ONE SIZE FITS ALL

A Better Idea: At one major aircraft manufacturer, production problems occur in all sizes. This flexible manufacturing system solves them all by handling parts up to 240 in long and replacing three machining centers – that’s right, three – in a difficult “one off” work environment.

Our story begins with a Tier One manufacturer of large aircraft structural assemblies that are required, on a daily basis, to modify semi-finished and partially assembled detail parts. The parts are required for large airplane structural assemblies and are needed just prior to final assembly. Over time, this company began to search for an all-purpose machine that could quickly be stepped up to run a wide variety of parts that support several different airplane programs.

To better appreciate the complexity of this challenge, it helps to understand the difficult work environment that comes with the territory of being a Tier One supplier.

Aircraft manufacturers often start the production of parts for a specific airplane order well before all of the detail components for that specific plane have been fully defined. This is great for the airlines, but it greatly complicates the process of building planes to a tight schedule. Furthermore, aircraft manufacturers must have considerable flexibility built into their processes to allow for last-minute modifications to parts that will be needed shortly thereafter in final assembly.

In this sort of environment, the shop must have the ability, as required, to modify or rework every detail component needed for a specific structural assembly. In other words, on very short notice CNC part programs must be created for virtually every part used in that subassembly, while special tools and fixtures must be readily available and easily set-up in machines so that the parts can be manufactured just-in-time before they are needed at assembly. This is truly a “one off” manufacturing environment, where the first part must be a good part with no margin for error.

So the overall objective of this Tier One supplier was to replace three small footprint machining centers with a single machine tool that could quickly be reconfigured with tools, workholding fixtures and programs to manufacture the parts needed to complete a specific airplane build (ship) set. Specialized workholding, developed over many years for legacy (older model, mature) aircraft, had to fit onto the new machine, as well as new workholding needed for the latest model airplanes. Setup time needed to be minutes or, at the very most, hours . . . not days.

This shop wanted to decrease bottom line operating costs by reducing the number of people needed to run the machines, shortening the setup time and improving productivity. The existing machines could only manufacture small parts that fit a 20 in x 30 in work envelope. But the new machine, to be effective, had to handle these parts plus other ones as long as 240 in. There was neither budget nor floor space available to replace the existing machining centers with new ones and purchase a new long X-travel machining center.

During its search for a solution, the aircraft manufacturer began exploring an opportunity presented by Bertsche Corporation (Buffalo Grove, IL).

Having already developed a long X-travel, raised-carriage, fixed bed machining center (Xi-Mill) for single or dual work zone long part machining used in other applications, Bertsche’s idea was to expand the capabilities of this existing machine design by developing a new type of machine – christened the X-Flex Center – configured with multiple pallet receivers instead of the typical T-slot tables. This would allow the Tier One supplier to load multiple workholding fixtures into the machine and have them ready to run on short notice.

Equipped with a setup changer, the machine could quickly be reconfigured by pulling new workholding fixtures from storage and loading them into the machine to match constantly changing production requirements and schedule changes. Using the dual work zone feature, where each work zone has its own dedicated tool changer, the operator was able to load newly tooled fixtures in one zone, while parts were being machined in the other.

In keeping with lean operating principles, this concept eventually evolved into a flexible manufacturing system (FMS) that didn’t require all the automation, expense and overhead to manage. The machine was easily overseeable, pragmatic and affordable, and it allowed the manufacture of a wide range of parts with just a single machine tool. As the project unfolded, however, many divergent new requirements were identified as desirable and this began to conflict with the original “as concepted” solution.

Long parts, short parts, 4-axis machining, smart fixture control, automatic fixture recognition, integral thermal growth compensation, video camera actual cutting display, different materials, aluminum, titanium, composite (CFRP) parts, sandwich materials, materials separation, special water filtration, materials separation and other considerations arose, with each requirement mandating additional machine functionality.

