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REDUCTION OF INSPECTION TIME MEDICAL MACHINING How to use precision GD&T to reduce inspection time for complex medical devices

Jan 4, 2022

Precision GD&T, if applied strategically to medical device components, can speed time to market by reducing inspection time across the three main areas of inspection: setup, in-process and final. This article explains how precision GD&T and profile tolerancing can reduce inspection time with the help of a case study. - Jim Stertz, Director of Technology, Lowell, Inc

With efficiency and speed to market being on top of mind, medical device companies are searching for the best way to quickly and accurately manufacture devices that function as designed. For years, inspection has been critical to a seamless manufacturing process, but it can often be headache-inducing, especially when it comes to complex features with tight tolerances. As new technologies fundamentally shape metrology and measurement science, they also create new opportunities to improve the inspection process and alleviate these headaches. At the centre of this change are precision Geometric Dimensioning and Tolerancing (precision GD&T) and profile tolerancing.

Precision GD&T applies geometric dimensioning and tolerancing to optimise the design and function of devices. It allows the maximum tolerances to be used for features while still conforming to the design intent. Profile tolerancing, inherent to precision GD&T, is often one of the best methods to optimise a design when compared to conventional linear +/- dimensioning. Simply put, profile tolerancing defines uniform upper- and lower-level boundaries around the desired physical geometry. It simplifies the tolerancing scheme and delivers results for device companies, including reduced inspection time, improved accuracy and reduced measurement error. This is accomplished by providing a means to more efficient, effective measurement and quality control.

A leading medical device company was looking for the best approach to manufacture a cervical plate efficiently in one such illustration. Lowell advised using precision GD&T, specifically profile tolerancing, in place of a more conventional linear +/- dimensioning scheme. The example in Table 1 reveals the significant time savings possible — in both dimensioning their drawing and in inspection-related activities — with precision GD&T. Profile tolerancing consumed about one-third of the time of linear +/- dimensioning.

Precision GD&T: New technology drives adoption over the past decade

Profile tolerancing dates to the 1930s but was relatively unused until a decade ago when new inspection software and PC-powered adoption. Equipped with the tools for profile analysis, manufacturers were eager to apply this cutting-edge technology to every medical device. In recent years, however, manufacturers have changed their approach based on lessons learnt. They’ve narrowed their focus to the most complex features of medical devices, which often require the most rigorous inspection. This shift allows medical device manufacturers to take advantage of this technology where it’s most effective and best optimises the production process.

Another change for precision GD&T has been the increasing availability of profile tolerancing software. Advances in 3D measurement tools with coordinate measuring machines (CMMs) and measurement analysis software combined with the nominal geometry of 3D solid models eliminate many of the challenges in the inspection department. Now, starting with the CAD model as its base, engineers, manufacturers and inspectors can take advantage of precision GD&T in place of 2D methods, such as micrometres and optical comparators. Most CMMs today include basic profile analysis software that generates 2D versions of the actual part deviation relative to the CAD model. Dedicated profile analysis software, such as SmartProfile® from Kotem Technologiesii, elevates these part deviations to 3D. Via CMM or dedicated profile analysis software, data gathered from a part is fit to the program for instantaneous manufacturing feedback at all stages of inspection.

Setup inspection: From hours to minutes with profile tolerancing

The impact of profile tolerancing starts with the fully dimensioned drawing that a medical device company shares with its manufacturer. For the cervical plate in the example, profile tolerancing cut the time required by more than half compared to linear +/- dimensioning, in this case from 480 minutes to 180. Time savings continue at the manufacturer level with setup inspection, also known as first article inspection. Setup inspection verifies that the manufacturing processes’ first part in a lot conforms to the drawing requirements. The example shows how dramatic the difference can be, with the profile eliminating more than 330 minutes from the linear +/- dimensioning process.

Setup inspection is an essential first step to ensure the process is set up correctly and that devices and components will be made accurately. Any adjustments to the machining process will be made based on this inspection, so accuracy is critical. Before the first part is made, the CAD models are uploaded into software to create the 3D profile. This ensures the CAD model is true to the engineer’s design intent. Figure 1 is an example of the complexity that a linear +/- tolerancing scheme creates for a theoretical ring with complex features. In this case, the complex features are a series of small arc radii. There are 50 angles, 150 radii, 50 peak to peak, 50 valley to valley and 1 outside radius — over 301 measurement results. Setup inspection for this part could take up to two hours, as each feature is inspected for size, form, orientation and location.

In contrast is Figure 2, where the drawing is converted to a 3D profile. There is only one measurement result. Inspection time for this part drastically reduces from about two hours to about five minutes. Beyond time savings, an additional benefit to profile tolerancing is greater confidence that parts conform to the requirements. Unlike linear +/- dimensioning, profile tolerancing harness powerful software to help machinists and inspectors compare a device’s dimensions more accurately against the drawing.

