Finding the Sweet Spot in Vertical Machining Centers
Article From: June 2020 Manufacturing Engineering, Jim Lorincz, Contributing Editor
Machine tool builders ‘build out’ for precision, productivity on proven machine technology
The 40/20 vertical machining center platform is rightly regarded as the workhorse of job shops. To help their customers deal with increased competitive pressures, machine tool builders are improving on this established 40 × 20″ design, relying on advanced spindle technologies, fourth and fifth axis table-on-table configurations and high-speed, high-efficiency machining strategies.
Machine tool builders also need to keep up with other advances, according to Dante Payva, sales manager New England for GF Machining Solutions, Lincolnshire, Illinois. Better cutter technology and CAD/CAM machining strategies require machining centers that can match them. “Putting these advancements on traditional mills just doesn’t justify their purchase,” he said.
This article focuses on a selection of proven vertical machining centers (VMCs) and what has made them popular choices for a wide cross section of industries and applications.
High-Speed, High Performance Machining
GF Machining Solutions’ Mikron Mill S high-speed milling machine and the Mill P high-performance milling machine both feature polymer granite base construction with ten times less growth than cast iron and superior temperature control. The Mill S also includes linear motors for high-speed acceleration.
“Linear motors don’t wear like ballscrews and they enable increased machine speed while maintaining accuracy and reliability. If you don’t have a heavy foundation, you’re going to put vibration into the cutting process, affecting accuracy and surface finish,” said Payva. “Portal design means that all axes are supported through their entire travel at all times. Spindle speeds to 60,000 rpm make the Mill S especially well suited for a wide range of applications, including those where detail work like electrode manufacturing is required.”
Applications for the Mill S can range from low- to high-volume production work, according to Payva, with Mikron technology having a long history of success with the mold and die industry. “Our spindles are designed to run 24/7 at 100 percent of rpm, which is very important for the mold and die industry where programs might run over 10 hours,” he said.
That’s where the linear motor technology comes into play. “Not every machine is built for the long haul with the ability to maintain accuracy without destroying itself,” said Payva. “Eventually, the agility of a machine like the Mill S must be able to achieve certain chip loads and speeds of the X-Y-Z axes to keep up with the high rpms of the spindle.”
Linear motor technology on the Mill S reduces cycle times by 30-40 percent compared with traditional machining strategies of running at lower feeds and speeds, according to Payva. “By using high-speed milling strategies like trochoidal milling where a chip is cut away rather than hogging it out, we can run circles around traditional approaches to aluminum machining,” he said. These and other milling strategies not only increase machining accuracy and reduce heat generation, they extend tool life as well. “In the past few years, the Mikron Mill S line has become a medical industry go-to option for manufacturers machining titanium, cobalt chrome and stainless steel,” said Payva.
The Mill P milling machine is a gantry-style machine with a larger work envelope up to 32″ (812-mm) diameter, which incorporates fully supported double Y-axis ballscrews above the cut zone. The machine’s rotary table sits on a polymer granite base and uses beefier spindles with both 20,000 and 36,000 rpm available. “We trade off a little on the rpms for more horsepower and torque. As a result, the Mill P can handle tougher materials,” he said.
Both machines are designed with automation in mind and not as an afterthought, said Payva. “We don’t just drop a robot in front of the machine, blocking operator access. Access for automation is through the back or sides of the machine without blocking the operator.”
Last year Payva held two seminars on trends in high-speed and high-performance machining. “When I asked our attendees (business owners, machine shop leads and machine operators) what they were looking for in a machine tool, they answered that they wanted speed, accuracy and surface quality. We laughed because they wanted it all, the biggest bang for the buck.”
Machine Architecture For Versatility
Okuma offers a full range of machine tools, but the most popular machine in Okuma’s entire machine tool lineup by far is the Genos M560 bridge-style vertical machining center, according to Wade Anderson, product specialist sales manager, Okuma America Corp., Charlotte, N.C. “The Genos M560 is mimicked in virtually every Okuma VMC model,” he said. “The M560 is a premium VMC designed to run everything from aluminum to Inconel and titanium. We don’t build a machine and call it our titanium or aluminum VMC. The M560 VMC, with its 15,000-rpm spindle, is designed to cut them all,” he said.
The Genos M560 is a little bigger than the typical 40/20 class VMC. According to Anderson, it shares the same skeleton structure design as the MB 56 V, Okuma’s premium VMC. One of the key design differentiators is bridge-style construction. “If you look at one of our large double-column machines and you scale it down, it’s very similar to how the M560 is built. If you understand how the M560 is designed and constructed, I can talk about all of our VMCs because they all have the same type of functional features,” he explained.
