Dispute - Checking a thin walled, round, hollow part with micrometer

Golfman25

Trusted Information Resource
#1
Here is the senario. Part is a thin walled, round, hollow part -- say .015 thick. Part is made to a specified dia. indicated on print. Parts submitted (by party A) for approval and are rejected (by party B) due to undersized dia. Part is being measured by both parties in a fixture holding your standard micrometer. Party A is a 30+ year experienced toolmaker with a real fine measuring touch. Party B doesn't appear to have a lot of experence -- he "rachets down" on the mic. causing the part to flex resulting in the undersized dia. ( a difference of .001-.002).

Party A says parts are good and that some "touch" is required so as not to flex the part and doesn't feel it is appropriate to influence the part during the check.

Party B says parts are no good and doesn't want any touch -- the "rachet down" will take the touch away even though it flexes the part during the check.

Parties are at a stand off. Any suggestions? Thanks in advance.
 
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U

Umang Vidyarthi

#2
Re: Dispute -- checking part with micrometer

Here is the senario. Part is a thin walled, round, hollow part -- say .015 thick. Part is made to a specified dia. indicated on print. Parts submitted (by party A) for approval and are rejected (by party B) due to undersized dia. Part is being measured by both parties in a fixture holding your standard micrometer. Party A is a 30+ year experienced toolmaker with a real fine measuring touch. Party B doesn't appear to have a lot of experence -- he "rachets down" on the mic. causing the part to flex resulting in the undersized dia. ( a difference of .001-.002).

Party A says parts are good and that some "touch" is required so as not to flex the part and doesn't feel it is appropriate to influence the part during the check.

Party B says parts are no good and doesn't want any touch -- the "rachet down" will take the touch away even though it flexes the part during the check.

Parties are at a stand off. Any suggestions? Thanks in advance.
You may ask for third party inspection to solve this problem.

2. You may check the dimensions with a 'Contactless micrometer' which eliminates the human error.

3. A cheaper solution is to use 'GO' - 'NO GO' gauge.

Hope this helps

Umang :D
 

Jim Wynne

Staff member
Admin
#3
Here is the senario. Part is a thin walled, round, hollow part -- say .015 thick. Part is made to a specified dia. indicated on print. Parts submitted (by party A) for approval and are rejected (by party B) due to undersized dia. Part is being measured by both parties in a fixture holding your standard micrometer. Party A is a 30+ year experienced toolmaker with a real fine measuring touch. Party B doesn't appear to have a lot of experence -- he "rachets down" on the mic. causing the part to flex resulting in the undersized dia. ( a difference of .001-.002).

Party A says parts are good and that some "touch" is required so as not to flex the part and doesn't feel it is appropriate to influence the part during the check.

Party B says parts are no good and doesn't want any touch -- the "rachet down" will take the touch away even though it flexes the part during the check.

Parties are at a stand off. Any suggestions? Thanks in advance.
In addition to Umang's suggestions, you might want to consider supporting the ID with a gage pin. This can create expansion issues if you're not careful, though.
 

Ron Rompen

Trusted Information Resource
#4
A little more detail is (IMHO) required to properly answer this question.

1) What type of micrometer (point, single ball, double ball, etc) is being used to verify the part?
2) Clarify 'ratchet down'. If the micrometer being used has a friction thimble, and is appropriate for the measurement (i.e. either a double ball or point micrometer), then bringing the anvils together until the friction point is exceeded is the correct practice.
3) As much as I respect the experience of the toolmaker, are his results repeatable by other parties (not just party B)? A GR&R study may point you in the right direction in this case.
 

ScottK

Not out of the crisis
Staff member
Super Moderator
#7
Do you have other tools available?

In my operation I would take both guys to an optical comparator and measure it. It likeley will prove the machinist is right.

Without an optical comparator avaiable I would go with the gauge pin in the the through hole as has been pointed out, but not an oversized one as that will stretch the part.

If you are measuring strictly for pass/fail criteria - make a go/no-go gauge and take out the measurement altogether.

It also sounds like the inexperienced inspector needs some further training from his boss.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#8
Any suggestions? Thanks in advance.
Yes...use a gage with limited or no gage pressure - an indicator or indicating snap gage or non-contact methodology. Eliminate the gage error. You have a rudimentary gage R&R failure here - you don't even need to do the test! It is staring you in the face! This is a good reason why micrometers are such terrible gages. Their gage pressure is uncontrollable - and is even worse in situations where people are in a hurry, because even if it has a slip or ratcheting thimble, impact from whipping the anvils onto the part cranks the pressure way up.

The question of function if fun...using mating parts as a gage? Some folks like that...:tg:
 
D

DrM2u

#9
Did anyone suggest a non-contact gage, like an air ID gage? A contact or non-contact CMM would also work well in lieu of an optical comparator. Also, there is the option of bore gages that use (3) rollers at 120° to stabilize the part and measure. There are some limitations on how small or large of a diameter these gages can measure. Another consideration is whether variable or attribute data is needed in this case. Like other posters have suggested, a GO/NOGO ID gage (plug gage) is a potential solution as well.
 

Golfman25

Trusted Information Resource
#10
A little more detail is (IMHO) required to properly answer this question.

1) What type of micrometer (point, single ball, double ball, etc) is being used to verify the part?
2) Clarify 'ratchet down'. If the micrometer being used has a friction thimble, and is appropriate for the measurement (i.e. either a double ball or point micrometer), then bringing the anvils together until the friction point is exceeded is the correct practice.
3) As much as I respect the experience of the toolmaker, are his results repeatable by other parties (not just party B)? A GR&R study may point you in the right direction in this case.
1) using a blade mic.
2) "Rachet down" -- meaning after the part is contacted by the mic., they turn the rachet until it stops. Done on a gage block, there is no issue. Done on this part, the part flexes and give a different reading.
3) People who know how to use a mic can get repeatability.
 
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