September 2011 / Volume 63 / Issue 9|
A Fine Grind
By Bill Kennedy, Contributing Editor
|Technology, experience and attitude breeds grinding success—on a very small scale. |
Courtesy of All images: B. Kennedy
An eastern Pennsylvania grinding shop is combining high-tech grinding equipment, an enthusiastic and disciplined approach to problem solving and more than 50 years of grinding experience to handle the challenges of grinding microscale parts.
In 1957, Meron Shegda picked up a used Cincinnati centerless grinder at an auction and began to take on freelance grinding jobs. In 1959, he founded M&S Centerless Grinding Inc. Beginning with a 1,200-sq.-ft. shop, the business has grown into an 11,400-sq.-ft. facility in Hatboro, Pa., employing 20 people, including 14 on the shop floor.
M&S Centerless Grinding specializes in cylindrical grinding, including OD and ID work, as well as parts chucked between centers. Meron Shegda’s son John, who began to run the company in 1990, said the M&S focus is on centerless grinding and its derivatives. About half the company’s business involves grinding microscale parts for medical applications, including implants, hypodermic tubes, device components and cardiac guide wires. A quarter of the shop’s work is what Shegda calls “ultraprecision” grinding—mostly for aerospace and high-tech customers—that involves processing parts up to 0.750" in diameter with diameter tolerances to ±0.5µm and cylindricities to ±1µm. General grinding makes up the rest of the shop’s business, including small parts and large parts, such as 14"-dia. mining industry bushings.
“We are all over the country and the world,” Shegda said. “Many of our customers are in the Minneapolis area and in Southern California. One of the medical jobs that is running right now we are exporting to China.”
Centerless grinding involves a specific configuration of grinding wheels and workpieces. Traditional centerless grinders feature two parallel spindles placed side by side, one spinning a grinding wheel and the other a regulating wheel. The cylindrical workpiece is located between the wheels, supported by a stationary work blade. The workpiece can be fed axially between the wheels in through-feed grinding, or the grinding wheel can infeed (plunge) directly into the workpiece. Infeed grinding is necessary when a part requires a step or similar feature. Centerless grinding, especially for through-feed applications, is fast and precise, and is beneficial where workpiece flexing would be a problem when grinding between centers.Tight and Thin
Compared to grinding larger parts, small-part and ultraprecision grinding “requires a different mentality,” Shegda said. “To go from holding a tolerance of +0.0001"/-0.000" to +0.00005"/-0.000" is a big leap. Then, when you look at going from 0.00005" to 0.00002" total tolerance, which we do, that’s an even bigger leap. You are testing the limits of the machine and what the process is able to achieve. It starts becoming about how good the equipment is and how good the people are.”
For example, in grinding pistons for a Minneapolis company, M&S holds cylindricity—roundness, taper and straightness combined—to 0.00003". “We are holding 0.00001" round, 0.00001" straight and 0.00001" taper, total deviation,” Shegda said. “It is a very high-end application. To be able to squeeze that out is very, very difficult.”
Grinding thin-wall tubing is an M&S specialty. The company recently ground 25-gauge stainless steel hypodermic tubing, 0.030" in diameter and 1" long, to a wall thickness of 0.0006". The tubes required a centerless infeed operation because the OD had a step; a small portion of the diameter remained at 0.030", but the rest was ground to 0.025". Although the wall was extremely thin, the ratio of diameter to wall thickness (about 47:1) was large enough that the tube was sufficiently rigid to grind. “The tube wasn’t squashing,” Shegda said. “A 0.001"-thick wall in a 0.200"-dia. tube is way less stable than a 0.0006"-thick wall at 0.025", but a 0.0006"-thick wall is so thin that even a grain of the grinding wheel breaking loose will dent it and make the part unusable.”
M&S developed a process to consistently achieve 0.0006" thicknesses.
“It was not just figuring out the kind of wheel and the feeds and speeds needed, but how to condition the wheel and how to work with the part that allowed us to be successful,” Shegda said.
