Stone grinding machine

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Possibly one of the first large steam-powered grinding systems. Picture by Johann Erdmann Hummel in the Alte Nationalgalerie Berlin, 1831. The illustration shows the granite bowl in the Lustgarten in Berlin.
Surface grinding line. The lined up tranches are polished by a single grinding head (half left) that is guided on a bridge.

The stone grinding machines are used for grinding and polishing natural stones in the stonecutter .

Demarcation

Stone grinding machines, which are used to cut gemstones from other minerals, are a special case ; they are not machine tools. These are drum mixers into which abrasives and water are added. After filling, they are set in rotation and grind the stones until they are polished.
Machines for smoothing the plastic running surfaces of skis and snowboards are also mentioned as stone grinding machines. However, they are not part of the stone grinding machines discussed here.

history

Before machines were used for grinding and polishing, grinding was a laborious and exhausting job. This work was carried out by hand and whetstones were used that were moved in one or both directions or a steel block was moved back and forth with loose abrasives scattered in it.

The stonemason Erhard Ackermann from Weißenstadt in the Fichtelgebirge is considered to be the inventor of the mechanical grinding and polishing of granite on an industrial scale, a hard stone . Grinding and polishing processes on individual pieces by machines powered by steam power took place earlier. The Berlin stonemason Cantian is known for this for polishing the large granite bowl in the pleasure garden at the end of the 1820s. With the development of industrialization , surfaces were ground and polished on a large scale. For this, steam and later electrically driven grinding machines were used. The principle of the articulated arm grinding machine has established itself on a large scale. For example, in the Sudetenland , one of the large quarry areas, around 800 such machines were in use. They were served almost exclusively by women. It was mostly the wives of the stonemasons and stone carvers who generated additional income for their families.

Most of the grinding processes are water-cooled. Today (2008) dry grinding only takes place when processing the edges of soft stone . Hand-operated, electrically or compressed air-powered hand grinding machines are used.

  • Historic stone grinding machines

  • Cantian grinding machine powered by steam (1828)

  • Floor plate grinding machine, probably powered by steam (before 1907)

  • A plate grinding and polishing machine around 1900

  • An edge grinding and polishing machine with a roller around 1900

Typology

Traditional articulated grinder
Multipurpose articulated arm grinding machine that can also mill

The stone grinding machines that grind natural stone are basically divided into:

Stone grinding machines are machine tools in which the tool rotates.

Abrasives

When it comes to abrasives that are used for natural stones, a basic distinction is made between loose and bonded abrasives.
Loose abrasives are: quartz sand , emery , pumice , kieselguhr (triple), steel sand , silicon carbide and corundum . Today (2008) loose abrasives are only rarely used for grinding. They are mainly used in gang saws , in which they are washed into the sawing joints of the stones with water and thus optimize the sawing performance when sawing the stone rough blocks into slabs and slices of size.

Types of binding

Bonded abrasives in abrasive tools for processing natural stone are: steel sand, silicon carbide, electrical corundum , boron carbide and diamonds .
Depending on the stone to be ground and the conditions of use, the types of bond used by the abrasives are different. There are mineral, ceramic, bakelite, plastic and metal-bonded grinding wheels or bodies.

  • Mineral-bonded abrasives: A distinction is made between magnesite bond and silicate bond. Magnesite bonded grinding wheels are manufactured using the cold casting process. They are only suitable for wet grinding, but cannot be stored for long, especially when they are not damp. They are approved for peripheral speeds of up to 20 m / s. The silicate bond consists of water glass and the production takes place at a temperature of 250 to 300 ° C.
  • Ceramic-bonded abrasives: They consist of a mixture of quartz , feldspar and clay and are fired at around 1,100 ° C. They are suitable for wet and dry sanding.
  • Bakelite-bonded grinding wheels: These are grinding wheels with a synthetic resin bond that are hardened at 180 ° C. They are suitable for both wet and dry sanding. The circumferential speed is specified as 45 m / s and, for fiber-reinforced disks, 80 m / s to 100 m / s, depending on the situation.
  • Plastic-bonded abrasives: Binding agents such as polyester , epoxy resins or polyurethane , which achieve high strength values ​​and are mainly used for fine sanding, are used. Almost without exception, they are only suitable for wet sanding.
  • Metal-bonded abrasives: The binders are bronze, steel and hard metal. Diamonds are incorporated into the metal bond. Bronze bond is for limestones , marbles and cast stone with marble aggregates, steel binding is for all hard rock and for special "wear-hard" natural stones used. The hard metal bond is used for sandstone and basalt lava .

Abrasive grit

The grain size used is indicated on the back of the grinding wheel in the form of numbers from 000, 00, 0 and from 1 to 5. The grinding tools for hard stone are finer in No. 3 and 4. The stone surfaces of natural stones are ground differently depending on the requirements. The number of grits, which is usually stated on the back of the grinding wheel, is based on the American unit of measurement, mesh , the number of meshes of a screen per inch (25.4 mm), which is also called the grading curve . The larger the number, the finer the grain. The smaller the number, the rougher the stone surface.

Grinding wheel

Different shaped systems are used when using the so-called grinding wheels. Slip rings, grinding wheels or grinding segments can be differentiated according to their shape. A special form are grinding rollers that grind stone edges.
To optimize the grinding effect, either pressure is applied or so-called grinding heads are used. So-called wobble, planetary, roller and universal grinding heads are used.

The characteristics and properties of the abrasive media mainly depend on the abrasive grain size, the type of bond and the structure.

literature

  • Frieder Bernhard (ed.): The stonemason and stone sculptor. Volume 2: Ulrike Ader: The work on stone. Callwey, Munich 2003, ISBN 3-7667-1330-2 .
  • Reiner Flassig: Tool, machine and equipment knowledge. In: Educational center for the stonemasonry and sculpture trade (Ed.): Steinmetzpraxis. The manual for daily work with natural stone. 3. Edition. Ebner, Ulm 1993, ISBN 3-87188-139-2 .