Author: Tony Tillett – Managing Director Status Metrology
Keeping your CMM machine calibrated regularly will ensure that the measurement data that you create is accurate. The definitive procedure for calibration of CMMs which is used by all CMM manufacturers is the ISO 10360 series. Most CMM machine calibration will be accomplished with the use of an artefact, but for more demanding calibration, a laser interferometer can be employed. Ultimately CMM machine calibration comes down to one thing: precision.
How Calibration Is Determined
A CMM measuring machine can have errors along 21 different measurement axes. This means a company needs to use accurate calibration data to make sure that any misalignment is determined, so that it can be either fixed, or integrated into the data. There are different levels of CMM machine calibration ranging from a weekly check, to precise calibration that is performed once or twice a year. Many modern CMM machines use advanced software to account for small misalignment’s, though the risk is that an error will not stay constant.
For most CMM machine calibrations an artefact will be used. The kind of artefact that is chosen for calibration will be based on the probe being used and the type of measurements being performed. It is important to choose an artefact that is similar in hardness to the material that will be measured, so that there isn’t any inconsistency due to probe and material deformation. Some of the more common forms of calibration artefacts are:
- KOBA Step Gauge
- Length Gauge Blocks (Square or Round Ends)
- End Bar
- Ball and Cone
- Ball Plate
- Hole Plate
- Swift-Check Gauge
There are a number of different CMM calibration processes. All calibration processes will measure an artefact along a predetermined measurement plan, so that the data points can be referenced against the known dimensions of the artefact. While no CMM machine will ever be perfect, the goal is to remove all errors that would prevent the machine from accurately measuring the parts being inspected.
When a higher level of calibration is required, a laser interferometer is used. This system uses a laser with a beam splitter that makes extremely accurate measurements based on reflected laser light. The reflected light creates an interference pattern, and by using computer software, the CMM machine’s movements are measured. There is no downside to this calibration system in terms of accuracy, but it will take a lot more time to perform than using an artefact. It also should be completed by an experienced calibration technician.
How To Choose The Right Procedure?
At Status Metrology we have been working with some of the best CMM machines on the market for decades and can advise on the best procedure for any given application. The Status laboratory maintains both ISO and UKAS certifications.
The Status Metrology free CMM Maintenance Guide provides useful advice about calibration and obtaining the best CMM productivity and is available for download free.
For more information: www.status-cmm.co.uk