Metrology for Medical Manufacturing


ADVANCEDMETROLOGYSOLUTIONSOFFERADVANTAGESTOMEDICALMANUFACTURERS Know Your Options When Selecting a Metrology System to QC Medical Parts Author: Mark G. Arenal, General Manager, Starrett Kinemetric Engineering, Inc.

The L.S. Starrett Company - Metrology Division 26052 Merit Circle, Suite 103 Laguna Hills, CA 92653

Dimensional Metrology Systems facilitate critical measurement of medical parts, but the selection of a metrology system can be challenging. A wide range of options exist for hardware and software which can be combined to create different types of systems, sometimes at widely different costs. The systems also vary in complexity, ranging from easy-to-learn manual optical comparators to fully programmable multi-sensor CNC Systems. Many of the higher-end products are electro-mechanical metrology systems which may combine optics with precision mechanical motion, linear encoders and digital image and data processing. These systems include optical comparators, vision (or video) metrology systems and a new product category of digital comparators. A digital comparator is a vision metrology system that functions like an optical comparator, but utilizes a DXF "digital overlay" instead of traditional overlays. Depending on the application requirements, each metrology solution has its advantages. QC demands for medical parts are extremely high and warrant a close look at the most advanced metrology solutions.

The growth of plastics and polymers is exponential. Plastics are used everywhere from in consumable materials to life-saving medical devices where product quality is paramount.

FORGOING GO / NO-GO Medical parts inspection demands are increasing with tighter accuracies, traceability and the need for 100% inspection. Many manufacturers have been accustomed to using go-no-go methods of gaging, such as the use of overlays in optical measuring or functional fit gages. However, there are shortcomings associated with these methods. With overlays, operator subjectivity is a significant factor. There are fine nuances to contend with, such as on-screen placement, imprecise or worn Mylar's, magnification, complex part shapes and especially the lack of detailed, variable data. Such issues force an inspector into the realm of judgment, which varies from operator-to-operator. Fatigue and data accuracy also comes into play. Functional fit gages also present their share of challenges. The gage development process is usually lengthy and expensive, with limited flexibility when parts change. And again, no variable data is attainable in the inspection process. Moreover, both overlays and functional fit gages provide no opportunity for a comprehensive audit trail and traceability process, which is paramount in medical manufacturing.

GR&R VIA ADVANCED METROLOGY SOLUTIONS Ultimately, the goal in any good inspection program should be to attain maximum gage repeatability and reproducibility (GR&R) and in the process, provide comprehensive, data for statistical process control (SPC) and traceability.

Reliable GR&R is possible with today's non-contact video and multi-sensor systems and their advanced software. Current display readout and software measuring technology such as Metlogix M3 provides full qualitative/ quantitative profile analysis functions where an inspector can compare a part profile against a nominal CAD model and obtain an actual graphic representation of any deviation from the CAD file. The system automatically finds and tracks the edge, continuously comparing it to a 2D profile, and superimposes the edge to a CAD model. In this scenario, an operator quickly, easily and automatically collects tremendous amounts of data that are all archived and documented with date, time, lot number, job number and so on, removing operator error from the equation. Not only can 100% inspection be realized, but inspection speed and throughput can also be dramatically increased due to automated systemmeasurement routines and depending on the application, palletized multi-part fixture inspection tables. Indeed, today's technology offers far more than error prone traditional methods.

The AV Automatic Vision Metrology Systems use software to control all features of Video Edge Detection and multiple channel Fiber Optic or LED illumination.


APPLICATION VERSATILITY New technology exists in the form of a large field-of-view benchtop vision measurement solution that is capable of being used in either a vertical or horizontal orientation, features a high-resolution digital video camera and precision optics for accurate, large FOV measurements. The latest system lends itself to an extremely wide array of applications from flat parts to turned, threaded and complex shaped parts such as orthopedic joint implants. A main operator interface displays a live video image with touch-screen enabled software measurement tools and graphical digital display of measurements. A part image can be resized using pan and pinch zoom. Measurements can be taken by simply touching a feature on the monitor screen. A wireless keyboard and pointing device can be used to enter file names and target key functions. Software includes 2D geometric functions such as points, lines, circles, arcs, rectangles, distances, slots, angles and skew, and utilizing the part design DXF/ CAD file digital overlay makes part inspection simple.

New, innovative HVR100-FLIP large field-of-vision (FOV) Bench-top Vision Measurement System is capable of being used in either a vertical or horizontal orientation, features a high- resolution digital video camera and minimal optical distortion for accurate FOV measurements of up to 90mm (3.65").