According to Anderson, the advantage of bridge-style structure is that the X axis actually rides overhead, leaving the throat distance for the Y axis to be shallow and constant. “No matter where you move the table, it only moves in Y axis,” he said. “The spindle moves in X, so there’s no table-on-table stackup. The result is more rigidity and better accuracy because there’s no stackup of tolerances and no stackup of moving components. No matter where you move the work or the spindle, the harmonics of the machine doesn’t change. That means [with] any cutting process I set up I can usually run the same type of cutting parameters regardless of where my part is on the table.”
Anderson went on to note that the machine’s spindle is equipped with four different cooling channels. “We isolate as much heat as possible from the machine tool with our Thermo-Friendly concept,” he explained. The spindle is designed with a continuous and intermittent duty rating of 199 Nm.
Okuma’s open-architecture OSP CNC gives its customers the ability to create their own applications (apps), recognizing that today, with fewer skilled operators, new hires may have only limited experience in running controls. These apps are a way to address the issue.
“We use the acronym WITY: What’s Important To You,” explained Anderson. With the Windows operating system, “our customers can develop their own apps or run their company’s process sheet; they can create a user interface that allows them to punch a few buttons and create hundreds of programs, all being done by macros on the backside of the control.”
Anderson cited an app created for a company that produces automotive axles where the GUI allows any non-programming operator to create a part, and load and cut it in just a few scans and clicks. One scan of the router loads the data needed to create a part program, a click of a soft button generates the part program and loads the control, and the third button starts the manufacturing process.
The newest addition to the Okuma M series of VMCs is the Genos M660-V, which features the ability to add fourth- and fifth-axis rotary tables and a 60 × 26″ (1,524 × 660-mm) table workspace. Two spindle options are available: a CAT-40 BIG-Plus 15,000 rpm or a CAT-50 BIG-Plus 12,000 rpm spindle.
Heavy Cutting C-Frame, Bridge-Type VMCs
Toyoda’s vertical machining center lineup includes C-frame, bridge-type and five-axis box-way VMCs. “Our VMCs are well known for their heavy cutting capabilities,” said Randy Ragnini, director of sales, JTEKT Toyoda Americas Corp., Arlington Heights, Illinois. VMC models include 40-taper, 40 × 20″ C-frame machines with 8,000-rpm geared headstock, 50-taper, 6,000 rpm geared head with 400 lb-ft (542 N∙m) of torque.
“Our C-frame VMCs are rigid box-way machining centers with high torque and high horsepower spindles,” Ragnini said. “We are best known for box-way, geared-head, heavy-cutting, high-horsepower machines. They have over 56 hand-scraped surfaces and the geared heads feature a five-spline shift gear shaft. When you switch gears from high to low, five splines support the shift gear rather than a single keyway, which is common in the industry. When that key breaks, the transmission is dead in the water. I haven’t seen one of our gear boxes break and don’t expect to see one break any time soon.
The strength of the design of our box-way machines is their heavyweight castings. If you strip down our C-Frame machines, they have the widest width to the outsides of the two box-ways to support the Y axis. The physical base of support for that Y axis is critical. We don’t minimize the width of the support.”
In addition to C-frame VMCs, Toyoda offers bridge machines with moving tables, ranging from 1.2 m in Y-axis travel and 2 m in X-axis travel with a double-column bridge to 10 m in X and up to 4 m in Y and everything in between. “The most popular size is the SP316 YM,” said Ragnini. “The Y axis is extended travel from 1,600 mm to 2,300 mm. It’s a five-face machine with an automatic head changer. We have a 90º head that can be changed automatically. Tools in both the horizontal and vertical spindles are changed off the same ATC. It’s the most popular bridge machine. It’s basically 120″, 3 m, in X travel, by 1.6 m in width on the table with the ability to do five-face machining.”
Toyoda uses THK heavy-duty linear guides on these larger machines for their load-bearing capacity and reliability. An important piece of auxiliary equipment on every Toyoda VMC is a spindle oil cooling chiller. The chillers are tied into a sensor embedded in the machine’s casting. “It eliminates the need to add linear scales for accuracy. We rely on the design of the cast iron and spindle for thermal control, regardless of the climate that the machine is running in,” said Ragnini.
Flexible, Kentucky-Built VMCs
Mazak, a machine tool builder based in Japan, is building vertical machining centers at its U.S. headquarters in Florence, Ky. These are not commodity machines, according to Rick Bramstedt, product manager, and Kenzie Roaden, product specialist. Designed to process large parts or multiple small workpieces in single machine setups, they provide shops options to achieve a balance of speed and torque.
According to Bramstedt and Roaden, a significant contributor to the machine’s productivity is a traverse rate of 1,654 ipm (42 m/min) in the X, Y and Z axes. Plus, a servo-driven automatic tool changer can be combined with an optional 60-tool magazine to bring even more versatility to the shop floor through reduced tool setups, which equates to increased spindle uptime.
