When people think about centerless grinding, they often think of a finishing operation or a thru-feed operation to reduce the diameter of a part. However, Glebar Co. President John Bannayan says he’s more than ready to offer a different look at the process.

“In general, some people just don’t understand where grinding fits in for medical devices, and that’s what Glebar is able to come in and explain,” Bannayan says. “While the common thought is part finishing, our experience with materials – from super magnets and fiberglass to shape-memory alloys – allows centerless grinding of finished products with very precise diameters. Because form grinding has the same technique that makes a check valve, this can be used to grind a carbon-fiber ball or anything similar. So from form and plunge grinding, users can go from a cylinder to a finished part in one shot.”

Grinding solutions have offered users improvements in throughput by up to 30x while enhancing the quality of products and efficiency of operations.

“Some of Glebar’s innovations have replaced alternative technologies such as lathes and OD grinders, with precision, high-volume production, turnkey systems,” Bannayan says.

Machine and vision

Centerless grinding, used for medical manufacturing, produces precision parts such as guidewires for stenting procedures and shavers used in arthroscopic surgery. The CAM.2 and the GT-610-CNC grinders find strong use within medical manufacturing, with the CAM.2 being the most widely used system for guidewire production.

“The CAM.2 is designed to address the growing cost of healthcare, particularly regarding minimally invasive procedures. Able to produce complex shapes in cylindrical components, it is a micro-machining center that can profile grind a 0.0090" diameter wire down to 0.0015" in diameter, maintaining length tolerances better than 0.0020" and diameter tolerances better than 0.0002" across an unlimited length of product,” Bannayan explains. “Since this machine is fully automatic, an operator can manage a five-machine cell, making this machine the industry standard for guidewire production.”

A newer powerhouse in the medical market, the GT-610-CNC, offers repeatability with the built-in CNC dresser, automating the process and helping minimize down time between dressing cycles. Built-in software wizards, configured for a part family, further reduce setup times and programming times. Surgical tools 0.1320" in diameter can be profile ground to better than 0.0001" in 9 seconds, producing mirror 3Ra to 5Ra surface finishes.

Bannayan says Glebar engineers took the same techniques used for grinding titanium fasteners for large aerospace producers – who are grinding six fasteners at a time versus one – and applied those to develop additional capabilities for the GT-610-CNC, which is not only used to manufacture fasteners but is also finding strong use in the production of shavers used in arthroscopic surgeries. These surgical tools consist of an inner and outer tube. The inner tube has a radial shape on its tip. A window in the outer tube exposes sharp teeth on the inside tube, which rotates at high speeds. While the teeth are typically produced on a different machine, tube form production occurs on the GT-610-CNC.

Arthroscopic shaver – grinding two mating metal components

Two tubes are assembled with as minimum a gap between the two parts as possible – as small as 0.0005" – so that the inner tube moves freely inside the outer tube without allowing debris to catch between the surfaces. Using the Glebar GT-610-CNC to in-feed grind the shape of the inner tube, engineers were able to maintain a ±0.0002" tolerance on the elliptical tip dimensions.

At the same time, they ground three diameter features to within 0.0002" per diameter, maintaining a TIR of 0.0001" and producing a smooth 9 Ra surface finish on 304V stainless steel with a ±0.001" wall thickness. Next, users removed 0.012" in stock for the majority of the part and 0.004" off the tip geometry. The entire fully automated process was done in less than 20 seconds, adjusting for part length variation, heat expansion, and a near zero tip geometry requirements. The process reduced previous scrap rates due to tip geometry imperfections, as well as eliminated an extra step in the manufacturing method which left unacceptable tool marks on the surface.

For the outer sleeve, a GT-610 thru-feed grinder is used to grind the OD of the blank tubes, maintaining a surface finish between 3RMS to 6RMS and maintaining a comfortable 2Cpk to 3Cpk on the outside diameter of the part. Material removal of 0.005" with a tolerance requirement of ±0.0005" was met easily in a fully automated turnkey process.

“The biggest challenge faced is getting those inner and outer tubes to have zero clearance between the surfaces. So we are grinding them, but not how one would normally think,” Bannayan says. “Normally an engineer would think of an in-feed grinding process, but we’ve taken it one step further. Because of the different lengths of the tubes, it makes it difficult to get essentially a zero tolerance on the curve and to get that shape.”

