Out of tolerance gauge blocks - How did these blocks go out of tolerance?

[dbulak]

I would like a second opinion from others on this matter. I just got back a set of gauge blocks fom being calibrated. Two of the blocks were found to be out of tolerance. One block was over the maximun tolerance limit and the other was under the tolerance limit. These blocks are never used during the year. The just lay in the box. My question is how did these blocks go out of tolerance? What phenomenon is taking place to make them go out of tolerance? Or did the calibration company not get good readings? Thanks in advance!

 

[Jerry Eldred]

Re: Out of tolerance gauge blocks

I don't know that this is "THE" answer. But what I would do is plot the history on the impacted blocks to convince myself whether the readings made statistical sense. If. for example, I had five years at one year intervals, and I saw a general trend, then a "JUMP" in the readings, that would personally be a red flag to me.

If you just switched vendors (i.e.: the blocks have previously been cal'd elsewhere), that may be a potential red flag.

If you have any instruments accurate enough and adequate resolution to reasonably resolve the shift, try some experimental readings to see if they correlate.

These are just some checkpoints I would do.

If you discern that your experimental readings don't correlate to the illedged shift, that is another red flag.

If these are critical or important, I might suggest sending them blindly to a different TRUSTED vendor, and have them re-calibrated. Compare the readings (to within uncertainties).

I'm not at all an expert with gage blocks. But there are a number of reasons their readings could be skewed.

There numerous "if this then that" options too lengthy for this post. Hopefully the above will give you some options and ideas.

 

[jfgunn]

Did you receive an accredited calibration for the blocks?

Did the vendor provide the measurment uncertainty for the calibration?

Did the vendor take this measurement uncertainty into accoun t when making the decision to say that the block was out of tolerance? For example, I had a gage block recently where the tolerance was 4 uin. Starrett got a reading of +4.1uin with an uncertainty of +/-2uin. They told me the block failed. Depending on how I use the block and its associated uncertainty, the block may or may not have "failed".

Also, what were the sizes of the blocks? Were they relatively long or short. Gage blocks can grow up to 1uin/in/yr depending on how they were manufactured, stabilized, etc...

If you have a little more info about the measurement data from this calibration and previous calibrations, I could try to provide a better answer.

I hope this helps a little.

 

[dbulak]

I guess my question is how can blocks grow and how can they shrink? What is this called?

 

[bobdoering]

I guess my question is how can blocks grow and how can they shrink? What is this called?

Physical growth (not measurement error): coefficient of thermal expansion?

 

[wmarhel]

Physical growth (not measurement error): coefficient of thermal expansion?

Depending on the environment, you can see changes in relatively short periods of time. The more precision the item, the more likely it is to experience some form of fluctuation caused by environmental factors. Hence the reason for a controlled environment and allowing items to stabilize in temperature before calibrating or using them for calibration.

Wayne

 

[rmf180]

We have established our own internal tolerance limits for assets. First, consider the use of your blocks. What do you do with them? If you use them to calibrate hand tools with .001" resolution and accuracy, then do you really need to worry about a tolerance of +.000004/-.000001?

As stated in one of the early replies, check out the uncertainty statement as well. Very often you will see that the uncertainty is as large as the reported error. Our calibration providers have removed the In/Out of tolerance statement from their cert and left the decision to the customer.

 

[Kevin H]

Were they hardened steel blocks? Starret, I believe used to make their hardened steel blocks from 52100 grade steel. Hardly a corrosion resistant steel, so you may have picked up minor corrosion products.
Also, they're quenched and tempered, so they're microstructure is tempered martensite. There is enough alloy content that you could have some retained austenite that over time would transform to martensite creating stresses and changing dimensions - the crystal structure for austenite is face centerd cubic, while martensite is body centered tetragonal. Martenisite is considered to be a metastable phase, not a totally stable one.

 

[BradM]

I guess my question is how can blocks grow and how can they shrink? What is this called?

Gremlinology. They sneak in and cut off some on some gauges, and glue some on other gauges!!

OK... enough of the horseplay. I guess there is some element of truth to the calibration gremlin. Kevin and some others hit upon my thoughts, which were the type of material the gauge blocks are made of. Too, are we talking about 1/4 inch gauge blocks, or 10 inch gauge blocks? I always had a terrible time with the long gauge blocks.

Also, did the same organization perform the verification both times? The others have covered it so well about competence and uncertainties. If there are differences among the two sources, you might be seeing a difference between them.

Let us know how this progresses; I'd be interested to know what you find.

 

[bwilk]

To get a decent answer to your question we are going to need a little more information on the gauge blocks, and their calibration.

What temperature are they stored at?
What grade are they?
What spec did they fail?
How much did they fail by?
Whats the Measurement uncertainty stated by the calibration provider?
Do you use gloves when you handle them?
What presevative do you use when you store then fro the whole year?

There are tons of factors that can explain changes in the value of a gauge block. Just need some more info to start narrowing it down.