Uncertainty of a Master part

[WolfMan]

Hey guys, if you have a master part that is used to set the gage to zero or used for comparison relative to parts made, what kind of tolerance would the Master have? Or how would it be calculated? I am assuming Master part would have an combined uncertainty of the Gage and the process used during calibration.

[PH8IL]

Our masters have the tolerances of the blueprint. Though our masters are just used for comparison not setting gages to zero.

[JEFMAN]

About uncertainty, I would :
Measure a Koba step in many locations (parallel to axes, along diagonals planes and 3d diagonals), repeat the measurements (all steps) and calculate the standard deviation of all deviations (S1).
Measure a calibration sphere with many points (150), and calculate the standard deviation of radius deviation of each hit (S2).
Measure the part in different locations on the cmm, and calculate the standard deviation of each dimension (S3i)
U=±2*SQR(S1^2+S2^2+S3^2) should be the best uncertainty accessible on the part, around the average of each dimension.

In the first and second steps, the average of deviations should be very close to zero.
Trying to calculate the uncertainty of the distance between a plane and a point should take into account the flatness of the plane...
It becomes quickly very hard if there are a lot of dimensions...

[Schrocknroll]

Yeah, what he said.

[PH8IL]

Makes my post seem like a childs in comparision lol.

[Ego Murphy]

Measurement Devices have "Uncertainty", expressed as measurement units of expected error in the measurement results returned by the device.

Physical Objects, on the scales we operate on (i.e. human-scale stuff and as opposed to the quantum realm of particle-physics or the universe realm of astrophysics), are fixed and immobile - they have no uncertainty.

What you are looking for is the Uncertainty, not of the part, but of your CMM. The CMM should return the exact same results, within it's stated uncertainty as found in it's latest calibration paperwork, on an undamaged and same-temperature "known quantity" master part.

/thread.

[WolfMan]

No. I am not looking for uncertainty of a CMM. The feature in question is not even measured on a CMM. My question is about a Master part. I made an assumption that a Master part is just like any other Gages like a ring ID gage or a Gage Block. In both cases used to set gages or as a comparison device. These gages typically come with uncertainties, fairly small yet they are there on the certificate. Master part is no different than any gage, there for should have uncertainty .

[WolfMan]

Crap, you’re right. It took me a while to digest it though.

[JEFMAN]

Physical Objects (...) have no uncertainty.

I must agree with that ...
Uncertainty comes only from the fact of observing a quantity.
Most of units come from definition of the time.
A second has no uncertainty, but all national laboratories (NIST, LNE, METAS, PTB, NPL...) gives a different value of it (very very little difference - relative uncertainty around 10^-16 !!!) coming from there own uncertainty.


The main problem with Ego Murphy sentence is this : the work of the machinist is better than this of the metrologist !!!!!

[Ego Murphy]

No. I am not looking for uncertainty of a CMM. The feature in question is not even measured on a CMM. My question is about a Master part. I made an assumption that a Master part is just like any other Gages like a ring ID gage or a Gage Block. In both cases used to set gages or as a comparison device. These gages typically come with uncertainties, fairly small yet they are there on the certificate. Master part is no different than any gage, there for should have uncertainty .

Read the certification of the gages carefully and you will realize that they merely express the uncertainty of the measurement device used to certify them.

I must agree with that ...
Uncertainty comes only from the fact of observing a quantity.
Most of units come from definition of the time.
A second has no uncertainty, but all national laboratories (NIST, LNE, METAS, PTB, NPL...) gives a different value of it (very very little difference - relative uncertainty around 10^-16 !!!) coming from there own uncertainty.


The main problem with Ego Murphy sentence is this : the work of the machinist is better than this of the metrologist !!!!!

And a sticky problem it is, for we depend upon the machinists to make the certification tools.