Aerospace Machining: It's About Accuracy And Time

This aerospace machining company found CNCs that make it possible to meet their customers' tight delivery requirements while maintaining high-quality finished parts.

Case Study From: 9/15/1998 Modern Machine Shop

In some businesses, looking for cost savings is not always the best option for profitability. The aerospace industry is one case where time is a more critical issue. Delays can wipe out any money saved in production by using less time-conscious vendors.

The parts are complex, and building a quality part at a competitive price and delivering it on time are prerequisites to compete in the aerospace market, which requires a company to have state-of-the-art machine tools that are highly accurate and highly reliable. Because of the complexity of the designs and the high cost of materials, most aerospace companies approach JIT in their delivery requests in order to reduce inventory costs. Likewise, machine shops want to be able to ship parts as soon as they are manufactured for the same reason. Any delays or missed schedules are expensive for both parties.

Established 35 years ago, DASCO Engineering Corporation is a well-established machining and assembly company that specializes in CNC machining of small- to medium-sized structural air frame parts and skins.

During the past year or so, the company has added four new NC machining centers, including three Mighty U.S.A. (Torrance, California) Viper three-axis bridge-type vertical machining centers and one Mighty vertical machining center. With its 83-inch by 41-inch table, the Viper 2100 VMC provides the size and durability, and features a heavy duty Meehanite casting, which makes it solid. In addition to its solid frame, the Viper 2100 uses large ballscrew assemblies for maximum rigidity and precision. The ballscrews are in the exact center of each axis, which effectively eliminates heat and lost motion. High torque AC servo units are directly coupled to the ballscrews, moving the axis faster and with more precision. Equipped with "state-of-the-art" AC digital motors, cutting time is minimized with its high reliability and precision movement. Likewise, its 30-tool arm-type automatic tool changer is described as one of the fastest in the industry.

The first job that ran on the Viper 2100 required around-the-clock operation for a whole month while it was machining titanium.

Because DASCO's manufacturing mix includes long and wide parts, three of the machines from Mighty are bridge-type machining centers. Two of the units are Viper model B4000XT extended travel machines with travels of 161 inches by 72 inches by 28 inches. The other bridge is a Viper model B3000 with a travel of 121 inches by 71 inches by 28 inches. Like the Viper VMC2100, column and base of the three machines are made of Meehanite high grade iron that is specially heat-treated to relieve stress and to ensure the structure is free of distortion for its lifetime.

An 60 degree slant beam evenly distributes 50 percent of the thrust force from the Y-axis to the massive column, allowing the spindle centerline to be only 8 inches from the beam surface. The oversized slideways ensure maximum cutting stability, and the overall design is excellent for heavy duty cutting operations. The spindle is supported with Class 7 precision bearings, and an automatic lubricating system in the gearbox facilitates continuous cycling of the lubricant oil. Oil, which is cooled by the spindle chiller, circulates through the spindle jacket to prevent heat generation inside the spindle bearing. In turn, these features ensure excellent heavy cutting capacity.

The Viper bridge VMC has a high-speed chain-type 42 tool automatic tool changer that is the pacesetter for the industry. The ATC unit uses a quality hydraulic indexing motor to store and retrieve tools and completely avoids any spindle or bearing damage while changing tools. A program controlled door on the tool magazine ensures complete protection from chips and dust during the tool changing operation.

Featuring large box ways, the machines also use square slideways instead of linear motion guides. Hardened to above HS 65 (HRc 50), all mating surfaces have Turcite-B to minimize stick-slip and wear out. Stick-slip, or stiction, happens when an element does not move immediately, and force has to build up to break the inertia and friction holding the slide in place.

The key to quality starts right at the beginning of the process. Customer data is received in the form of IGES files, from which the company develops and verifies NC programs, with all machining operations working off their customers' original data.

Like most machining companies in the aerospace market, DASCO takes inspection seriously. In addition to using verification software before machining, it uses a three-axis coordinate measuring machine after machining. It has one Mitutoyo that handles ports up to 40 inches by 120 inches by 26 inches and a Checkmaster for smaller parts (36 inches by 26 inches by 24 inches).

DASCO found that Mighty CNCs made it possible to meet their customers' tight delivery requirements while maintaining high-quality finished parts. MMS

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