Important information about CMM calibrations: Myth vs. Fact
There are many misconceptions out there in the manufacturing industry when it comes to Coordinate Measuring Machines (CMMs). One of the largest is calibrations. We at Summit want to take a few minutes to try and set the record straight when it comes to what’s needed for a CMM calibration.
Statement: CMMs need to be calibrated every year per ISO standards.
CMMs are a gauge and like every gauge need to be calibrated. However, there is no set frequency in which a gauge is required to be calibrated by ISO, AS, or any other standards.
“The organization shall establish, implement, and maintain a process for the recall of monitoring and measuring equipment requiring calibration or verification.
The organization shall maintain a register of the monitoring and measuring equipment. The register shall include the equipment type, unique identification, location, and the calibration or verification method, frequency, and acceptance criteria.
NOTE: Monitoring and measuring equipment can include, but are not limited to: test hardware, test software, automated test equipment (ATE), and plotters used to produce verification data. It also includes personally owned and customer supplied equipment used to provide evidence of product and service conformity.
Calibration or verification of monitoring and measuring equipment shall be carried out under suitable environmental conditions (see 7.1.4).”
The organization shall determine if the validity of previous measurement results has been adversely affected.
It is entirely up to the company on when their equipment needs to be calibrated based on the risk to product if it is to be found out of tolerance. The last line above expresses this from the AS9100 standard.
Example. You have a gauge that is calibrated every six months. When it comes back from calibration it is found to have not passed. Now it is a requirement that all products measured with that gauge are looked at for possible non conformances. So, the longer you go between calibration the more possible product you have that could be at risk. This is why it is important to look at your before and after calibration data provided to you. If the machine is stable and does not require corrections every year, then it is possible to extend the calibration period. If it is always failing, then it would be recommended to tighten up the calibration period.
Statement: CMMs need to be calibrated back to the OEM specification
It is up to the company to specify what their required tolerances are for their product. In this way, the same model CMMs running different product lines could use two different acceptance tolerances. This way you could extend the time between calibrations on the machine running product with looser tolerances, because the risk of it being beyond your acceptance criteria is lower. If you don’t know what to use for acceptance criteria, Summit can help. It might be that the OEM stated accuracy is what we want to use, but it might not be.
Statement: CMMs can only by calibrated by the OEM
CMMs can be calibrated by anyone that has access to the error map. Depending on the CMM, the error map will exist either on the PC driving the CMM or directly on the CMM controller. Sometimes both can be true. In this case either the Error map stored on the controller can be updated or a text file can be updated that is specific to the software being used. Both CMM manager and Aberlink software are like this. The drawback to updating software specific error maps is that if the machine has multiple programming languages on it, you would need to keep multiple maps updated, which is why we recommend just updating the CMM controller whenever possible.
Just because a company has access to the error map, it doesn’t ensure they can do the work. It is always important to look for an accredited calibration company. Using an accredited calibration company ensures they meet the industry standards. An accredited calibration company will have to be audited each year to verify compliance with any changes in the standard, verify the standards are traceable to NIST, and demonstrate how they perform the work. They are also required to do proficiency testing with another laboratory to show statistically that they produce the same measured results after calibration.
Statement: CMMs calibrations must be tied into Service agreements
The largest two CMM companies like to tie in their software upgrades with CMM service and calibration. If you go this route, you will have some kind of savings on your CMM calibration from the OEM, but it is most certainly not required. It is possible to do the software upgrades with the OEM and then have a third party do the CMM service and calibration. This will save costs and eliminate lengthy OEM lead times.
One other thing to consider is whether the yearly software upgrades are worth it? Most companies upgrade every other year or every third year. Some years there are great new features released and others not so much. We typically tell customers to review what the changes are and if they really need them. Sometimes it’s beneficial to stay with a stable software package and not risk introducing bugs or other errors into inspection programs by updating every year.