Ballbar analysis is a proven method for determining machine tool capability and is the most practical, convenient and comprehensive tool for assessing the contouring accuracy of CNC machines. Ballbars have been commonplace at precision machining operations for more than 20 years, but one Midwest-based medical device manufacturer 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 the volumetric accuracy of those machines.
A demonstration of the QC20-W telescoping ballbar, not to be confused with the fixed length ballbars used for CMM (coordinate measuring machine) calibration, provides a simple, rapid check of a CNC machine tool’s positioning performance to recognised international standards (e.g. ISO, ASME etc) allowing users to benchmark and track the performance of their machines and to quickly diagnose problems that require maintenance.
Nemcomed, Inc. (Fort Wayne, IN), a division of Avalign Technologies (Lake Forest, IL), is a full-service supplier of implants, instruments, cutting tools, specialty instruments, cases and trays for medical device OEMs. The Avalign strategy is to be a “one stop shop” for OEM providers to the orthopedic, spine and trauma sectors – aiming to supply everything a physician needs to perform implant procedures.
Ensuring that machine tools and processes are capable of producing parts to spec is a goal shared by both their customers and their 450+ employees, as well as federal regulatory agencies. “Obviously, we have to meet FDA and ISO requirements,” says Eric Arnold, a manufacturing engineer at Nemcomed. “But we also have special customer requirements, as well as personal interest and pride, knowing that our products may end up in someone’s body. As potential patients ourselves someday, we want to make the highest quality parts possible.”
The company manufactures approximately 1000 different parts that are either implants or the tools used during implant procedures. These parts include implants for knees, hips, shoulders, wrists, elbows, fingers and the spine that are configured in “family sizes” with five to six sizes of each part. Many of these implants run typical lot sizes of 30-40 pieces with complex features, such as curved or spherical components. The tools built here include pliers, cutters and wire cutters. All of these parts and tools are supplied to the top orthopedic OEMs.
The Fort Wayne plant also makes proprietary products that are developed through internal research and development and licenses them to customers. Its Flex-Shaft and Self-Retaining technologies, for example, are patented and used in a variety of spinal procedures and hip, shoulder and knee arthroplasties. The Flex-Shaft is also applicable to surgical screwdrivers, taps and drills.
These parts are generally made of stainless steel, titanium or cobalt chrome and start as bar stock or forgings (implants). Raw materials are processed through a cut-off cell, then move on to a mill or turning center, depending on complexity.
As a manufacturer of medical devices, Nemcomed must comply with both the FDA 21 CFR Part 820 Quality System Regulation and the SO 13485 Medical Device Standard. To qualify machines, the shop had been using a traditional wired QC10 ballbar from Renishaw Inc. (Hoffman Estates, IL). “We test the X-Y, Y-Z and X-Z planes and the QC10 required a setup for each, so our setup time was about 1.5 hours,” said Arnold.
The Fort Wayne plant acquired a new QC20-W wireless ballbar two years ago that immediately had a positive impact on part quality and the company’s bottom line.
This linear displacement sensor-based tool retains the principle of using a CNC circular program and powerful software to quickly diagnose and quantify machine positioning errors, including servo mismatch, stick-slip errors, backlash, repeatability, scale mismatch and machine geometry.
The tool also provides an overall circularity error value and also adds new capabilities. “The wireless ballbar requires just one setup – less than 15 minutes – for testing in all three planes,” notes Arnold. “More important, it doesn’t disturb our production setup, so we don’t have to reset the machine when we go back into production mode. We remove the ballbar, insert a cutting tool and get back to making parts within minutes.”
Wireless operation is ideal for Nemcomed’s small machines. “Machine tool makers understand how much lean operations such as ours value floor space, so the new machine tools are designed with smaller footprints that result in less interior space to maneuver a wired ballbar. This makes wireless data transmission a big advantage,” explains Arnold. Being able to fully close the doors on the machine during the tests also improves plant safety.
“Shortly after receiving the new ballbar, we had a machine go out of spec so we tested it, as well as having the laser interferometer folks come in and test it,” recalls Arnold. “Our results were identical, so we learned then that the ballbar would allow us to test our CNC machines quickly and with an extraordinary level of confidence.”
Being able to test quickly and accurately with this sort of confidence attracted a large, quality-focused customer that 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,” states Arnold. “Instead, we showed them the results of the ballbar and interferometer tests and they agreed that ballbar testing would meet their verification requirements. The QC20-W basically paid for itself after testing just two machines.”
This wireless ballbar also brings the unique advantage of testing in three orthogonal planes through a single reference point with a single setup. This allows the accompanying software to produce a representative measurement of volumetric positioning accuracy by correlating all three.
Arnold explained how this benchmark of volumetric accuracy is valuable to Nemcomed because positioning errors can be compounded by simultaneous multi-axis motion during contouring. (Note: Volumetric accuracy is also important with large machining volumes and parts, where tool path deviation is amplified across longer machine travels.)
In August 2010, Nemcomed completed a 10,000 sq ft addition to the Fort Wayne facility to consolidate another plant’s operations and add space for new machine tools. These included five new Citizen Swiss machines and a 5-axis Fanuc Robodrill to complement its lineup of Mori Seiki and Mazak 5-axis mills, Fadal 3-axis mills, Brother and Fanuc wire EDMs and a Samsung 3-axis lathe.
The shop uses the ballbar on 20 machine tools, including all of its CNC mills and wire EDM equipment. Maintenance engineers monitor results by using a predictive maintenance program that tracks a three-month timeframe for early detection of errors for optimum efficiency in scheduling maintenance and repairs.
The ballbar comes with a system case that provides spaces for the most popular accessories, making it easy to transport. “We can take this to any of our four global manufacturing facilities, set it up quickly and get the machine qualifications we need,” says Arnold. “Knowing the machine tool’s capability before it cuts parts allows us to minimize scrap and machine downtime. That gives us high part quality and productivity while keeping our manufacturing cost down. That’s what lean manufacturing is all about: greater customer value.”
The company completed another 14,000 sq ft expansion in November 2011 and, as business continues to grow, Arnold said use of the ballbar will also grow.
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