Single-spindle productivity: high speed, five-axis machining on single-spindle machine tools helps this aerospace shop meet the demand for rapid, single-piece work flow

Modern Machine Shop, March, 2004 by Mark Albert

Factories full of multi-spindle gantry mills used to be one of the main characteristics of the aerospace industry. These machines were busy cutting three, four or five aluminum parts at a time. Typically, parts were roughed on one machine and then finished machined on another, as batches moved slowly but steadily toward the assembly department, shipping or an inventory storage area. Considerable handwork was required along the way.

Most aerospace shops don't look like that anymore. They are much more likely to took like GKN Aerospace in St. Louis, Missouri. There, large single-spindle machines operating at much higher spindle speeds and feed rates have become the main machining resource for large aluminum aircraft components. Single-piece workflow has largely replaced batches. Handwork is on the way out.

A few years ago, when this plant became part of Boeing after its merger with McDonnell Douglas, the plant resembled the typical shop full of multi-spindle gantry mills. Batches of airframe components--mostly aluminum--were processed in several departments and traveled quite a distance through the plant until complete. Work in process was not considered a big problem.

The plant started to take on a new look when GKN Aerospace acquired the facility in January 2001. GKN Aerospace is part of GKN PLC, an international corporation that is now a major global supplier of automotive components and systems as well as a supplier of advanced composite and high performance alloy structures for aerospace. In this sector, GKN Aerospace is a prime contractor to Airbus, Boeing, Lockheed-Martin and several jet engine manufacturers. With the acquisition of the St. Louis facility, GKN became a core strategic partner to Boeing Military Aircraft and Missile Systems Group. The U.S. headquarters for GKN Aerospace were established in St. Louis at the same time.

Time Of Transition

"During the due diligence period of this acquisition, our organization recognized that much of the equipment in the factory was outdated technology," says Mike Beck, COO and site executive of the St. Louis site. "The business model that we acquired needed to be transitioned from a feeder shop into a worldclass, state-of-the-art company focused on high variety and lower volumes."

This transition was needed because the aerospace industry as a whole has been in transition. Military and commercial aircraft are being produced in smaller numbers today. Single components tend to be larger and more complex because designers are developing monolithic structures that replace assemblies of smaller parts. Tolerances are tighter, too, because manufacturers want to eliminate costly handwork and shimming. Both geometry and surface finish must be much better.

These conditions have prompted aerospace shops to adopt lean manufacturing. GKN Aerospace is a good example of lean manufacturing in action. Lean manufacturing is an umbrella term for a variety of techniques that focus on eliminating wasted time, wasted motion and wasted material. As a production strategy, lean manufacturing emphasizes streamlined work flow and minimal work in process. "We want to be able to make parts in the exact quantity and timeliness that meets our customers needs--no more, no less," says Steve Pickett, vice president and general manager of the Metal Structures division.

At the time of the acquisition, however, the plant had more than 50 three-spindle, gantry-type machines. These machines represented the mainstay of its aluminum machining capacity. It was clear that these machines no longer suited the production strategy required by low volume, high variety work.

According to Mr. Pickett, the company responded with a capital modernization program for Metal Structures that would eventually result in an investment of $17 million during the next 3 years. This investment included the purchase of new machine tools as well as the restructuring of the plant into product cells instead of segregated departments. This meant moving or repurposing equipment so that workpieces could be completed within a self-contained area of the factory. Part handling, setups and queue time could be reduced or eliminated.

The centerpiece of this modernization program is the firm's acquisition of three high speed, five-axis machines built by Henri Line in Granby, Ontario. IMTA Inc. (Rockford, Illinois), which handles sales and marketing of Henri Line machines in North America, facilitated the engineering and installation of these Powermill machines. The first of two vertical machines was installed in June 2002; the second in November 2002. A horizontal version arrived in May 2003 and will be online late in the first quarter of 2004.

One Spindle Outpaces Three

The impact of the vertical machines on production flow has been considerable. The two machines essentially replace the output of five of the three-spindle machines, says Mr. Pickett. The single-spindle machines have the advantage because of their high-power/high speed capability, the flexibility of five-axis machining and options for efficient setup.