Dimensional Data Collection Through Coordinate Metrology: A Cost-Effective Way to Improve Moldmaking Operations

Advances in data gathering sensor technology and improved measurement and inspection software have made the use of coordinate measuring machines (CMMs) a viable and cost-effective method of improving moldmaking operations.

A CMM takes measurements by moving a sensing device called a probe along various axes of travel. The precise position of the contact is recorded and made available as a measurement output of the position or displacement of the probe. The CMM makes numerous contacts, or hits, with the probe until a useful database describing the surface of the part has been constructed. Because 3-D contoured shapes - such as those found in molds and molded parts - require a large amount of dimensional data to define them adequately enough for precision inspection, automated measurement is the most cost-effective and accurate method available.

Automated 3-D measurement requires a coordinate measuring machine capable of scanning, and the use of CMMs for scanning in the moldmaking industry has grown in recent years. Scanning is simply a way of automatically collecting a large number of data points to accurately define the shape of a part. Data collected in this manner - when transferred to a CAD system - can be used to create electronic part models for reverse engineering, construct machining paths, build and upgrade databases, and used to create a CMM inspection program off line. Scanning also is a fast, efficient method of archiving dimensional data for later use.

For example, a moldmaker may design tooling on a CAD system, machine it, test it to find that the molded part doesn't meet design specifications, then handwork the mold to refine its shape. The dimensions of the modified mold can be digitized using a CMM and the information archived in a CAD database for later use if the mold has to be repaired or replaced.

Continuous Scanning Accelerates Data Acquisition

Continuous and stitch scanning are the two methods used today. A conventional CMM uses a probe head that records a single data point with each deflection of the probe tip. The probe head must be lifted from the surface of the part, moved slightly and contact must be reinitiated for every data point that is collected. Breaking contact with the part like this is called stitch scanning. It is very effective for applications where fairly smooth 2-D surfaces prevail, requiring a minimal number of data points. Stitch scanning also is effective where surfaces are completely defined, and data is used solely for verification purposes to compare actual part dimensions to design dimensions.

In moldmaking operations, continuous analog scanning is a more viable alternative to data collection. Analog scanning probes maintain contact with the part surface, measuring changes in its surface elevation as a proportion of the measuring tip's deflection over its full range. An uninterrupted stream of data is sent back to the system's computer.

Analog probes act as small, accurate auxiliary, three-axis measuring machines whose readings complement those of the CMM. In the scanning process, the probe stylus is in constant contact with the part surface, and the measuring machine control system ensures that a consistent gaging force is maintained by detecting any deviations and regulating them immediately. High-resolution electronic transducers continuously record the displacement of the probe's three axes, which is combined with displacement of the CMM's three axes to provide a complete picture of where the probing is taking place. CMMs use two different types of continuous scanning methods, depending on whether the geometry of the workpiece is defined or undefined.

Open-loop scanning is a high-speed technique used for measuring theoretically known or defined shapes on parts whose geometry, characterized by few curves and surfaces, allows the probe to maintain contact with the surface. The machine motion is controlled by the workpiece nominal geometrical data recorded in the CMM part program. The probe records the magnitude of the error between the actual surface and the nominal. Prismatic features such as holes, planes and cylinders can be scanned in a few seconds with high repeatability and accuracy, and a complete description of their geometry, including dimension, location and form can be generated. Acquiring a larger number of data points ensures higher repeatability for diameter and position, and a much better definition of the true shape of the part's features. In general, the more points gathered the greater the repeatability of the results.

Closed-loop scanning is a high accuracy technique particularly useful for digitizing undefined, convoluted shapes. Molds - or any part that includes complex, very small and/or 3-D curved surfaces - are candidates for closed-loop scanning. In this scanning method, the probe detects changes in the surface directions of the part and adjusts itself to maintain contact with the workpiece.

Technology Advances Improve Scanning Functions

Advances in data gathering technology over the last three years have made scanning an even better way to gather data. The advances fall into three categories: controllers, data gathering sensors and software.

There is a demand for controllers that can execute more complex routines than in the past to provide advanced data gathering capability for reverse engineering applications. New controllers can react more intelligently to changes in direction of the scan, such as when the probe encounters a hole in the workpiece. When a surprise in the measuring path like this occurs, new controllers can automatically set up an action, such as a spiral search, to come back into contact with the part and continue on.