SELECTION TIPS For the best outcome, it is prudent to consult with an expert in this field. In addition, the following tips and considerations aid the selection process. 1. Measurement complexity . A 2D optical comparator may suffice for simple measurements and visual comparison, but more complex medical parts may require a multi-sensor metrology systems with CMM style touch-probe capability. 2. Measurement throughput . A manual system may be an option for prototypes, occasional and short run measurements by the QC department, but an automatic, CNC system is a more economical solution if complete production runs are to be verified.

3. Tolerance requirements . A Vision metrology system with a granite base may be required for meeting right tolerances, crucial for manufacturing medical parts. Tight tolerance may also indicate the need for higher magnification optics. 4. Size and weight of parts . A benchtop optical comparator or video metrology system may be appropriate for small parts, while a larger, floor-standing system can be helpful for large and heavy parts. 5. Harshness of environment . Most optical comparators are designed for typical machining environments sometimes found in machine shops. Varying temperatures, vibration, dust and oil particles can be present and need to be taken into consideration. 6. Man-made interface . Various interface options are now available for both optical comparators and video- based measurement systems including digital readouts, touch-screen PC solutions and rack-mounted computers with monitors/ software. 7. Software features . Capabilities can range from a simple two-axis readout to 3D multi-sensor capability with a rich set of measurement software tools, CNC control, statistical packages, DXF CAD file import and export, touch-screen operation with Windows Operating Systems and network connectivity.

The QC of medical devices is critical to ensure compliance to federal regulations.

The HDV Horizontal Digital Video Comparators combine the best features of a horizontal optical comparator and a vision metrology system.

3 Bulletin 3025 - Advanced Metrology Solutions Offer Advantages to Medical Manufacturers 01/18 The L.S. Starrett Company 2018 ©

VISION METROLOGY SYSTEM CALIBRATION When you invest in a Vision metrology system, calibration is a vital part of a scheduled preventative maintenance program. Some metrology system manufacturers offer calibration services which provide a calibration certificate, plus cleaning, lubrication and a complete functional test of all subsystems and critical components. These include the stage and optical video probe, which are part of all video metrology systems and also the touch probe and laser probe that may be part of a multi-sensor video metrology system. If any problems are found, they may be corrected on-site by the manufacturer.

The industry standard interval for calibration of a metrology system is one year; however, the actual need for calibration depends on machine usage. A calibration interval longer than one year may be possible, if for example, the machine is being used only a few hours per day during a single shift. A calibration interval shorter than 1 year may be required if the machine is used during more than one shift and tight tolerances, such as those on medical parts, have to be met. An appropriate calibration interval should be specified by the user's Quality Department policy. Calibration is also called for if the machine has been moved, if it has undergone temperature extremes, has been subject to mechanical shock, or if it is producing suspect readings. Measurement errors higher than factory specifications can reflect a mechanical or software issue.

About the Author Mark G. Arenal is the General Manager of Starrett Kinemetric Engineering, Inc. in Laguna Hills, CA, a subsidiary of the L.S. Starrett Company headquartered in Athol, MA. He oversees the development and manufacturing of precision optical and video measurement systems which are used in many facets of manufacturing including in the medical sector. Prior to this, Mark was involved in the creation and development of two companies in the measurement and precision motion technology space. The 150mm linear calibration artifact should be available. It consists of a 24 x 175mm borosilicate glass sheet with a chrome on glass pattern that has been optimized for machine vision recognition. It is accompanied by a calibration certificate which lists the position error of each fiducial. To verify Z height accuracy, precision gage blocks are placed on the X-Y stage, and their height is verified using a touch probe or dial indicator. Some vision metrology systems have a Z ("E1") height error spec of 2.5 + 5L/1000, where L is height to be measured. With such a system, the height of a 50mm gage block should be measured with an error less than 2.75μm. # # # # # For example, to verify X-Y accuracy, a 150mm linear calibration artifact with seven precision fiducials at 25mm intervals is placed at six specified locations on the stage, and the vision system's metrology software is then used to make ten runs for each location. The system is considered to be within factory specifications if the measured position of each fiducial is within the manufacturer's error spec. Some vision metrology systems have an X-Y ("E2") error spec of 1.9 + 5L/1000, where L is distance to be measured. For fiducials separated by 150mm, the measured separation should not have an error larger than 2.65μm.

Bulletin 3025 - Advanced Metrology Solutions Offer Advantages to Medical Manufacturers 01/18 The L.S. Starrett Company 2018 © 4

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