Inspection Requirements & Engineering Drawings - Measuring Shaft Dia.

N

Nick-C

#1
Hi All

Sorry if this is a basic question, but here at my company we are having a disagreement about something that is quite fundamental and I would like a second opinion please.

Our inspector is currently in debate with our Design team because of an item that passed FAI that has since been found to have a problem.

There is a diameter that needed to be measured and that was logged as OK in the inspection report. I have since checked the part and if you measure it in say 2 places around it's circumference you are likely to record it as OK. If you check in say 6 places, or if you look for the lowest / highest measurement for that dimension then you will see if falls below min tolerance.

The inspector explains that it is unreasonable to expect him to apply his own judgement in how to measure each dimension. He states that the drawing should specify how the part is to be measured if anything more than a simple, single measurement is required.

The design team believe the inspector should take the initiative and think it through more, and that the drawings shouldn't need to tell him exactly how to measure each dimension.

This is causing us issues now as parts are being dimensionally approved but on later inspection you can find reasons that they should have been rejected.


What are your thoughts on this please?


Thanks


Nick
 
Last edited by a moderator:
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W

Walnut

#2
Re: Inspection Requirements & Engineering Drawings - Measuring Holes

You don't indicate what size the holes are that are causing the issue but if they are small enough Gage Pins are the easiest Go/ No Go test of hole size and shape. You can purchase them oversize and undersize to set your tolerance limits. If the hole is larger purchase a plug gage. Same idea.
 
N

Nick-C

#3
Re: Inspection Requirements & Engineering Drawings - Measuring Holes

Hi and thanks for the reply.

The dimension is 6mm +/- 0.1 and it's actually a shaft not a hole. I think a moderator has edited the title of the post.

The debate is mainly based around whether the drawing should define how, and to what level of detail the inspector should perform his measurements.

Thanks again

Nick
 

Michael_M

Trusted Information Resource
#4
Re: Inspection Requirements & Engineering Drawings - Measuring Holes

Hi All

Sorry if this is a basic question, but here at my company we are having a disagreement about something that is quite fundamental and I would like a second opinion please.

Our inspector is currently in debate with our Design team because of an item that passed FAI that has since been found to have a problem.

There is a diameter that needed to be measured and that was logged as OK in the inspection report. I have since checked the part and if you measure it in say 2 places around it's circumference you are likely to record it as OK. If you check in say 6 places, or if you look for the lowest / highest measurement for that dimension then you will see if falls below min tolerance.

The inspector explains that it is unreasonable to expect him to apply his own judgement in how to measure each dimension. He states that the drawing should specify how the part is to be measured if anything more than a simple, single measurement is required.

The design team believe the inspector should take the initiative and think it through more, and that the drawings shouldn't need to tell him exactly how to measure each dimension.

This is causing us issues now as parts are being dimensionally approved but on later inspection you can find reasons that they should have been rejected.


What are your thoughts on this please?


Thanks


Nick

They are both correct.

The diameter has to fall within the tolerance regardless of where you measure it. For the most part, most diameters are measured once and that is the number recorded. If the engineering team put a circularity/roundness call out on the print, this would 'require' the diameter to be measured in multiple places.

The inspector could have measured the part in multiple places and the engineering team could have draw the print so there is only one interpretation. Neither side did this and it looks like a finger pointing contest.

Can you fix the issue, apply a 'lessons learned' and move on?
 
I

iamtroll

#5
Re: Inspection Requirements & Engineering Drawings - Measuring Holes

I agree with Michael but more on the inspector's side. He could have used more initiative, but with the correct tolerancing there would have been no question. Since +-.1mm is a fairly large tolerance for most machining processes, he probably made an incorrect assumption that a singe measurement would give him enough information. So not knowing your processes and being a Quality Mgr., I tend to like the engineers to give me as much direction as possible to help me make decisions about measuring rather than leaving it to the discretion of my inspectors.
 

Golfman25

Trusted Information Resource
#6
Ah, the old engineering vs. quality conundrum. Been thru that battle too many times.

IMO, it depends on the process used to make the part, the tolerance and the expected use. Do you want to check the parts to be good or bad because on a lot of parts you can do both.

In our business, parts are naturally out of round with very tight diameter tolerances. We generally use an average of several diameter measurements to get the recorded measurement - basically what a cmm does. If roundness is a concern, we expect a separate callout. That's what we have been doing for years.

But as people retire or leave, we are finding more an more issues which need re-clarification. In many instances, we would prefer measurement techniques used to be documented on the print for critical dimensions. It is difficult to do because engineering rarely considers how to measure something.

Good luck.
 

Kales Veggie

People: The Vital Few
#7
I think that Engineering is to blame here. It is typical to measure only one or two places for a diameter, not more.

If Engineering wants more detail, they should specify roundness, which is typically specified for a shaft.

It appears appears that Engineering is trying to blame someone to cover up their mistake.
 

Kingsld1

Involved In Discussions
#8
The problem with having engineering decide measurement methods is that in most cases they are not metrology experts (or even good amateurs). They'll specify CMM's when calipers are needed and calipers when CMM's are needed. They may not even know what kind of equipment is available int the inspection area.

If one was to get technical about it, The inspector should be doing a MMC check using a ring gage (Full form gage) and a LMC check using a two point measuring instrument such as a micrometer. Reality is that most inspectors will only use one or the other method to do the check.
 

John Broomfield

Staff member
Super Moderator
#9
Sounds like a design review problem to me. Not including the required competencies in process design review (a 7.3.1 or 7.1 nonconformity).
 
P

PaulJSmith

#10
Seems like there's enough blame to go around for everyone involved.

Rather than playing that game, though, it's probably more productive to spend your time solving the problem and ensuring it can't happen again.
 
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