Glebar engineers’ approach is to have the machine load the part, measure the length of the tube, and then move the whole work rest laterally (a patent-pending process) against the face of the wheel to produce the desired shape. With this design, machinists are using parts of the grinding wheel that previously weren’t accessible.

Typically, processes to produce arthroscopic shavers would require a plunge grind and a turning operation. However, with the Glebar GT-610-CNC, users are able to combine the two operations.

Enhancing this process is the machine’s inclusion of a profile inspection system, Glebar’s P4K, originally designed for guidewire production. It’s a laser gage with a linear motor on it, or as Bannayan calls it, a CAT scan of any kind of cylindrical-shaped part.

Users are able to scan the tube, or multiple tubes, and feed that data back to the machine, with the machine automatically changing not only the diameter position but also the dress profile. Therefore, when there are very complex cylindrical shapes machinists need to dress, they can feed that information back from a gaging system and automatically modify the dress on the grinding wheel.

More time-consuming traditional methods would be for the operator to run a part, take it to an optical comparator or another vision system, apply an overlay, take it back to the machine, make machine adjustments, and then run another part, repeating the above steps again, as needed.

“The vision system we have on the machines is a mature product, designed for a range of applications that takes a diameter measurement every 30 millionth of an inch,” Bannayan explains. “The machine operator is able to define the points of measurement with diameter, length, surface finish, and more. When producing the arthroscopic shavers, machinists look at diameters and points on a software-generated overlay of the system, and those points are fed back to the machine. The machine can reshape or resharpen the grinding wheel to get back to a production run faster.”

Medical market glance

“We have always been in a range of markets with our grinding machines. With guidewire manufacturing, doctors were performing too many stenting procedures without a confirmed need. So, the idea came about to mill a slat on a 0.002" diameter guidewire at the tip so a chip could be bonded to it. That chip would measure pressure and temperature at the point of blockage, feeding back that information to the doctor, offering numbers to prove if there is a blockage that needed to be addressed. However, to produce that slat we needed to grind a very small flat with a very high tolerance of ±0.0001" so the chip could be assembled on the guidewire tip. It’s the requirements as miniscule and precise as these that keep our engineers continuing to advance the technology available on our grinding machines. This is also what pushes us to develop machines that are very intuitive so they can be run by the range of skilled workers entering the market while holding micron tolerances and high-quality.” ~ John Bannayan

Skill sets and technology

Manufacturing is facing a shortage of skilled workers, and finding skilled grinders is even more of a challenge. Addressing these obstacles is what has led Glebar engineers into advancing the machine technology while simplifying from a process standpoint.

“I think the education system is failing us in the manufacturing sector,” Bannayan says. “There are not enough people going into this trade in general, and it’s becoming harder to get young people involved in this sector. In our company, we have employees that came to us knowing zero and we have trained them from the ground up.

Bannayan says machine technology can ease the struggles. Machinists are able to operate four or five machines with ease. The technology enables the operator to load bundles of flexible wire, and then allows the system to singulate the product, strip it out of the bundle, and feed it into a machine. With this more automated process, the need for higher skill sets is reduced.

Glebar officials work with two local technical schools and some of their biggest successes have been veterans returning from service.

“We have hired some veterans who have taken a few CNC machining classes locally, and then we train them on our systems. Our success has been great, with some of them travelling around the country installing and servicing our machines as well as training machinists on our machines,” Bannayan says. “These technicians are able to do a complete rip-down and set-up of our very complex machines. I know if the U.S. government trusts them with nuclear subs, we can certainly trust we have a qualified person working on our machines.”

Glebar grinding machines have been running on computers and touchscreens since the late 1980s, and have subnets and IP addresses. This has kept the machines current with the move toward connected plants as well as the ability to troubleshoot from any location. Similar to how the installed inspection system goes through a company network and back to the machine, the system is configurable for each customer’s use, with Glebar technicians able to get device feedback on machine issues and failures.

Additional work the staff at Glebar finds satisfying is getting involved in projects from the start. This gives them the ability to look at the pre- and post-process grinding operation to see how they can take cost or production time out of the equation. Customizing the machines to add attachments or functions so a part can remain on the grinder for all stages is one way to achieve savings for customers, as well as increasing production time and quality.