

I do understand for certain Key dimensions, high tolerance, and/or high risk dimensions will need to be looked at differently. We do have exceptions for that. But what is that "good machine" practice when running a part to make sure you don't run 40 parts with a bad dimension.
This is what we have come up with:

1. First part produced - inspect 100% of the dimensions for that operation.
2. Every part produced – inspect 100% of tight tolerance holes (less than +/-.002), tight tolerance dimensions (less than +/-.005) and key/high risk dimensions (defined by planning)
3. If the order quantity is 10 or less: Inspect every part produced - inspect 100% of the dimensions for that operation .
4. If the order quantity is 11 or more: Inspect every 3rd part produced - inspect 100% of the dimensions for that operation .
5. Last part produced - inspect 100% of the dimensions for that operation.
6. First part produced at shift change - inspect 100% of the dimensions for that operation.
7. Exceptions may include complex CMM inspected parts.

 

[BillPSU]

It still depends,
If the inspection process is extensively long I would not inspect all dimension especially if the dimensions are machine controlled. An example is a bolt hole pattern on a cnc mill. I would not reinspect this pattern after first piece. Many turning dimensions are not rechecked if a part has a rough turn and finish turn again on a cnc lathe. I am talking about the shoulder positions and chamfers and radii again machine contolled. I would probably inspect on of the dimensions and assume the remainder were machine controlled. Would I inspect the OD's sure this is a simple project usually. However even that sometimes is more difficult then you think. I once worked at a company called McNally's in Pittsburg, KS. It was a combination job shop and product shop which built large parts. There was a large vertical boring lathe. Table diameter 16 feet turning capacity 21 feet. Try measuring an 216" +/- .06 OD. Think pi tape with temperature measuring.