Don't Take Grinding For Granted

Until the global economy scenario changes, the immediate future for subcontracting job shops in developed countries may look bleak. It appears that the more complex work requiring short-batch deliveries will stay in the developed countries, ­or at least in Western Europe, because more communication is needed between supplier and customer. Sending complex workpieces out to Asia involves an OEM having to overcome not only language difficulties, but culture, too.

If the production work is disappearing, maybe it is time to consider what else is possible. You read about the increasing complexities involved in producing medical equipment parts in "never-easy-to-machine" alloyed stainless steels, and the aircraft industry is always finding more difficult alloys to machine. Sometimes the traditional turning and milling does not do the work satisfactorily.

Most job shops have a surface grinder and a cylindrical grinder. However, some of these machines are quite old. Now there are complex techniques involving grinding with impressive metal removal rates in equally complex alloyed steels and nonferrous alloys. One of the problems with conventional grinding wheels is that if you try to take significantly deeper cuts at high feed rates, two things happen. Either the wheel tries to return to its original granular materials or you have a picturesque blue workpiece that is locally heat treated to the detriment of its original metallurgical characteristics.

Saint-Gobain has been spending about 3 years looking at the grinding process and has developed the use of highly porous, permeable grinding wheels with either a vitrified or an organically bonded structure. One of the company's objectives was to develop an aggressive grinding wheel technology to creep-feed grind those sticky, long-chipping, nickel-based alloy parts in a machining center. The wheel used has a widely spaced, aluminum oxide grain held in a vitreous matrix. The high porosity of Saint Gobain's Vortex grinding wheel absorbs large quantities of grinding coolant and diffuses it into the cutting zone. The result is that heat is removed quickly from the chips, and clogging of the wheel's cutting edges is stopped.

If your shop wants to pick up some aerospace work, take note of one example where a subcontractor, using Vortex wheels, creep-feed ground a gas turbine stator vane slot 8-mm deep by 10-mm wide (0.32 inch by 0.4 inch) producing a three-fold increase in Inconel parts before the wheel needed replacing. The shop in question was using a 500-mm dia-wheel on a Blohm Profimat CNC grinder.

Similarly on automotive work, a 400-mm diameter Vortex wheel was used to produce a 6-mm (0.24-inch) groove in a steering part. The Vortex wheel produced 150 percent more parts and reduced production cost per part by 27 percent. Saint Gobain also claims that about 20 percent less power is needed for a Vortex wheel to produce a given metal removal rate when compared with conventional wheels. The Vortex wheels can be used on surface grinders and cylindrical grinders to produce jobs with higher metal removal rates and no burning.

Another grinding process in which European job shops are watching with interest is electrochemical grinding. Like electrochemical machining, the tool is electrolytically charged. In this case, the grinding wheel ­is charged to produce smooth and burr-free edges, so secondary-finishing operations can be eliminated.

Conductive grinding wheels usually have a matrix of copper, abrasive, resin, a DC power source and a reservoir of electrolytes. During grinding, the electrolyte is sprayed through a nozzle, so that the wheel fills up with the solution. This electrochemical cell oxidizes the workpiece surface, so that only a low grinding wheel force is necessary. The claim is that surface distortion is removed and grinding wheel life is extended. Electrochemical grinding eases the machining of metals having high strength-to-weight ratios such as Hastelloy, Inconel and Stellite. In the United States, it is worth talking to Everite Machine products that has already sold a number of electrochemical grinding machines to U.S. aerospace and medical OEMs.

Permeable grinding and electrochemical grinding are just two examples of relatively new, well modified machining processes that are worth considering if your job shop wants to explore diversification as a way to avoid losing work to overseas competition.