Large-Part Machining Tests Horizontal Machine Tool Capabilities
Manufacturing Engineering Magazine February 2019
By Jim Lorincz

Versatile platforms handle processing with maximum efficiency

Horizontal machining centers (HMCs) are versatile four-axis and, increasingly, five-axis machine platforms that maximize processing of multi-sided large parts by minimizing part handling. They’re used in industries as diverse as aerospace, automotive, power generation and general engineering.

With their near-universal production capacities, HMCs offer many choices to manufacturers, making them less susceptible to the cyclical ebb and flow of any one industry, such as oil and gas. HMCs are readily automated. They are typically equipped with twin pallets for tombstone-mounted parts, a single workpiece, or they can be automated with pallet pools of six or more pallets. For unattended lights-out production, linear flexible manufacturing systems (fed by stockers and tended by rail-guided, overhead gantry robots, or pedestal-mounted six-axis robots) are available. Single large workpieces range from engine blocks to oil field drill bits, pump housings and manifolds, and families of tombstone-mounted components.

Matching Machines, Large Parts

When it comes to machining large parts, rigidity of the machine and its fundamental construction are critical to process efficiency, accuracy, and consistency, according to Sal Swierczek, assistant sales manager, Kitamura USA Inc., Wheeling, Illinois.

Kitamura’s HXiG series of HMCs range from 250 to 1250 mm table size with 8,000 rpm or 12,000 rpm four-step, gear-driven spindles that deliver 430 to 900 ft-lb of torque for cutting exotic materials. These HMCs feature oil-cooled dual ballscrews on boxways that are in-house induction hardened and ground, according to Kitamura’s patented process to sub 4 µm surface finish for superior mating, accuracy and long-term reliability. Craftsmanship in handscraping techniques allow for consistent accuracies of ±0.002 mm full stroke and ±0.001 mm repeatability.

The first consideration, of course, is that the workpiece fits the machine’s work envelope, whether loading one part on the table or loading multiple parts to increase throughput. Key machine selection criteria include machine capacity (including overall machine size), spindle power, rigidity, the number of tools, magazine size selection, and tool taper and sizes. Kitamura horizontal machines have a standard two-station APC [automatic pallet changer] with an option to increase the number of pallets with pallet pools in the field.

The ability to automate answers one of the challenges facing shops today, and that is the difficulty in finding skilled operators and a workforce that can handle lights-out machining.

Horizontal machining centers offer much-needed flexibility in part processing. Compared with a three-axis machining center, full four-axis machining centers minimize part handling, reducing the number of times a part has to be flipped from six to two. With the fourth axis on the machining center, the operator can approach the part from the front and both sides and then reposition it for final processing.

Machines for Large Parts

For large part machining, Methods Machine Tools Inc., Sudbury, Mass., offers two horizontal machining center lines: Niigata and KIWA Japan. The KIWA Japan line is known for high-speed, high-reliability machining typically for job shops and Tier One and Tier Two automotive shops. The Niigata product line is designed for heavy-duty cutting applications where low-frequency machining and heavy metal removal rates are required.

The Niigata product line includes the five-axis HN50E-5X and HN63E-5X, the four-axis HN50 through HN1600, the HN800 through HN1600-BAR and the HN800 and HN1000-FC facing and contouring machines. Machine platforms are 500 mm through 1,600 mm in table size. Spindle taper sizes are CAT-50 BIG Plus, HSK100A, with spindle options of 6,000, and 8,000, 12,000 and 15,000 rpm.

The industry right now is trending toward five-axis platforms in horizontal machining centers, which Niigata offers in both 500 and 630-mm sizes. In the aerospace and energy industries, Niigata focuses on low-frequency machining applications, which benefit from the natural dampening characteristics of the machines. Low-frequency machining is becoming a popular phrase when it comes to high-torque machining of titanium and Inconel for better tool life and longevity of the spindle.

The ability to dampen vibration of cutting forces is naturally inherent in the machine’s castings and metal-to-metal contact surfaces that are hand-scraped for mating surface finish. Typical applications in the energy and power generation fields include flanges, fracking heads and blisks.

In large part machining with HMCs, diametric accuracy of the travel in each axis and rigidity are especially important to achieve precision and repeatability. The beauty of five-axis machining is that you don’t need an elaborate fixture or custom workholding unless you’re machining a unique part that might require a window-type fixture or some special method of clamping.

Application-Focused HMCs

Today, demand for large parts is definitely coming from aerospace, which is a big driver right now, both in the jet engine family of parts and structural components. Other target markets include mold and die, outer space and defense and industrial gas turbine sectors.

Our horizontal machining centers, featuring 1 m and larger pallets and companion table configurations that can accommodate weights from 1,000 to 10,000 lb [453-4,535 kg], are considered our large part machines. It’s important to note that our machines in this size range all stemmed from customers driving the development. We didn’t create the machines and then search for markets. They are all derived from a customer’s requirement for a specific application.

To be considered for large-part manufacturing, Mitsui Seiki machines all share the same principal technical features. There are two categories in my mind—large parts that also require large amounts of material removal and parts that are simply awkward and require lighter cuts. Mitsui Seiki operates in the large-part sector that requires heavy-duty cutting of tough materials. As such, our machines have a lot of mass, stiffness, large box guideways, large-diameter ball screws for ample thrust, robust axial torque and special servo motors to move the part in the work envelope and accommodate the necessary cutting forces.

All of these features, plus control and construction precision are required to deliver the proper performance for the application, according to Hudson. He added that the fixture is an important element in large-part manufacturing, and is often as substantial and weighty as the workpiece it’s holding.

