First Article Inspections - 'how to measure'

[Candi1024]

Hello! We are completing a first article inspection, but some of the dimensions shown on the drawing we cannot measure in house the way they are referenced.

If a drawing shows a measurement and we cannot measure that exact distance due to part shape, are we allowed to create other reference points to mathematically determine what the actual measurement is?

And yes, I am researching outside labs to complete this. But I need to answer these questions as well.

Thank you!

 

[Jim Wynne]

More information is needed. You ask about being "...allowed to create other reference points..." but who decides what's "allowed"? Is the FIA being done for a customer, or is this strictly internal to your company?

 

[Candi1024]

It is for a customer. As for as "allowed", I want to follow NIST measurement standards.

Also we are 13485

 

[Cari Spears]

Can you put it up on a comparator? Can you stack up gage blocks for reference? Without knowing the configuration of the part, I'm afraid we're not going to be much help.

 

[Mike S.]

This is somethig that ideally should have been questioned during contract review. Since it wasn't, you could always ask the customer how they measure it and/or get their approval for your proposed measurement method. Maybe the customer doesn't realize they spec'd something that can't readily be measured.

 

[Jim Wynne]

It is for a customer. As for as "allowed", I want to follow NIST measurement standards.

Also we are 13485

There are no NIST measurement standards for this sort of thing. You should ask the customer how they will measure the feature/dimension and get any necessary dispensations from them.

 

[Candi1024]

Here is an ugly sketch with made up dimensions, but you get the idea. All units are in MM with a 1mm tolerance.

 

[Jim Wynne]

Here is an ugly sketch with made up dimensions, but you get the idea. All units are in MM with a 1mm tolerance.

Oh, my, the dreaded zero-baseline dimensioning. As advised earlier, talk to your customer. It might be a good candidate for an optical comparator.

 

[Candi1024]

Oh, my, the dreaded zero-baseline dimensioning. As advised earlier, talk to your customer. It might be a good candidate for an optical comparator.

So back to my original question, the engineers want to use squares and such to extend the dimensional lines and attempt a measurement. I can see how this might work.

For measurements that start at the center of a hole and extend to an edge, they want to use the hole diameter to determine radius and then measure from the end of the part to the edge of the hole, then add the two measurements together. My instincts tell me this is wrong, but I don't have anything to back that up.

I'm all about just sending it out to someone with the right equipment!

 

[Jim Wynne]

So back to my original question, the engineers want to use squares and such to extend the dimensional lines and attempt a measurement. I can see how this might work.

It might work, but before taking such measurements as gospel I would want to see at least two different people do the measurements independently.

For measurements that start at the center of a hole and extend to an edge, they want to use the hole diameter to determine radius and then measure from the end of the part to the edge of the hole, then add the two measurements together. My instincts tell me this is wrong, but I don't have anything to back that up.

This is a common measurement method, especially when the tolerance is liberal.

I'm all about just sending it out to someone with the right equipment!

It's all about risk.