Besides the machine variables, additional parameters exist in belt grinding that influence process behaviour and the output. This includes the force with which the belt is tightened and the form of embodiment of the contact element as well as the engagement of grinding fluids such as greases or cooling lubricants. The boundary condition of belt stiffness influences the operating behaviour of the belts as a tool — specific parameter.
The amount of belt tension has a large influence on the process and the output when smooth contact wheels are employed. In the case of helical toothed contact wheels, an increase in belt tension may result in somewhat higher material re
moval rates, but it also brings about a higher surface roughness. A further disadvantage of high belt tension is increased stress on the contact wheel and the machine (bearings, clamps), which must therefore be designed for stability. This leads to increased acquisition costs.
Increased belt expansion, which is also used as an argument against excessive clamping forces, is not as noticeable in newer carriers as before and is thus less significant. However, for the reasons mentioned, one should choose a belt tension that is only so high that the belt is guided smoothly by the contact wheel and does not proceed in an uncontrolled fashion.
In most belt grinding methods, the abrasive belt is supported in the engagement zone by a contact element. In peripheral grinding, this is the contact wheel, in side grinding it is the contact shoe.
Contact wheels consist mostly of plastic or light metal bodies on which a rubber, plastic or textile support pad is placed.
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The variety of forms of these pads stretches from smooth coatings to toothed (straight, helical and arrowhead) implementations to individual plastic discs attached to the body. The most common support pad hardness levels are [N. N.14]:
The right choice of a suitable contact wheel contributes significantly to the optimisation of belt grinding, since the contact wheel formation has a decisive influence on the machining cycle and the grinding output.
In Fig. 6-71, the effects of contact wheel hardness on the material removal rate and surface roughness is illustrated. With increasing support pad hardness, de — formability and thus the contact surface is reduced. The normal force per unit area (cutting pressure) is thus large with a hard contact wheel and small with a soft wheel. Accordingly, the individual cutting edges are pressed more deeply into the workpiece material and remove larger chips. In this way, the material removal rate and surface roughness go up.
The surface pressure between the abrasive belt and the workpiece can be considerably further influenced by the form of the contact wheel pad. Toothed support pads lead to a much more aggressive belt engagement [N. N.14]. Of decisive importance for this is the ratio of the cleat width to the void width, the influence of which on the material removal rate and surface roughness is shown in Fig. 6-71. The kinematic circumstances furnish the reason why the support pad profile is transferred to the workpiece in a defined imaging ratio. This may lead to an increase in surface roughness or to an increased waviness, but in this way surface effects or special micrographs can be specifically produced [BUCH90].
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Grinding with constant contact force
Fig. 6-71. The influence of the contact wheel on belt grinding [N. N.14]. |
Contact wheels are toothed not only straight, but also helical. This assists in noise-reduction on the one hand, on the other it also leads to a more even force transmission into the contact zone.
Higher material removal rates can also be achieved if a smaller contact wheel diameter is selected.
Abrasive belts are manufactured for various machining tasks in various degrees of flexibility. The desired adaptability of the belt to the respective workpiece form is crucial. Belt stiffness has an essential influence on the material removal rate. With increasing flexibility, or dropping stiffness, the abrasive grains spring farther back upon making contact with the workpiece and thus remove smaller chips. In this way, the material removal rate and surface quality increase correspondingly [BUCH89].
Due to the higher tumbling work which flexible belts are exposed to, stress on them is greater, and their tool life time is shorter than the one of stiffer belts. For this reason, abrasive belts are used that are as hard and stable as possible.
In belt grinding as in other grinding methods, grinding fluids basically have the following functions:
• lowering grinding temperatures by reducing friction between the grits and the workpiece material,
• cooling the workpiece and the tool by removing heat,
• avoid clogging and
• binding harmful grinding dust.
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If these tasks are fulfilled sufficiently by a cooling lubricant, longer tool life time, lower surface roughness and more humane working conditions are the result. The use of cooling lubricants only brings about optimal results however when the type of cooling lubricant supply (spraying, flooding), its chemical composition and its viscosity is adjusted to the respective machining task [TOEN70]. For this purpose, various cooling lubricants are available for belt grinding, the effects of which on the material removal rate and surface roughness is elucidated in Fig. 672.
Fig. 6-72. The influence of the cooling lubricant type on the material removal rate and surface roughness in belt grinding [N. N.14]
In dry grinding, there is initially a higher material removal rate than when cooling lubricants are used. But the high frictional and thermal stress blunt the grains more quickly and the tool life time becomes shorter.
In the case of usual material removal rates in the domain of precision machining, the use of oil may result in a smaller material removal rate, but the surface quality is also improved. Moreover, abrasive wear progresses much more slowly due to diminished friction and lower temperatures, so that longer tool life times and tool life volumes are possible. This trend continues with the use of emulsions or greases. However, grinding results vary with the use of different cooling lubricants as well as in dry or wet grinding.
When deciding upon a cooling lubricant, one has to examine, for example, whether the high cost of furnishing the large amounts of oil required for oil flooding is justified by the technological advantage of this type of cooling lubricant supply, or whether a cooling lubricant spraying device is more economical.