Robots for grinding?
Robots for grinding?
The author W. Brian Rowe is Guest Academic Editor of this issue of Industrial Robot and Professor Emeritus, Director AMT Research Laboratory, Liverpool John Moores University, Liverpool, UK.
Keywords Industrial robots, Machine tools, Machining
In 1960, a university lecturer visited Austin Motors where I worked. He claimed all the technology was available to eliminate operators on the shopfloor and what were we doing about it? Only a few years later, the visionary, D.T.N. Williamson described the Molins Systems 24 concept for the fully automated factory. New high speed programmable machine tools had been produced to make this concept possible.
The concepts required for the automated factory have been around a long time. However, for precision grinding, the reality has been slow in arriving. Fifteen years ago, there were relatively few CNC grinding machines. Today, most grinding machines have programmable or CNC capability but skilled operators are still employed.
The reason for the lateness of fully automated grinding is that the process generally takes place at the high precision end of production. If the parts produced have to be very accurate, the machine tool has to be even more accurate. When the size tolerance for a part is 5 microns, a machine error of 10 microns can clearly cause problems. Of course, it is always possible to overcome errors which are precisely repeatable. This is achieved by calibration and compensation. Many machine tools already incorporate this principle to overcome machine errors. A bigger problem arises with variable errors arising from variable cutting forces and temperature fluctuations.
So what are the prospects for robots in precision grinding? Robots for precision machining seem to contradict Koenigsberger's principle of machine tool design for stiffness, that you have to close the force loop surrounding the process. Clearly, robots which have a broken force loop are incapable of precision grinding. Or are they? No doubt, someone will know different. Is there a way robots can be employed for precision grinding with economic material removal rates? What are the requirements? Does there have to be a lengthy trial and error planning process even for low accuracy grinding operations?
Special high speed,high-powered grinding machines achieve high removal rates as in drill flute production. Intelligent CNC grinding machines with in-process gauging achieve very close tolerances on hardened parts such as bearing rings and fuel injector needles. It seems hardly likely that robots will compete with either of these two types of situation.
Researchers are now actively examining the possibility of planning and operating robots in a more routine way for fettling and deburring. The accuracy required is not very high,but the flexibility and manoeuvrability of an industrial robot is ideal for tracing the complex paths which have to be followed. The problem is that the forces are variable and the robot arm is not an ideal machine tool element. The challenge is to provide a stable control system which can sense the position of the grinding tool relative to the workpiece, compensate for errors in position and damp out vibrations. It should also be possible for the part-programs to be programmed off-line to reduce machine set-up time.
