A knowledge of the geometry of screw threads is key to the successful manufacturing of medical screws. OEMs also should understand the thread-gauge inspection process.

Jim Speck

October 1, 2007

11 Min Read
Part Geometry: The Art of Orthopedic Screw Inspection

PART INSPECTION

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Inspection for bone screws is a complicated process. Using a thread gauge measurement approach may enable a streamlined manufacturing process.

Controlling the manufacture of bone and pedicle screws requires a deep understanding of the mechanics of screw thread technology. The geometry, inspection, and control of medical component screw threads are key to gaining an advantage in today's competitive medical products market.

Part geometry is often measured and controlled using optical comparator overlay charts combined with micrometers. However, device companies may benefit from a measurement approach that sets specification limits at the manufacturing site. Thread gauges can be used for medical applications from bone screws to pedicle screws. This approach enables a faster, more efficient process for Swiss machining precision threaded parts.

Screw Threads

It may not be coincidence that screw threads share a structure with the DNA double helix. The helix is a geometric form at the root of ancient science dating from the time of Archimedes. It speaks volumes that this time-tested geometric form is also the foundation of life science. The helix-like shape is also found in orthopedic hardware such as in the threads of bone screws. Although there are many proprietary thread forms, certain geometric features are common to all bone screws. These geometries are as follows:

  • Major diameter.

  • Minor diameter.

  • Pitch diameter.

  • Helix angle.

  • Lead and trailing flank angles.

  • Root radii.

  • Taper.

  • Lead.

  • Circularity.

With such geometric commonality, it is useful to understand how the bone screw thread forms into the site. It is also critical to know what geometric features, techniques, and equipment can prove useful in manufacturing process control.

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Figure 1. (click to enlarge) The interaction of screw parts. D is the major diameter, d is the minor diameter, and L is average interflank length.

Figure 1 shows how the leading and trailing flank angles interact. These angles partially define the area that contains the site material when the screw is torqued into place by the surgeon. Radially, the major and minor diameters also geometrically define and control this area. Figure 2 highlights the interflank contained area (cross-hatched section).

Measuring and Controlling Geometry

Traditionally, manufacturers have used optical comparator overlay charts combined with micrometers to measure and control general bone screw geometry and major diameter. However, a measurement approach that sets specification limits is increasingly being used because of its accuracy and efficiency.

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Figure 2. (click to enlarge) The interflank contained area geometrically defined by the minor (d) and major (D) diameters.

For example, an orthopedic hardware manufacturer may want to reduce the time for tooling changes and to limit the decisions made by operators. Such a firm could switch from using an optical comparator chart and a 0–1.00-in. digital micrometer to a two-station indicating roll gauge. The manufacturer could thereby accomplish three goals, as follows:

  1. To decrease time needed to inspect each bone screw.

  2. To have true numerical values of part screw-thread dimensions automatically stored on the company server.

  3. To make process statistics available for analysis and continual threading process improvement.

Using the measurement approach, the manufacturer could set the screw-thread geometry specification limits into the screw-thread indicating gauge at the manufacturing site. The manufacturing operator could insert the threaded component into a gauge station, and the gauge would thus indicate the actual screw-thread measurement.

In the screw-thread indicating gauges, the key screw-thread features, which have maximum influence on screw performance, are isolated and controlled. A mathematical calculation of the volume-contained area from Figures 1 and 2 could be written as

Ac = 0.7854(D2d2)L    (1)

where Ac is the interflank area, D is the major diameter, d is the minor diameter, and L is average interflank length.

Given that the contained volume has a direct influence on the holding strength of the installed and tightened screw thread, this equation shows that major and minor bone screw diameters have an exponential effect on the interflank-contained area.

During the manufacturing process for machining screw-thread forms in bone screw manufacturing, minor diameter is a feature that has a tendency to vary at a faster rate than other geometries. This is because the smallest sectional area of the machine tooling generates the minor diameter. By design, bone screws require hard-to-machine materials. This means that the difficulties involved in machining threads are exponential.

One way to alleviate these difficulties is to control manufacturing using the measuring approach. By using an indicating gauge to inspect the minor and major diameters, an orthopedic manufacturer can acquire a near-real-time measurement of these exponential geometric factors. Process statistics enable the bone screw geometry to be closely controlled. For example, minor-diameter is best measured with low-flank-angle minor-diameter rolls that clear bone screw flanks. The rolls are particularly useful on twin-lead bone screws. Major diameters are similarly measured with major-diameter rolls that quickly report numerical data for both diameter and circularity. By measuring these in tandem, manufacturers can ensure that optimal bone-screw-thread geometry is achieved.

Reducing Gauge Costs

Gauge efficiency means identifying the information needed to optimize the part quality while reducing both the fixed and variable costs of bone screw inspection.

In a bone screw application, the pull-out strength of the screw in situ is indicated by the constrained area bounded by screw-thread diameters (both major and minor) and by the thread's leading and trailing flanks. Because of the complex geometry, it is useful for manufacturers to be able to measure these parameters directly. Using modular gauge frames and interchangeable gauge rolls can minimize fixed costs. Gauge use or variable gauge costs can be minimized when key thread-geometry inspection reports are available in a timely manner.

Another example would be a medical hardware manufacturer that supplies small-diameter unified screw threads. In this example, the existing fixed-limit ring gauges were prone to excessive wear rates, which resulted in out-of-tolerance gauge measurements and, thus, out-of-tolerance threads. The screw-threaded component is manufactured from a high nickel alloy with difficult machining characteristics. Adding to the screw-thread manufacturing problem is the fact that production lot sizes are relatively low but still require frequent machine setups. The company was experiencing numerous discrepancies caused by different split-ring gauge settings and wear of the ring gauges during use. It needed a method to improve control and inspection.

