The primary reason for installing and operating coordinate measuring machines (CMMs) on the shop floor is to keep process quality under control, preventing the production of defective parts.

Time is a critical factor. To reduce measurement time to a minimum, only the most critical features should be inspected in a typical process control operation. Nearly all CMMs feature good dynamics and are able to perform positional moves and pick up points very quickly. As a result, measuring programs generally require a short execution time.

The measurement cycle itself represents only a small part of the time required to execute the "floor-to-floor cycle"-from collection and loading of parts on the CMM to their removal once inspection is completed. It's not uncommon to find situations in which the loading/unloading operations take considerably longer than the measurement cycle.

Some CMM manufacturers provide turnkey measuring systems that optimize the floor-to-floor cycle at various automation and integration levels. These measuring systems normally include the CMM, loading/unloading systems, fixtures and software. They are specifically designed and built to meet the technical requirements of various applications depending on the type of parts to be measured, part weight, sampling rate, specific production, and the economic requirements of small, medium and large firms.

Part fixtures for dimensional control

Accuracy and reliability of measurement on the shop floor may be affected by several factors. Environmental factors such as temperature, vibrations airborne contaminants, material type and the machining process itself all influence the reliability of measurement results. Incorrect positioning of the part on the measuring machine may also lead to incorrect measurement. As a rule, parts to be measured need to be mounted on suitable fixtures. These fixtures must:

• Ensure repeatable positioning of the part both in the measurement volume and on the fixture itself.
• Allow quick loading of the part.
• Ensure part stability during the measurement cycle.
• Allow access to all features to be measured.
• Prevent damage or deformation of the parts.

In the case of free-form parts with low rigidity, such as sheet metal panels, fixtures also have to force the part to take on its nominal geometry. For this reason, fixtures for car body components are generally more complex than those for mechanical, prismatic parts. The choice of the right fixture is critical for reliable results and to minimize part change time.

Fixtures for measurement and inspection operations come in two general types:

• Dedicated fixtures specifically built for a given part or family of parts.
• Modular fixtures that are reconfigurable and can be re-used on several parts since they are made of modular elements. A fully flexible fixturing system that can be reconfigured in few minutes.

Loading/unloading systems for metrology applications

The loading/unloading systems that are the core of every integrated measuring system can be subdivided into these categories:

• Manual systems
• Semi-automatic motorized systems in which every function has to be activated by the operator
• Automatic systems that require no operator involvement

Depending on the workload of the CMM, these systems may be configured with a single, double or multiple pallet. CMMs integrated into production lines generally have highly automated workhandling systems.

A simple, manual system is generally used on small- to medium-sized measuring machines to quickly replace the pallets. These systems consist of a base plate permanently fixed to the CMM bed. The plate mounts three spheres ensuring highly repeatable isostatic support onto which the operator places the pallet. Plates and pallets are available in various sizes to suit different types of parts. Both dedicated and flexible fixtures can be mounted onto the pallet.

If part handling is difficult due to part weight, but the part is not heavy enough to require the use of an automated system, the pallet may be pushed instead of lifted by the operator by means of a transfer system on roller guideways. This solution allows the operator to perform pallet or part change in a comfortable and ergonomic position, even where access is restricted, such as when the CMM is inside a cell. When needed, two pallets allow the operator to prepare a new part for inspection while the measuring machine is performing the inspection operation on another piece.

Motorized single-pallet loading/unloading systems also are used frequently to position very heavy parts on the CMM. In this way, damage is prevented to the bed or to the CMM itself. This solution is often used for loading parts on CMMs located inside enclosures. Typically, these systems allow accurate positioning, repeatability of the pallet and a cycle time of less than one minute, excluding measuring time. If it is necessary to further reduce the floor-to-floor cycle time, similar systems are also available in double-pallet and multiple-pallet versions.

With these types of systems, it is necessary for the operator to be close to the measuring machine each time the part has to be replaced. If the CMM workload is high and the measurement cycle is relatively short, the operator must be present at all times. If an operator has to be employed at several workstations, multiple-pallet systems are generally used. These systems allow loading a number of parts in a single stage, even parts of different types. Once the inspection cycle begins, the measuring machine automatically performs the inspection of parts in the order given.

Loading/unloading systems can have multiple pallet and linear transfer. For logistics requirements, an alternative system might be a multiple-pallet solution on rotary table.

Another inexpensive solution is to create a buffer of parts to be measured, represented by closed-loop loading/unloading systems. This system is very simple to use and has the advantage of minimum required floor space. It can be used for a large variety of parts because it is suitable for components weighing a few ounces to 500 lb.

There can be direct integration between the measuring machine and a numerically-controlled lathe. A bridge robot handles the movement of the parts between the two operating units. In these cases, measurement data is used to correct the machine tool in real time. Software processes the measurement results and calculates the machining program corrections to account for tool wear or misalignment.

There is a close interaction between the measuring machine and the other operating units (in this case, a lathe, a machining center and a dedicated machine tool) in a flexible manufacturing cell (FMC). A mechanical gripper mounted on a rail picks up incoming parts from a motorized conveyor and moves them from one workstation to another. Based on the data obtained from the CMM operating software, the mechanical gripper deposits the parts back on the conveyor, or in a scrap bin.

A by-pass system can be integrated in the main line for manufacturing prismatic components. Among the key elements of this turnkey system supplied by a CMM manufacturer are:

• Stop and synchronize devices for the parts on the key conveyor.
• Transfer units from or to the conveyor.
• Part fixtures.
• The measuring machine in an air-conditioned enclosure.
• System operating software handling all automation activities of the inspection cell and its interaction with the manufacturing line.

Process management for turnkey measuring systems

Unlike installations with a standard measuring machine in which only traditional CMM activities need to be coordinated, the synchronization of loading/unloading systems must be managed. Although it isn't a prerequisite, it's better to separate the handling of measurement and inspection activities from automation activities in order to maintain optimum control of the two activities.

As a result, turnkey measuring systems usually have specific software that controls all automation devices and synchronizes their activities with those of the measuring machine. In case of high-level integration, the software also interacts with other programs, typically production line software.

Operating software is generally customized to the specific application and is able to provide the operator with a simple and intuitive graphic interface, information in real time on system status and on the quality of the measured parts.

Sidebar: Quality Tech tips

• Accuracy and reliability of measurement on the shop floor may be affected by several factors.
• In the case of free-form parts with low rigidity, such as sheet metal panels, fixtures also have to force the part to take on its nominal geometry.
• Separate the handling of measurement and inspection activities from automation activities in order to maintain optimum control of the two activities.

Links

• Hexagon Metrology