Quality Scan: Portable or Fixed-Position CMM?
By Jonathan Dove Applications Operations Manager North America
By Doug Schueneman
Technical Services Manager
Manually operated, articulated-arm CMMs are used in many manufacturing operations because they can be moved quickly and easily to the shop floor to perform fast, accurate measurements. Articulated arm CMMs use an anchored, jointed arm with an attached probe tip at the moveable end. The length of each arm section is combined with the encoder angle at each joint to compute the probe's position.
Fixed CMMs, as the name implies, are located in one position. What fixed CMMs give up in mobility and portability, they return in accuracy and repeatability. Most fixed CMMs are also direct computer controlled (DCC) systems that automate the inspection operation.
Both types of CMMs can be used to monitor and identify evolving trends in production to eliminate the manufacture of out-of-tolerance parts, and to accurately detect variation in part dimensions caused by changes in process tooling, providing data to establish optimal tool setup and subsequent tooling changes.
The decision on what technology to use depends on the following factors:
- Size, configuration and transportability of the workpiece.
- Number of features that require inspection and accuracy required.
- Magnitude of the process control problem that must be corrected.
- Investment objectives.
Large workpieces such as complete auto bodies, auto body sheetmetal panels and subassemblies, airframe components, molds, dies, and welding fixtures present special handling challenges. It's not only their size that makes these parts difficult to measure, but also the variety of different tolerances often found on them. Large parts are often extremely heavy, bulky, and awkward to transport, adding time and labor cost to the inspection cycle if the parts must be moved from the shop floor to an inspection area.
Because large parts, by their very nature, require more time to move and set up, the use of portable CMMs to inspect them in place on the shop floor helps reduce inspection-cycle time.
Some parts have features that are located in difficult-to-reach areas. Because of its flexibility, the articulated-arm portable CMM makes it easier to inspect these features, reducing overall inspection time. There are also some parts, such as welding fixtures, that must be inspected in place where they are constructed. These parts are prime candidates for articulated-arm portable CMMs.
The number of workpieces, the number of features, and the accuracy required are other considerations in the process of deciding whether a portable-arm CMM or a fixed-position CMM is the best choice for an application. If there are more than one or two parts in a batch to be inspected, and if those parts have several features that need to be checked, it may be more cost-effective to inspect them using a fixed DCC CMM than a portable-arm CMM. The time savings possible by running an automated program on a fixed CMM may more than compensate for the time required to move the part to the inspection station.
In some cases, a combination of the two types of CMMs may work to the user's best advantage. The portable-arm CMM is a good investigative tool, and can quickly isolate process problems. Early in the manufacturing cycle, a portable-arm CMM may provide the level of accuracy necessary to determine if the process is in control--if holes and slots are in the correct position, for example. Its use can, in fact, relieve the burden on a fixed-position CMM, improving overall inspection efficiency. Later in the manufacturing cycle, such as finish-machining operations, it may be appropriate, and faster, to inspect the part on a fixed-position CMM.
Equipment cost is also a factor. For example, a portable-arm CMM with a 15' (4.6-m) measuring volume is considerably less costly than a fixed-position CMM of the same measuring volume. Total inspection operating costs, however, must be factored in, and those costs include inspection cycle-time costs. A DCC CMM may prove less costly to operate over the course of a production run. In the end, the final decision on which technology to use depends on a careful examination of total inspection costs.
This article was first published in the July 2005 edition of Manufacturing Engineering magazine.