What are the advantages and disadvantages of using CMM for true position measurement?

True position tolerance zone is zone (can be a circle, cylinder or two-parallel planes) where the feature of interest lies inside he tolerance zone. The tolerance zone is "relation tolerance", meaning, the tolerance zone position is based on specified datum references. In many cases, true position is measured to verify a hole or shift location/position in a part. For features that have function only for mating, such as a hole to fit a bolt, there will be a bonus in case the the hole is made bigger form its nominal diameter (MMC). In case we want to save materials, we can apply LMC condition. for example, if we have a hole that need to be applied a boring operation, if the hole made smaller than its nominal, there will be a bonus (LMC)

When the CMM is used the programmer/ operator must fully understand the cmm results. If you don’t know what the data represents then you may give a false positive or negative. Are you just checking a single line circle or a cylinder. When do you do one over another? My measurement decision is based on the size of the hole and the bore depth. If the bore depth is less than the diameter then 1 circle, 2x then a cylinder with 3 circles, 3x cylinder with 5 circles. This is my starting point and will vary based on the part. CMMs are great for collecting the data especially if it is a bore pattern with datum mobility. Also CMMs check projection well, whether your callout is standard position or fixed fastener condition.

The most important advantage of using CMM (tactile-CMM) for true position is that this type of CMM is well-understood. Hence, the uncertainty estimation of a measurement obtained by using this CMM can be reliably estimated. One of the fundamental reason is that, with tactile CMM, the interaction of the tactile (stylus tip) of a CMM with a measured surface can be modelled with confidence. Hence, the expected error from this CMM measurement can be predicted.

CMM position checks are slow compared to functional gauges made to virtual condition. First piece approvals and audit checks are quicker. Machine uptime is improved. A full CMM audit can be done on the fly, for documentation, if required.

Some CMM disadvantages for true position measurement: - Since it is a contact (tactile) measurement, if the feature size is too small, such as very small holes, the probe tip cannot access the feature and then cannot measure the true position. - In addition, the Hertzian force of the probe tip to the surface may leave some marks on the surface. this may cause problem for a part with surface appearance is important. - With point-by-point sampling, the measurement with CMM will relatively take longer time compared to optical instrument. - There is a low-pass filter to the measured pints due to the diameter of the probe-tip. Hence, high frequency features cannot be picked up by the CMM.