Simply achieving a certain level of tolerance, however, doesn’t guarantee the next tighter level is possible. Shedga noted that over the course of a few months developing ways to grind the 0.0006"-thick walls, he probably spent a week trying to determine how to get from 0.0008" to 0.0006". “We’d try different ways to do it, but the wall would break through. The part would just blow apart in the machine if I tried to go just a little bit further with it.” When reaching the limits of what the machine and process can do, he said, “you have to get really creative to figure out ways around that.”
The creativity includes a disciplined approach—like a “science experiment,” Shegda said. Taking a pad and pen onto the shop floor, he writes down every variable in the process, changes only one variable at a time and observes the results. “We might change the truing or dressing of the grinding wheel, or we try running faster or slower or feeding harder or feeding slower. We try all kinds of different things, and look for the process to get better or worse.”
It is almost as worthwhile to have a negative result as a positive one. “If you can make some kind of change happen, even if you make something worse, you now have something that is affecting the process,” Shegda said. “If we can change that aspect and make it go a different way, it will make it better.”
The company’s longtime and recent experience expedites process development. “We’ve learned quite a bit over the last 50 years, especially the last 10, because of the different challenges that have been presented to us,” Shegda said. “Grinding that 0.0006"-thick wall, we used some techniques we recently learned in grinding plastic, and they worked.”A True Machine
Differing machine configurations enable the shop to handle varying part requirements. M&S grinds a lot of tiny, intricate parts on CNC centerless DedTru machines from Unison. A DedTru machine basically consists of a centerless grinding fixture placed in a surface grinder. In a variation on the traditional centerless arrangement, the pair of wheels is turned 90°, putting the regulating wheel on the bottom and the grinding wheel on top. A spring-loaded pressure wheel holds the workpiece on the blade and against the regulating wheel. Then the grinding wheel is free to engage the workpiece.
Shegda described a typical micropart ground on a DedTru machine: a 0.058"-long, barbell-shaped locking pin that secures a clip onto a heart valve. “The tiny pins were a big stumbling block for the device manufacturer,” he said. The pins were ground from Elgiloy Co-Cr-Ni alloy rod, heat-treated to 50 HRC. The 0.030"-dia. barbell ends had a relatively open tolerance, about ±0.0005", but the diameter of the shaft between them was +0.0001"/ -0.0000" (0.0191" to 0.0190" diameter).
The most difficult part of the project was grinding the corner transitions from the shaft to the barbell ends, which required a 0.001" maximum radius. “To hold a 0.001" radius is very difficult with a grinding wheel,” Shegda said. “We ground the features on the end of a rod, then cut them off and figured out a way to face them.” Holding the parts and achieving a uniform faced surface is a proprietary process developed by M&S.
The DedTru machines employ toolroom-type wheels that are 7" to 8" in diameter and ¼" wide. Much of the time, a rotary diamond disc dresses a form on the wheel, and the wheel is plunged into the part to create the desired feature. But for certain features that require light feeds, M&S dresses the wheel to a shape that enables it to be applied like a lathe tool and interpolates the required form. Caution is required for heavy cuts, however, where the wheel may wear and reduce the accuracy of the feature being ground.
M&S generally uses conventional abrasive wheels, including aluminum oxide, silicon carbide and different grades of ceramics, with different grits for different applications. “We run the gamut, from grits as fine as 1,200 to as open as 46,” Shegda said. “Most people think when you are using a 46 or 54 grit you are taking heavier cuts. But there are times that you want to use a coarser wheel, not so much with tiny parts as with plastics, because the coarser wheel prevents loading.”Equipment Decisions
A number of factors influence the shop’s acquisition of new grinding equipment. The first consideration, Shegda said, is the market potential for the type of parts being ground. Second is the machine’s flexibility: Can it do just one or two things, or can it do a number of different things? Third, the equipment must represent leading or groundbreaking technology.
“Because we are a technology leader in the industry, we are able to solve problems that others struggle with,” Shegda said. “We need to have the best equipment to be able to remain a leader. The micro work is a big growth area for us, especially in medical. So we have to gear our machinery acquisitions toward that too. Our focus is to get better at going smaller.”
