Machine Design

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GRANITE BASE
Hembrug has been successfully using different kind of materials for machine structures, like cast iron, concrete, epoxy granite but also natural granite. After extensive tests and studying the physical data it turned out that natural granite is an excellent construction material for ultra precision lathes and offers some significant advantages. Natural granite is completely stress free after many millions of years of natural stress relieving, has very good damping properties and a low coefficient of thermal expansion, thus improving the thermal stiffness. Moreover natural granite has a long-time stability without shrinkage, is corrosion free, non magnetic and the material allows the accurate machining to close tolerances and high figure accuracies, which is absolutely necessary for ultra precision machine structures. This apart from other physical properties such as high E-modules and low density.
Natural granite base:

• Good damping properties
• High thermal stability
• High static and dynamic stiffness
• Free of stress
• Corrosion free

All these aspects make natural granite ideal for construction material in the Mikroturn machine series, especially where an accurate stable datum is required such as machine base and headstock housing.

HYDROSTATIC SLIDES AND SPINDLE

Hembrug has more than 40 years of experience in the design and manufacturing of ultra precision machines with hydrostatic bearings and guideways. Compared to conventional bearings and guideways, the Hembrug hydrostatic system offers many advantages.




It is obvious that main spindle bearings and slide guideways are the most important machine elements in a lathe, determining for the greater part the lifetime of the machine and the accuracy of the workpieces to be machined. The accuracy and the stiffness of the entire machine is to a large extent dependent on the main spindle. Consequently, the bearing arrangement is of essential importance.The hardened and ground main spindle is a pre-mounted unit (cartridge type) and has hydrostatic bearings. In the front, there is a combined radial and axial hydrostatic bearing and in the rear, a hydrostatic radial bearing. A pneumatic labyrinth ring maintains an adequate, contactless seal against oil leakage. The inlet oil temperature is controlled within 20 degrees C for good thermal stability. This bearing arrangement guarantees a spindle run-out of 0,1 micron for the horizontal machines and a spindle-run of 0,2 micron for the Vertical machines. The location of the axial bearing is very close to the spindle nose, thereby minimizing spindle growth errors in Z-direction. The favourable short overhang of the spindle nose highly contributes to the stiffness between cutting tool and workpiece.

Proof of a wear free hydrostatic main spindle
The below mentioned charts shows the advantage of a hydrostatic main spindle having run for 16 years with any metal contact. The main spindle of the in 1993 manufactured Super-Mikroturn CNC still has a roundness of 0,091 micron in 2009. Our horizontal machines are standard specified with a roundness of 0,1 micron when sold.


 

The hydrostatic system in the Mikroturn Machine series is far superior to any other conventional system. All slides are sized to be very rigid and to have hydrostatic, flat guideways. Each slide is hydrostatic guided over the full travel in both the horizontal and the vertical plane, resulting in a good absorption of the cutting forces. These hydrostatic guideways, in combination with the symmetrically placed ultra precision ball screws, provide the slides with ample stiffness needed for finish hard turning and at the same time with enough sensitivity for moving the slides from standstill over the smallest increment of 0,01 micron.
 

Tool life
The chart shows the advantage on tool life when using a Hembrug Mikroturn machine with a hydrostatic main spindle and  hydrostatic guideways. Compared to machines with a conventional bearing system tool life is almost doubled.
(VB=Tool wear) 









SIEMENS 840D OR FANUC CNC-CONTROL
The Mikroturn® Series is equipped with the highly sophisticated CNC-control (Siemens or Fanuc) and has a refined resolution of 0.01 µm. This modern numerical control features a very high computing power and is a highly flexible system employing the latest technology in digital interfacing with the drive electronics. It is equipped with a 32-bit CNC-microprocessor. The digital drive concept form a closely interlinked control and drive system with a range of functions that goes far beyond the performance of today's systems. Moreover, this future-oriented digital drive system offers increased reliability, high dynamic response and precision and constant quality.