When performing a calibration of displacement, you have several options for a measuring standard. Two of the more common standards are hard standards like gauge blocks and laser systems. Each standard has its advantages and disadvantages, which we’ll explore some here.
A hard standard like a gauge block or micrometer standard is a physical item that is precisely machined to a specified length. Those standards generally come with a certification that shows what the actual length of the standard is. By stacking shorter standards together, you can do longer calibrations.
Advantages of hard standards
- Simple to use. Just place it on the machine and check it like you would a part.
- Inexpensive. Hard standards are less expensive than laser systems.
- Replicates the measurement process. This lets you see measurements in the way you use the measuring tool.
Disadvantages of hard standards
- Thermal expansion. Standards are typically calibrated at a specified temperature. If they are being used during a calibration at a temperature different from the standard, they could expand/contract which would have to be corrected for.
- Physical damage. If the standard gets damaged on either end, it will affect the accuracy of the length.
- Stacking error. By stacking multiple standards together during a calibration, they could be misaligned to the machine axis, and thus give a reading that is longer than expected.
- Squareness error. If the standards do not seat squarely against the stops of the machine being calibrated, the reading will be longer than expected.
In contrast to hard gauges, laser systems are non-contact. A beam of light travels from the laser head, reflects off an optic connected to the moving element of the gauge being calibrated, and returns to the laser head. Using optics and electronics, the laser system can determine how far the component has moved. The laser system has its advantages and disadvantages as well.
Advantages of laser systems
- Thermal correction. Laser systems often have probes that measure the machine temperature, as well as the ambient temperature. This allows them to accurately correct for the change in temperature from the standard.
- No physical contact. Since the laser isn’t a physical item, it can’t be damaged through use.
- No stacking error. The laser can measure any position between the head and its design length. So, unlike hard standards, there is no stacking or accumulation of error when performing a calibration.
- Squareness error. By aligning the laser to the machine travel, the beam remains completely straight and parallel. This will prevent any squareness errors from affecting the calibration.
Disadvantages of laser systems
- Difficult to use. Laser systems require some setup in order to be used. You have to mount the head, align the beam to the machine, mount the optic in order to get a reading, and typically have a computer connected to record readings.
- Expensive. A laser system will be much more expensive than a set of hard standards. This cost may be more than a company can justify spending.
- Different measurement process. A laser system measures differently than the machine, so systematic errors such as squareness or flatness between measuring surfaces do not get taken into account by the laser system.
Outside laser calibration + internal hard standard
Many of our customers will have us come in with our laser system to do a calibration. We check the machine at each error correction point, make adjustments, then validate the readings every inch over the whole length of the machine. For spot-checking, the customer then uses one or two hard standards to verify that the machine is operating as expected.
Doing this allows them to have the advantages of laser calibration, without the expense of purchasing a system and dealing with the upkeep on it.