Just Say VMC
Advanced technology is not an oxymoron
By Jim Lorincz
The most straightforward of machines, the VMC has morphed into variations in design that can accommodate the most intricate workpieces for medical devices, the longest workpieces for aircraft, and produce the smoothest surfaces for critical mold and die applications.
There isn’t an industry or manufacturing segment that hasn’t been affected by the versatility of VMCs. The automotive industry as well has benefited from the ability of VMCs to fulfill high-volume production requirements for such parts as transmission components, engine blocks and heads, and valve bodies.
The route to design changes in machine configuration hasn’t followed a straight line, however. Machine tool builders have responded to their customers with “done in one” machining of parts, eliminating the need for multiple fixturing and secondary operations fed by hand or by complicated and expensive material handling systems. As a result, VMCs routinely sport high-speed spindles, tilt spindles, trunnions, and rotary tables mounted on trunnions for precision contouring machining.
In a cellular configuration or fed by palletizing automation systems, VMCs provide the capability to machine one-off parts or groups of parts for production machining, often extending the number of tools that can be brought to bear on families of parts.
“I can’t think of a machine tool category that has more competitors than the vertical machining center segment,” says Chuck Birkle, vice president CyberTech Div., Mazak USA (Florence, KY). “The solution is to provide machine tools that improve the customer’s competitiveness, that are more productive. The typical ways of doing that are to provide automated loading and unloading, increase spindle utilization, and find cost-effective ways of adding more axes to the machine,” Birkle says.
“Whenever you add more than three axes, you give customers a chance to reduce dependence on forgings and castings that are increasingly difficult to get these days. They can machine from a solid block of material,” he explains.
Mazak’s most recent addition to its line of five-axis machining centers is the Variaxis 730 five-axis VMC. It features a 50-taper spindle for heavy-duty roughing and machining of castings, forgings, steels, and stainless. In a cutting test comparing it with a 40-taper Variaxis model, the 730-5X achieved cycle time reductions of 24% in machining an identical workpiece.
Lean manufacturing principles and VMCs seemingly go hand in hand with techniques developed by Datron Dynamics Inc. (Milford, NH). “Over the last several months and quarters, we’ve seen a definite change among our customers who are now looking to lean manufacturing techniques to remain competitive,” says President Walter Schnecker. “Our customers after all are competing with China, a country with an endless supply of labor.”
Datron Dynamic’s machines employ high-speed spindles to 60,000 rpm with small tools typically one quarter of an inch (6.35 mm) or under on a 30 × 40" (762 × 1016-mm) machine bed.
“What our customers can do with that real estate is what really matters,” says Schnecker. A cutlery plant simultaneously produces batches of knife handles on four different machines with a single operator due to the large bed size and the untended capability of the VelociRaptor VMC. Datron’s VacuMate workholding is a nonmechanical way of quickly securing the knife handle fixture so that batches of 28 handles can be finished with custom engraving.
Five-axis machining presents unique challenges when it comes to keeping track of the spindle and tool tip in space, and those challenges can affect the fixturing on a five-axis VMC. Mitsui Seiki USA (Franklin Lakes, NJ) has addressed the issue with its exclusive advanced five-axis dynamic fixture compensation function available in the Fanuc 31i control on its Vertex 550-5X VMC as well as other models.
Scott Walker, Mitsui Seiki USA president, explains that four and five-axis simultaneous machining of complex shapes, particularly simultaneously contoured aerospace and medical device parts, can sometimes be difficult to run in production because part form accuracy and surface finish are often the direct result of how well the part is fixtured.
In many applications, the part datum must be aligned perfectly at the “programmed center points,” which takes into account the orbiting points of the rotary axes, otherwise the part program must be reposted to offset the error in setting up subsequent parts. The results are usually the same: either the manufacturer has to utilize very elaborate fixtures, or the operator must be diligent enough to set the new parts perfectly back on center.
