Correct Calibration Method for Dial Depth Gage

Calgirl319

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What is the correct calibration method to calibrate this type of depth gage (photo attached)? I was trained to zero out on surface plate than measure different sizes of gage blocks taking a low, middle and high measurements, but my gage reads in .001 and my calibration tolerance is +/-.0005. I can’t meet that with this method. Also, what if the two surfaces are not flush (see 2nd picture) Doesn’t this effect the measurement? As I learn more, I don’t think I’m getting trained right lol!
 

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Ninja

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Howdy,

A few things:
Resolution of 0.001 (arguably 0.0005 for "in between hashes") and a cal tolerance of 0.0005 is not a great approach.
Best tolerance I would trust to is +/- 2x resolution...
Gages have error...calibrating to resolution limit is saying "This thing has to be 100% perfect all the time"...which never happens.

What you show (with the depth gage filed down like that) is a point measurement.
That's not a bad thing, height gages typically are a point measurement...it's just a matter of which point.
No big deal if the surfaces aren't flush, as long as the point measurement you get gives you reliable and valuable data...
 

Calgirl319

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Ninja,
I just realized the shop guys modified and grinded down the end to make it more pointy, which is why it's failing calibration. I am going to pass it because what they use it for the tolerance is large and not to critical also if I scarp it and give them a new gage they will just modify the new one. Thank you!
 

Ninja

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Best tolerance I would trust to is +/- 2x resolution... so your saying a +/-.002 tolerance?

Yes, that's what I mean...though it is a preference.
If "Pass calibration" is +/- 0.002... is the gage still good for the job it is used for?
If so, why set the Pass/Fail criteria tighter? What would be the point?
 

Calgirl319

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Now the big question...are you sure that's all they use it for?
:) Our gages are assigned to users, so I will talk to him and let him know not to let anyone borrow it and to only use it for that particular operation.
 
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