The VCN-570C is equipped with the MAZATROL SmoothG control. The user-friendly control offers EIA/ISO programming as well as MAZATROL Conversational programming that makes it easy to generate simple or complex programs for drilling, milling or tapping operations, according to Mazak.
The control incorporates various advanced programming functions that allow it to offer ease of use and ensure high-speed, high-accuracy machining performance, according to Mazak. Some of these include variable acceleration control that calculates optimal acceleration for a combination of axes, intelligent pocket milling that engages a high-efficiency toolpath when milling part cavities, and smooth corner control that makes cutter path adjustments to help shorten cycle times.
The most recent Mazak VMC addition is the Kentucky designed and built VC-500A/5X VMC, and one of the VC Series machines that features a trunnion-style tilt table. The 500A/5X VMC is designed for processing small, complex parts via full five-axis machining. The VC-500A/5X features the MAZATROL SmoothX CNC for programming complex parts, and the control’s advanced functions ensure the shortest cycle times for simultaneous five-axis operations, according to Mazak. Axis travels of the VC-500A/5X accommodate part sizes up to 19.88″ (505 mm) in diameter and 12″ high (305 mm). Rapid traverse speeds for the three linear axes are 1,181 ipm (30 m/min).
In addition to its VCN and VC VMCs, Mazak offers its VTC Series of vertical traveling column machines, also manufactured in Kentucky. VTC Series machines feature fixed-table designs and offer both CAT 40 and CAT 50 spindle connections along with multiple table sizes. There is an optional table partition that is removable and allows the flexibility of a standard machine and that of two pallet changer style machines. Spindles range from 6,000 rpm to 20,000 rpm.
Fast, Affordable VTX Drill/Tapping Centers
The Tongtai VTX series of drilling and tapping centers from Absolute Machine Tools Inc., Lorain, Ohio, provide high-precision, high-volume machining capability, characteristics that are especially valued due to the low availability of skilled labor or during periods of significant economic disruption. Absolute offers assistance in engineering and building custom-tailored automation, robotics and turnkey machining systems.
“Cobots and robots are becoming ever more important to customers as they evaluate the current economic impact of employees having to be home because of the COVID-19 virus. Shops still need to make parts, and, for that part, remain in business, and these VTX machines will help in the absence of human labor,” said Courtney Ortner, chief marketing officer for Absolute Machine Tools. “Absolute can easily integrate its line of cobots to offer an affordable partner to ensure production throughput remains constant.”
The Tongtai VTX series of drilling and tapping centers are for high-precision, high-volume machining applications. The machines have a 87 × 63″ (2,210 × 1,600-mm) footprint. Rapid traverse speeds are up to 2,362 ipm (60 m/min) in all axes with acceleration of 1.2 g. The fixed column/moving table type design, with Meehanite cast iron frames and roller-type linear slideways, provides rigidity and fast, precise movement, according to Absolute.
Two types of high-speed ATC systems are available. The 21-tool turret-type ATC on VTX-5 and VTX-7 models offers a tool change time of 1.4 seconds tool-to-tool and 1.9 seconds chip-to-chip. VTX-5A and 7A versions have a 20-tool arm-type ATC system that reduces tool change time to 0.8 seconds tool-to-tool and 1.3 seconds chip-to-chip. A Mitsubishi M80A control is standard and is suitable for applications ranging from production to high-speed machining. A FANUC OiM-F control with a 8.4″ (213-mm) LCD is also available.
VMC for Die/Mold Accuracy, Surface Finish
The latest vertical machine tool from Methods Machine Tools Inc., Sudbury, Mass, is the high-speed OKK VB53a VMC for die-and-mold machining. It is designed to minimize vibration and maximize accuracy, aiming at delivering superior surface finishes for high-end die and mold applications and precision parts machining, according to Methods. Features include a rigid machine base, wide-size linear roller guide and fine-pitch, high-resolution ball screw that increases machining precision. A 0.05-μm resolution linear scale enhances positioning accuracy. OKK’s “Soft Scale Cube” technology features thermal sensors that monitor and compensate for any displacement in real time.
An externally located, isolated oil cooler removes a common source of machine vibration and results in better die and mold quality. A coolant-sheltered work environment protects the machine casting from chips and coolant to avoid thermal distortion. The Y-axis shutter reduces chip accumulation and operational downtime, while coil-type conveyors clear out chips quickly and efficiently. An automatic grease lubrication system ensures there is no waste oil contamination of the coolant.
The OKK VB53a features a FANUC F31i-B control together with Hyper HQ control technology, which is critical for efficient and accurate manufacturing. Hyper HQ control consists of a 64-bit, high-speed RISC processor, including a look-ahead multiple block (multi-buffer). It automatically detects the corner on parts from the NC part program and controls the feed rate so it does not exceed the machine’s permissible acceleration rate.