M&S has just added a Royal Master TG series centerless grinder that can grind diameters as small as 0.001". “That’s actual diameter, not tolerance,” Shegda said, adding that the shop will use the machine to grind very thin wire in through-feed applications.
Another Royal Master machine, a GenX model acquired a few years ago, differs from a pure centerless grinder because it replaces the regulating wheel and work support blade with a stationary wire support, and feeds the wire through the machine like a Swiss-style lathe. The machine can hold a linear tolerance of ±0.005". M&S added a loading system that enables automated grinding of cardiac guide wires.
The inspection equipment at M&S is unconventional in that all of it is portable. “We find that inspection during the manufacturing process is the most efficient way,” said Mike Petrucci, quality manager. “We do first-article checks on every operation of every work order, and anything required beyond that we do at the machine.” A final inspection is too late. “You want the process to yield good parts. We find that if the operators have the tools available, especially with some of our long cycle times, they can run parts and simultaneously check them 100 percent.”
When large volumes of parts are running quickly, the shop often has one person feeding workpieces and making compensations on the grinding wheel while another manually catches parts to avoid damaging their surfaces. “The person catching parts can also be checking them,” Petrucci said. “As the wheel wears, the part gets bigger and we have to compensate the wheel a little bit.” Adjustments can be made as the process continues.
The operators are ultimately responsible for their parts, and they understand they are the first line of defense on quality. “They take a lot of pride in seeing that they exceed the customer’s expectations. They are typically as picky, or pickier, than I am,” he added.
M&S employs a variety of inspection equipment. For tight-tolerance ODs, it uses a laser micrometer because it instantly displays accuracy to within 0.00001". “For really tight tolerances, we can dual-point the laser between two certified measuring pins,” Petrucci said. “That tightens up the range and then we can get to within 0.000002" to 0.000005".”
The shop uses a Mitutoyo Roundtest measuring instrument to check roundness and comparators to check other dimensions. Petrucci said today’s comparators are much more versatile than the shadow-box comparators of old. “Now they have software enabling measurement radiuses and angles, and touch probes that can take 10 or 12 points on different surfaces and get a really good indication of a radius—usually within 0.0001".”
A portable surface-finish tester is also available, but surface finish usually isn’t a critical issue at M&S. “Although important, surface finish is probably the least-problematic factor in 90 percent of our jobs,” Petrucci said. “For the majority of parts, we far exceed finish requirements. To get the tight tolerances, we have to have a good finish because a rough finish affects the roundness or size. For example, high-end pistons call for an 8µin.-Ra finish, and we provide a 2.3µin.-Ra finish.”
Petrucci, who came to M&S from General Motors, is familiar with big-company QA demands and facilitates the shop’s relationships with large organizations, such as Boston Scientific and Medtronic. He also headed the M&S ISO 9001 and 1345 certification efforts.
Shegda said ISO certification will support rapid growth that began at the end of 2010 and that he expects will continue through 2011 and beyond. That growth, he said, “scared the hell out of me, quite honestly, because that is when you start cutting corners and problems can happen. You are trying to make everybody happy, but the processes and procedures can get circumvented by how busy you are. I felt like we needed a very rigid structure to ensure that no matter how busy we got, it made us do things a certain way.”Leap of Faith
During the recent recession, M&S continued to build for the future. “Medical was insulating us to some degree, and we only fell off 12 or 13 percent in 2009, which wasn’t huge, but most of our costs are fixed,” Shegda said. “Still, in a year when we were not hitting our sales targets, I added a bunch of overhead, bringing on a quality manager and sales manager and going for ISO certification. Our biggest expense is our labor, and I didn’t let anybody go. If I let people go, I would have to rebuild, and we wouldn’t have been able to take advantage of the opportunities I was seeing.”
Beyond high-tech grinding machines and micron-level measurements, a positive attitude reinforces M&S’s capabilities. “We really have a great group of people, and we have an awesome culture,” Shegda said. “I provide the vision and energy—that is the most important job that I have.” CTEAbout the Author: Bill Kennedy is a contributing editor for Cutting Tool Engineering. Telephone: (724) 537-6182. E-mail: firstname.lastname@example.org.
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