To machine the long parts, the machine needed two T-slot tables, one for each work zone and the two used together for very long parts up to 240 in. It needed a fourth axis trunnion-style table, designed to accept smart fixtures up to 72 in long, fixtures with retractable clamps controlled by the part program. The machine tables needed to be reconfigured from T-slot mode, to 4-axis trunnion mode, to pallet mode and back again.

To meet these requirements, a zero point clamping system was incorporated into the base table, which allowed the pallet receivers to be decked with various long part handling special tables. The zero point clamping features meant that different tables could be quickly installed, with a high degree of repeatability (0.0002 in repeatability for a 120 in long table). Large and heavy special tables could be craned overhead, lowered into place, clamped and locked down to do production-ready machining in minutes.

For further flexibility, a vacuum workholding system was added to allow any of the pallet receivers to use both mechanical and vacuum workholding. Each work zone was equipped with a CNC part program addressable connection port, so that special fixtures with complex part clamping (part program actuated) could be controlled by the programmer.

Being a very large machine with two different work zones and machining parts that only 24 hours later were used in final airplane assembly also meant there would be zero tolerance for error or delay. A video camera view port was provided to allow the operator to see what was happening in one work zone, while the other zone was being setup for the next part.

A further complication revealed itself late in the project. Constantly changing and reconfiguring the worktables with different workholding fixtures also meant that the system was prone to operator error. To address this problem, a radio frequency identification system (RFID) read/write station was incorporated into each pallet receiver location. By means of reading the tooled fixture pallet tag, it was now possible to compare the “workholding fixture” installed in a given pallet location to the part program pulled down to the CNC. This insured that the program, part and tool matched. Additionally, the same system was used to scan the tools in the tool changer and could be used to verify the correct tools were available to machine the selected part before the actual machining cycle began.

Switching from aluminum parts to CFRP composite parts required a chip and water filtration system to convey both chips and sludge-like material. The waste material needed to be separated from the coolant. First, the chips from composite sludge, then the coolant suspended CFRP fines had to be filtered from the coolant. Material separation and disposal was not just an operational issue, it was also a facility and hazardous waste disposal issue. The machine was equipped with a flush and wash feature that allowed the entire work zone to be washed down and cleaned before the next material was machined.

For chip collection and removal, a conventional chip conveyor was used to transport chips out of the machining area. The chips were separated and put into a dumpster. The cutting fluid carrying liquid and suspended composite particles was pumped to a cyclonic separator, where the sludge was separated and fleece paper filtered for separate disposal.

For long part machining, “hole pattern” to “hole pattern” accuracy was critical and thermal growth of the part relative to the machine was factored into the design. Knowing the temperature of the part (so that appropriate length compensation could be applied) was critical when parts needed to snap together at assembly. To this end, each work zone was equipped with a material sample station. A sample piece of the material was mounted, its temperature measured and the information fed to the CNC for corresponding machining thermal positioning adjustment.

To interface with the shop floor managing network, a front-end PC equipped with bar code reader allowed the operator to scan in work order information and, together with pallet loading instructions, send the right part program for the part(s) to be manufactured as needed to meet daily production requirements. The operator loaded workholding-tooled pallets, as directed, to a specific pallet receiver location, while the system verified that the correct tools were available in each tool changer. The operator could then arm (enable) the work zone in which machining was to next take place and wait until the part(s) in the other zone had been manufactured. This process repeated throughout the day with nearly 100 percent spindle and operator utilization in a “one off” demand-driven environment.

To ensure the correct tools of size and length were being used, each ATC was equipped with its own tool length setter. Each tool could be measured, with offsets adjusted prior to machining. A spindle probe, one for each work zone, allowed fixtures and parts to be probed for location with corresponding work coordinate adjustments. A special marking tool allowed information to be written on a part in easily readable script.

While not every manufacturer is a Tier One supplier with these same complex problems, having such a large flexible machine to make both long and short parts makes sense for even the smallest manufacturer. As a result, any manufacturer who needs flexibility and reconfigurable worktables can quickly adapt his machine tool to meet a wide variety of such machining needs.