Quality Magazine

Quality 101: The Many Faces of Automatic Gaging

October 23, 2008
This system is suitable for checking high-volume production of cylindrical parts immediately after a machining or grinding operation. Source: Marposs Corp.


Automatic gaging systems often are thought of as dedicated solutions suitable for only long-run, high-volume applications. While that may have been true in the past, today’s technology makes it possible to add a great deal of flexibility to these systems, making them a good choice for many lower volume and family-of-parts applications.

The major benefit of an automatic gaging system is consistency because virtually all operator-related variables are removed from the process. There are, in fact, three variations on the automatic gaging system theme: flexible, re-toolable or dedicated, depending on application requirements.

Knowing the differences is key to a successful implementation.

While parts must be clean and dry because of optical measuring sensors, this noncontact system can be changed over for checking virtually any shaft-like part within the measuring envelope. Source: Marposs Corp.

Overview

An automatic gaging system is a stand-alone device that measures part attributes without operator intervention. The system may be used for final inspection of a finished part or inter-operational inspection of an in-process part. It does not have to be part of an automated production system.

Flexible systems can be computer numerical control (CNC)-based and measure part attributes within a given volume, much like a coordinate measuring machine (CMM), but much faster. They also can be optical based, which require no part-touching details. Both are suited for large family-of-parts applications requiring high precision.

Re-toolable systems require a change of mechanical elements to handle different, but similar, parts. These are suited for small families of parts at lower volumes.

Dedicated systems, as the name implies, are designed specifically to inspect a single part, usually at high volumes during long production runs. They often are used for 100% final inspection of critical parts. These are the systems that typically come to mind when automatic gages are mentioned.

Regardless of the system configuration, parts to be inspected are typically delivered by a robot or other automated handling device and positioned by an integral handling mechanism in a cradle, nest or other fixture for inspection. After measurement, the part often is laser marked or pin stamped and then returned for pickup by the robot or handling system. Automatic gaging systems also may be manually loaded, but this is less common than the automated process.

When used for final inspection, an automatic gaging system will typically have some provision for sorting parts based on test results. This can range from simple good part/scrap part decisions, to sophisticated classification systems based on measured dimensions.

Inter-operational gaging systems usually generate feedback signals to the machine controls, as well as rejecting out-of-tolerance parts. Final inspection gages also may generate compensation signals. Both types can provide statistical information to support quality and process control systems.

System Components

A typical automatic gaging system consists of a programmable logic controller (PLC) or some other station controller to sequence the material handling components, gage motions, interlocks and other mechanical functions; a display or amplifier to collect gage transducer outputs and process them to generate dimensional and geometric values; and auxiliary equipment to laser etch, pin stamp, barcode or otherwise mark the part for tracking.

The amplifier is typically a PC running proprietary signal-processing and data-analysis software that filters the raw signals and then combines various transducer readings to generate dimensional information. The amplifier also may perform statistical analysis and generate tool compensation signals to be executed by machine controls, if desired.

The amplifier also drives the operator interface, which may range from good part/bad part lights and simple LED columns to sophisticated onscreen graphic displays.

This system has contact-type gage sensors that are not affected by coolant or dirt so parts can be loaded immediately from the machine tool without cleaning and drying. Source: Marposs Corp.

Choices in Gaging Technology

The choice of gaging technology is largely driven by application and environmental conditions:

  • Optical gages have the most flexibility, allowing quick changes to accommodate different parts. They have a low tolerance for environmental contaminants, however, and are sensitive to variations in part finish.

  • Air gages have a somewhat higher tolerance for environmental contaminants and are less influenced by part finish than optical gages. However, correct function requires a small clearance between the gage and part, which can make them dirt sensitive and difficult to load.

  • Linear variable differential transformer/half bridge transducer (LVDT/HBT) technology is used in electronic contact-type gages. The choice between the two is largely a matter of customer preference because the performance is comparable. These technologies have the best environmental survivability as evidenced by their widespread use on in-process and jump-on gages. They also stabilize very quickly, which can reduce cycle time.

    Because these are contact-type gages, they may leave minute marks on the part, which can be an issue in ultra-precision applications such as aerospace and medical components.


  • Getting the Most Value

    Because they have traditionally been used in high-volume, long-run applications, automatic gaging systems are often overlooked as viable solutions for other kinds of production. This is a mistake because modern technology has made them a viable choice for many mid- and even low-volume applications.

    But that flexibility is a double-edged sword because it also complicates the component selection, configuration and justification processes for an automatic gaging system. Getting full value from this technology requires in-depth knowledge of its capabilities and limitations, and that means the system supplier needs to be part of the project team from the very beginning.

    A potential implementer need also keep in mind that floor space must be allocated for the gage. Also, it must be integrated into upstream and downstream processes, and cycle times need to be coordinated. And, most importantly, the gage needs to qualify and validate the process after the machine tools are in place.

    An automatic gaging system can contribute significantly to both the productivity and quality of a manufacturing system-but only if it is the right system.