Training programs were necessary. The new and refurbished equipment requires a competent workforce. To ensure the availability of skilled operators, GKN Aerospace has linked with a local junior college which has a machinist training program.
They have a close relation with the school which is a potential source of not only machinists but maintenance personnel as well. There is a cooperative interchange between GKN Aerospace and the school faculty to ensure that course work matches the company's needs.
Deburring is essential for both ease of assembly and ensuring proper air flow over external mold-line surfaces. At GKN it's done in a vibratory media system. A recent addition is a unit that can take larger parts such as a wing spar measuring 8" X 12" X 20' (203 mm X 305 mm X 6 m). Part areas with very critical dimensions are masked to prevent dimensional changes.
Machine tools. The key machine is the Cincinnati Milacron (Cincinnati, OH) five-axis gantry. A gantry is the optimum machine to use for many of the F/A-22 parts.
These machines are undergoing a major refurbishing to bring them up to the standards needed for F/A-22 part production. All parts related to accuracy were reworked or rebuilt. Changes include the replacement of the original control with GE Fanuc 150i units, new drives and servos, and a full rebuild of the gearboxes, transmissions, and the 3600-rpm spindle packs. In the reworked machines quality, has improved 30% and there has been a 70% reduction in maintenance problems. The production Cpk is now averaging 1.3. The current goal is a runout of 0.0001" (0.003 mm).
This is an ongoing program. There is still a lot of older equipment to evaluate to see if it fits the core competency. The evaluation is expected to take another two years.
Among the newer machines are three units supplied by Henri Line' (Granby, Quebec, Canada). That company has supplied two vertical and one horizontal milling machine.
Mazak (Florence, KY) machines using their Integrex spindles are one of the more recent equipment additions. They have improved both manufacturing speed and part quality. An Integrex head allows access to more areas of the parts, which means more can be done in a single setup.
Before production begins, GKN Aerospace engineers have to match the part to be made to a particular machine. Most of the simple cuts are made on a five-axis gantry that cut multiple parts in one setup. However, the gantry is limited to cutting angles of 20º. For more complex shapes, the parts are machined on a single-spindle machine with a 40° cutting angle such as the Mazak or Henri Line' units.
The Henri Line' machines have 90-hp (67.5-kW) spindles operating at 24,000 rpm. With a 110º B axis they can really crawl over a part, and do five sides of a complex part in a single setup. When necessary, a proprietary version of shrink-fit toolholding is used.
Flood cooling is used in about 99% of the operations. It's preferred over other cooling because of the need for chip evacuation. Dry and mist lubrication is not currently used. In some cases access slots are milled in the parts to ensure complete chip removal.
Assembly required simplification. One of the benefits from increased accuracy by the GKN Aerospace operations has been to simplify assembly by the customer. In the past, setup jigs and fixtures were used to position parts for assembly, or to perform some drilling and machining to accommodate the assembly. These preassembly operations are no longer needed because of the accuracy of the GKN Aerospace detail parts.
Waterjet cutting provides two savings. First, there are no heat affected zones (HAZ). If the part were cut by other means, the HAZ had to be machined off. Also, there is a saving in metal. Parts can be nested more closely so there is less excess material. Normally, these parts are made to near net shape and a finished machining cut is required. Company engineers are looking for other opportunities to integrate this process.
Finish and match. Finish is an important issue on the wind-swept parts because the aircraft operates at supersonic speeds. Both the surface finish and the blends (joints between parts) are critical. To avoid mismatches, the finished surface of one part is feature probed and the data recorded. Those data are used to modify the toolpath when machining the adjoining piece. That way any tolerance buildup that might influence fit is eliminated.
Cutting tools. Monitoring is a big feature that helps ensure accuracy. Each tool carries a bar code that gives the tool's description and operating history (i.e. how much it was used, its expected life). This helps ensure that the proper tool is automatically selected for a given job.
Many of the tools are of an earlier McDonnell Douglas design that uses a special carbide alloy. It's a proprietary carbide material developed several years ago, but which still outperforms some newer materials. Virtually no inserts are used. Cutters are chiefly carbides and steel, some with specialized coatings.
Part holding. One way GKN Aerospace has found to minimize fixture problems is to make tapped holes in sections of the part that will later be cut away. These holes are used to bolt the part to the worktable. Since the bolt comes from behind the part, there is nothing on the part's surface to block the spindle's path. The cutter can reach five sides of the part, and only a single setup is needed.
In analyzing error GKN Aerospace engineers found wrong diameter cutters, wrong cutter radius, and wrong set length. These problems have been virtually eliminated by a three-step program.
First, the tools and checking gages needed for a particular job are positioned near the machines where they will be used. In addition, the operator has a card for each tool to simplify the checking of tool type and sequence of use.
Second, before the tool begins to cut it is programmed to make a pass around an aluminum test post. If the diameter, radius, or set length is off, the tool will hit the post and stop the machine.
Third, a Blum laser-checking system is used on Line' machines. It checks the size and position of the tool, and shuts down the machine if an error is found. This laser system is being considered for use on all machines.