Quality control managers in the automotive and aerospace industries are responsible for ensuring that manufactured parts meet customers’ requirements, specifications, and tolerances. To do so, they rely on coordinate measuring machines (CMMs), which are the most precise metrology equipment available for quality inspection.
Getting this level of precision, however, comes at a price (often in the form of inconvenience). The CMM may not be available to practice first article inspection (FAI), or, worse, it may be totally loaded because a fault has been found at the end of the production of a part. Then, lots of back and forth between the CMM and the shop floor is necessary to locate where the issue has occurred.
In those situations, quality managers are handcuffed by the technology and limited in the execution of their work. What if they had a secret weapon to deploy when the CMM is loaded? What if they could have access to a portfolio of alternative solutions that they could rely on to improve the quality inspection?
In order to make sure that you are never restricted in the exercise of your duties, we’ve compiled the following list of different metrology tools available to QC managers, with the pros and cons of each.
A Look At Options
Hand measuring tools are among the ways to approach the issue of monitoring. The most common hand measuring tools include micrometres, slide callipers, indicators, and gages. These tools are mainly used for simple inspections and basic measurements such as measuring a diameter or a thickness or any other dimensions that do not justify or require a report.
The pros of hand measuring tools are: simplicity of use; a basic level of technical expertise needed; high precision; quick use for simple measurement and features.
But there are reasons as well not to rely exclusively on such tools, including repetition: because the measurement depends on the operator’s manipulations and difficulty selecting the most suitable tool, because each measurement requires a different tool. As well, they can be hard to use for complex parts.
The next option is to rely on fixed CMMs, which are definitely a better choice than hand measuring tools for complex parts. Indeed, they can measure any type of feature with a high level of precision. Because of that, they are the no.1 metrology equipment choice for quality control managers. They are so popular that they are often loaded by different operations.
Fixed CMMs are also flexible, allow access to automated reports and show ability to measure and inspect any types of feature as well as provide unbeatable precision.
However, they also have some drawbacks, including that the shape of measured parts is limited due to the size of the measuring table, high cost of utilisation and a high level of technical expertise needed for equipment fixed to the ground and that requires a rigid setup.
There is also a subcategory in laser trackers that are often used to measure parts of large dimensions. While fixed CMMs are limited by the table surface and portable CMMs are limited by their measuring volume, laser trackers can measure parts like aircraft wings or auto frames, as well as large toolings. Their disadvantages are that they require a rigid setup and are sensitive to instabilities in the environment.
That brings us to portable CMMs, an alternative solution when a part cannot be moved from the production floor to the measurement lab. These include the advantages of fixed CMMs, as well as portability, which enable one to move the CMM to the shop-floor, another building, or a supplier’s facility.
On the plus side, they show portability: the measurement tool goes to the measured part (rather than the opposite, simplicity of use and the ability to measure directly on the production floor.
However, they may be sensitive to vibrations and not adapted to unstable shop-floor measurements as well as requiring a rigid setup, and noting that operator experience and skills can affect the measurement accuracy.
There is also a subcategory here in optical portable CMMs.
This subcategory offers the same advantages of portable CMMs, with an extra: a rigid setup is no longer required. This means that everything (i.e., the tracker, the measuring tool, and the measured part) can move during the measurement.
Therefore, it reduces the pressure placed on operators. In addition, their level of expertise does not need to be so high because fewer errors will be caused by extra manipulations and alignments. In short, optical portable CMMs are perfectly adapted to shop-floor measurements.
Finally, there are metrology-grade 3D scanners complete our portfolio of alternative solutions to rely on to improve quality inspection.
Like portable CMMs, most 3D scanners can be moved around on the production floor, but they also have the capability to measure in a complex production environment—just like optical portable CMMs—that is often influenced by temperature variations, vibrations and inexperienced operators.
However, it is through the information density they can acquire and analyse that 3D scanners distinguish themselves from other measuring equipment. Because of that, they are the preferred solution for FAI, where each dimension measurement is critical. The complete part (dimensions and aesthetics) must be inspected and approved during the FAI. Therefore, it is risky to omit defaults with a probing unit that only measures a restricted number of points (or samples of points).
This occurs through speed of acquisition and density of information analysed, the short time required to characterise a complete part and via efficiently digitising complex shapes with a very large number of points and without contact.
However, there are some drawbacks too such as the need for the measured part to be in the line of sight of the scanner and as an overkill solution to inspect simple geometrical features like pins and holes.
So the secret weapon for quality control teams, therefore, is to have different measurement options to inspect parts. Because each solution has its pros and cons, being able to rely on different tools according to the type of inspection being performed or the shape of the piece being measured is the key to performing excellent quality controls.
APMEN Metrology & Design, Nov 2016