A longstanding risk in setup inspection — especially for small, complex geometries — is misinterpreting a dimension or measurement result. This could give inaccurate feedback, resulting in a harmful adjustment to the manufacturing process, including changes to the tooling, tooling path or the machine program itself. When unnecessary changes are made, driven by false negatives or false positives, the functionality and design of the part can be compromised. Profile tolerancing helps reduce this risk.

In-process inspection: Profile tolerancing maintains efficient, accurate production

An in-process inspection ensures the manufacturing process, once verified in setup inspection, is maintained throughout production. Devices coming off the line are checked at intervals to ensure process stability. Profile tolerancing simplifies these steps of inspection. With traditional linear +/- tolerancing, a machinist or quality engineer must scan lines of dimensions and measurements on a printout and compare them back to a drawing to see if there is any deviation (see Figure 3). Even with printouts from advanced programs such as PC-DEMIS™iv, reports can be as large as 20 pages if there are multiple critical dimensions. This creates the need for the machinist and inspector to review potentially hundreds of dimensions printed in excruciating detail. While this mountain of data is reviewed, parts continue to be made. This creates a potential gap between identifying an error and removing nonconforming parts from the line.

This gap is reduced with profile tolerancing, which provides visual feedback of an inspected part via a 3D CAD model. The example in Table 1 shows the difference: 120 minutes in linear vs 20 minutes in profile for dimensional inspection, with an additional 17 minutes saved per round of inspection results interpretation. With the 3D CAD model, as in figure 4, a machinist can see at a glance if a dimension or feature is nonconforming, decreasing the time between identifying an error and halting production to amend the process. This reduced inspection time helps overall process efficiency, which can improve time to market.

In Figure 4, areas in or nearing deviation from design are marked in red and blue for quick identification. An area of red indicates a deviation into the plus material condition, and the blue areas indicate a minus condition. With this visual feedback, a machinist working with an inspector can quickly diagnose the problem as an incorrect offset, fixture problems or excessive clamping pressure.

Final inspection less paperwork and faster turnaround with profile tolerancing

The typical final inspection process for medical devices requires the review of a random sample of the lot to ensure the products are made correctly and conform to requirements. Statistical sampling tables determine the quantity of parts needed for testing. These tables are based on acceptance sampling, which indicates how many products need to be reviewed to reliably predict an entire batch conforms to requirements.

Traditionally measurement, testing and inspection of the final elements would be completed manually against linear measurements. That data is then used to create the associated inspection report paperwork. This entire process is greatly improved by utilising CMM technology. Instead of manually preparing an inspection report, CMM data is printable for use in an inspection report and can be submitted for external review. In the example from Figure 1, the time savings in the final inspection stage of the example showed a total reduction of inspection time by 60 minutes when using a profile, with an additional 17 minutes saved per round of inspection results interpretation. Medical device companies who use profile tolerancing have also seen this project data streamlines their internal acceptance process. A manufacturer will compile the profile data into a project file shared with the customer. This first-hand data can be reviewed in much less time than traditional linear +/- data.

Define a precision GD&T strategy to reduce Inspection time

Precision GD&T can be strategically applied to medical device components to speed time to market by reducing inspection time. These time savings are seen across the three main areas of inspection: setup, in-process and final.

With expertise in cardiovascular and orthopaedic implants for spine, trauma and extremities, Lowell has more than 10 years of experience in applying profile tolerancing to complex medical devices. Everything starts with design intent — how the component or assembly is designed to function — then the latest technologies are utilised to create high-quality, manufactured products.

Image Gallery

  • Table 1: Comparing time estimates for steps in linear +/- dimensioning vs profile tolerancing

    Table 1: Comparing time estimates for steps in linear +/- dimensioning vs profile tolerancing

  • Figure 1: An example of the complexity that a linear +/- tolerancing scheme creates for a theoretical ring with complex features

    Figure 1: An example of the complexity that a linear +/- tolerancing scheme creates for a theoretical ring with complex features

  • Figure 2: A drawing that has been converted to a 3D profile

    Figure 2: A drawing that has been converted to a 3D profile

  • Figure 3: With traditional linear +/- tolerancing, a machinist or quality engineer has to scan lines of dimension and measurements on a printout and compare back to a drawing to is if there is any deviation

    Figure 3: With traditional linear +/- tolerancing, a machinist or quality engineer has to scan lines of dimension and measurements on a printout and compare back to a drawing to is if there is any deviation

  • Figure 4: Areas in or nearing deviation in the design is marked in red and blue for quick reference. Red indicates a deviation into the plus material condition and blue indicates a minus condition

    Figure 4: Areas in or nearing deviation in the design is marked in red and blue for quick reference. Red indicates a deviation into the plus material condition and blue indicates a minus condition

  • Jim Stertz
Director of Technology
Lowell, Inc

    Jim Stertz

    Director of Technology

    Lowell, Inc

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