Tech Talk
What is the material "Granitan"?
In precision machine tool engineering it is essential to have a machine bed with high thermal stability and good damping of natural resonance. A great advantage of Granitan, compared with a cast iron or welded construction, is the short manufacturing time of less than 10 working days. Follows is a ten day illustration of the day-to-day rundown on the manufacturing of a typical machine bed.
1st day: Preparation of Mould
Before the mould is assembled its walls are sprayed with a special paint, which acts as a release agent. The paint bonds with the Granitan as it sets and forms a firmly adherent surface layer. The mould is then assembled. The prefabricated cast-in parts are attached to the mould walls and to the bottom of the mould, where they bond with the Granitan, thanks to its excellent adhesive properties. The dimensional accuracy of Granitan is less than a tenth of a millimetre (0.004").
2nd day: Casting
The stone is automatically mixed with epoxy resin in the casting machine, using a special formula. The mould is then filled layer by layer and vibrated throughout the casting process. The acceleration of the 10-tonne vibrating table is so severe that it compacts the stone mixture to a pore-free structure. The operators work on platforms next to the table for the simple reason that human joints would be unable to withstand the G forces generated during the process.
3rd and 4th days: Removal from Mould and Curing
After eight hours the base is usually more or less set. Large components are cooled slowly under controlled conditions in a special kiln. The bed, together with the mould walls, are then hydraulically forced off and lifted away from the base of the mould. For most machine beds, the shape and smoothness of the upper surface are of paramount importance so the beds are cast upside down. As a result, the upper side of the bed is at the bottom during the casting operation and has to be inverted when removed from the mould. The fact that the mould can be fully dismantled means the side walls can be forced off individually.
5th to 7th days: Painting
The beds are given a coat of two - component paint; this may be either standard design or one specified by the customer. If required, it can be smoothed and polished. The final curing of the machine bed also takes place during this period.
8th day: Casting of Slideways
The slideways are cast in a single operation to an accuracy of several microns. The slideway surface is extremely hard-wearing and has excellent glide and stick-slip-free running properties. The knobs are optimized, so that the frictional force corresponds to a practically linear function over the entire speed range. This permits high-accuracy electronic position control.
9th day: Casting-in Bearing Points
The bearing points and guide busshes for the recirculating ball-screws require a positional accuracy of a hundredth of a millimetre (0.000.40"). These parts are positioned in relation to the slideway using gauges, and are undercast.
10th day: Scraping and Measuring
On the 10th day, the slideways are measured, corrected if necessary and recorded. The longitudinal slide can then be assembled and the top table scraped in. The longitudinal table is then mounted onto the machine base, the height measured, and then finish ground to fit the height specifications.
