When two turrets aren’t enough, manufacturers opt for multitasking milling and turning lathes to manufacture orthopaedic components.
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Manufacturing accurate and complex orthopaedic components places tremendous demands on machining technology. To stay ahead of the competition, manufacturers must continually examine effective options, such as multitasking milling and turning lathes, for precision machining. Multitasking milling and turning technology involves putting at least two tools in the cut to perform multiple processes on a single machine to produce a finished part. The ability to mill, drill, turn, thread, bore, or groove complex, contoured medical devices during one setup is beneficial to manufacturers. Successful milling and turning operations require a close examination of processes to yield optimal productivity, profitability, and part quality. Multitasking Increases Productivity
More recently, machine tool builders began making computer numerical control (CNC) lathes with back-working spindles. These tools allowed the back side of a part to be completed on one machine. Some builders made true-opposing twin spindles with two turrets and a milling capability. The turrets had 12 stations, six of which could mill at about three horsepower (hp).
These platforms helped manufacturers reduce cycle time by as much as 50% and enabled them to produce a high-quality product with less handling of the workpiece than the first CNC lathes with back-working spindles. Complete parts could be produced in one machine. This twin-spindle and two-turret configuration became popular because it helped manufacturers increase production and reduce cost.
In a few years, builders were adding a y-axis to this platform. The y-axis significantly increased productivity by allowing shops to work on one machine to produce more complex parts with features that were not on centerline.
The next significant change in machining occurred as U.S. manufacturing became part of the global economy. U.S. manufacturers had to compete with countries such as China, Mexico, India, and Turkey. The cost of goods sold in these countries was lower than in the United States because labor was considerably less expensive. Manufacturers in the United States simply couldn’t compete based on a labor rate that was (and remains) as low as $0.50 to $2.00 an hour. As a result, much manufacturing shifted overseas. In some cases, the cost of raw material in the United States was more than a given country’s finished product, and the United States was reduced to producing high-quality devices at low volume. Although twin-spindle, twin-turrets multitasking machines are a potential solution for some product applications, short-run production jobs present a challenge. Due to the long setup time on short production runs, cost can also be an issue.
To address the changing manufacturing environment, the U.S. machining market required improvements in multitasking technology. Machine tool builders developed a twin spindle with a b-axis tool spindle, which replaced the 12-station turret. The turning machine has a tool spindle with 10, 15, and 20 hp, the ability to change tools like a vertical mill, and y-axis capability.
This technology also added the tool magazine to the machine, allowing for 40, 60, 80, 100, or more tools. This multitasking design reduces setup time because the tools stay resident in the tool magazine and do not need to be removed. For example, complex orthopaedic parts can be machined complete, including intricate angle features, because a b-axis tool spindle can change to any angle as easily as a new program position.
Lower Turret Technology
Although this technology reduces setup time, with only one b-axis tool spindle, the true multitasking advantage of maximizing with two tools in the cut is lost and cycle times increase. Introducing a lower turret can address this problem. In addition, there are machines with lower turrets that have a milling capability and a y-axis.
The lower turret technology reduces cycle and setup times when manufacturing small lots of complex parts. Intricate medical components can be manufactured effectively with this multitasking technology.
Three Turrets Equal Efficiency
Manufacturers need to reduce costs even more to increase productivity and keep the manufacturing in the United States. Improvements in multitasking lathe technology offer significant benefits. Machine tool builders now make turning systems with a twin spindle (20 to 30 hp), and three turrets with all turrets y-axis capable. The three turrets provide 36 to 72 tool stations that are capable of 10 hp milling at 40 N•m torque on all stations.
This current technology allows the production of a family of parts with complex features. Using three tools simultaneously in the cut can produce dramatic cycle time reductions. Because the bar stock becomes the fixture, no additional fixtures are necessary to produce complex shapes with tight geometric tolerances.
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| This solid titanium backbone implant is about 1-in. wide. Machine rigidity and spindle torque enable the processing of angular cuts and complex features. Image courtesy of Methods Machine Tools. |
For example, an orthopaedic manufacturer producing a backbone implant from solid titanium bar stock benefits from multiturret machining. The manufacturer produces a family of backbone implant sizes, ranging in thickness from 0.65 to 1.3 in. They have some similar features, including serration, but the holes, milled window, and different angle surfaces require different tool sizes.
With a multiturret machine, no tools need to be changed when manufacturing the various-sized parts. In the backbone implant example, all of the tools are ready on the machine for the entire family of implants. This allows the operator to simply change the machine program and run a different size part from the family. The other advantage with this method is that it’s not necessary to inspect a first-piece article each time an additional part is run, because there is no new setup. The program and tools are qualified from the original first-piece inspection.
