The highest levels of precision in metalworking technology today can only be achieved through a delicate balance of sound mechanical design, a powerful computer control and a highly-responsive servo system.

Today's world-wide competitive market has resulted in a quality-driven consumer. The customer demands products that will work right the first time, and he expects that they will provide levels of reliability that were unthought of just several years ago. As countries with lower-cost labor enter the industrial race, price competition is becoming more fierce. These factors have driven the industrial powers to focus on their strengths. They have begun to concentrate on more reliable end products and shorter development cycles of a more sophisticated nature. Statistical process control and continued process development is a way of life in many manufacturing facilities. There is a need to constantly improve part accuracy and finish, while at the same time, reduce scrap and manufacturing costs in order to remain competitive.

These changes in market requirements have created the need for a new generation of milling machines that are more accurate, faster to set up, more reliable, and have the potential to continue to be upgraded as applications require. The machine tool builder is faced with a series of performance trade-off decisions which is further complicated by the consideration of final cost of the product. Some builders have decided to target their machines for high performance while others have opted for lower accuracy and cost, or simple prototype work. Still other have designed their machines for precision applications. Here, we will address the requirements of precision milling, primarily because the technological advances in this area provide so many opportunities for future competitiveness.

Precision is defined not simply as the measured static linear positioning accuracy in a single plane, but as the machine's consistent ability to impart the required accuracy to the work-piece. The mill must be able to support the complexities of accurate contouring, generate superior surface finishes, and enable the user to deal with a rapidly changing list of materials and applications.

A precision milling system can be subdivided into three general areas. The mechanical design and manufacture, the CNC functions, and the capabilities of the servomechanisms. We will look at the critically important areas in each of the categories.

Machine Configuration

The basic configuration of the machine has a lot to do with its potential for precision. Knee-type machines cannot be expected to impart high accuracy because they do not provide adequate support of the moving saddle and table. Machines with quill-supported spindles do not keep the spindle rigidly contained, which can result in spindle droop in horizontals, or vibration sensitivity. Fixed bed, travelling column designs address some of these problems, but have the tendency to wear and reduce the capabilities of dynamic performance due to the need to move the column mass. The fixed-bed, vertical-spindle configuration, when approached with care in the early design stages, affords the highest potential for optimum performance. Spindle overhangs and compound stage stack heights must be held to a minimum in order to reduce the effects of geometric errors and optimize stiffness.

Mechanical Accuracy ... Behind The Covers

Unlike basic machine configurations, there are many areas that contribute to machine accuracy which cannot be seen. As accuracy requirements, spindle speeds and feed rates increase, the quality of the components and the care taken in assembly become more critical. The quality of the design and manufacture of machine tools can be measured by the attention that the builder pays to the principles of roundness, flatness, straightness, parallelism and squareness.

Roundness of ballscrew and spindle support pockets as well as rolling elements of linear guideways have a direct contribution to bearing heat, life, and system rigidity. Guideway surfaces, table tops, motor and spindle housings, and mating sections of castings require grinding, or in some cases, manual scraping, to provide adequate bearing surfaces. As these surfaces compress with their mating counter-parts, geometric distortion takes place, resulting in alignment errors. Some of these errors can be easily measured on the final product, but others cannot. One example is the alignment of the ballscrews and drive motors. Precision applications require higher servo responses which, in most cases, eliminate the use of couplings which provide a reduction in torsional stiffness. This necessitates that motors and drive housings be hand-scraped and fitted to the appropriate castings.

Guideways should be spread as widely as possible to provide maximum rigidity to the moving members. The straightness and parallelism of these way surfaces will affect the overall geometric accuracy. The straightness of the ballscrews and their alignment during installation can also have a direct effect on geometry. The variability of these parameters throughout the axis travel can also result in premature wear and stiffness reduction. The straightness of moving machine members has its greatest influence on accuracy when we consider the compounding effect of adding other perpendicular axes to the machine.