GD&T Fitting Software Improves Spinal Implants

Lowell Inc., Minneapolis, MN, has been manufacturing orthopedic spinal implants since 1995. Founded in 1964, this private company with 70 employees uses 100 CNC machines to make hundreds of different variations of these critically dimensioned parts that must conform precisely to their design drawings. A key aspect of the process is the custom tooling they use to make the parts. Since the performance of the custom tooling – with their small arc radii and small surface areas – affects the success or failure of every part, getting the fabrication and qualification processes for the tooling under control was very important to Lowell. The company attributes SmartProfile interactive Fitting and GD&T software to 10 time improvements in the useful lifetime of the hundreds of custom tools Lowell uses to make its customers parts. Tools that used to last for 50 parts now produce thousands.

Like any manufacturer with its focus on the end products they produce, Lowell initially concentrated its energies on them and had its tooling made by an outside supplier. These intricate tools have tolerance radii of 0.0008", 0.0005", and 0.0002" and thickness of the tool profile geometries of 0.0150" to 0.0200". Numerous problems with tooling performance, including breakage and inconsistency in producing conforming parts, forced Lowell to bring tooling production in-house. They invested in new equipment such as a high-end Swiss tool grinder and created new, more detailed drawings of the tooling.
 

Importance of Metrology
Since Lowell determined that paying more attention to its tooling directly affected the quality and quantity of the spinal implants they manufactured, the investment in better equipment for making the tooling seems rather obvious. In addition, the best way to know if the tools are to design specification is to measure them properly. This is where the Lowell experience is different than might be expected. They do more than simply measure the tooling.

Lowell uses a variety of measurement systems and gages, but two of them are most important for inspection of their custom tooling. One is an OGP Avant ZIP 400 video measuring machine from Optical Gaging Products, and the other is a Brown & Sharpe CMM. The OGP machine uses analysis of magnified images of part details to measure dimensions, angles, and radii. The CMM measures larger features via touch probing. Together the systems provide the necessary measurements to confirm tool designs, but the interpretation of measurement results was inconsistent. It required a skilled engineer to interpret the large number of individual measurement results for each tool. The engineer could recognize which of those measured values was important and to what magnitude, but it is the cumulative relationships of all the measurements that is important. This is where the investment in SmartProfile software comes into play. It does more with the measurement data and makes it easier to validate tooling against design drawings.

SmartProfile is a 2D/3D interactive best fitting-software application that is used in a GD&T environment where part acceptance or rejection is required. It takes point clouds of data from part measurements performed on any measurement system, merges that data with the nominal CAD model of the part with GD&T tolerances, and automatically performs a results evaluation based on those tolerances.

As part of its project to improve its custom tooling, Lowell created detailed CAD files of the tooling. As part of its evaluation process, SmartProfile can import any number of popular CAD formats, including IGES, STEP, VDA, STL, and DXF files. Since it accepts measurement data from virtually any measurement system, SmartProfile readily fits into Lowell’s custom tooling process.

A Lowell engineer at a computer workstation uses SmartProfile to dimension the CAD model, define datums, and apply GD&T tolerances. Measured data from the OGP ZIP 400 video measuring system is imported and overlaid on the model. The imported data points may be aligned to the model manually or automatically. SmartProfile then compares the measured data with the nominal model and determines whether the measured feature is within tolerance. Colorful graphic displays, with whisker plots, confirm tool quality at a glance.

The key advantage of SmartProfile for Lowell is that it can evaluate surface profiles or geometric tolerances against Datum Reference Frames, with compound datums, pattern datums, datums at MMC/LLC, and composite tolerance zones. These extensive analyses are very important to Lowell, since their intricate custom tools have numerous exacting profile tolerances for their extremely small and complex geometries.

“Accurate confirmation of the tools is key to what SmartProfile provides,” says Jim Stertz, Lowell quality assurance manager. “Now the toolmaker gets confirmation of what he is making. Without SmartProfile we would still be guessing.”

Stertz learned about SmartProfile from Productivity Quality (PQI), the local OGP representative. During the course of assessing Lowell’s requirements, PQI performed several demonstrations. “Seeing demonstrations of SmartProfile by PQI on site – right then and there I knew we had to get it. There was no decision making beyond that. I could see its value,” Stertz says.

At the same time, the Lowell staff learned more about GD&T and what it could do for their processes.
Since SmartProfile compares measured data to the part CAD file, Lowell simultaneously updated its design drawings. The tooling design and new inspection processes, as well as new tool grinders, were implemented in parallel so they came into use at the same time.

SmartProfile is now a critical part of Lowell’s custom tooling process. Before implementing SmartProfile, two highly trained people evaluated the raw measurement data for every part. Today, six to eight Lowell employees use SmartProfile to evaluate tooling more accurately, and inspection takes 50% to 80% less time.
 

Optical Gaging Products Inc.
Rochester, NY
ogpnet.com