Machine tool 101: Part 7, Machine tools of the future

Manufacturing Engineering, Jul 1994 by Aronson, Robert B

The push for cutter innovation is in opposing directions. Those with consistent jobs want cutters designed for specific materials and speed ranges while those with more general work want one tool to do more with less changing of tools and inserts.

"A lot can be done to improve tools including insert geometry, materials, and coatings as well as holders," says Knobeloch. "As the ranges of available spindle speeds and part feeds get wider and wider, holder design gets more critical and there are more problems with resonances and vibration in the holder. Ideally, we need to have holders that can work from 100 to 40,000 rpm."

One benefit of better inserts and stiffer turning machines is the process called hard turning. Recently, it's been replacing some grinding operations as well as opening up new applications. Instead of rough cutting, then hardening, and finishing by grinding, hard turning is done on a lathe after the hardening operations. It works with materials over R sub c 48 but is more effective in applications at hardnesses above R sub c 60.

MIGHTY MATERIALS

Machine tool builders, particularly cutting tool makers will be faced with a new set of problems as composites, particularly carbon-fiber-reinforced materials become more popular. They are already extensively used in aerospace and to some degree in automotive work. These applications frequently involve hand lay-up, castings, or tape, not parts machined from solid materials. Finding ways to efficiently machine parts from these materials will bring new challenges.

Another material change might be a move from aluminum to magnesium as the material of choice for lightweight parts. This, however, means machining in an inert-gas atmosphere, because magnesium is flammable.

The basic castings and fabrications used for machine tools are also under evaluation. Artificial granite and various composites are now used for machine bases and as a filler and damper in some machines to reduce vibration. A lot of alternate "artificial" materials are being proposed for conventional cast iron and steel for machine tool bases and columns. "Unfortunately," cautions Knobeloch, "as some of the newer materials solve some problems, they create new ones. For example, some with very low coefficients of thermal expansion absorb moisture. Although they don't cause expansion problems from heat, they distort because of absorbed water. The perfect material would have no response to the environment...neither temperature nor humidity... have high static rigidity, and excellent damping."

THERMAL TRAUMAS

Heat is a major enemy of accuracy and speed. It causes thermal expansion of critical machine tool elements, such as spindle bearings and ballscrews, that lead to distortion and limit speed. Machine tools handle heat in a fairly crude way by cascading cooling fluid over the tool and workpiece. Now there are several more sophisticated methods to prevent, or at least reduce, heat problems.

As mentioned earlier, machine tool designers remove some of the expansion problem by designing the elements so thermal expansion does not greatly influence tool position. For example, axial expansion is more easily compensated for than radial.