Hydrostatic bearings in surface grinding machines have programmable variable pressure to achieve flatness and straightness levels within 0.5 micron/m machine table stroke.

The latest 'groundbreaking' development in adding programmable variable pressure to hydrostatic bearing technology by Okamoto of Japan, the world's largest manufacturer of grinding machines, enables some four times greater levels of rigidity to be obtained over conventional surface grinding machines that use hydrostatic bearings. As a result, flatness and straightness levels within 0.5 micron can now be held per metre stroke of the machine table in the latest UPG-84NC Ultra Planarization surface grinder while supporting table loads up to 700kg. Now available from the 600 Centre of Shepshed near Loughborough, the UPG-84NC is the most compact of the latest UPG machine range of 13 machines which can be specified having table lengths up to 4000mm and all are capable of generating the same degree of flatness.

The technology was originally developed by Okamoto's special purpose application engineering team to create a new order of 'mother' machine capability with minimum inertia and vibration when the table reverses.

The idea was to create new orders of flatness and straightness for production grinding its own surface grinding machine tables, beds and bearing guide rails.

The latest UPG range is ideal for grinding tasks such as dies, master granite surface plates, metrology equipment, LCD and semiconductor production.

All the machines in the range have been completely re-designed using totally enclosed stainless steel guarding giving easy access to the grinding area.

The programmable Okamoto 'non-contact' hydrostatic slide system reduces the oil supply to the bearing by one-third of a conventional hydrostatic system and improves the strength of the guideway structure of the machine giving improved table loading performance.

As a result, it achieves a consistent level of accuracy throughout the grinding plane giving a longitudinal straightness of 0.5 micron/1000mm and 0.4 micron/800mm on the cross axis.

In addition, it is possible to establish a minimum 10Nm input increment and work within a resolution of 2Nm in both axes.

Traditional hydrostatic slideways vary the thickness of the oil film layer formed on the surface of the guideway according to the load on the table.

Maintains Okamoto, by controlling this oil film layer grinding accuracy can be improved by some 400%.

Also, by avoiding high levels of hydrostatic oil pressure combined with traditional temperature control techniques, this significantly reduces running costs of the machine.

Automatic compensation is applied to the Okamoto slideway bearings that have six faces all under variable hydrostatic control through the Okamoto developed software in the Fanuc 31i Model A control system.

The hydrostatic oil, hydraulic oil for the table longitudinal axis drive and the coolant fluid are all temperature controlled on the UPG to within +/-0.1 deg C and an ultra-accurate hydrostatic spindle is also part of the standard machine specification.

A recent Okamoto grinding demonstration on a larger UPG 408 NC machine, able to accept a table load of 5.5 tonnes, illustrated how a coating roller die some 2000mm long could be ground to a flatness of 0.15 microns.

Normally a die of this type, which is used to apply glue to postage stamps, is ground, lapped by hand and polished to achieve a flatness of 0.3 micron.

The UPG-84NC has a table size of 850mm by 400mm and an electro-permanent magnetic chuck size of 800mm by 400mm by 100mm.

Grinding wheel size is 305mm by 38mm by 127mm driven by a 5.5kW motor and distance between wheel and table surface is between 50mm and 550mm.

The X-axis longitudinal table speed is variable between 0.3 and 25m/min, vertical and cross-feed rate is between 1 and 790mm/min.

The Fanuc 31i-MA control provides two-axis simultaneous movement in Z and Y and interpolation in the X-axis.

Options include automatic dynamic balancing of the grinding wheel, linear glass scale feedback for the vertical axis, gap elimination and a table longitudinal axis crowning.