Downgrading Grade 0.5 Metric Gage Block Set

Started by Hawaii596, 10-01-2009 -- 11:00:02

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Hawaii596

10-01-2009 -- 11:00:02
I have a set of Grade 0.5 metric gage blocks that the technician requested we lengthen the cal interval on.  So I looked up the history and out of the last 8 cals, it has been out of tolerance twice.  so my gut reaction is no, we can't lengthen the interval.

After further extensive discussion about it, I learned a few things (not being a dimensional type).  The T.U.R. when they were cal'd was 1:1, so it may have been an uncertainty issue, as no blocks were discarded/changed out; and in both OOT cases, the OOT blocks were "magically" back in tolerance the next cal.

I also learned that we only use these as a back up with one of our inch blocks doesn't have a close enough size for a need, and so use the nearest metric equivalent block.  AND, our other sets are grade 3, and we have no requirement for a grade 0.5 set.

The thought it, then, to have these blocks cal'd to a lower grade than 0.5, as we don't in any way need that accuracy anyway.  We would label them and properly document that they are cal'd to a lower grade (maybe grade 2, making them still the best blocks we have).  Then later in, if we want - and the blocks would still meet it, we could always have them cal'd back to an 0.5 in the future.

Are there any industry rules that I'm not thinking of that would disallow downgrading blocks like this?  It is not part of our scope of accreditation, so there shouldn't be any implication there.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

step30044

#1
10-01-2009 -- 14:31:49
I would first find a new vendor to calibrate my gage blocks. If the vendor supplied data for the blocks that were found to be out of tolerance that is standard practice in my experience. The expectation being that the owner will apply corrections based upon the data. If the blocks were never used for an accuracy better than the Grade 2 I would say you would be fine

MIRCS

#2
10-01-2009 -- 14:47:35
What's really the problem.

You have the report. How far out were the ones OOT and then back in tolerance?? That could just be a temperature fluctuation during the cal from one to the next.

What was out of tolerance........a few blocks, the entire set??  What are they made of??  We have a Grade AA croblox set here that only has 4 blocks out of tolerance since 1967.

As long as you have the values from the RoC then nothing should be wrong. Parallelism issues could be a problem, length not so much.

Hawaii596

#3
10-01-2009 -- 15:48:14
The primary issue is that we don't use Grade 0.5 and don't want to spend the $$ to have that level of cal done.  We only do low end dimensional work in a non-controlled environment; and this metric set is a back up that is rarely used.

I know having blocks drift in and out of tolerance, then back in is a red flag (the block didn't get smaller then all of a sudden get bigger - or vice versa).  But because we don't need Grade 0.5 accuracy, we don't want to have to deal with the associated responsibilities that go along with having more standard than what is needed.

I haven't pulled the history yet - going to do that next.  I want to see what has statistically/numerically occurred to see if it seems to indicate poor cal quality as the culprit.

But...  what I really want to do is downgrade the blocks.  If we could dispose this set and replace it with a Grade 2 or 3 set, we would do that.  What we want to do is use this set as (probably) grade 2.  If I'm convinced that the OOT's on that set as held to Grade 0.5 are minimal compared to Grade 2 specs, and that it appears to be an uncertainty-related quasi-drift (i.e.: the blocks didn't actually drift, just a poor cal), then we can send them out for a Grade 2 certification and if they come back in tolerance at Grade 2, there is no issue.

I believe (not sure) that they went to Starrett (but I'll have to check the history to confirm).

Anyway, back to topic...  I want to be sure there is not some technical reason why we shouldn't have them certified to Grade 2, and appropriately label and document to assure they are only used as Grade 2.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883

step30044

#4
10-01-2009 -- 17:24:26
I don't understand:? There should be no problem in down grading the accuracy if they have never been used for calibrations that don't require the Grade 0.5 accuracy.

I would think that temperature fluctuation would not be the issue as the uut and standard should be allowed to obtain temperature equilibrium. This maybe a factor due to operator error. MIRCS is dead on if you have the values from the RoC then you should be fine.

MIRCS

#5
10-02-2009 -- 08:25:26
My comment about temperature is that for Grade 2 blocks, 0.5 degrees F is almost the entire positive tolerance and almost twice the minus tolerance.  So in reality if the have equalized and there is a 0.1 degree shift then it could cause an OOT condition.

Yeah downgrade em......no problems

Hawaii596

#7
04-02-2010 -- 09:35:22
We finally decided to put them in inactive status.  We have multiple sets of inch blocks and pretty much never use the metric blocks.  We also couldn't find a lab willing to do a downgraded cal on them (we didn't look very hard, as we really aren't using them).  so, off to the environmentally controlled storage room with a big red sticker and fresh coat of preservative they go (along with database entry, of course).  Thanks for the inputs, all.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind."
Lord Kelvin (1824-1907)
from lecture to the Institute of Civil Engineers, 3 May 1883
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