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

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#11
There have been many good answers, including at least one diving into the physical make up of the blocks, including effects of corrosion.....

Other influences have been mentioned.....including coefficient of expansion/contraction

But I have a couple of questions that I didn't see, but may have been asked already.....

How - exactly - are the blocks used?

How often are they used?

How are they stored?

Hope this helps.
 
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D

dbulak

#12
I finally got around to gathering some more information. The set is from Starrett and is a grade 2 (A+) steel blocks. Any thoughts would be appreciated.


BLOCK DEVIATION
YEAR BLOCK SIZE
0.108 0.18

2009 -5.5 7.1
2008 -1.1 -1.3
2007 -4 -3
2006 -5 -3
2005 -5 -2
2004 -5 0
2003 -4 1
2002 -5 -2
 
B

bwilk

#14
Well your not going to like this answer, but here we go.

The tolerance for a Grade 2 set of gage blocks is around 4-6 uin depending on exactly what procedure is used by your cal provider. From the looks of the data your cal provider is using 5 uin as the tolerance because 5 uin was in spec in 05, 06, and 5.1 was OOT in 09. The tolerance for the 2 different blocks is the same regardless of procedure.

Here is the fun part. The BMC for gage block comparisons for most accredited labs is about 3-8 uin. It depends on their standards, and comparison meathod. This means that either of those two blocks could acutally be in or out of tolerance. Due to the Measurement Uncertainty being as large as the deviation from nominal, it is immpossible to say for sure if the gage blocks are in or out of tolerance.

Different labs handle these problems many different ways. Some will call the UUT in or out of spec without considering the MUC. Some will not call the UUT in or out, and just tell you the measurement. Some will fail all UUT's that miss the spec - the MUC. All of these, and coutnless others are acceptable ways of handling complaince statements. There are requirements of the lab in how they handle compliance statements, but these vary slightly form one accrediting body to the next. There most likely is a note somewhere on the cert, of the qoute that states the labs MUC. Some labs also have an explination of the compliance statement process on the cert.

In the end the lab that made the measurements probably made good measurements. There is a chance that either of those blocks do, or don't meet spec's of a grade 2 set. There is a chance that alot of the other blocks may or may not meet grade 2 specs either. What matters is if any one of those numbers plus the MUC of your cal provider could have a negitive effect on one of your measurements. If you only use these to calibrate hand tools 7 uin is not going to effect anything anyways. if you do gage block comparisons of your own then you may have a problem.

I have to add this part.

Your cal house most likely did a fine job here. All calibration labs will run into simular issues on grade 0,1,2 block sets. There is no way to maintain a 4 to 1 TUR on some types of equipment anymore. This is exactly why accreditied labs are important, and they report MUC to their clients. That way you do have all the numbers needed to see how the measurements effect your process.
 
D

Duke of Metrology

#15
If you used an accredited lab, there is an uncertainty that in some cases exceeds the block tolerance. New blocks will change 80% of their final size within the first 3 years of manufacturing. foreign Blocks will change more due to the manufacturing/Heat treat process. If you change vendors , usually your numbers will vary some. In all cases, If you know the error- dont worry about the tolerance. there is no need to replace anything. You would use the cert value to correct the value of the stack. Attribute gages are calibrated to determine the observed value so you, the user can use the block, add or subtrace the error and determine a corrected value for your stack. Easy stuff. Use the blocks.
 

mdemirer

Starting to get Involved
#16
Hello,
I have a question concerning to lapping process of the gage blocks. As you know lapping is a hard work and takes too many time..especially if gage block is really bad situation..Do you know an automatic lapping device for gage block lapping?

Thanks,
Demirer
 

Jim Wynne

Staff member
Admin
#17
Hello,
I have a question concerning to lapping process of the gage blocks. As you know lapping is a hard work and takes too many time..especially if gage block is really bad situation..Do you know an automatic lapping device for gage block lapping?

