Measurement: The ABCs of In-Line Gaging
July 26, 2011
In-line gaging is a hot issue among quality professionals. But, like most rapidly evolving technologies, the concept means different things to different people. With that in mind, let’s begin with a few definitions.
For purposes of this discussion, an in-line gage is a gage that is on the production floor, close to the manufacturing operations being performed on a workpiece, located physically inside the cell, line or other system configuration. In most applications it is used to check 100% of the parts being produced and may provide compensation feedback to the machine tools in the process.
It is loaded with an automated device, typically the same robot or gantry that loads and unloads the machine tool, and after being measured the workpiece may be returned for further processing. In this case it is an interoperational gage that checks specific dimensions and/or geometries to verify that the workpiece is ready for subsequent operations. It may also be a final inspection gage, used to determine if the part meets design specifications for its intended use.
It is not an in-process gage that measures a part while it is being machined and is interfaced with the control to stop the processing when the specified dimension is reached.
And, of course, it is not an off-line gage, which is manually loaded and located on a gage table, metrology laboratory or other shop floor area away from the manufacturing operations.
There are a few other terms that also need to be defined:
A dedicated gage is one that is available in a range of predefined sizes and configurations and is used to inspect a single part type or a very narrow range of parts.
A modular gage is made up of standard components, such as pencil probes, contact arms and work rests mounted on a base that allows mounting details to be randomly located and re-located as necessary to inspect a wider range of parts.
A reconfigurable gage is one with easily changed or repositioned arms, contacts and other features that allow it to accommodate a range of similar parts with little time spent between changeovers.
A flexible gage is one that can inspect a range of part types without having to be reconfigured.
Unfortunately, there is no industry agreement of these definitions and, therefore, a good bit of ambiguity exists when they are used by various manufacturers and operators. The aforementioned definitions are used by us both internally and externally to differentiate the functionality provided by each category of gage. They are independent both of the particular gaging technology being applied and of the manufacturing processes being monitored.
Selecting the Right GageIt is very important to begin by defining exactly what the gage is expected to do. For example, specifying a flexible off-line gage for an application in which a dedicated in-line gage provides the necessary functionality can add both expense and complexity to the installation, with little or no offsetting benefit. The opposite, of course, also would be true.
There are a number of questions that have to be answered when selecting an in-line gage, and not all of them are intuitively obvious. Here is a brief checklist of things to consider:
Production rate. Can we measure fast enough to keep up?
Production size. What is the gage cost per part? Can the gage last for life of the production run?
Number of part types. Can one gage measure all parts?
Differences and similarities between part types. Is it better to make some checks on all part types, or all checks on one part type per gage?
Lot size-changeover frequency. Can the gage be manually retoolable or must it be flexible?
Part material. Can the gage contact the part or must it use air?
Part configuration and size. Are measurements best performed by putting gage in/on the part, or the part in/on the gage?
Measurements needed to control. Do all dimensions need checking?
Part tolerances-geometric, dimensional, surface finish, flaw detection. How precise does the gage need to be?
Checking parts for function or for process control-what datums to use? The machine tool producing the part may not use the same datums that the end product uses.
Process environment-temperature, temperature variation, cleanliness, light conditions. Does the gage need temperature compensation or a washer/dryer? Can inspection be done using visible light?
Operator skill. Is the gage too complicated?
Process control requirements. Is feedback to the machine tool required? If so, how often?
Statistical or data retention requirements. Is a computer needed?
Location in the process, line or cell layout. Is the gage accessible and convenient?
Ergonomic requirements. Does the gage require difficult or repetitive movements?
Price. Is the price within the budget?
Delivery requirements-timing of the project. Can the gage be delivered in time?
Correlation requirements to another measurement source. Different technologies may give different results.
Part condition-surface finish, temperature, cleanliness, wetness. Does the gage need part temperature compensation, a washer/dryer, noncontact technology?
Automated process or manual. Is automatic part handling needed?
Audit checks or 100% inspection. Are the checks safety related, or only for process control?
Maintenance requirements. Does the operator have the time and skills required to maintain the gage?
The answer to each of those questions is important and has a direct bearing on gage type and gaging technology best suited for the particular application. Failing to consider and evaluate all of these points (and perhaps others) will likely lead to a less than optimum solution.
For example, there often is a trade-off between time and cost. Using multiple gages to measure a part generally takes longer than doing all the measurements in one gage. The difference may be between performing the inspection in a few seconds or a few minutes. However, a single gage that measures everything in seconds is probably a custom design that may well cost twice as much as two or three simpler gages.
The possibility of purchasing components and building your own gage also should be evaluated. Most major suppliers of gaging technology today offer the “pieces and parts” required, and the savings in engineering cost can be substantial. Naturally, in-house design capabilities will be a major factor in this decision.
During this phase of the process it also is important to evaluate a supplier’s product line to be sure it includes all of the potential gage types and measuring technologies that may be suitable for the application. It is the only way to be sure that the solutions ultimately offered are the best available and not simply what’s available from a limited product line.
Selecting the Right TechnologyFinding the right balance between flexibility, accuracy and precision, ease-of-use and cost is a very important and often difficult task. Different technologies offer a variety of characteristics to help solve this dilemma.
Electronic contact-type gages are one of the most widely used categories available today. They are precise, robust, flexible, easy to apply and offer a wide range of measurement. They typically provide a good, economical solution for parts that are likely to be less than perfect when measured because they are able to tolerate contaminants such as coolant, swarf and chips, as well as surfaces that are not extremely smooth due to tooling marks.
Air gaging is usually the best in-line solution for parts that cannot be touched, and for bores and other details that are too small to accommodate a mechanical gaging probe. While extremely accurate and precise on smooth surfaces, air gaging is an averaging technology so it may not perform as well as a contact-type gage on rough surfaces. It also can be affected by contamination.
Air-type gages also have an added operating expense because they require a constant supply of clean, dry air. Therefore, it is not possible to operate a precision gage on shop air.
Air-type gages also are among the most dedicated because they have a very small measurement range. For example, while this technology can produce excellent results measuring a bore to ±10 microns, it may not be usable at all where the tolerance is ±100 microns.
Optical gages can bring a very high flexibility to the gage solution because of their noncontact characteristic. Optical gages also are often the only solution for particular parts, such as orthopedic joints and other medical devices, which must be perfectly smooth. These gages can detect scratches, blemishes and other surface imperfections that are so minute they can only be seen by the human eye.
One rapidly emerging optical technology takes multiple photographs of a surface under white light while the part is moving using a field of view as small as 3 x 3 millimeters for metal parts, or 1 x 1 millimeter for highly polished ceramics. A computer analyzes the images in real-time to provide a 100% surface inspection. While this technology cannot presently provide dimensional data, applications combining it with “shadowcasting” and other optical technologies are under active development.
The Bottom LineThere is no one-size-fits-all solution for in-line gaging applications. A thorough, methodical evaluation of the application, combined with a realistic analysis of the economics, and an in-depth examination of a potential supplier’s product lines and experience to make sure you will have access to all potentially applicable technologies is the best way to arrive at an optimal solution. Q
Quality OnlineTo learn more about inline inspection, visit www.qualitymag.com to read the following articles:
“Improve Inline Inspection”
“Inline Probing for Process Improvement”