On the wall of the lobby of Stryker Instruments, Kalamazoo, MI, is a motto: “First, Be Best. Then, Be First.” In its manufacturing, one of the strategies for being best is not only to invest for the future, reduce piece cost, and improve quality but also to support its product development effort with short-lead prototype machining, reducing time to market for its products. So, beginning in 2004, Stryker invested in a Mazak Variaxis 500 5- Axis Machining center with Palletech manufacturing cell, and has expanded it twice since then by adding a machining center and more pallets.
A global leader in the medical technology industry, Stryker, with over 15,000 employees worldwide, provides a broad range of products and services to the orthopedic industry, including implants, powered surgical tools and operating room equipment. Its turnover in 2006 was $5.1 billion.
According to Stryker, the Variaxis cell is actually part of a vision the company had for its manufacturing and the capacity it foresaw itself needing in order to remain first in its field. The investment has returned dividends in the form of reduced cycle times, reduced outside costs, high quality surface finishes, and much higher machining availability.
“We use the Variaxis cell exclusively to machine aluminum handles for our heavy-duty powered surgical instruments, which are like electric hand tools which surgeons use,” said Keith Cannan, associate unit manager at Stryker. “Our System 6 handle is about the size of an electric hand drill. The cell machines the handle for these in two operations from a 2” x 6” aluminum block.
After machining, which includes high speed milling, drilling, and tapping, they are sent out for polishing and anodizing. The handles include pockets for small motors, electronics package containers and other features, some of which may be unique to a specific handle. Product diversity is almost unlimited.
“The previous version of the handle had several different individual components which are now integrated into the handle of System 6. So with more complexity in the new handle design, we knew we needed to be able to machine five sides in a single setup. Mazak Variaxis provides that capability, and with GibbsCAM, we are producing part programs that save time while performing flawlessly,” Keith said.
The Variaxis vertical machining centers with 400mm tables, capable of multi-faced simultaneous machining are especially well-suited to machining light alloys. Two Variaxis 500’s feature 12,000 spindle rpm and one Variaxis with has a 25,000 rpm spindle. Each is equipped with a 1000 psi high-pressure coolant system. The machines in the Palletech system provide Stryker production planners plenty of tool capacity with 60- and 120-tool magazines. Each machine is equipped with an ethernet interface to accept direction from the cell controller.
The rigid, built-in tilting rotary table on the Variaxis machines allows workpieces with complex contours and features to be finished in just a single machine setup. Table tilts up to 150° (A-axis) and rotates up to 380° (C-axis).
While the cell easily maintains geometric tolerances, which typically are +/-.0005”, just as important, according to Keith, is the appearance of the part once it reaches the doctor’s hands. The surface must have no imperfections.
“What we like is that the machine can position the workpiece wherever needed to allow the tool to machine all the pockets, details, and surfaces on every handle,” said Pat Barnes, production supervisor, “saving about 10% in cycle time compared to the previous method.
And the machining accuracy of the Variaxis leaves a surface finish of a quality that has allowed us to reduce an outside finishing cost by 50%.”
Once the programs are perfected, Stryker plans on moving to true simultaneous 5-axis contour machining—which the Variaxis machines are fully capable of. “Currently, even though we are yet utilizing the machines for true 5-axis simultaneous machining, the Variaxis machining accuracy and consistency clearly helped us take cost out of the process by eliminating the need for some hand polishing,” Keith said, “while decreasing cycle times.”
Pallet system key to flexible production.
Serving the machines in the cell is the Mazak Palletech system, a modular, flexible pallet delivery system which permits Stryker to operate the cell unattended for long periods of time. A high-speed, AC-driven pallet loader robot keeps pallet transfer time to each machine very short, so no machine is ever kept waiting for parts. Pallets are loaded with blanks or first operation parts in the system’s two load/unload stations.
The production schedule is controlled through the scheduler software which determines which pallet goes to which Variaxis. At Stryker up to 24 pallets can be scheduled and queued through the cell, each with two workpieces on board. Plus, Stryker can mix parts and the cell controller can tell the machines what style is on which pallet, so no time is lost to changeover and production demands are met. Production continues uninterrupted.
We foresee taking the current 1.3-hour total cycle time for the handle to less than 1 hour. Stryker produces 12 different handle designs in the System 6 family up from only two in the previous version. “The Mazak Palletech cell makes it possible for us to produce this variety in these volumes,” Pat said. “Without it, I don’t see it happening. Palletech is the key to us being able to mix work orders and run lights-out.”
Palletech palletized machining systems are pre-engineered automation that can serve one machining center with a few pallets – or up to eight machines with (100) pallets. As Stryker adds machines, it can also add pallets and load/unload stations as needed, matching capital investment with business growth.
As Stryker has found, Palletech cells are an effective strategy for both low-volume and high-volume requirements. Resident pallets and part fixtures make lots-of-one manufacturing economical, and tombstones with multi-part fixtures efficiently serve higher production needs.
“With three Variaxis vertical machining centers, two loading stations, and a 24-pallet Palletech system, we are setup to have 9 hours of lights-out cutting capability, and with some fixture changes, we will bump that up to 12 hours,” said Keith. During unmanned machining time the system operator can engage in CAM program development for new or potential products without interfering with production demands.
“Currently, we run three shifts, a first and second, 10 hours each per day,” Keith said. “From 1am to 5 am there is no one on the floor, and that’s where the Palletech helps us to maintain production. The third shift is actually Friday, Saturday, and Sunday 12 hours a day,” Keith said. There is only one dedicated operator per shift for the highly productive cell.
Machining cell as R&D tool.
Rapid product development is essential at Stryker, and the company also makes use of the Variaxis cell in that effort. Alongside production of more than 1500 handles a month, it uses the cell for R&D tryouts.
“We actually have some R&D time scheduled for the cell,” Keith said. Due to the cell’s flexibility and easy programmability, Stryker R&D works cooperatively on the first shift with TJ Olmstead, the dayshift production machining operator. R&D designers and machinists sit around a table and discuss design goals and manufacturing capabilities. The Palletech automation makes it possible for Stryker to schedule the prototype without the need to reorganize other work.
This approach dramatically compresses time to market for Stryker’s innovations so that within a month prototype parts can be produced. “Previously, it would take us 3 months to produce a part,” Keith said. “MAZATROL CNC is easily programmed and our knowledge base with Mazak technology is growing, saving us development time and allowing us to be responsive to the needs of the medical market.”
The System 6 has been in the market for about a year, and Stryker has continued to improve the successful product. It has even produced bespoke designs on the Mazak cell. “If R&D wants a special, they walk out here and talk to us at the Mazak cell directly. No red tape,” Pat pointed out, reducing lead times significantly.
Keeping the cell up and running requires only timely maintenance. “Mazak service has been very helpful in showing us how to take preventive maintenance actions to keep our uptime where it needs to be,” Keith said. “Some time ago, the cell had effective uptime of around 85%, which was really unacceptable. Today, with aggressive maintenance schedules based on hours of operation and not the calendar, our cell uptime is consistently better than 99%,” he said. “Unscheduled downtime is basically zero so we can plan production accurately.”
The experience with the Variaxis cell has proved beneficial to Stryker in production flexibility, product quality, piece cost, R&D responsiveness and employee effectiveness. It has opened up new potential for Stryker in responding to the needs and preferences of its customers. Stryker knows what works, and it will soon take delivery of a fourth Variaxis and it will also be part of the cell. “We have a few Mazak CNC mills on the floor that are 17 years old and are still pumping out production, still performing at acceptable levels,” said Keith. “We will continue to rely on Mazak for a major part of our manufacturing strategy for the future.”