Harry you said something that interest me. We do receive documents stating the thickness and we are only varifying upon receiving that our material is what they have already stated it was. We are not using our measurements as statements of thickness but only for ourselfs.
OK, I'll try an answer for your question.
Then I will pose a question to you.
If the reading does not matter, then don't calibrate. So if what value your instruments give you upon acceptance doesn't matter, then no; no calibration required. However, if you have a purchase order or a material requirements specification, how can you accurately verify that the material fulfills those specifications?
Hence Harry's valid question (along with the others). How you are using the values from the instruments will help guide your decision process.
* I'm a calibration guy. I believe in it.
However, a calibration program needs to be managed, and things need to make sense. Instruments that aren't used/don't matter need to be done away with. That which is left, probably matters and needs to be verified. Now, what level of verification and how often, falls under the "manage" issue.
Now comes my question...and you don't have to answer; just wondering. Why don't you want to calibrate the instruments? Is it a financial thing? I ask because maybe if you let us know your reason, we might be able to provide some suggestions.