Hardinge Inc. : DESIGN, MANUFACTURING, AND PERFORMANCE VERIFICATION OF A SUPER-PRECISION TURNING CENTER.

DESIGN, MANUFACTURING, AND PERFORMANCE VERIFICATION OF A SUPER-PRECISION TURNING CENTER.

Machine Capability of a Super-Precision® (SP) TurningCenter may be, in general, defined by three major characteristics: productivity, accuracy, and reliability. This means structural and kinematical components of the SP machine must be stiff, accurate, and reliable for the machine tool to be adequate for the task. For this reason, Hardinge Inc. has established assembly procedures and developed theoretical and experimental tools to design and deliver to the market a family of cost effective machines that will perform as required by customer machining processes.

Hardinge Inc. has a long tradition of design and manufacturing of high precision and stiff spindles. A special assembly room and high tech instruments, such as spindle analyzer and spectrum analyzer, are used to insure the right preload in the spindle bearings and high precision of spindle axis run out (less than 0.000010" at the collet seat).

The base with high static torsion stiffness is the key to lathe stiffness in cutting point. Bases for Hardinge Inc. T-series machines have the optimal stiffness to mass ratio due to a wide base with stiff inner structure and rigid pockets connected to the outer walls.

The dynamic stiffness is defined by static stiffness and the amount of energy dissipated under load. It requires two variables: displacement and resistant force to dissipate as much energy as possible. Harcrete® (polymer concrete), which has high damping characteristics, is cast in the pockets of the base near outer walls that will have the highest displacement under cutting load. This approach provides maximum effect of added mass for structure damping.

Distances between the X and Z axes rails, slides dimensions, and linear ball trucks size are optimized to provide the required static stiffness of X and Z slides in the cutting point. The geometrical accuracy of X and Z slides is delivered by grinding of the rails support areas and specially developed assembly procedures. The position accuracy of slides is assured by using an oversized ball screw and direct drives that, together with linear guides, provide a low friction to drive system stiffness ratio. This low ratio is the main reason for positioning accuracy and repeatability of the slides.

In the case of an SP machine, high structural and kinematical stiffness allows successful use of linear scales as position feedback devices on both axes. With this type of feedback, high stiffness of drive system, and CNC control ability for position and straightness compensation, the achievable geometric accuracy of T-series machines can be compared with the accuracy of diamond turning and grinding machines. The cylindricity of parts cut on this machine is in the range of 0.000020"-0.000040" at 5 inch length.  These results were obtained on a production SP turning center that has a main spindle with 51 mm through bar capability, 12 station top plate with live tools, second spindle, and has 20 inches Z axis and 7 inches X axis stroke.