A common question that we are asked is: what is the resolution of a CT scan for determining gaps or voids or true surface locations? We are also asked: How can you measure a gap between two parts that may be closer than 1 voxel in size? The answer lies in sub-voxel surface determination and in the number of voxels that are involved and continuous surfaces as detailed below.
Modern Additive Manufacturing (AM) methods often employ subtractive machining to achieve precise dimensional and surface finish characteristics. This is due to current AM limitations in build precision and surface quality. These hybrid AM / subtractive components also demand a hybrid approach to measurement. Internal features on AM parts are difficult or impossible to non-destructively measure using solely traditional metrology methods and so industrial computed tomography (CT) must also be employed. CT scanning not only allows for precise measurements using metrology CT, but allows for detection of internal defects that are not otherwise able to be seen. However, well machined portions are best served through other measurement methods due to precision requirements and data characteristics. This can necessitate a hybrid measurement approach. However, traditional CT scanning and service providers are not trained as metrologists and do not follow particular standards related to metrology – especially ISO17025. Also complicating measurement is the design and datum reference frames used for additive parts that typically follow a traditional machined component print layout. A layout method following best practices from the casting industry is suggested here. This article explores the methods (GD&T), issues, limitations and current solutions associated with good metrology practices for AM and hybrid AM components.
COVID 19 has literally shut down a majority of the world’s production. As we have talked with our customers over this time, we have found that many have reduced their workforce or have implemented rolling furloughs. The problem is that once you let that skilled person go, they may actually find other work before you can hire them back. So, all of the training that was invested in them is gone. This seems especially true of Quality staff. During each normal business cycle and when things get tight, the quality staff is the first to be reduced. A new staff is then later brought on once the cycle has completed with the requisite experience or they are slowly trained up to a proficient level. This is logically the case as production requirements are lower during these cycles and removing production personnel would immediately and directly hamper the ability to generate revenue.
If we lived in a perfect world the parts you manufacture and the gages used to check them would both be exactly to nominal specifications. At that point a gage would be obsolete and so too would be the metrologist, but we all know that this world is far from perfect and that the best we can ever do in the study of measurements is to provide our best estimate of where the actual value lies along with a provision of uncertainty of measurement that can be traced all the way back to the international system of units.
First Article Inspections (FAI) are used to ensure that parts off of new or modified tooling or processes conform to the part design requirements. This even includes changing the location of manufacture! Yes – this can make a difference sometimes (usually due to different equipment, the same equipment setup slightly differently, environmental conditions, different manufacturing standards, etc). This includes any time the Form, Fit, or Function could be impacted.
Radiographic testing is a widely accepted method for finding voids and defects in materials. Makers of castings and solder connections on printed circuit boards have used radiographic testing to detect voids in 2D images and Computed Tomography (CT) in 3D images with excellent results that are well documented. As CT scanning continues to evolve and emerge as the inspection method of choice new challenges arise. Many times customers are unaware of the difference and apply old specifications on their drawings that may no longer apply. Or they do not know the full capabilities of the newer technologies or have not yet defined the specifications for their application.
Industrial CT scanning has gained prominence in dimensional data collection technologies with the advent and continual development of additive manufacturing. Now, CT scanning processes are being added to ISO17025 scopes of accreditation and the emergence of Metrology based CT is gaining recognition and acceptance across many industries.
Many measurement and reverse engineering requirements require more than simple caliper or micrometer measurements. Whether you are conducting a First Article Inspection (FAI), troubleshooting a manufacturing issue or reverse engineering a legacy part for improvements, one or multiple of the modern data collection methods may work – but what is best and what are you compromising on by using any particular method? Do you do this work yourself or hire out an experienced service provider?
Contact measurement devices constitute the majority of what is traditionally used for dimensional metrology. These consist of devices that must be in physical contact with the object to be measured. Below is a listing of general types of contact measurement devices. This covers the vast majority of what is used today and is a good overview on what is possible and what to expect from data taken with these instruments. We freely use these types of devices in our service business where it makes sense for any particular project due to quality, cost and timing.
Non-contact or touchless dimensional measurement devices are the modern new comer to an array of contact devices that have been traditionally used. Contact devices are discussed in this link on Common Contact Measurement Devices. All of the non-contact devices generate a point cloud or voxel data set as their most raw form of data. The exception is the optical comparator which has been around since the late 1920s. It does not expressly produce digital data, but some of the variant equipment that evolved from this equipment does.