• Understanding ballbars
• Why should a designer care
• Keeping contouring accuracy

Medical parts that are machined must often be held to close tolerances. That means that the machine tool itself must also be kept in spec. Ballbar analysis is a proven method for determining machine tool capability and is a practical tool for assessing the contouring accuracy of CNC machines.

In theory, if a CNC machine is programmed to trace out a circular path and the positioning performance of the machine was perfect, then the actual circle would exactly match the programmed circle. In practice, many factors including wear can cause the radius of the test circle and its shape to deviate from the programmed circle. Accurately measuring the actual circular path and comparing it with the programmed path gives a measure of the machine’s accuracy. This is the basis of all ballbar testing.

So how does ballbar testing work? One ballbar kit can include, say, a 100-mm long ballbar assembly and 50, 150, and 300-mm long extension bars. By assembling the ballbar with different combinations of extension bars it is possible to carry out ballbar tests with 100, 150, 250, 300, 400, 450, 550, or 600-mm radii. First, a center pivot is placed on the machine bed. A “setting ball” is then placed on top of the pivot. A tool “cup holder” is placed in the tool holder. The cup holder is a hollow shaft, the same diameter as the setting ball. The machine table is moved until the setting ball and the tool cup holder line up. The center pivot is then clamped down. The setting ball is removed and the machine table is moved 100 mm (or a distance the length of the extension bar). Next, one end of the ballbar is placed on the top of the pivot. The other end of the ball bar goes against the end of the tool cup holder. The machine traces out the programmed circle, which is plotted on a laptop, letting users see any deviations.

Precision medical machining

Ballbars have been commonplace at precision machining operations for more than 20 years, but a Midwest-based medical device manufacturer, Nemcomed, which supplies implants, instruments, cutting tools, specialty instruments, cases, and trays for medical device OEMs, is showing how the latest wireless ballbar technology makes a big difference in doing fast capability checks on small machines, as well as establishing a benchmark on machine volumetric accuracy.

“Obviously, we have to meet FDA and ISO requirements,” says Eric Arnold, manufacturing engineer at Nemcomed. “But we also have special customer requirements, as well as concern because we know that our products may end up in someone’s body. [As] potential patients ourselves someday, we want to make the highest quality parts possible.”

Regulatory environment

As a manufacturer of medical devices, Nemcomed must comply with both the FDA 21 CFR Part 820 Quality System Regulation and the ISO 13485 Medical Device Standard. To qualify machines, the company had been using a traditional wired QC10 ballbar from Renishaw. “We test the X-Y, Y-Z and X-Z planes and the QC10 ballbar required a setup for each, so our setup time was about 1.5 hours,” says Arnold.

The switch to Renishaw’s new QC20-W wireless ballbar resulted in a positive impact on part quality and the bottom line. The new ballbar retains the principle of using a CNC circular program and software to quickly diagnose and quantify machine positioning errors including servo mismatch, stick-slip errors, backlash, repeatability, scale mismatch, and machine geometry, as well as providing an overall circularity error value. “The wireless ballbar requires just one setup — which takes less than 15 minutes — for testing in all three planes,” says Arnold. “More importantly, it doesn’t disturb the production setup, so we don’t have to re-set the machine when we go back to production mode. We remove the ballbar, insert a tool, and get back to making parts in minutes.”
Wireless operation works well for Nemcomed’s small machines, says Arnold. “Machine tool makers understand how lean operations must carefully use floor space, so new machine tools have smaller footprints,” he explains. “This results in less interior space to maneuver a wired ballbar, so wireless data transmission is a big advantage. Safety is also improved by the ability to fully close the doors on the machine during the tests.”

Surgical implants and instruments

Nemcomed manufactures knee, hip, shoulder, wrist, elbow, finger, and spine implants as well as pliers, cutters, and wire cutters. Each implant comes in five to six sizes with typical lot sizes 30 to 40 pieces. Many implants have complex features, such as curved or spherical components. The company also makes proprietary products and licenses them to customers. Its Flex-Shaft and Self-Retaining technologies, for example, are used in spinal procedures and hip, shoulder, and knee arthroplasties. The Flex-Shaft works with surgical screwdrivers, taps, and drills.

Parts are stainless steel, titanium, or cobalt chrome, and start as bar stock or forgings (implants). A cut-off machine processes the raw materials, which then move on to a milling machine or turning center, depending on complexity.

“Shortly after receiving the new ballbar, we had a machine go out of spec so we tested it, as well as having laser interferometer technicians come in and test it,” says Arnold. “The results were identical, so we learned then the ballbar would let us test our CNC machines quickly and with confidence.”

A little later, a large customer required verification of machine calibration. “The expense of bringing in the laser interferometer for every machine would have made it cost-prohibitive to supply this customer with parts,” Arnold explains. “We showed the company the results of the ballbar and interferometer tests, and it agreed that ballbar testing would meet its requirements. Basically, the QC20-W paid for itself after testing just two machines.”

Volumetric testing

The new ballbar design allows testing in three orthogonal planes through a single reference point, with a single setup, which allows the accompanying software to produce a representative measurement of volumetric positioning accuracy by correlating all three. A benchmark of volumetric accuracy is valuable because positioning errors can be compounded by simultaneous multi-axis motion during contouring. Volumetric accuracy is also important with large machining volumes and parts because tool path deviation is amplified across longer machine travels.

In 2010, Nemcomed added five new Citizen Swiss machines and a 5-axis Fanuc Robodrill. It already had several Mori-Seiki and Mazak 5-axis mills, Fadal 3-axis mills, Brother and Fanuc wire EDMs, and a Samsung 3-axis lathe. The company uses the ballbar on 20 machines, including all of the CNC mills and wire EDM machines. Maintenance engineers monitor results in a predictive maintenance program in a three-month timeframe.