Understanding Gauge Calibration
When in front of a part that is out of tolerance, machine operators often tend to point their finger at their machines or the machining process. While in most cases they are probably right, sometimes the fault might be on an unusual suspect: the measuring tool.
Although its function is to give consistent and precise measurements, gauges are not immune to wear over time, especially if we consider that they are often used in harsh environments, and a worn instrument will inevitably deliver inconsistent results. Like anything else, tool wear and its causes cannot be predicted. Luckily there is a way to prevent the problem: gauge calibration.
Calibration helps operators understand how much wear has occurred since the last time the tool was calibrated and is the only way to ensure that the measurements are accurate.
Here are the answers to some of the most common questions regarding calibration.
HOW FREQUENTLY SHOULD A GAUGE BE CALIBRATED?
Shops generally decide how often to calibrate gauges based on the quality of the instrument (tip: always invest in quality tools), the manufacturer’s recommendations, how heavily the tools get used and the impact that a failure would have on the production run. Based on these factors, calibration intervals usually range from one month to over a year.
“I get this question about once a week,” said Scott Robinson, head of tech services/quality control at The L.S. Starrett Co. “Customers will call in and explain that the equipment paperwork gives a general calibration cycle of once a year. We explain that it’s important to consider the environmental and operator training factors, and whether the tool is in control of one person or is in a tool crib that is accessible to anyone. There are so many variables that come into play to establish an interval that should determine the best schedule.”
Machinists, however, should inspect their tools daily to make sure that they are fit to function.
“Ideally, every time an operator picks the tool up, he should perform a field check issue, especially if the tool is issued from a tool crib,” said Dexter Carlson, chief inspector and head of the calibration laboratory, The L.S. Starrett Co., Athol, Mass. “Every tool has some sort of a field check that can be performed. A red flag should pop up if there are inconsistencies in any of the results from the field check.”
SHOULD GAUGE CALIBRATION BE CONDUCTED IN HOUSE?
Shops tend to conduct calibrations in-house as they prefer to have control over the process. But is it always a good idea?
Several factors can influence this decision:
The environment where the calibration takes place is the most important element of the process. It needs to be controlled, quantified and repeatable. Does your machine shop have what it takes to recreate those conditions? If not, the calibration might not be reliable and problems might occur.
Conducting in-house calibrations requires significant investments, both in terms of time and money, especially when it comes to getting accreditation. Although outsourcing the process will not be as much of an investment, in some cases, it might be more convenient.
Time is an important factor to consider when it comes to calibration. Can the tool be away from the shop floor for a significant period of time? If there are multiple tools, can the shop send them all off to a calibration service at once? If not, in-house calibration might be the best option.
In general, in-house calibration seems to be the more favorable option, especially in larger facilities.
KEEPING TRACK OF THE CALIBRATIONS
Regardless of the frequency and location of where a calibration takes place, the most important aspect of the process is keeping detailed records. Having historical data for the tool will help shops understand how much wear has occurred between calibrations and will help them evaluate how often the gauge should be recalibrated.