The Vertex 550-5X is equipped with the new Fanuc 31i control which optionally incorporates the advanced five-axis dynamic fixture compensation function. The operator can simply apply a work coordinate offset adjustment, and the control will continuously track and update the part programmed vector points dynamically in real time. In some cases, a spindle probe can be utilized to pick up the necessary part datums and apply the five-axis work coordinate offsets automatically.
Mitsui Seiki’s advanced function continuously calculates the offset vector for current position and offsets X, Y and Z-axis position accordingly in real time. As a result, not only can the machine be used for five-sided machining, but also four and five-axis simultaneous contour machining.
The roles of workholding and automation figure prominently in the ability of VMCs to be custom-tailored to the end user’s requirements. Hermle Machine Co. LLC (Franklin, WI) has developed its stable machine designs for its C series machining centers using a modified gantry design with power transmission through three staggered guideways, a mineral base, and three axes through the tool. Utilizing a swiveling A axis and a rotary C axis, the C 40 U dynamic machining center performs five-axis and five-sided machining for such workpieces as tools and molds, and parts for the medical device, optical, aircraft, and automotive industries.
Adapting an RS 6 robot cell to the C 30 U dynamic five-axis machining center creates a manufacturing cell. To minimize nonproductive time and further reduce costs where near-net-machining only is required, an NC-clamping yoke with 20 positions offers a cost-saving solution. The device offers high precision to five-sided machining in one clamping operation. Tool change times can be spread over 20 workpieces, allowing one operator to run multiple machines due to long run times.
Handtmann CNC Technologies Inc. (East Dundee, IL) produces five-axis VMCs that are constructed in a modular design. Douglas Gale, Handtmann vice president, explains that fully untended operation is possible in their PBZ line of extrusion mills. These systems can be integrated with automated material handling systems capable of handling stock lengths of virtually any length for automotive, aerospace or any business that machines aluminum extrusions, composites, or AL monolithic parts from solid bars.
“The beauty of the system is that the manufacturer can fully machine the parts in one setup, eliminating sawing and secondary operations that can add up to handling parts more than a dozen times using conventional machining,” Gale says.
Handtmann also produces a full line of single and dual-spindle gantry machines. The Gantry TS twin spindle system can run mirror-image or identical parts simultaneously or use both spindles on one part. For end users who don’t want to buy a machine dedicated to steel and one dedicated to aluminum, Handtmann machines feature an automatic spindle change system that allows the machine to convert from a high-speed 30,000 rpm HSK 63A spindle to a high torque 9000-rpm HSK 100A spindle in two-a-minute change time. In fact, the user can be simultaneously machining and changing spindles on both heads.
Enshu USA (Schaumburg, IL), which until 2000 was better known in this country for its large C-frame mold making machines, is a producer of VMCs that are used in high-volume-production applications, particularly in the automotive industry.
According to Enshu’s Mike Germann, typical applications include transmissions, engine blocks and heads, valve bodies, and differential cases, all parts that fit well into the a work envelope of 1 m × 530 mm. The company began as a supplier to the Japanese motorcycle industry, and still devotes half of its production capacity there to being a manufacturer of engineered parts to the automotive industry.
“We follow a modular path in our recommendations to manufacturers so that we can match production to the customers’ anticipated schedules, ramping up and considering takt time to achieve the volume needed,” says Germann.
Reliability of the system is a keystone of Enshu’s approach to machine and system design. “Our design philosophy targeted 5000 hr mean time between failure, and we have compiled an extensive library identifying the performance of components.”
Design innovations to produce a more stable, vibration-free machine structure were at the heart of the Driven at the Center of Gravity (DCG) concept, introduced by Mori Seiki USA Inc. (Irving, TX), in both its VMCs and HMCs. Confirming the design concepts, which originate with more than 400 developers in Japan, is the work of the Digital Technology Laboratory (DTL; West Sacramento, CA).
According to Adam Hansel, DTL director, DTL analyzes machine designs, verifies designs through computerized techniques including CAE and FEA, tests performance in a virtual environment, and recommends changes to developers.
