Criteria for taking Dimensional Measurements of a Painted Hole in a Part

A

Aviation Alvin

#1
Here's my dilemma. I was recently issued a Supplier CAR from a Customer because a screw hole diameter was undersized by .003" - and therefore out of drawing tolerances. Our measurements for this particular part were undertaken during an in-process stage of production before primer and paint, and at which time they were well within specified tolerances (screw hole requirement was for .218” - .229”). After primer and paint, the hole closed to .215”.

The obvious solution is to ensure that the hole is oversized to the high end of the range, and then measuring again after prime and paint to ensure that it falls within the allowable range. The only problem that I have is that I’m taking a measurement against a structurally unstable material (prime/paint) versus against the structurally sound material (aluminum). If I were to initially oversize the hole outside the allowable range and then close the hole with primer and paint to bring it within tolerances, theoretically then I’d still be within compliance. For obvious reasons that makes me nervous. :notme:

When I asked our engineering department if the dimensions stated on the drawing are applicable to the finished part (after prime/paint) or to unfinished part (processed raw material only), they stated that all dimensions are to the raw material state (in-process) and that this was an accepted standard. When I asked for a documented industry standard that addressed this, they couldn’t supply one (ASME Y14.5M-1994 doesn’t cover it). The drawing didn’t stipulate if the measurements were for a finished product or not (flag notes or otherwise).

Does anyone know of an aviation industry documented standard that dictates the criteria used to meet dimensional measurements as indicated on a drawing?

Thanks so much for the help, and for a forum where so many professionals are willing to share their expertise and knowledge.

:thanx:
 
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Jim Wynne

Staff member
Admin
#2
Re: Criteria for taking dimensional measurements of a part

Does the drawing specify finish (paint) requirements? If so, then drawing dimensions apply after finishing--they apply to the part you are supposed to supply. In my experience, dimensions always apply after processing unless otherwise specified. If you control the hole size and the paint process, you should be able to hold a .011" tolerance spread. If you think that the tolerance is too tight, ask the customer for some relief. Note that a .003 miss for a clearance hole is almost certainly nitpicking on the customer's part.
 
A

Aviation Alvin

#3
Re: Criteria for taking dimensional measurements of a part

Thanks Jim for your speedy reply. The drawing has a Bill of Materials (BOM) block that references all raw material and processes that are to be used for the production of the part. The chem. conversion, primer, and paint processes are listed here, as well as the paint and primer types.

Since my initial post, we've resigned to the fact that we'll have to build to the upper allowable tolerances and then measure post paint to ensure that we fall within tolerances. We’ll also utilize our Chamfer gage when we have to ensure that painted screw countersinks holes are within the .010” allowance.

I agree with your view of the Customer nitpicking on the over-tolerance measurements, but I have very little recourse - or a valid argument - when it doesn’t meet the drawing requirements. We've sent a lot of previous parts to this customer with no issue, but this shipment seemed to have generated a flow of paperwork (new, junior QC Inspector possibly?).

Thanks again for the help and guidance.


Cheers,

Al
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#4
When I asked our engineering department if the dimensions stated on the drawing are applicable to the finished part (after prime/paint) or to unfinished part (processed raw material only), they stated that all dimensions are to the raw material state (in-process) and that this was an accepted standard. When I asked for a documented industry standard that addressed this, they couldn’t supply one (ASME Y14.5M-1994 doesn’t cover it). The drawing didn’t stipulate if the measurements were for a finished product or not (flag notes or otherwise).
I think there is your opportunity for improvement. As Jim states, if there is no specification, then as supplied makes the most sense. But, I always say: if you have to ask, then stick it in the print and be done with it. Otherwise, you will be back to visit this problem again in the future...

I do hate building up to a final size from plating, painting, coating, heat treating, etc. Need wide specs to live comfortably with that.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#5
Re: Criteria for taking dimensional measurements of a part

I agree with your view of the Customer nitpicking on the over-tolerance measurements, but I have very little recourse - or a valid argument - when it doesn’t meet the drawing requirements. We've sent a lot of previous parts to this customer with no issue, but this shipment seemed to have generated a flow of paperwork (new, junior QC Inspector possibly?).
This is a chronic situation. More of a reason to get it in writing!
 

Jim Wynne

Staff member
Admin
#6
Re: Criteria for taking dimensional measurements of a part

Thanks Jim for your speedy reply. The drawing has a Bill of Materials (BOM) block that references all raw material and processes that are to be used for the production of the part. The chem. conversion, primer, and paint processes are listed here, as well as the paint and primer types.

Since my initial post, we've resigned to the fact that we'll have to build to the upper allowable tolerances and then measure post paint to ensure that we fall within tolerances. We’ll also utilize our Chamfer gage when we have to ensure that painted screw countersinks holes are within the .010” allowance.

I agree with your view of the Customer nitpicking on the over-tolerance measurements, but I have very little recourse - or a valid argument - when it doesn’t meet the drawing requirements. We've sent a lot of previous parts to this customer with no issue, but this shipment seemed to have generated a flow of paperwork (new, junior QC Inspector possibly?).

Thanks again for the help and guidance.


Cheers,

Al
There's always danger in "getting away with" nonconforming conditions, even over long periods of time. As you suggest, all it takes is one newbie to upset the apple cart. Hole sizes--especially clearance holes--are famously problematic when painting or plating is involved. It's often difficult for a supplier to know when size really matters, and designers who mindlessly apply tight tolerances (often defaulting to block tolerances to avoid thinking) are the root of the problem. Nonetheless, if hole sizes are targeted on the nominal and controlled (especially punched holes) it's usually fairly easy to control coating thicknesses within finished hole-size tolerance.
 
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