THEORY OF SHARPENING
THEORY OF SHARPENING |
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A pulpstone, manufactured from either Ceramic bonded abrasive or Cement bonded abrasive, consists of abrasive grains held together by a bonding material. Pulpstone "sharpening" is actually a misnomer, it does not sharpen the individual abrasive grains as such since these grains are much harder than the sharpening burr. Pulpstone sharpening is, in effect crush dressing and is accomplished by fracturing the softer bond material called bond posts, which surrounds the abrasive grains. This causes the worn grains to be removed, uncovering new sharp abrasive grains.
Fig.1 Grit and Bond
A pulpstone is sharpened for the following reasons:
After a period of time the abrasive grit of the stone becomes rounded and dull due to the rubbing of the wood against the stone. As stated earlier the sharp teeth of the burr fracture the bond posts holding the abrasive grit in the stone surface. This bond post fracturing causes the dull grit to fall away from the stone surface leaving new sharp abrasive. The amount of sharp abrasive presented at the stone surface is controlled by the depth of penetration of the burr teeth into the stone surface. The deeper the penetration the greater the number of bond posts that will be fractured thereby removing a greater amount of abrasive grit.
Studies have indicated that in making news grade stone groundwood, each grit on the stone surface dissipated 1/1000 of a horsepower of energy. A normal news grade pulpstone contains approximately 2OOO active grits per square inch of stone surface thereby requiring 2 HP per square inch to make news grade pulp. Grinders do not have motors large enough to provide 2 HP/sq. inch. The average grinder installation is able to provide about 1 HP/sq. inch. It is therefore necessary to reduce the active grits on the stone surface to 1000/sq. inch. Putting grooves into the pulpstone surface with a burr will reduce the active grits to a range suitable for making the proper quality pulp.
Fig.2 Pattern Cross-section
In the grinding process the lands and grooves of the stone, as well as the individual grits, pass rapidly over the wood surface. Each time a land passes over the wood fibre it compresses the surface of the wood and each time a groove passes over the wood fibre it relaxes the surface of the wood thereby causing localized heating of the wood. This heat softens the lignin, which binds the wood fibres together. When the lignin has been adequately softened, the fibres will separate from the surface of the wood.
A pattern having a narrow land area will provide increased unit or specific grinding pressure. This higher pressure increases the deforming pressure on the wood surface causing increased localized heating of the wood. Increased softening of lignin will cause longer length fibres and fibre bundles to be separated from the wood. This results in a pulp of higher freeness.
Fibre length is controlled not only by the degree of softening of the lignin as discussed earlier, but it is also controlled by the amount of regrinding of the fibre after the fibre has been separated from the wood. The grooves of the stone will carry fibres out of the grinding zone thus preventing regrinding of these fibres. Well defined, deep grooves will contribute to producing longer fibre pulp. Increasing the angle of lead of the burr pattern can lengthen the grinding zone acting upon the fibres. This lengthened grinding zone will produce more regrinding of the fibre producing a shorter fibre pulp of lower freeness.
Fig.3 Lead angles and grinding zone length
The pores of the stone become filled with pitch and wood debris. As the pores serve to dissipate heat and control stone temperatures, a severe plugging of these pores can result in the stone surface cracking and burning. Sharpening not only exposes new abrasive grits but also exposes new pores. These new pores can absorb cooling water and regulate stone temperature.
By placing grooves in the surface of the stone, the total surface area of the pulpstone is increased. This greater area contributes to improved heat dissipation at the stone surface.
It is important to have adequate water in the grinding zone. Without this water the stone surface would burn and no longer grind. The grooves transport water from the showers into the grinding zone. Groove depth will control groove volume and the amount of water entering the grinding zone.
Fig.4 Deep and shallow grooves
