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 that your company needs to use accurate calibration data to make sure that any misalignment is found, 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 misalignments, 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 you choose for your calibration will be based on the probe that you are using and the kind of measurements you are making. 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
Many CMM machines will require a custom artefact. The advantage to this is that the artefact material can be identical to the material that you measure, so that any deformation in the probe and material will be the same. It is also important to maintain a steady temperature in your measurement lab, so that the expansion or contraction of the probe and artefact won't create discrepancies in the measurement data.
There are a number of different CMM calibration processes. Depending on what you are planning to measure, you can employ a different artefact and method. 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 done by an experienced technician, who is aware of how to do this kind of calibration as efficiently as possible.
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 you on the best procedure for your application. If your company needs to get a calibration schedule together for your quality control department, we can help you do it. We can offer you industry leading calibration services, or we can train your staff in the specifics of CMM machine calibration.
Our laboratory maintains both ISO and UKAS certifications, so you can be sure that we are very keen on details. If you have more questions about what we can do for you, give us a call on 0115 939 2228, or please visit our website.
Our free CMM Maintenance Guide is full of useful advice about calibration and getting the best productivity from your equipment. Click here to access your copy.