Tool calibration is an important part of a machine shop's quality system. Calibrating tools and maintaining records of calibration may seem like a daunting task, but it doesn't have to be. You don't even need to spend a lot of money. All you need to get started is an excel sheet, or word document, and a set of gage blocks!
First, you will want to determine what the needs of your organization are. If you are a small machine shop working with simple parts you will obviously need a less critical calibration system, but if you commonly machine tight tolerances up to +/-.001" or smaller, then you will want to ensure your tools are calibrated with certainty. All inspection equipment has a accuracy and range of measurement. For example, standard calipers have an accuracy up to +/-.001" but aren't useful for checking a dimension that needs to be accurate within +/-.0005". The accuracy of your tools will help determine which kind of gage blocks you will need. You will want to follow the 10:1 and 4:1 Test Accuracy Ratio (TAR), which simply means you'll want the accuracy of your reference to be 4-10x the accuracy of the instrument being calibrated.
We typically use gage blocks that are highly accurate to calibrate our inspection equipment. You can get a set of gage blocks online from your local tool provider, or if you are looking for a more entry level option for calibration, then you can buy individual gage blocks from retailers online. Be sure to have your gage blocks regularly calibrated by a professional calibration business who have special machines to ensure the blocks are within specifications. Once you have the correct gage blocks, then you can start calibrating your tools.
It's important to work in a clean and stable environment. Some tools and reference blocks will have a standard temperature that they are required to be calibrated in. Temperature is an important factor to think about when building your calibration system. If your shop is constantly warm at 75-80 degrees Fahrenheit, while your inspection room is a cool 68 degrees Fahrenheit, then there may be discrepancies found between parts measured at each location. Many materials tend to expand in higher temperatures and shrink in lower ones. We found that certain parts can shrink as much as .002" when subjected to a colder environment than found in our shop. Ultimately, you should keep the temperature in mind and consider setting a standard temperature that you conduct inspection at.
You will want to have a space to record the results of your calibration. You can do this easily on excel, but pen and paper work fine as well! Below I have shown an example of a table I created on excel to document the results of calibration for a standard 1-2" micrometer.
I will begin by inspecting different sized gage blocks with the micrometer to see if the micrometer reading (actual value) matches the gage block (planned value). You can either record the error witnessed or the actual value. For example, if I was measuring a .50" gage block and the micrometer measured .50005", then I would document the recorded error as +.00005". It is important to measure a variety of different sized gage blocks along the entire length of the tool's range. For example if you had a 6 inch caliper, then you would want to inspect gage blocks at 1inch, 2 inch, 3 inch, 4 inch, and 5 inch. This ensures the part is reading accurately at all intervals. There is no need to document 0 but the tool should zero out and repeat at zero when fully closed.
All instruments will have a tolerance level which determines how much error is permissible for the instrument. You can do a little research online to find what the tolerance for your tools are. If the recorded error exceeds the limit of error for the tool, then it should be sent to a professional for repair.
Once you have any discrepancies documented between the actual and planned dimensions recorded for your inspection equipment, you will want to assign your tool an asset# and label that the tool has been calibrated. An asset# is a number used to easily identify the tool in your quality system database. Assigning each tool an asset# is an easy way to keep track of which tools are ready to use and which ones need calibration. Personally, I use a label maker to create labels to identify each tool. I simply put the asset #, the date the tool is due for calibration (typically a year from the calibration date, but can be more or less frequent based on usage), and the initials of the calibrator. After you put a sticker on the tool and file away your readings, your tool calibration is complete.
As a disclaimer, this post is designed to be an entry level guide to tool calibration and your quality system may require a more advanced understanding of the calibration standards. Ultimately, a good calibration system can save you time and money by decreasing scrapped parts caused by out of spec inspection equipment. It also increases your customer's confidence in your capabilities. Good luck calibrating!
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