"Critical" or "Inspection" dimensions -- useful or waste of 9

[kmckee]

I'm in engineering, designer of the parts, creator of the drawings. Our procedures currently require 100% inspection of our first article parts as they are delivered from our vendors. We want to develop a procedure for inspecting the balance of the lot. Of course 100% inspection would require too much time and relying solely on our vendors' inspection reports is risky.

It has been suggested that we identify either on the drawing or on a separate internal inspection report which dimensions are critical. My idea was to perform a random dimension inspection of an agreed upon number of dimensions but this would be a policy created by the QC department. QC would like us to mark on the drawing which dimensions are critical and to these they would limit their inspection. Understand that we are verifying that the vendors' reports are accurate, not inspecting the part for form, fit and function.

I've read the engineer's forum on this topic and I would like to hear opinions of inspectors before taking my stand either way.

I look forward to your thoughts and opinions.

Ken

 

[DennisP]

If the dimensions REALLY are critical, why not require SPC with some minimum CPk level on an ongoing basis from the supplier. As a general rule, if relying on a supplier's measurements is deemed too risky, then maybe you need new suppliers. I'm a supplier quality engineer and would not want to work with suppliers I couldn't trust to make and report measurements accurately. One of our biggest problems problems is actually measurement correlation. Different methods will sometimes yield different results. Be sure the supplier understands the intent and it will result in significantly fewer problems.

 

[PSE]

Any time our design group creates a new part, it goes through our Initial Sample Inspection Report (ISIR) process whether it is purchased from a supplier or fabricated internally. Initially, a full layout is done (all features/dimensions) on a randomly selected sample from the first lot. If or when the component changes over time, we will then do a partial ISIR verifying the changes. These reports are then signed off by engineering for acceptability.

We will also work with our suppliers along the lines that DennisP indicates by looking at capability studies and what we can tweak to improve results. We utilize a supplier certification procedure that can eventually result in a dock to stock passing of the component. We still reserve the right of random lot inspection for verification or the occasional supplier audit.

One of the challenges you may face as the designer is once you have released the component, it then essentially becomes the property of (assuming there is one), the procurement group. They may go with someone completely different from who you initially worked with. My best advice is if you have this type of business structure, to get procurement, potential suppliers, and QC all together as part of the design/release process. Communication flow up front can save a lot of headaches later on.

Regards,

 

[dingy2]

The amount of inspection is a quality function and I would not suggest that you mark your drawings with "major","critical", etc. "Critical" usually means that the characteristic is confirmed using SPC. Make sure that your drawings have appropriate tolerances and proper use of GD & T though.

I would suggest that you have a meeting with the Quality Engineer (someone in that function) and discuss the fit, function and appearance requirement on the part. From that information, the Quality Engineer will come up with a list of dimensions or attribute characteristics that the Supplier should confirm on a regular basis.

The Quality Engineering should then have a meeting with Purchasing (should never go directly to the supplier) about the needs of your company with Purchasing forwarding the information to the supplier. If the suppllier has any questions, they can directly contact the company's Quality Engineer.

Have the supplier provide an inspection report on each shipment reflecting an agreed upon sample size and the actual results of their inspection. Do NOT spend too much time in receiving doing your supplier's function (receiving inspection). This report should be forwarded to the Quality department and it is up the that department whether or not to confirm any characteristic shown on the report.

Hope this helps.

Dave D.

http://www.qmsi.ca

 

[AmeristarQA]

Im my opinon even though you are only verifying a suppliers inspection report, if you are checking the part you should verify with the critical dimensions. Also because dimensions critical to function are usually held with a much tighter tolerance and will give you a good indication of your suppliers gage capability. QA departments typically like to have those inspection dimensions identified so that there is some consistency from one inspector to the next. What we do is the Quality Manager/Engineer/Qualified Person will just circle the dimensions on the print required for inspection, then sign and date the print so everyone knows who is determining this. This has helped us tremendously.

 

[KENAT]

I've seen variations on what AmeristarQA said and agree with dingy2. Every major product at my last place had a 'quality plan' part of this was determining inspection requirements on the various parts drawings. This was done by either annotating a drawing (and somehow controlling it) or occasionally listing the dimensions with their location on the drawing in the plan.

I don't believe this information belongs on the drawing. The drawing should give the requirement (dimension & tolerance in this case). It shouldn't normally say how to meet that requirement or how to verify it's been met. Generally doing so would contravene ASME drawing standards. The only type of inspection which I've normally seen detailed to any extent on drawings is NDT where there is a safety consideration and then this is usually done in accordance with the appropriate spec.

 

[AmeristarQA]

I think whether it needs to be on the drawing is up to the manufacturer and their procedures. For us, the print and the operations routing is the only work instructions we use. It really comes down to what works for your company.

 

[KENAT]

By not on the drawing, I mean not on the relased 'master'. Annotating a copy of the drawing for the QA file/plan is fine.

If you claim to follow ASME drawing standards it probably shouldn't be on the drawing.

If you don't follow these or equivalent industry standards then yeah, it's up to the manufacturer and their procedures.

KENAT, probably the least qualified checker you'll ever meet...

 

[eddiewdi]

Just for information to the subject.

API 19th Edition states page 60 clause c.

"The manufacture shall specify and verifiy critical dimensions.

Acceptance criteria for critical dimensions shall be as required per the manufacture's written specification."

 

[underspec]

Doesn't "critical" basically makes or breaks the part for form and function? Choose the tightest toleranced dimensions imo.

 

[KENAT]

There was a related question on another post, can't remember which forum.

I will say that you should probably take into account process capability as well as end effect.

If the part is dimensioned & toleranced correctly then meeting the required tolerance on pretty much every dimension should arguably be 'critical' to correct funcion...

KENAT, probably the least qualified checker you'll ever meet...

 

[MikeHalloran]

If anyone chooses to not bother inspecting every dimension on a first article lot, that's a _business_ decision, not a technical one. Not my problem.

All of this ranked dimension hoo-hah is just an attept to justify charging Engineering's budget for business decisions that turn out badly.



Mike Halloran
Pembroke Pines, FL, USA

 

[tab1999]

kmckee,
Does your QC department have a Procedure on random sampling?

Look at this site.

http://www.randomizer.org/form.htm

" There is no Shortcut to anyplace worth going "