Hold It, Cut It, Check It
Article From: July 2019 Manufacturing Engineering, Ed Sinkora, Contributing Editor
Innovations in workholding, tooling and measurement for medical manufacturing are helping meet the challenges of medical manufacturing.
The phrase “medical manufacturing” is broad umbrella and not every product is produced with the tightest tolerances or the toughest materials. But the more demanding applications are driving interesting innovations in workholding, tooling, and measurement that are worth a look.
Hold on Tight
In any machining operation, maximizing tool life and imparting surface finish quality starts with secure workholding. And in medical manufacturing, the target can be a mirror finish on a part made from stainless steel, cobalt-chrome, titanium, or ceramic. Luckily, companies like Hainbuch America, German-town, Wis., and Destaco, Auburn Hills, Mich., offer solutions that deliver the required rigidity without damaging the part surface. Take Hainbuch’s latest micro mandrel from its MANDO Mandrel line. It uses segmented clamping bushings to hold the ID of a part down to 5.5 mm in diameter (the previous limit was 8 mm), and the bushings are specially treated to prevent clamping marks.
The MANDO T212 conquered a recent challenge to turn the ball profile of the acetabular cup used in a hip joint implant. Michael Larson, marketing director for Hainbuch, explained that the part’s ductile stainless steel required high cutting forces, yet even minimal clamping marks would scrap the part. The MANDO T212 held the ID with only a 4-mm clamping area and without a tail-stock, such that the customer was able to apply high cutting forces without marring the ID and then ultrasonically clean the part.
Larson said Hainbuch has also introduced the MAXXOS mandrel with “a hexagonal shape for aggressive machining on difficult-to-machine materials such as titanium. The shape eliminates any initial twist or movement when machining.” As noted above, stability is key for maximizing tool life (by reducing vibration), and Larson noted that it is especially important with titanium. Another interesting innovation from Hainbuch is a chuck made using carbon fiber reinforced plastic (CFRP), the MANOK Carbon Fiber.
Larson said it also uses a hexagonal clamping head (collet) unique to Hainbuch that delivers “20 percent more clamping power than the round collets used in quick-change collet chucks. It also has superior resistance to chip and dust contamination, which is very critical when machining porcelain.” The MANOK Carbon Fiber is 58 percent lighter than the standard MANOK plus, 22 lb (9.98 kg) vs 52 lb (23.6 kg), enabling the machine spindle to accelerate and decelerate faster, which increases productivity.
Larson added that medical lot sizes tend to be small, requiring frequent changeovers, so Hainbuch’s systems enable the user to quickly change the size of the clamping mechanism, or the interface to the machine, so it can be moved to another.
Tom Mikrut, Destaco’s commercial sales director for the Americas, echoed the need for flexibility in the medical market, explaining that “going forward, more people envision having shorter runs and two, three, or four different components on one piece of equipment.” This drives the need for versatility in workholding, indexing systems, and conveyors, all of which Destaco provides to one degree or another.
For example, while previously a customer might have been happy with a Destaco Camco indexer that moved a part in and out of a machine between two fixed positions, today he’s more likely to invest in a servo-driven unit that can be set for different positions.
“Destaco’s Camco GTB Servo Positioners are still zero backlash, fully pre-loaded, and with the same strength as the purely mechanical units,” explained Mikrut, “but you can change the position at any point throughout the day.” The company offers the same technology on conveyors as well. “Camco precision conveyors are becoming very popular in the medical market for assembly, because machines are becoming more elaborate and need more stations for inspection, rejection, and assembly,” Mikrut said. He added that medical products are more complex, with more parts to put together, so there is a move from rotary to linear motion and precision conveyors to give users more stations in the same footprint.
Mikrut said his company has also introduced a complete line of stainless-steel clamping products to address the need to prevent rust when fixturing stainless steel medical components. More recently, it introduced the E-Clamp, an automatic, electrically activated clamp.
“Medical manufacturers prefer them,” said Mikrut, “because they’re cleaner. They would like to get away from pneumatic lines and compressors completely if they can. You can adjust the force and orientation right on your iPad and the clamp communicates to the PLC whether it’s open or closed and what position it’s in. You can also easily adjust these factors as you go, within the application.” So far, the automotive world has more widely embraced that communication function, but Mikrut expects greater interest in the medical community because it “answers the question ‘When will I be at risk?’ The clamp can tell you at the first instance that something is going wrong and needs to be checked.”