Both long and short production runs can be made with great efficiency because setup time isn’t required. The three turrets with multiple tools resident means no tool changes are needed—the operators can change the bar size or workholding and start making the part.
There have also been improvements in the manufacturing of the milling heads used on these machines. The manufactures are using better bearings; in some cases, they use four bearings instead of three. Some manufacturers are also using Timken roller bearings for greater rigidity. These bearings have high-pressure coolant-thru capability and offer quick-change systems.
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How to Implement Machine Multitasking
When deciding whether to implement multitasking technology, there are some basic helpful guidelines to follow.
Choose the Right Personnel
Selecting the appropriate operator or engineer to execute the multitasking machining process is critical for success. The multitasking process may be a new manufacturing method for an organization and its personnel, who may be accustomed to using more than one system to complete a part. The right personnel will be able to follow a different strategy to machine a complete part on one machine.
Change Your Part Manufacturing Approach
The most important element of the multitasking process is the transfer of the part to the second spindle. As with most current processes, the operator should ascertain whether there are good features to use for the geometric tolerance or the drawing. For example, look at what features must be done on the same side to hold the print dimensions. With a multimachine process, one operation is generally reviewed at a time. With multitasking, the whole part must be examined.
Upgrade or Get New Software
In most instances, you will have to upgrade your current computer-aided manufacturing system. Software has been developed to use the codes that builders invent to control the process. Most builders use wait codes (M100-M199), which are used to synchronize the two programs (left and right spindles). When a wait code is commanded by one control, the control hangs up (waits) until a matching wait code is commanded in the opposite control. This avoids interference between the two programs, such as crossing over from one spindle to the other or conflicting turrets between the two while working on the same spindle. With programming software, you can use the wait codes to optimize the process and verify that there is time to add a second operation while another is in process.
Research a Quality Machine Tool
All machine tools are not created equal. Proper research is required before a purchase is made. Ensure that the machine offers true multitasking. This includes putting two or three tools in the cut simultaneously. It is a good idea to see a machining demonstration using a familiar sample part, because there are a number of mill and turn machines with impediments that prevent full multitasking for your application. If you cannot put two tools in the cut, then you are not multitasking. This means cutting on the left and right spindles simultaneously, not just two tools on one spindle. Otherwise, you will not obtain the cycle time reduction that is so critical in multitasking. Selecting a quality machine tool is an important investment.
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Applying Multitasking
There are many advantages to multitasking, the biggest of which is the improvement in process and cycle time to produce a complete part (see the sidebar "How To Implement Machine Multitasking"). Milling and turning lathes can be used to make complex parts. The different machine configurations—two turrets, three turrets, three turrets with a y-axis, or a b-axis tool-changing machine—enable multiple opportunities.
This does not mean that a horizontal machine or a five-axis milling machine can be replaced with a milling and turning lathe. However, when it comes to productivity and profitability, multitasking is an attractive option. So how do you know when to look at a milling and turning process? The simple answer is that it can be applied to any part with turning, drilling, or boring; outer diameter or inner diameter threads; and a few mill features, such as a square or hex; and a bolt hole pattern.
Some uses for multitasking may not be immediately apparent. Let’s look at a hypothetical example. You need to produce 20,000 pieces per year of a bone strap. The part is a stainless-steel extrusion made to shape.
The part includes many special threaded holes to accept the various bone screw head shapes, a special angle locking form in the hole, and radius features on the sides. A special round boss on the two ends and burrs present challenges due to various angle intersections.
This description sounds like a typical vertical or horizontal machining application. However, it could be manufactured in a multitasking turning center. For comparison purposes, see
Table I for a description of the machining processes.
The multitasking process gives a faster cycle time versus horizontal machining applications because there are more tools in the cut simultaneously. A manufacturer would need to run the horizontal process a full third shift to meet production needs. The tooling cost is about the same for either process, but the fixture cost and the change over time on the horizontal would be much more than the multitasking process.
To maximize profits further, manufacturers can reach lights-out (unattended) operation using simple automation such as magazine barfeeders, multipallet machines, and robotics. In addition, machine tool manufacturers supply software features such as tool-load monitoring and tool-life management to make lights-out running successful.
Conclusion
More than 30 years ago, the first CNC machine was introduced. Some manufacturers did not see the advantages to this technology and continued using traditional and manual methods.
Today manufacturers are at a crossroad and must embrace new technology to rise to new levels of productivity and profitability. In the case of manufacturing accurate, complex orthopaedic components, multitasking milling and turning lathes are a beneficial technology. As multitasking technology continues to advance, it is vital for orthopaedics manufacturers to stay on the pulse of these improvements to achieve optimal efficiency in their precision machining applications.
Richard Parenteau is director of applications development and a product manager at Methods Machine Tools Inc. (Sudbury, MA).
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