Outside micrometer anvils parallelism

amstel1

Starting to get Involved
#1
Hey guys, I would like to hear from you what you think about this, please check the attached photos.

Micrometer on the photos is Mitutoyo with interchangeable anvils. Does this actually pass parallelism test?

Only way I can zero outside mics by myself is to place them on the stand, make sure they are vertical and place rod standard in between. Usually good anvils stand straight and flat against rod standard and they are repetitively on zero. But many are not, like the ones on the attached photos, you can move it around and get different measurements(which can be beyond our lab tolerance).
I sent few of those to two local vendors and they both are telling me that I cannot measure parallelism(which I am not, I am just setting zero) like that and they both claim that they pass parallelism test on their certs.

In my opinion, this is beyond calibration procedures and more of a common sense, as anvils are clearly bent and they shouldn't be used. My practice is to dispose them and get a new ones, but I changed so many of them that I need to question if I am doing the right thing.
Technicians on the floor who are using them are not taking good care of the tools and it is very likely when they try to measure components they are not really careful and gentle as they slam interchangeable anvil against component. That is at least my theory how they got bent.
There is over 500 techs across three shifts and talking to them individually is not an option.
 

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John Predmore

Trusted Information Resource
#2
@amstel1 asked one question "Does this actually pass parallelism test?", although you clearly think it does not.

The question you didn't ask is what part(s) might be damaged to cause this problem you describe.

You stated the "anvils are clearly bent", but in the preceding paragraph you sent a few (I think you refer to anvils) to local vendors and they both told you the parts pass parallelism test on their certs. A contradiction appears!

How could the anvils appear bent in the assembly but individual parts pass inspection for parallelism? Maybe parallelism is not the right characteristic to focus on. The local vendors may have been trying to tell you that, based on what I read of your report of their feedback. Without seeing the part or the drawing of the part, I can imagine that parallelism of tiny planar features would be difficult to measure.

Out-of-roundness would be a better measure of the condition of a bent shaft. I am guessing from the pictures of your lab you have the knowhow to measure runout when rotated in a chucked spindle. Then you could quantify all your bent anvil shafts and decide how far out is unacceptable.

Another possibility to consider is maybe the damage causing the problem is not to the anvil but to the socket where the anvil is inserted. Based on the size of span, I am guessing your sample parts are heavy, and you imply some of your 500 techs are "not really careful and gentle" in their use of the micrometer. You might also have a scenario where there is no damage to the socket, but a metal chip or other debris inside the socket can cause variability in how the anvil sits in the socket, and that could manifest as a shaft off kilter.

Perhaps the frame itself could be twisted or deflected a tiny amount (again, assuming heavy parts and careless operators), and a tiny angle would cause the axes of the spindle and the anvil to be slightly out of alignment. To check alignment, I would try to fashion a way to put a laser pointer pen in the socket where the anvil goes. Or if the laser pointer idea doesn't work for you, a solution used for thousands of years would be a plumb bob hanging from the upper arm to the lower socket (invert the micrometer if that geometry works better for a plumb bob). Then you would have visual verification of the alignment of the two axes.

Obviously, I don't know the cause of the problem, but I hope logical, impartial thinking about your situation might offer a few fresh ideas.

The second question you didn't ask explicitly is how to prevent damage when you have 500 techs who are "not really careful and gentle". Voltaire said "No problem can withstand the assault of sustained thinking." I believe once we foresee a problem, clever engineers can devise a way to error-proof or minimize against it. I'll leave that second question to be continued...
 

amstel1

Starting to get Involved
#3
Thanks John.

Whole set(s) was sent out to 3rd party for certification, not only anvils. How they inspect it there I am trying to find out, but one of the answers was "Typically parallelism is not performed on large micrometers as that requires an optical parallel long enough for the minimum measurement of the mic" . Which I understand, but then I wondered how else do they check the parallelism on large outside micrometers with interchangeable anvils if they state on their certificates that it passed parallelism. Using certified rod standard or gauge block and just trying to zero it would clearly show that something is off. If you actually continue and perform the linearity test with these anvils, measurements would be "all over the place".

For instance, in my lab, on those "bent" anvils zero measurements with certified rod standard can be off for 0.001"-0.003" (in-house specs are 0.0002"-0.0005" or more, depending on the size). It is not barrel misalignment or linearity, because other good anvils are parallel, sitting flat and on zero with good contact on each side of the rod.

Also, to make sure that it is not the outside micrometer frame bent, debris or something else in thread hole I do the same zero test with brand new anvils and they would have full, flat contact with rods and they would be right on zero, also linear across the scale.

My way to error-proof or minimize the operator's errors are tight tolerances and frequent checks. I can provide them training how to use tools, they will listen and see, but they just won't care as there is really no accountability in regard to tool damage. I do also track all the expensive repairs and I bring them where they need to be brought.
Here and there I would be asked from senior managers or directors why am I buying so many parts, but when I explain why I get no response.
 

amstel1

Starting to get Involved
#4
Anyway, my lab resources are limited and I can only check zero, linearity and do some cleaning on outside micrometers. Everything more than that is going out for certified third party calibration/repair. These bent anvils brought my attention and I decided to ask here to hear more opinions.
 
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