When it comes to engine parts machining, one of the most important aspects is something that tends to get far less attention than its importance merits: the fixturing. The fixtures that hold the workpiece in place during machining must not only provide accurate positioning so that the component can be cut with micron-accuracy (~20 microns), but they must be sufficiently robust to keep that component in place as the cutting tools engage the metal with terrific forces. This goes to explain why fixtures are (1) long-lead time items, (2) expensive, (3) designed to work on particular parts (i.e., are "dedicated").
Consequently, the excitement of Pulak Bandyopadhyah, group manager, GM R&D Plant Floor Systems, and Yhu-tin Lin, staff research engineer at the GM Technical Center, is understandable as they describe the flexible fixture that they and their GM R&D colleagues have developed, as it provides the ability to make rapid configuration changes so that it can be setup to accommodate the machining of workpieces of varying sizes, such as 4-, 6- and 8-cylinder engine parts. Consider: they're performing in excess of one-billion machining operations per year at GM, so that means a lot of fixtures.
So what they've created is a fixture that is based not on hydraulics, as is typically the case in production machining, but on magnetic force. That's right, they're using an electromagnet to hold the components that hold the part in place. The pull-resistance is on the order of 12,000 lb.; the sliding resistance is on the order of 3,000 lb. Which means that what gets put in place stays in place. The rare-earth electromagnet (sourced from Tecnomagnete; Sterling Hgts., MI) is one that the magnetic force is generated once the power is removed, so that if there was machining and a power failure, the workpiece and fixture components wouldn't fly away. (Actually, there is a series of 1-in. magnets that are put together to form one large magnet.)
In the test setup—and plans are to put the fixture into a plant during the first quarter of '05—the fixture is a tombstone mounted on the table of a Makino A99 horizontal machining center. The backside of the fixture has a series of slots into which the fixture components (clamps, guide rails, supports) are fitted. When a fixture is to be constructed on the front of the plate, the machining center's spindle is used to select the appropriate pieces from the slots in a way analogous to selecting cutting tools from a toolchanger. In effect, the fixture is put together through instructions from the CNC control. According to Bandyopadhyah and Lin, this use of the tombstone fixture to store the tooling elements will not be used in the production setup, as the select component-rotate table-insert component-rotate table-select component. . . routine is far too time inefficient. For production, they're looking at a five-minute fixture change. Compare that to the months it takes now.
Apparently, they've been trying all manner of ways to fixture parts in place and while people have thought about the magnetic approach, it had seemed as though it wouldn't be up to the rigors of machining. Clearly, that challenge has been overcome.