According to VNC senior engineer Neil Hathway, the unique thing about British Aerospace’s new programming system at its Brough machine shop, near Hull, is the way it ties together a variety of software. `We have not developed anything special,’ Hathway says. `It is not rocket science. But it does allow you to arrive at a streamlined, structured way of generating machining programs.’
The new programming system, which incorporates toolpath verification, NC program simulation and statistical process control techniques, took BAe 18 months to implement, but has already saved the company £1.5m. Hathway says it has helped avoid 12 or 13 fatal spindle-damaging collisions on Brough’s high-speed machining centres.
`The spindles cost between £12,000 and £13,000 to refurbish,’ he says. `Each major collision causes four days of downtime. This is where costs mount up. As we run the machines 24 hours a day, when one goes down we have to outsource the work at short notice and we get charged astronomical rates.’
The Brough machine shop has six high-speed machining centres – four Matsuuras and two Mazaks – which operate at speeds of up to 12m/min and 20,000 rpm. They are used mainly for making complex, three-axis aluminium alloy parts for aircraft sub-assemblies such as fuselage and wing parts. The parts are machined from solid, typically requiring about 60% of the metal to be removed. Cycle times vary from fifteen minutes to two or three hours.
`The ceramic bearings on high-speed machining spindles are fragile,’ says Hathway. `We don’t just have to avoid collisions but also to avoid a depth of cut which would cause too great a force on the spindle. The programmers have strict guidelines because it is imperative to get it right when we do the actual machining.’
The streamlined Brough programming procedure starts with the solid model CAD description of the part to be machined. This is created on Catia. Next – again using Catia – a `tool path’ is generated for the part and a verification exercise is carried out using CGTech’s VeriCut package. The tool path is then processed to make it specific to the individual machine tool it is going to run on, incorporating all the quirks of the machine’s specific numerical controller. Finally, the program is brought into a quality area where a machining simulation process is performed, using Deneb’s Virtual Numerical Control package. `We are using VNC as a total quality tool, which we think is unique,’ says Hathway.
Brough now has four Deneb seats running the VNC software on Octane workstations from Silicon Graphics. `They are used to guarantee the quality of the program to the shopfloor,’ says Hathway. `VNC’s total machine simulation mirrors the functions of the machine controller. Theoretically, it could replicate the entire operation of the machine. It also verifies the workpiece setup, the origin, where the operator locates the metal billet, the fixturing of the part and the tooling.’
Hathway adds: `We used to have a specific set of fixtures for each job, which could cost more than £1,000 each. Now, 90% of the parts will go onto a modular fixture and any part can be run on any of the machines.’
Once the program has been loaded onto the machine tool and the metal cutting has taken place, the machined part is inspected on a computer-controlled co-ordinate-measuring machine. Further machining is often needed to adjust a part’s final dimensions and finish.
The new system has improved programming efficiency. `In the past, a job would often go through four, five or six iterations before it could be approved,’ says Hathway. Sometimes the number of iterations would run into double figures, whereas now, the average is 1.4, he adds.
Statistical process control (SPC) is used to record and find the sources of errors, and eliminate them. `We measure key characteristics and look for trends,’ Hathway says. `Collision is the obvious error to eliminate because it is very costly.’
`SPC has enabled us to improve the percentage of right-first-time programs from 32% to 70% over the past 18 months,’ Hathway claims. `It demonstrated that what we were doing was worthwhile and we should continue with it.’ It has also helped to identify training needs.