Thanks,
Demirer
I don't think there is such a thing as a dedicated gage block lapping machine, but if there were it would probably be difficult to justify the cost unless there were a lot of blocks that needed lapping.
 
J

Joe222

#18
Different labs handle these problems many different ways. Some will call the UUT in or out of spec without considering the MUC. Some will not call the UUT in or out, and just tell you the measurement. Some will fail all UUT's that miss the spec - the MUC. All of these, and coutnless others are acceptable ways of handling complaince statements. There are requirements of the lab in how they handle compliance statements, but these vary slightly form one accrediting body to the next. There most likely is a note somewhere on the cert, of the qoute that states the labs MUC. Some labs also have an explination of the compliance statement process on the cert.
I know some labs do not consider MU when making statements of compliance, but 5.10.4.2 does state "When statements of compliance are made, the uncertainty of measurement shall be taken into account".

Some AB's let them get away with it anyway.
 
B

bwilk

#19
I know some labs do not consider MU when making statements of compliance, but 5.10.4.2 does state "When statements of compliance are made, the uncertainty of measurement shall be taken into account".

Some AB's let them get away with it anyway.
Thats not really how it works. 5.10.4.2 does state that it needs to be taken into account, but it does not state how.

Consider the fact that most US AB's are part of the ILAC MRA, and that ILAC's own G-8 document states


An agreement between the client and the laboratory or a code of practice or specification may
state that uncertainty can be ignored when judging compliance. Similar considerations as for

shared risk
(above) apply in such circumstances.

This would mean that "ALL" ILAC MRA members do accept this type of compliance statement unless it is explicitly stated in a policy by that paticular AB that they will not allow this, or it is not acceptable by a paticular labs clients.

The clients are the true enforcers of all the important aspects of 17025, and other quality issues. Not the assesment teams. The AB's are only there to evaluate minimum compliance to the document, and technical competence. It is the clients job to request specifc methods of test, or quality processes.






 

Wayne

Gage Crib Worldwide
#20
What phenomenon is taking place to make them go out of tolerance?
Gage blocks can grow up to 1uin/in/yr depending on how they were manufactured, stabilized, etc...
I guess my question is how can blocks grow and how can they shrink? What is this called?
[
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.
New blocks will change 80% of their final size within the first 3 years of manufacturing. Foreign Blocks will change more due to the manufacturing/Heat treat process.
It seems that bwilk's 11th Feb 2009 5:05PM post has the most likely correct analysis of the original situtation, but all the above quotes center on growth of hardened steel. This is a concept that just baffels some of my customers. I even lost a customer once after explaining the concept to her. She inquired of her engineers and they told that the thought of hardened steel growing was preposterous. Well I guess that just goes to prove that engineers don't always know everything.:D

As I explained to my customer, the growth is usually noticed in larger sizes because it is an inch per inch type growth. What happens is when the steel is hardened the modules are trapped in stasis. Slowly the molecules relax and as they relax they cause an expansion of the material. The gradual relaxation process takes about a year. The larger size the more the change is noticed. The tighter the tolerance the more the movement is noticed.

To combat this phenomenon gage makers require that their hardened steel be triple cold stabilized. The process requires the gage to be cycled three times through a freeze/thaw process. This procedure will reduce the probability of gage growth. Note the use of the word ‘reduce' instead of ‘eliminate'.

The probability of growth is most likely noticeable:

1. If the gage is over an inch in size. Because the growth is compounded as size gets larger it will be more apparent. On smaller tools the growth will most likely be considered within the range of measurement uncertainty or be explained away as being from different technician, equipment or laboratory variation.

2. In the first year after it has been hardened. Most of the movement is aged out of the steel after the first year. Things like temperature variation and gage use will encourage faster aging. Being unused and in a box may encourage slower aging to a small degree. Any growth detected after the first year will most likely be considered within the range of measurement uncertainty or be explained away as being from different technician, equipment or laboratory variation.
 
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