Advanced metrology for medical manufacturing

Dimensional metrology hardware and software for medical parts can be combined to create different types of systems with widely different costs. The systems vary in complexity, ranging from easy-to-learn manual optical comparators to fully programmable, multi-sensor CNC systems.

Higher-end products tend to be electro-mechanical metrology systems that 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 digital comparators – a vision metrology system that functions like an optical comparator but uses a DXF digital overlay instead of traditional overlays.

Depending on the application requirements, each metrology solution has its advantages. Quality control (QC) demands for medical parts are extremely high and warrant a close look at the most advanced metrology solutions.

Forgoing go/no-go

Medical parts inspection demands are increasing with tighter accuracies, traceability, and 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, with overlays, operator subjectivity is a significant factor. There are fine nuances to contend with, such as:

• On-screen placement
• Imprecise or worn Mylars
• Magnification
• Fatigue
• Data accuracy
• Complex part shapes
• Lack of detailed, variable data

Such issues force an inspector into the realm of judgment, which varies operator-to-operator.

Functional fit gages also present 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.

Neither overlays nor functional fit gages provide opportunities for a comprehensive audit trail and traceability process, which is paramount in medical manufacturing.

Repeatability, reproducibility

Any inspection program should 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 non-contact video, multi-sensor systems, and advanced software. Display readout and software measuring technology, such as Metlogix M3, provides full qualitative/quantitative profile analysis functions where an inspector compares a part profile against a nominal CAD model, obtaining a 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 automatically collects tremendous amounts of data that are archived and documented with date, time, lot number, and job number, to remove operator error from the equation. Not only can 100% inspection be realized, but inspection speed and throughput can increase with automated system measurement routines and, depending on the application, palletized multi-part fixture inspection tables.

Selection tips

To achieve the best outcome, consider the following when consulting with an expert in this field.

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 system with coordinate measuring machine (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 more economical if complete production runs are to be verified.
3. Tolerance requirements: A vision metrology system with a granite base may be required for meeting crucial tolerances for manufacturing medical parts. Tight tolerance may also indicate the need for higher-magnification optics.
4. Part size, weight: 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. Environment harshness: Most optical comparators are designed for typical machining environments, but varying temperatures, vibration, dust, and airborne contaminants can be present and need to be taken into consideration.
6. Man-made interface: Various interface options are available for 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 2-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 L.S. Starrett Co. – Metrology Div.
www.starrett.com
IMTS 2018 Booth #135532

About the author: Mark G. Arenal is the general manager at Starrett Kinemetric Engineering Inc. in Laguna Hills, California, a subsidiary of the L.S. Starrett Co. headquartered in Athol, Massachusetts. He can be reached at 949.348.1213 or marenal@starrett.com