Cut it Fast
Orthopedic implants made of cobalt-chrome and titanium are generally the most challenging medical applications for cutting tools, according to Walter Stuermer, director of technical operations at Millstar, a division of Cole Tooling Systems, Orion Township, Mich. “Grinding is a great way to finish some of these components, but we’re trying to eliminate as much of the grinding as possible,” he added. Stuermer pointed to innovations in roughing and semi-finishing combo tools and “the ability to incorporate the different geometries for the finishing application.”
However, it is not just the tool geometry, or the use of nano, fine-grain carbide. “I can’t overemphasize the importance of the coating in extending tool life and its consistency and reliability over time. It’s the third leg,” explained Stuermer. Millstar partners with coating experts, typically using physical vapor deposition (PVD) to achieve the best combination of heat resistance, hardness, and wear resistance.
Getting the fine surface finish required on a part like a femoral knee implant takes a good ballnose end mill. But this leads many people to either machine slowly with a two-flute tool or to problematic machining with a four-flute tool, as Stuermer explained. “In theory, a four-flute ballnose should be twice as fast as a two-flute version. But traditionally two flutes stop short of center on a four-flute tool. To achieve a superior surface finish, you have to take a fine step-over. With a light finishing pass, the two flutes that stop short of center will easily be beyond the depth of cut, so it is really only cutting with two flutes. If the feed rates are set twice as fast, thinking you’re cutting with four flutes, the finish will suffer.”
What is more, the small chips produced by this cut tend to get caught in the gap between the end of the flute and the center, then drag across the part, leaving marks. The solution, said Stuermer, is a “true four-flute” ballnose tool where all four flutes come to center, which Millstar calls the BM4T. “You can double the feed rate, because you are using all four flutes of the tool. The other advantage is that you’re not driving the chips across the part.”
Stuermer also observed that medical manufacturers have hundreds of machine tools with smaller, less rigid spindles than machines commonly used in other industries. This limits the size of the tooling they can use effectively. To address the need for increased productivity without changing machines, Millstar has introduced a new line of ABL milling tools. These tools feature a large radius on the end face, typically as big as the diameter, creating what is sometimes referred to as a lens shape.
“You can take twice the step-over and still get the same surface finish,” said Stuermer. “We’re also experimenting with combining this shape with a barrel form on the side.” He added that although the achievable surface footages in these materials may be low compared to easier-to-machine materials, “the new geometries and coatings still make dramatic improvements, like the difference [in moving from] 300-400 sfm finishing in titanium to 700-900 sfm.”
Dan Doiron, milling product manager for Emuge Corp., West Boylston, Mass., also highlighted the importance of tool coatings. Its TiNox-Cut material-specific end mills feature a high-heat-resistant, PVD-applied, multi-layer TiN/TiAlN coating. Doiron said the series was developed especially for machining titanium alloys, nickel-base alloys and stainless steel. The product range consists of solid sub-micro grain carbide end mills with variable flute spacing (for vibration dampening); optimized geometry for long chipping, tough materials; and h5 tolerance shanks with a roughened surface (non-polished) for greater gripping force.
“Roughing and semi-finishing and finishing profiles are available,” and finishers are available with a flute length-to-diameter ratio of up to 4:1. Doiron added that its Top-CUT VAR general-purpose end mills are an economical option for “nearly all materials and milling strategies due to their special geometry properties.” They’re made with premium solid micro-grain carbide and feature variable helix flute angles for vibration dampening, a tapered core diameter for added stability, and high-performance AlCr coating for added wear and heat resistance.
Check it Out
It’s no surprise that the medical industry is “very much focused on quality, both from a competitive standpoint and in terms of wanting to deliver a good product to the customer,” as Mark Arenal, general manager for The L.S. Starrett Co.’s metrology systems division in Laguna Hills, Calif., put it. “In most cases, there are also FDA quality and safety standards that they need to conform to.” Achieving good quality entails both in-process and post-process measuring, and optical comparators have long been a key instrument for such checks. Arenal said Starrett’s relatively new HDV (Horizontal Digital Video) product “takes the optical comparator to a whole new level.”
With a traditional comparator, the technician optically magnifies the area of the part he wants to inspect and compares the profile to a Mylar overlay with a drawing of the part at the same scale as the lens magnification. (The light source is behind the part, facing the lens.) But “this is very subjective and has the potential for errors,” explained Arenal. “We have digitized this process, so the operator can import a DXF file of the part and overlay it electronically on the live video screen.”
