In order to achieve a good and permanent polish on the surface of the marble or granite, it is therefore imperative that the surface irregularities should be ground to such an extent that the light rays falling on it are reflected to a large extent. A properly ground (honed) surface will result in a permanent polish since no artificial coating is used to enhance the reflection of the light rays.

In most modem stone processing plants, bonded abrasives are used for grinding and polishing natural stones. Earlier, the use of silicon carbide powders was made for grinding and polishing granite with the help of a rotating steel plate. Pressure applied by the steel plate on the free particles of silicon carbide resulted in the grinding action. Water was used as coolant. After the honing was completed with 400 mesh silicon carbide, a copper plate was used to lap the surface with 800 or 1000 mesh silicon carbide. In case these fine mesh powders were not available, coarser mesh of SIC was used until it was ground down to the finer mesh. The final polish was achieved by lapping the finely honed surface with the help of lead plate and tin oxide. This lapping action imparted a high gloss to the granite.

The most common type of bonded abrasives are made by binding Silicon Carbide powder in a magnesite bond. This is the most economical and efficient method of grinding and polishing of granite and marble.

Since granite is a hard rock, special grinding and polishing heads are used for rapidly grinding and polishing the slabs of granite. The principle of these heads is to increase the contact pressure between the abrasives and the stone, so that the abrasives can rapidly grind the stone. All the grinding heads utilise the principle of line contact between abrasive and stone, to ensure a faster grinding action. The most common type of grinding heads in use today are the oscillating heads (or the rocking heads) and satellite heads (or planetary heads).

The rocking heads consist of 2, 4 or 6 rotating abrasives, which are made to oscillate about their own axis while they are grinding the stone. As a result of the oscillating movement, the abrasives generate a line contact with the stone. (See fig. 2A). This increases the specific pressure on the stone tremendously and results in high material removal.

The planetary head on the other hand has a number of abrasives which are rotating about their own axis as well as revolving around the central axis of the head (See fig. 2B). The abrasives are also inclined at a small angle, so that they generate a conical shape, such that there is only line contact between the abrasives and the stone. Due to the line contact, there is high specific pressure coupled with high grinding velocity. This action is able to grind the stone very rapidly.

Both the above systems are efficient forms of grinding and polishing granite. However, the planetary head has a higher incidence of maintenance due to the high rotating speeds of the abrasives and subsequent shocks on the bearings. The oscillating heads incur low maintenance cost and are also more efficient in the utilization of electric power.

The following tabular column elaborates on the method of using bonded abrasives for soft and hard natural stones:-

There are five major stages of application of abrasives which are as follows:

1. Selection Of The Sequence Of Abrasives (For Polishing Granite)

It is very important that the initial grinding be carried out thoroughly with the rough grits in order to make the surface of the granite absolutely flat. It is important to remember that the quality of the polish depends entirely on the quality of the flatness achieved by the initial grinding and honing process. Many a times, operators ignore the first two stages and concentrate on the application of the polishing abrasives in an attempt to get a high gloss, but this will not help unless the grinding and closing of the pores of the granite is carried out successfully.

2. Selection Of The Bond Of Abrasives

The selection of the bond of the abrasives depends on the sawn surface of the granite,the quality of granite and the production rate required. In general, soft bond abrasives will give a faster rate of production and are normally used for polishing monuments & tiles. Hard bond abrasives last longer and are extensively used in the polishing of rough gang sawn slabs where the wear and tear of the abrasive is excessive.

3. Required Polishing Pressure

The grinding pressure determines the rate of material removal. However, excessive grinding pressure may cause burn marks, scratches, breakage of granite slabs/abrasives. Normally hard and porus granites like Red Granite require higher pressure as compared to softer granites like black granite. The pressure also depends on factors like Conveyor Speed, Bridge Speed, type of machine, type of abrasive etc.

4. Adequate Water Supply

Adequate flow of water is a must for good grinding, honing and polishing. A copious supply of water will ensure good surface preparation and will inhibit burn marks and scratches. The finer mesh abrasive should preferably have a supply of fresh water to ensure scratch free polish.

The belt speed in the Line Polishing Machine and the number of passes in the Bridge Polisher will determine the quality of polish as well as the rate of production. In Line Polishing machines the belt speed should be restricted to a level where a clear and acceptable polish is achieved. Higher speeds than required, may cause uneven polish, bum marks, scratches, dullness etc. Likewise, the Bridge polisher should also have adequate number of passes to ensure complete grinding and polishing.

Ceramic Polishing

The polishing of Ceramic Tiles involves a different bond as well as a different application of abrasives. Ceramics are much harder than granite and therefore require a suitable bond for grinding and polishing. At the same time it is necessary to achieve the optimum cost per sq. mtr. to ensure the economy of polishing.

Life of Abrasives

The cost per sq.mtr. is a very important criteria when evaluating the performance of abrasives. The individual price of the abrasive is of little consequence since the abrasive which has a higher initial price may also give substantially higher life and hence only the cost per sq. mtr. is of significance. It is also important to remember that very hard abrasives which give a low cost per sq. mtr. may not be grinding or polishing at the required rate of production, thus giving a low level of productivity. Hence, when comparing the performance of the abrasives the cost per sq.mtr. as well as the rate of production has to be kept in mind.

To achieve excellent results, the application of the correct grades from coarse to fine has to be masterfully accomplished. The technicians (Machine Operators) should have a good understanding of the various types of natural stones and must vary the grades of the abrasives according to the sawn surface of the stone.