Automation Made Easier

While setting up automation with any type of machine is complicated, HMCs seem easier to automate because of the volume of their production output.

The speed of recovering the dollars invested is always the primary concern in HMC purchasing. Popular sizes are 500/630 mm with a minimum 60-120 tool capacity for small job shops, and 240 tools and up for major factories. Greater tooling capacities allow for more complex jobs with minimum setup time, especially with the multi-pallet cell configuration.

Productivity begins with the highly rigid T-base foundation with rib construction. YCM combines this foundation along with a one-piece cast spindle headstock and a double-wall column design. The direct-drive spindle features precision ceramic ball bearings, which allows for a higher cutting dynamic and higher spindle speeds.

High-precision roller-type guideways on all axes allow for rapid rates of 2,362 ipm (60 m/min) and cutting feed rates up to 1,575 ipm (40 m/min), according to Gay. The precision pallet indexing system accommodates a six-pallet APC for high throughput of a single component or families of parts.

All of these features allow utilizing the latest in cutting tool technology, improving part processing and lowering total cost of ownership.

Large Part HMCs Built in America

DMG Mori, Hoffman Estates, Illinois, manufactures four models of horizontal machining centers in its Davis, Calif. factory. The NHX 4000 and NHX 5000 are third-generation, 40-taper machines with 400 and 500 mm pallets; the NHX 5500 and NHX 6300 are second-generation, 50-taper machines with 500 and 630 mm pallets.

Their horizontals are designed for both the heavy cutting and high accuracy requirements of our customers in the energy, aerospace, industrial equipment, and contract manufacturing industries

Typical workpieces include big valves and turbine parts for the energy industry; engine, differential and transmission housings in industrial equipment; and structural components in aerospace. Materials range from titanium or stainless steel to cast iron or aluminum—all depending on the application.

Capability in Construction

Okuma’s horizontal machining centers, like the MA-600HII, are popular choices for automotive, die mold, energy and aerospace industries. They deliver accuracy and stability to machining large parts because of their consistent thermal stability. Machines are tested for their thermal profile and data about possible thermal deformation are fed back into the control in real time for any necessary compensation through Okuma’s TAS (thermal axis stabilizer) software.

The MA-600HII is a 50-taper machine with a 6,000 rpm spindle and standard 40-tool magazine that can be upgraded to 60-tool and even up to 285- or 400-tool, so it is ideal for job shop or production manufacturing. The MB-8000H is virtually the same platform with an 800 mm table, and the next size up is the MB-10000H with a box-in-box design rather than a column.

Okuma machines are hand scraped on each mating surface for increased accuracy and productivity over a longer timeframe for decreased costs and maximum profitability, said Burrell.

Typical oil field components like pump housings benefit from Okuma’s unique Turn-Cut capability. Turn-Cut is a programming option available on Okuma HMCs that allows the machine to create bores and diameters that include circular and/or angular features. This enables users to turn features on large, unbalanced parts on the same platform where standard CNC machining functions are performed. Turn-Cut eliminates the need to purchase special-purpose machines, tooling, fixtures or add-on components.

Turn-Cut does not affect the performance or longevity of the machines. When heavy milling is required, the Okuma HMC equipped with Turn-Cut is still as capable as ever, according to Okuma. Most importantly, Turn-Cut allows for processing work without the need to use a secondary machine for a turning application. Parts such as valves, pipes or manifold-type parts are ideal candidates for Turn-Cut.

Full Fourth Axis, Automation

Mazak Corp., Florence, Ky., offers a full line of HMCs in addition to its large bridge-style vertical machining centers, five-axis verticals with turning (INTEGREX) and its VARIAXIS five-axis trunnion-style machine, according to product specialist Jared Leick.

Large-size part machining with 50-taper spindles is done on machines with 800 to 1,600 mm tables/pallets, including our Quill series HMC with 1,250 to 1,600 mm tables, as well as our HCN-6800, with a 630 mm table for large engine blocks for automotive and energy components.

The 50-taper line includes three additional models in the series (HCN-8800, HCN-10800, and HCN-12800), with 10,000 rpm standard and optional 8,000 rpm and 6,000 rpm 50-hp spindles for higher torque machining of titanium, stainless and other exotic metals. For machining of large aluminum structural parts for the aerospace and semiconductor industries, a 16,000 rpm spindle is available

Mazak offers full four-axis HMCs, which account for about 60% of all HMCs sold by the company, he said. Four-axis HMCs with standard 0.0001o of positioning allow rotating a two- or four-sided fixture for drilling compound angles, indexing 90o and rotating back. Full four-axis operation enables contouring and can be used to reduce cycle time, making processing easier, especially for complex parts.

The Mazak HCN-5000 HMC is now available with the company’s MPP system, a basic automation solution in a compact footprint. Shops that lack the floor space necessary to accommodate a conventional linear horizontal pallet stocker now have the ability to automate for continuous production.

MPP offers the convenience of a larger Mazak PALLETECH System while taking up to 30% less room. Servo motors on each axis, as well as a two-level pallet arrangement that keeps each pallet the same distance from the center of the stocker, ensure fast and stable loading/unloading.

Designed to expand as production needs grow, manufacturers can reconfigure the system’s standard 10-pallet stocker to house 16 pallets for increased output. With the large, heavy part operations of the HCN-5000 in mind, Mazak made this latest MPP its biggest version in terms of part size capacity. The system houses 500 sq. mm pallets that accommodate workpieces up to 800 mm in diameter and 1,000 mm high.