what lenght of calibration cycle will an iso auditor except, or is it less hassle and cost effective to just buy new ones each year
Welcome to the Cove.
What is an iron and steel plate gage?
I understand Jim's query to mean WHICH type of thickness gage is used. I'm aware of several types of fixed dimension go/no go gages plus a few spring loaded ones for measuring go/no go attribute plus, of course, instruments ranging from hand held verniers and micrometers up to air and laser gages which measure thickness characteristics at discretion and accuracy of up to 50 millionths of an inch.
Offhand, I'd say that the first task should be regular and frequent "verification" by checking readings against Standard bars to assure an attribute gage still reads in the allowed tolerance range. Actually "calibrating" the gages to the point of adjusting the gage to meet the tolerance range is strictly an economic decision.
An instrument which measures characteristics (i.e. not just go/nogo, but the actual measurement) needs BOTH frequent verification and calibration when required (certified traceable to NIST).
In my own machining company, we verified gages and instruments against Standards every shift they were used. Suspect gages and instruments were sent to independent specialists for confirmation. Decisions whether to replace or calibrate/repair were on a case by case basis.
Believe me, setting arbitrary time spans for verification/calibration/replacement regardless of the frequency in which the gage or instrument is used can be wasteful and costly. A micrometer which sits in a box on a shelf in the tool cage for a year without use probably does not need to be sent out for affirmation of its status, whereas the one in an operator's pocket used on a daily basis may need to be replaced as frequently as monthly or even more often, especially if the spindle and anvil are in continual use with abrasive surfaces (like hot rolled steel sheets or plates.)