Six Key Considerations When Selecting A Gantry CMM

Choosing the right gantry measuring machine is a crucial driver of efficiency in sheet metal inspection. By Sea Chia Hui, regional product manager, stationary, Hexagon Manufacturing Intelligence Asia Pacific

Widely applied in the automotive and aerospace industries, gantry coordinate measuring machines (CMM) are essential tools for the inspection of large sheet metal parts. With increasing pressure on manufacturers to drive productivity while maintaining the quality of their products, it is vital that their gantry CMM is reliable, durable, and aligned with their manufacturing needs.

Naturally, the key features a manufacturer look for in their gantry CMM varies—their particular operating environment and business needs will influence the desired specifications and their ultimate purchasing decision. However, there are some absolutely key factors to consider when selecting a gantry CMM to ensure high quality and productivity.

The Required Measuring Range Of The Cmm

Calculating the measuring range needed is a logical place to start when assessing requirements. The crucial factor here is the dimensions of the parts to be inspected, but there are other issues to consider. For example, inspecting parts with difficult to reach areas might require probe extensions and fixtures. These additions could mean that the minimum required measuring range is much larger than the largest workpiece’s dimensions.

A good rule of thumb for sizing a CMM is to double the height, length, and width of the largest workpiece—these measurements should suggest the X, Y, and Z measuring ranges required from the gantry CMM. The CMM supplier should provide machine specifications that illustrate the range of measuring volumes available.

The Required Accuracy Level

All-purpose gantry CMMs like the Dea Alpha 2.0 are suited to the dimensional inspection of large workpieces such as castings and machined parts. Image Source: Hexagon

The level of accuracy offered is a key differentiator of gantry CMMs on the market. Some gantry CMMs are designed for application in the aerospace industry or ship engine constructions, where the need for large measuring volume is matched by the need for maximum precision.

In contrast, manufacturers in the automotive industry might require less accuracy and may opt for a machine that offers a potentially attractive tradeoff: less accuracy but higher speed.

For manufacturers with a particularly large volume of parts to inspect, this opportunity to drive higher throughput might be the smarter decision.

When calculating the required specification for a CMM, it is good practice to apply a ratio of specified maximum permissible error to part tolerances. This involves analysing the required uncertainty in three categories: diameters and distances, position tolerances, and form tolerances. The manufacturer should supply information detailing the machine’s maximum permissible errors in regards to part size, orientation and form, and other geometrical properties.

Temperature Control

The temperature of the measuring environment can influence the measurement uncertainty due to the effects of thermal expansion. As such, temperature variation and how the CMM is equipped to limit its effects are essential considerations.

A CMM manufacturer should ideally specify key information like the temperature range and temperature variation per day/hour/metre within which the CMM can achieve its performance specifications. In many cases, gantry CMMs are implemented in air-conditioned, climate-controlled installations to maintain a consistent ambient temperature.

The size of gantry CMMs means that the machine beams can be susceptible to temperature stratification. The structural design of some CMMs aims to counter these effects; in these cases, the machine specification will identify features like thermally-insulated covers, internal forced-air circulation, and multisensor temperature compensation.

Access To The Measurement Area

Many CMMs are built with central pillars to hold up the cross beam and reinforce the structure, while some are designed without them to maximise access to the parts in the measurement area. It is important to assess which design is appropriate, considering the size of the material handling tools used to load and unload the parts.

Accessibility does not necessarily need to be sacrificed for a higher measuring range. For example, the Dea Alpha 2.0 pictured can reach a range of 5,000 mm in the Y-axis without the need for central pillars. Potential machines’ specifications should help buyers determine what is possible within their required measuring range.

Avoiding Deflections

When dealing with large, heavy parts, it is important to consider the possibility of increased probe deflection, which can seriously compromise measurement accuracy. If the part is particularly weighty, the CMM might need a special foundation inertial block to support the weight and limit deflections.

Controlling The Effects Of Vibration

Floor vibration will impact the accuracy of a CMM’s measuring performance. It is crucial to consider this factor if vibration poses a problem in the operational space and have a professional performing a seismic vibration study at the installation site. The results of this assessment will allow buyers to cross-compare with the stated maximum vibration that the CMM can withstand while still performing to its specifications (the supplier should be able to provide this information).

In many cases, a gantry CMM requires a foundation that is pneumatically isolated to absorb the vibration that would otherwise come up through the ground underneath the machine. These isolators filter the vibrations to a level the system can withstand. Other optional active and passive vibration damping systems are available. In some cases, such as Hexagon’s large-scale bridge CMM Global eXtra, machines come equipped with a sturdy granite base, negating the need for special foundations.

Software Capabilities

At a minimum, the CMM will come supplied with software capable of the most basic measuring routines. To ensure the software package is capable of performing the necessary measuring tasks, buyers need to assess parts to be inspected and the complexity of their geometry.

The more numerous and intricate the features of a large sheet metal part, the more likely it is that users will require sophisticated software capable of complex measuring routines to ensure the inspection is comprehensive and accurate. As such, it is crucial to not just consider potential suppliers’ hardware offering, but assess their software solutions and expertise too.

These are just some of the factors to consider when selecting a gantry CMM. Of course, buyers should not have to answer these questions alone. A good supplier is not just there to provide a product; they should support customers with their experience and expertise. The quality of the supplier is therefore another crucial consideration. To maximise the success of the CMM’s installation, and indeed the productivity of manufacturing output, a supplier who is willing to partner with the buyer on providing the CMM that fulfils their needs is crucial.