Gage Block Calibration
Consider these steps before going through the process.
Gage blocks are the most used physical representation of length available to companies requiring the highest order of precision for their work. Their versatility makes them indispensable when ‘master’ dimensions are required for checking work or for setting or calibrating instruments. To the uninitiated, they look like rectangular or square blocks of metal with two polished faces, but their simple appearance does not reveal the precision to which the length between those two polished faces has been refined.
The unique properties of gage blocks means little unless they are well maintained and calibrated at intervals appropriate to their use. Calibration of them is an exacting process with the highest level of work being performed by NIST and a few industrial facilities. But the cost of achieving this level is beyond the reach of most budgets so companies invariably turn to commercial laboratories for the service where the uncertainties involved are higher but the cost is considerably lower.
My laboratory calibrates a significant number of gage block sets each year and, as a result, we see a lot of mistakes made in preparing them and in specifying what is expected from the process. This column and the next will deal with things you should know or do in order to get what you need from such facilities.
Your Calibration Source
Check to ensure the lab you’re considering is accredited to a recognized standard such as ISO 17025 and ask for a copy of their scope. Read it carefully and compare what they are listing to what you are intending to send them. Check their uncertainty to see if it is within a range you can work with. Ask for a sample report to compare what they are reporting to what you need. If it is really just a certificate without individual values for each block get another source. Are the blocks you need calibrated ceramic or carbide? Does your source have similar masters for doing the work or are they all steel only? These are just some of the items you should be considering.
Too many gage block users ask that their blocks be calibrated in accordance with a specification for new gage blocks. Assuming the blocks have been wrung together since they were last calibrated or were new, this requirement cannot be met because the surface texture of the blocks will not comply with a new product specification. And for 99% of users it won’t matter as long as they are free of burrs and significant changes in geometry.
If you want the laboratory to make quality decisions such as replacing any block that is out of tolerance, you’d better state what that tolerance is. If you want the lab to replace blocks that are worn beyond a certain point be prepared to wait for replacement blocks to come in for your set. Also remember to ask how their uncertainty will be factored into such decisions. You might find it simpler to just get their readings on the blocks and you make the acceptance decisions based on your work.
Shipping Blocks to the Lab
We regularly receive sets of blocks that were inadequately packaged and damaged in transit. Someone from your quality department should supervise the packing to ensure it is done right. After all, these are the corporate jewels you’re dealing with and they must be protected. Padding inside the lid of the box so the blocks do not rattle around is a critical first step. Then tape the lid shut using fiberglass reinforced tape because the usual catches on the cases are not that reliable. Overpackage the set in foam rubber or bubble wrap and place it in a suitable cardboard carton. Place this in another carton with shock absorbing packing all around it such as the popular plastic ‘popcorn’ or flexible foam materials. If you’re worried about the environment, use real popcorn—unbuttered of course and hold the salt.
Some folks feel more comfortable hand-delivering their blocks to the lab for calibration. In such cases, a lot of the packaging noted above can be dispensed with. But if you’re going to use your company’s truck, assume that the set won’t get better care than it would by an independent carrier and package it carefully.
It’s now a matter of waiting for their return complete with a brand new calibration report, a subject I’ll deal with in the next column.