This is where the closed-loop and open-loop scanning modes overlap. In newer systems, each scan path helps define the next scan path, extrapolating the recently acquired information to predict what the next pass is going to look like. If contact with the part is lost, the system shifts to an unknown mode. Once contact is re-established, the system shifts again, this time back to a defined scanning path.

Controller advances are an answer to users who would like to simply place a part in the CMM measuring volume, stake out four corners of the part and walk away, letting the machine controller handle the entire measuring routine. CMM manufacturers are working toward this "set it and forget it" mode.

Advances in smart controller technology are coming rapidly now as a result of a proliferation of open architecture controllers using inexpensive PC chips. This lets developers use the "C" programming language rather than proprietary languages, which, in turn, allows more motion control experiments in less time than in the past. This makes it very easy to try new controller ideas. Because these newer controllers are PC-based, they can use a network to pass data directly to a computer for analysis at speeds up to 1,000 points per second. The traditional RS-232 interface has a limit of about 25 points per second. The ratio between throughput and accuracy is always a concern in scanning operations. Generally speaking, the faster the data is collected, the less accurate it is. However, that is changing as well. Improved accuracy and throughput ratios are the result of improvements in CMM drive mechanisms. Stiffer drive mechanisms that can react instantly to rapid direction changes have improved the accuracy and repeatability levels of scanning CMMs.

Today, users are accepting current accuracy levels and are looking for advances in throughput. They want data as quickly as they can get it. The focus is on speed with reliable accuracy.

Optical Sensors in the Spotlight

In the area of data gathering sensors, the balance is shifting from contact sensors to non-contact sensors. Non-contact sensors provide greatly improved reliability and accuracy compared to sensors of several years ago. They also offer the advantages of speed, plus they can be used in applications where contact with the surface of the workpiece would cause damage.

As the cost of CCD camera chips comes down, optical sensors have become a practical choice for scanning applications. Optical sensors can be affected by the surface finish of the workpiece, but for most reverse engineering applications, non-contact sensors are the best solution since they can rapidly gather large amounts of data accurately and can digitize details that may be too fine for a traditional ball probe.

New coordinate measuring machines offer multi-sensor scanning capability for operating flexibility. These systems are multi-purpose CMMs that can handle touch trigger probes and analog scanning probes - which perform both tactile scanning and precision single-point probing - as well as laser triangulation scanning probes. This flexibility broadens the capabilities of the traditional CMMs. They now are able to locate workpiece features on prismatic parts using traditional point-to-point CMM techniques, use analog probes to scan form elements and complex geometries on free form surfaces such as sheet metal, and use non-contact scanning sensors to maximize data collection rates and inspect sensitive or very small parts.

Advanced Software Simplifies Data Analysis

Today, measurement and inspection software has been refined to the extent that no computer programming knowledge or experience is needed to run even the most sophisticated programs available.

Virtually all CMM software consists of menu-driven, user-friendly programs. Software for any particular routine is very easy and flexible to use and can be customized to fit individual applications using plain English rather than specific programming language tied to a complicated operating system.

Because measurement of contoured surfaces requires the collection of so many data points, there are special scanning software functions that aren't found in non-scanning software packages. For example, scanning software has a filtering ability to distinguish between changes in the surface direction and changes in the surface finish. A special filtering technique called "point smoothing" compensates for surface aberrations such as dirt and scratches. Software features for digitizing let the user mark the perimeter of the scan and lay out obstacles such as holes to be automatically avoided.

New measurement and inspection software links seamlessly to CAD/CAM systems and provides interactive graphics for the generation of mathematical models and the verification of sculptured surfaces. Easy-to-use reverse engineering functions available with CMM software allow the generation of CAD/CAM data directly from the part scanning files. The surface data points generated by the system can be directly imported into a CAD workstation to create a mathematical representation of the part geometry.

The art and science of metrology is constantly improving as manufacturers require more sophisticated, yet easier to use measurement and inspection systems. Current continuous scanning technology can provide a new perspective for moldmakers who are looking to use the power of dimensional data to improve processes and the overall quality of moldmaking operations. Stay tuned - there's more to come.