This approach offers advantages; one is eliminating most (or even all) the subjectivity of the operator. “Now the operator can be confident that a part is good or bad,” Arenal added. “You get discrete data of where a part is in conformance and out of conformance. It’s easier to do reverse engineering. And you don’t have to manage the overlays.” Starrett offers various degrees of automation, “from the operator making the decision about whether the part is good or bad to the system deciding that and running in a full CNC mode.” Another benefit is the ability of the DXF overlay to track the live part as it moves through the field of view for cases where the part will not fit in the field of view.
Users include orthopedic implant manufacturers for knee and hip joints and bone screws, to name a few. “They use it for in-process measurement to improve their process and to reduce scrap,” explained Arenal. “Depending on the number of features you measure, it can be very fast. In the case of a knee, we hold the knee joint on a fixture on a rotary table and rotate that knee through a variety of section views and then measure the profile at each stop along those cross-sections. This allows us to effectively get a 3D measurement of the part.”
To measure features on the surface of a part (as opposed to the edge profile), Starrett offers a variety of lighting techniques, including dark field illumination, coaxial illumination, and segmented LED illumination. Arenal said “the whole idea is to illuminate the feature effectively so it stands out and you can grab the feature detail. Our tools give the user options to see difficult details on parts.” For example, on a molded part with black-on-black features, the edge of the feature will only be pronounced enough visually by casting light at an oblique angle. “That’s where surface illumination techniques come in,” he said.
While the tools are powerful and the software interface intuitive, such cases require an experienced operator to get the right information. “This is a level of technology that’s evolving, but it’s important that an engineer or operator knows the part they’re verifying and that they’re actually seeing the right points, given the variability of small features, which are sometimes in close proximity to other details,” said Arenal. “On the more challenging applications we haven’t eliminated the need for a qualified technician to set up the part program to begin with.” It’s also possible to integrate touch probes for measurements where that’s the best method, and laser sensors for contour, profile, and surface finish measurements.
Arenal added that all Starrett systems are “usable for what we like to call ‘walk-up metrology.’” “Walk-up metrology” means two things to Starrett. First is the ability for any mill/lathe/grinding machine operator to “take a part out of his machine, put it on a nearby Starrett measuring system on the shop floor, click the right part program, and run that part right there, rather than taking the part down to the quality lab and waiting for a first-article inspection.” The operator can quickly make any needed adjustments to his machine or process. “It’s a much more efficient way to keep production moving,” he said.
Second is the ability to quickly measure a few features on a new part without any pre-programming. This last capability rests on the “discoverability” of Starrett’s software. “It’s almost like the experience of using a smartphone, with common functions across all machines,” explained Arenal. “Everything is a touch screen.” For example, if the entire bolt circle of a small hub is visible, the operator can touch each diameter on the bolt circle and then touch the center of the hub. They have then created a bolt circle and can use easy construction tools to create angles between each hole in the circle and the center, according to Arenal. “I can then switch between the graphics screen, or the feature in detail, or the actual live video view,” he said.
Combining Assembly and Testing
Sometimes the solution is to get assembly and testing technology from the same vendor, as was the case when a maker of vascular closure surgical devices experienced intermittent product failures in the field. The manufacturer was using air-actuated assembly presses to assemble the vascular closure devices that lacked accuracy, process monitoring, and traceability. There were failures of the surgical device’s release mechanism and often a seal wasn’t properly installed, leading to the male/female clips being unjoined but still progressing down the assembly line. This was according to Glenn Nausley, president of Promess Inc., a Brighton, Mich., manufacturer of electric assembly presses and other assembly products and controls.
“These issues were costing the manufacturer thousands of dollars in reworks and end-of-line batch checking,” said Nausley. “Promess offered our Electro-Mechanical Assembly Press, EMAP, with integrated force versus position monitoring as well as a TorquePRO system for functional testing of the final assembly.”
After Promess application engineers found the proper forces for the housing to be assembled and the necessary torque for the release mechanism functional test, the EMAP allowed the customer to press to the same force within ± 0.5 percent every time. “With the TorquePRO now conducting a functional test on every vascular closure device, the manufacturer was able to ensure 100 percent in-process verification that every assembly was done correctly. This eliminated the need for end-of-line batch checking and gave them individual part traceability,” Nausley said.