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Hi.
I read about this in the NPL Good Practice guide No. 80 - Fundamental good practice in dimensional metrology.
And I found it interesting, so I decided to give it a try. I just wanted to see how you guys would go about doing it?
Which measurements would you take, and how would you evaluate them? What evaluations would be most relevant?
I read about this in the NPL Good Practice guide No. 80 - Fundamental good practice in dimensional metrology.
Interim checks
The regular performance verification check of a CMM should be a key component of
the company quality system and records should be kept of the results obtained.
Unfortunately these tests may only be performed once or twice a year – so what do
you do to ensure you have confidence in the machine on a daily basis? The answer is
to perform interim checks. These checks are also described in the ISO 10360-2, but
essentially involve regularly measuring an object that is similar to the regular
components inspected by the machine – this may be a special test piece, a calibrated
master part, ball plate, hole plate, machine checking gauge, or ball bar. By measuring
a series of features on this artefact each day, or perhaps first thing on a Monday
morning and recording the sizes on a control chart, one builds up a visible history of
the machine’s capability.
Trends may be observed on the control chart indicating that the results are within
some specified limit and likely to stay within the bounds for some time, but the real
benefits are observed when sudden changes are highlighted. Sudden changes are often
indicators of nasty things happening to the machine – damage to an air bearing, a
collision with a forklift or crane, or simply dropping a heavy object - sometimes
people are embarrassed to admit they have damaged an expensive item and may not
say anything about it. It is up to the operators to ensure that they are confident in the
results from the CMM and getting the result they expect from a daily interim check.
If the CMM fails the interim check then the operator is encouraged to undertake a
performance verification test to see if the system really is out of specification.
The regular performance verification check of a CMM should be a key component of
the company quality system and records should be kept of the results obtained.
Unfortunately these tests may only be performed once or twice a year – so what do
you do to ensure you have confidence in the machine on a daily basis? The answer is
to perform interim checks. These checks are also described in the ISO 10360-2, but
essentially involve regularly measuring an object that is similar to the regular
components inspected by the machine – this may be a special test piece, a calibrated
master part, ball plate, hole plate, machine checking gauge, or ball bar. By measuring
a series of features on this artefact each day, or perhaps first thing on a Monday
morning and recording the sizes on a control chart, one builds up a visible history of
the machine’s capability.
Trends may be observed on the control chart indicating that the results are within
some specified limit and likely to stay within the bounds for some time, but the real
benefits are observed when sudden changes are highlighted. Sudden changes are often
indicators of nasty things happening to the machine – damage to an air bearing, a
collision with a forklift or crane, or simply dropping a heavy object - sometimes
people are embarrassed to admit they have damaged an expensive item and may not
say anything about it. It is up to the operators to ensure that they are confident in the
results from the CMM and getting the result they expect from a daily interim check.
If the CMM fails the interim check then the operator is encouraged to undertake a
performance verification test to see if the system really is out of specification.
Which measurements would you take, and how would you evaluate them? What evaluations would be most relevant?
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