Machine tool for large parts does CMM double-duty

  • 07-May-2010 12:05 EDT
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Martin Thorden, Volvo Aero Connecticut Engineering Manager, stands near the Zimmermann FZ42 portal milling machine.

Volvo Aero Connecticut specializes in the machining of large components—none bigger than the 11.5-ft-diameter fan casing for General Electric’s 127,900-lb-thrust GE90 engine that powers the Boeing 777. The company claims it is the largest fan casing in the world.

Other large products include fan and compressor structures, compressor rotors, low-pressure turbine cases (LPT), and military parts, usually from aluminum and titanium but also from Inconel and Waspaloy. Parent company Volvo Aero produces numerous titanium fan cases for Rolls Royce engines.

With the large workpieces, high material removal rates, and challenging geometries inherent in aerospace metals manufacturing, coupled with substrates such as titanium that are often difficult to machine, there is always a need for powerful machining with superior accuracy. Such conditions present substantial hurdles at aerospace supply companies as they seek to maintain that delicate balance between these seemingly opposing concepts.

As part of its program for the GEnx engine, to be used on the Boeing 787 Dreamliner, Volvo Aero Connecticut recently added a Zimmermann FZ42 machine tool to its Hartford, CT, shop complex. The five-axis, five-side portal milling center joins other five-axis mills, vertical turning machines, four-axis machining centers, and a deburring robot in the shop.

Key features of the FZ42 that led Volvo to make this acquisition include:

• The machine’s rigid design

• Very large working areas of 177 x 197 x 59 in.

• High dynamic stability for machining very heavy and large workpieces

• Fast linear feed rate of about 2362 in/min.

• Large swivel ranges within the rotational axes

• Slim profile milling head with patented Zimmermann Multiple Spindle Technology (MuST) changing system.

In addition, this FZ42 carries a high-accuracy package that provides optimum temperature control of the structural machine parts through the use of a special fiber-reinforced compound in the massive side columns. Also included are built-in cooling ducts in the portal and Z-axis slide, independent cooling circuits for the A- and C-axes and spindle, ground surfaces on the guideways for the Z-axis and side columns, and a double-pitch measuring system on the A- and C-axes.

During the course of manufacturing, thermal expansion of both machine components and workpieces can substantially impact the positioning accuracy of the machine, the stability of the workpiece, and the resulting effect on finished-part dimensions. The unique combination of temperature control and mechanical accuracy in the machine construction were deciding factors for Volvo in its acquisition of the FZ42.

Said Martin Thorden, Engineering Manager at Volvo Aero Connecticut: “These features, combined with the onsite machine setup provided by Zimmermann, were very important to us, especially on this new GEnx project.”

Thorden noted the importance of the control on the machine, a Sinumerik 840D CNC from Siemens.

“We see a very big benefit in how well we are able to control the tool with the CNC,” he said. “We have been able to use the control to take on additional tasks that were previously handled by our CAD/CAM system.”

Specifically, he cited the unconventional approach Volvo took in machining the big fan cases. The machine was installed without a rotary table, which resulted in more than $500,000 in direct cost savings plus material-handling time. As a result of that decision, the company believed it needed a top-of-the-line control to probe and accurately machine all the features on the part.

After machining, Volvo can transition the machine tool into a coordinate measuring machine for inspection, according to Thorden. That is because of the superior accuracy provided by the CNC, as well as the special 90° angle heads supplied with the machine (part of the MuST spindle technology from Zimmermann).

Onboard the machine tool, a massive 120-position tool carriage holds various HSK63 and HSK100 tools that are used to machine the substrates. Inside a series of four side stations, within the machine workspace, four additional specialty tools are stored and used for various operations in the machining and measuring process.

All motors and the drive package on the Zimmermann FZ42 at this Volvo Aero Connecticut facility are made by Siemens.

In a typical machining sequence, the milling head on the machine provides numerous other advantages, according to Thorden. For example, all roughing and subsequent finish passes on any material can be achieved in one setup, which reduces the preparation time and overall production costs by as much as 10%. There is also no need for additional machines and the corresponding material-handling time.

The head design allows for tasks other than milling by using the specialty heads stored on the side stations. Changing the spindle, instead of the milling head, enhances repeatability and further reduces the company’s spindle interface costs, according to Thorden.

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