Centerless Grinding Forms a Part

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.

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.

"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.