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Table I. (click to enlarge) Sample results of a gauge repeatability and reproducibility (R&R) study. The most important number for assessing variation is the comparison to tolerance
(approximately 10.4% in this example).

When the company adopted functional segments and pitch diameter rolls to inspect the critical component threads it increased throughput on the manufacturing floor and in receiving. To verify the accuracy of thread measurement data generated by this equipment, a gauge repeatability and reproducibility (R&R) study was conducted on a large-diameter threaded component. The results of this study are shown in Table I.

An analysis of variation shows the gauge R&R results approaching 10%; however, it is better to have variation approaching 5%. A major benefit of the measurement approach in this medical application is that it limited discrepancies in thread-size reading. That means rework and calibration of thread gauges were also reduced. In addition, numerical thread-size values for Go (or functional size) and No Go (or pitch diameter) on specific parts in a production run and on geometric threaded sections located on individual parts became available.

Internal Medical Thread Inspection

A final application of the measurement approach involved an internal buttress thread form. This particular application is found on the manufacturing floors of many orthopedic hardware manufacturers. The buttress thread form uses leading and trailing flank angles of 45° and 7°. The angular differential provides the surgeon with performance advantages in load applied and decreased radial stresses. This thread with the 45°/7° thread flank angles has demanding characteristics and the very short thread length of engagement results in each pitch of thread being critical. The component also usually has an interrupted thread design. (Its appearance is similar to a tuning fork. The internal buttress threads are on the inner surfaces of the two prongs.)

The company's initial thread inspection practice used a Go and No-Go work plug set. Again, high Go plug wear rates caused both a high scrap rate of the plugs and uncertainty in the true functional diameter of the internal buttress threads. A conventional and standardized thread inspection procedure for commercial-quality screw threads is available in ANSI/ASME System 21. However, attribute gauges such as the above thread working plug and ring gauges are difficult to standardize because of wear of the thread flank and major diameter of the Go member. There is also uncertainty in the individual thread-pitch diameter sections' ability to contact the No-Go member. Depending on the level of inspection intensity, variables for thread gauging, which are specified as ANSI B1.3M Systems 22 and 23, can resolve this uncertainty. The indicating gauges generate actual numerical values of screw-thread sizes for the manufacturing operator, quality assurance personnel, and management.

In the internal buttress thread application above, a three-station variables system measures pitch diameter with a cone and vee set of indexing segments. This cone and vee geometry is recognized in ANSI specifications B1.3M, B1.2, and Federal Standard H28. The examples above illustrate the adaptability of cone and vee geometries. The thread gauge element geometry can closely indicate a medical screw thread's pitch diameter and also pinpoint the minimum material location and conformance to the manufacturer's screw-thread specifications.

Functional diameter is measured with full form segments, simulating the Go working plug gauge but with much faster part entry and exit from the Go member. Not having to turn on and off components, the gauge repeatedly reduces wear caused by component-and-gauge frictional contact.

The key principle of variable thread gauging is that if leading and trailing flank angles, lead and helix angles, and lead and circularity of each thread diameter are true with no variance, the differential between pitch and functional diameter equals zero. The following equations express this principle for internal and external threads:

fdIT = pd – Σ tgv    (2)

fdET = pd + Σ tgv    (3)

where fdIT is internal thread functional diameter, fdET is external thread functional diameter, pd is pitch diameter, and tgv is the sum of thread geometry variance.

For ideal screw-thread form control for both internal and external threads, a variables measurement should result in no differential. In real-world precision threads, many manufacturers have found that analysis and control of the thread form are the vital factors that comprise a screw-thread lot's average and peak thread form differential. Thread form control enables an efficient increase in quality and a reduction in thread manufacturing cost. The decrease in cost occurs because the manufacturing process, which causes the screw thread form geometric variance, also causes suboptimal thread tool life.

The internal buttress thread example demonstrates this principle. The buttress has 1-mm-pitch threads that show a differential of more than 70% of the screw thread's tight tolerance for minimum and maximum pitch diameter. Analysis indicated that the threading tool was deflecting, or being pushed away from its intended path and that the tool's diameter was not generating the specified thread lead angle. Both thread flank and thread lead angles were experiencing high variance.

In controlled experiments, the company used a thread gauge to run pilot production with different tool diameter and tool holder changes. The experiments enabled the firm to measure pitch and functional diameters quickly and precisely, and then calculate the differentials.

The final setup resulted in much better form control, as indicated by much smaller differentials; increased tool life; and reduced CNC downtime for tool changes. A good industry rule of thumb is for differentials to be targeted to 40% of a pitch-diameter tolerance. The former high-differential tooling setup focused cutting forces on small areas of the threading tooling, thereby increasing their threading stresses.

A final step for this application measured only the major diameter of the internal thread. This measurement was accomplished by using segments of three-pitch length of engagement with flank angles. The measurement is a key piece of information because the major diameter is the first component that wears down on the screw. The final step reduced, by a designed number of degrees, the ring setting's maximum major diameter.

Conclusion

In each of the three applications outlined in this article, the thread inspections generated numbers that were used for analysis, as well as process and part improvement. In addition, the master setting gauges used to zero the gauges could be set to minimum, maximum, or mean dimensions to the user's specification. Because the master is used specifically for gauge setting, the process provides a stable, traceable, and reliable reference.

Medical device makers need a thorough understanding of the thread-screw inspection process. Gauges designed for medical component screw threads increase speed of thread inspection, provide increased knowledge of screw-thread quality, and put the optimum performing threaded component in the hands of the surgeon.

Jim Speck is an application engineer for the Johnson Gage Co. (Bloomfield, CT). He can be contacted at [email protected].

Copyright ©2007 Medical Device & Diagnostic Industry

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