Therapeutic Equipment

Lots of mission-critical metal parts have complex features and are made of difficult-to-machine materials, but medical components stand out because they are frequently implanted into humans or used in surgical instruments. To effectively produce complicated 3D forms, tiny part features and intricate component faces with aesthetically pleasing surfaces, some machine tool builders target equipment to the medical market.

One such builder is Grob Systems Inc. in Bluffton, Ohio, which hosted a Medical Technology Day last fall that featured live machining demonstrations on the G150 5-axis universal machining center of medical parts such as a titanium bone plate and cobalt chrome knee femoral. Three linear and two rotary axes enable five-sided machining and simultaneous interpolation in the company’s smallest horizontal machining center in its modular G-Series universal line.

Workarounds are available to produce challenging medical parts without 5-axis machining, whether simultaneous or 3+2, but having 5-axis capabilities enables manufacturers to reduce operations and allow more automated manufacturing, said Derek M. Schroeder, universal machines sales and proposal manager for Grob. More automation combined with simpler and fewer operations enables U.S. medical manufacturers facing high labor costs and a lack of skilled workers to more effectively compete with low-labor-cost countries.

As the supply chain continues to be rearranged, especially from the U.S., medical manufacturers should strive to be independent of China, where the price of an implant slashed down up to 80% due to centralized procurement, according to Stephan Kowalski, sales for universal machines, Center of Excellence Medical at Grob-Werke GmbH & Co. KG, Mindelheim, Germany. This will affect the manufacturing of medical devices enormously. “You really need to think about your manufacturing line in the medical world and you really need to gain advantages because there will be winners and losers in the future.”

One of the advantages gained from 5-axis simultaneous machining of medical parts, he noted, is the ability to apply short cutting tools, such as barrel tools, to enhance cutting stability and part accessibility.

Cellular Approach

Various automation options are available for medical manufacturers, such as pallet changing systems that Grob produces in an array of sizes, but dedicated robot work cells make more sense for those with high production volumes, Schroeder said. Unlike a pallet system where the operator has to clamp and unclamp parts manually, a drawer system feeds a robot cell, and the operator only has to move parts in and out of trays. “You have less for the operator to do.”

A robot cell from Grob that’s well suited for medical production is the GRC-R12, which further increases the productivity of the G150 HMC, Schroeder said. “Our customers see a big advantage in having the automation and machining center come from the same builder because it enables a better support model to keep the cell running at high efficiency longterm. Also, we prioritized minimizing the cell’s footprint to save valuable floor space.”

The cell has a 6-axis Fanuc M-10iD/12 robot with a reach of 1,441 mm (4.7'), capable of handling a maximum load of 12 kg (26.5 lbs.). The drawer feed system can have up to six drawers that can be unloaded and reloaded while the robot cell and machining center are in operation.

When appropriate, Kowalski added that a robot cell can be positioned in front of a pallet storage system. “When you go on further, you could even load this all with an AGV [automated guided vehicle] or with a gantry system. We develop tailor-made solutions for the customer and his requirements.” Those requirements can include other operations such as measuring, cleaning and marking.

Having horizontal kinematics when 5-axis machining provides benefits, Schroeder said, such as effective chip evacuation. “You don’t have chips piling up on the part, where you’re recutting chips.”

In addition, because of how the Z-axis moves, pushing the cutting tool horizontally into the chuck or workpiece, an HMC is the most rigid when cutting the part, Schroder explained. “Typically, what you see in the market with verticals, they are the least rigid out at the part. Rigidity equals tool life, better surface finish.” He added that horizontal kinematics also allow the use of long tools with large parts, but medical parts are usually small.

Kowalski emphasized that it is critical to optimize cutting tool costs, especially when machining medical parts made of challenging workpiece materials, such as titanium and cobalt chrome. “For that you need a rigid machine, you need short tools.”

Whole Lot of Holes

Numerous medical components require the drilling of holes — frequently with high depth-to-diameter ratios — that must be straight and concentric. Traditionally, medical manufacturers have machined these parts, including gundrilling, on Swiss-style machines, according to Anthony Fettig, CEO of Unisig in Menomonee Falls, Wisconsin.

The manufacturer of deep-hole drilling machines does not want to replace Swiss-style machines at medical shops, Fettig noted, but create a complementary relationship in which a Unisig machine with a robot is in a work cell with several Swiss machines. “A Swiss machine is a great CNC machine. It’s very productive.”

In that scenario, the Swiss machines produce complex part features from the raw bar stock while the robot loads and unloads the gundrilling machine to create accurate holes with depth-to-diameters ratios up to 100:1 or deeper, he explained. The gundrilling machine has an advantage when drilling deep holes because of the counter rotation of the workpiece and the cutting tool. For example, a tool might spin at 24,000 rpm while the workpiece rotates at 4,000 rpm, combining to 28,000 rpm to provide a high penetration rate and high cutting speed and impart a fine surface finish. “Also, the counter rotating characteristics make straighter holes with better concentricity. Swiss machines just don’t have the type of performance built in. It’s very slow to gundrill on a Swiss machine.”

The machine that Unisig targets primarily for the medical market is its series of UNE6 gundrilling machines, Fettig said. The number in the name indicates the maximum diameter of the hole the machine can drill in millimeters, or 0.236". The smallest diameter possible is about 0.8 mm (0.031") and most drilled holes range from 2 mm to 3 mm (0.079" to 0.118"), he added.

The medical parts drilled on the machine include implants, such as bone screws for orthopedic surgery, and surgical instruments, Fettig said. “Both of those spaces have applications for our machines.”

In addition, gundrilling can be performed to drill stepped holes, he added. In this application, one hole is drilled from one end of the part, the part is flipped around and then a larger hole is drilled from the other end. “That’s way more productive than drilling a very small hole all the way through the part and then chasing it like a counterbore with a bigger drill or reamer.”

To sequentially produce a stepped hole in a one-piece flow works smoothly with automation and a twin-spindle machine, Fettig said. With automation, the operator does not have to transfer parts from one spindle to the other because the robot does that. Plus, a twin-spindle machine doubles the production rate compared to a single-spindle machine, and with part pallets integrated into the machine, lights-out manufacturing is possible.

Robot Lives in the Machine

He explained that Unisig integrated a robot into the machine early in the design stage. “We designed the machine around the robot automation, so it’s very efficient in its construction.”

Because a medical job shop might run multiple jobs a day, Fettig said Unisig developed a control system and specialized software that avoids the need for customers to be robot programmers when switching from one job to another. He added that a robot control running on CNC code is quite intimidating and it is counterintuitive to try and track all the complex jumps and subprograms. “We took all that away by putting it into the machine controller, which is more graphic rich and instructional in asking questions about what you’re trying to do, and then it tells the robot where to go.”

To achieve success consistently when gundrilling on a twin-spindle machine, Fettig said six items must be properly aligned: the two tools, two workpieces and two guide bushings. Unisig holds the cylindrical projection of the two spindles, guide bushings and workpiece spindles to within about 10 μm (0.0004"), and that factory alignment is maintained unless something drastic happens, like a machine crash. End users just have to periodically replace components, such as a bushing, to compensate for wear.

The medical market has some unique demands, and careful consideration is needed when procuring machine tools, but lessons learned serving other industries can prove helpful when targeting medical. For example, Kowalski noted that Grob came from the automotive world and much of that experience was transferrable to the medical realm. “At first it seems not so, but then you can transfer it and really make our customers more competitive.”