Vernier Caliper Accuracy Issue - Inside measurements were slightly off

[tekno9000]

While calibrating one of our digital vernier caliper, I found that the inside measurements were slightly off.

We define accuracy as per manufacturer – 0.001”

However, least tolerance specified on our drawings - 0.005”

What of the following option is the best one?

1) Increase our accuracy level requirement to .0002”
2) Use the vernier for outside measurements only.

Does any one have any other suggestions besides these and help me choose the best alternative.

Thanks.

ISO 9001 certification.

[20110131]

If cost is a priority then replace it.

1. Buy a new one. $30-150 is not a major sum for piece of mind.
2. Repair it, (but this costs more than many calipers cost).
3. Limit usage, by marking, and not using the feature.
4. Derate the specification to allow for derate item performance.This method will impact you TAR/TUR so maybe it is more headache than good.
5. Calibrate item, and use with correction information.

I overwhelmingly suggest replacing the item, and then retiring or moving it to a operational process.

[apestate]

No caliper currently being produced is advertised with an accuracy of better than +/- .001"

A four-measurement (OD, ID, depth, step) typical standard caliper jumps up to +/- .002" at 12" overall range. The same accuracy applies to each measurement capability of the instrument.

MSA covers what to do when you have an accuracy or capability that causes an overlap with a tolerance limit. Basically if your measurement is at the limit, you can't make the correct decision of whether to accept or reject the part. At this point you can switch to a higher accuracy instrument to make the correct discrimination.

When I calibrate calipers, I typically find that they are at least .001" inaccurate at some point. In case I'm using a caliper to measure a tolerance with a total of .005", I'm not going to rely on measurements within .001" of either limit.

[Kales Veggie]

In Reply to Parent Post by tekno9000

While calibrating one of our digital vernier caliper, I found that the inside measurements were slightly off.

We define accuracy as per manufacturer – 0.001”

However, least tolerance specified on our drawings - 0.005”

What of the following option is the best one?

1) Increase our accuracy level requirement to .0002”
2) Use the vernier for outside measurements only.

Does any one have any other suggestions besides these and help me choose the best alternative.

Thanks.

ISO 9001 certification.

I would take one step back and review the calibration method used for internal measurements.

[Ron Rompen]

With a tolerance of ± 0.005", your measuring instrument should have an accuracy of 1/10th (i.e. ± 0.0005") which is well beyond the range of a caliper, whether vernier or digital?

Calipers (in a skilled users hands) can be very accurate for small dimensions, but far too often they are misused, abused, and 'persuaded' to give the 'right' measurement by inexperienced users.

[tekno9000]

This is a digital vernier caliper.

We define accuracy as per manufacturer – 0.001” - is derived from Mitutoyo calibration certificate.


Thanks to everyone.

[Ron Rompen]

The inherent accuracy of the caliper is one thing, however once you take into account the variation of the operator(s) then the result will be somewhat different.

If you haven't already done so, I would strongly urge that you perform a proper GR&R study on the instrument, to determine whether or not it IS sufficiently accurate (when all factors are considered) to be used for this measurement.

To answer your original question, you several options (increase calibration tolerance, designate for exterior measurement, etc) however the FIRST thing you should be looking at is the potential impact that this error has on manufactured parts and your customer. If the parts are potentially out of spec, then you need to determine what action(s) are required, including possible customer notification. Be sure to document this, as it is an auditable requirement.

[tekno9000]

In Reply to Parent Post by george1weiss

If cost is a priority then replace it.

1. Buy a new one. $30-150 is not a major sum for piece of mind.
2. Repair it, (but this costs more than many calipers cost).
3. Limit usage, by marking, and not using the feature.
4. Derate the specification to allow for derate item performance.This method will impact you TAR/TUR so maybe it is more headache than good.
5. Calibrate item, and use with correction information.

I overwhelmingly suggest replacing the item, and then retiring or moving it to a operational process.

George,

Can you please throw some more light on sr. no. 4.

Thanks.