“The aim of DCG is to achieve true high speed and reduce noncutting times, while reducing vibration during acceleration and deceleration,” Hansel explains. NV4000 DCG and NV5000 DCG vertical machines have found ready acceptance with manufacturers of electric appliance parts, aircraft parts, die mold applications, and general engineering manufacturers. For example, since its introduction, there have been more than 2000 installations of the NV5000 DCG globally.
The latest addition to the line of DCG verticals is the NV6000 DCG machine, which contains five ballscrews, two on the Y axis, two on the Z axis, and one on the X axis. The new machine was developed to address growing demand from the LCD and automotive segments of the die and mold industry where improved surface finish, contouring accuracy, and tool life are highly valued.
Higher spindle speeds and the latest version of its patented Super geometric intelligence (SGI.4) are enabling Makino Inc. (Mason, OH) to hard mill mold cavities. “Hard milling involved finishing a mold cavity in the hardened steel state rather than machining in the soft state, heat treating, and then reworking the cavity in a hardened state,” explains Makino’s William Howard.
The newest version of Makino’s patented super geometric intelligence technology, SGI.4 provides higher processing capability with higher machine acceleration/deceleration rates and finer resolution feedback to deliver high-performance machining.
SGI.4 anticipates what the servo lag or following error will be for upcoming toolpath changes. It then moves the machine tool to that point ahead of the actual feedback from the servo drive, compensating for servo error to achieve a more accurate toolpath.
Makino engineers have designed a patent-pending hybrid automatic tool-length measuring system available on the Hyper 2J, V22, and V33 machining centers to measure the position of the tip of the rotating machining tool at the submicron level. The tool tip is measured with a low contact pressure-contact sensor while the spindle is stopped. Another measurement for position is taken when the tool is rotated at machining speed using a noncontact sensor. The control unit then determines these two measurements and calculates the position of the tool tip. The obtained tool tip position is then used as tool data during the machining process.
Medical device manufacturing continues to generate innovative machining solutions with the introduction of new and unusual machining concepts. NTC America Corp. (Novi, MI), well known for its flexible manufacturing systems for the automotive industry, has introduced what it calls a “zero metal contact machine.” The Zµ3500 does away with linear guide ways, roller bearings, ballscrews and spindle bearings. Instead the Zµ3500 uses temperature-controlled and filtered hydrostatic oil in the hybrid spindle bearing, static pressure guides, and the linear motor cooling system virtually eliminates thermal distortion saturation time.
According to Larry Jones of NTC, applications for the Zµ3500 include medical devices, small high-precision molds for stents, electronics, and possibly aerospace applications. “It’s hard to imagine that you could push that table that weighs 500 lb (226.8 kg) with one finger; with the oil film there’s no inertia to overcome. When you change direction, there’s no lag, slip, or stick,” he says.
To demonstrate the micromachining capability of the Zµ3500, Jones points to the block of RC 40–50 mold steel 50 × 50 × 10 mm in which micro-diam 20 µm spires were cut with an aspect ratio of 100, meaning that the spires are 100× higher than the diam.
Chiron America Inc. (Charlotte, NC) is addressing the medical device machining market with five-axis machining capability and a combination of milling, drilling, and turning capability in its newest machine line. “The medical device market is looking for very good accuracy,” explains the company’s Stanley L. Pearson. “To achieve the required accuracy we’re using a tilt spindle for machining, and have added turning capability on the same machine, an FZ08 KST model on which we’ve added a V slide and a full rotary C axis. We can put a milling head on the left side of the machine or a turning spindle. The part can be fed with a bar feed, chucked, or held between centers and finished completely.”
The tilt spindle allows machining the backside of the part (the sixth side) and completing the part in one setup machining. “We’ve taken the same concept and put it on our larger Mill 800 VMC. It will allow us to turn parts to 65 mm, about three times larger parts than with the FZ08. With the Mill 800 we can handle larger medical devices like hip replacement stems, again turning and milling and drilling in one clamping,” Pearson says.
This article was first published in the June 2006 edition of Manufacturing Engineering magazine.