Three factors affecting the surface roughness of grinding machine

Three factors affecting the surface roughness of grinding machine

 

There are many factors affecting the surface roughness of grinding machines, which are summarized in three types, factors related to the grinding wheel, factors related to the material of the workpiece, and factors related to the processing conditions. This paper elaborates on these three factors and it is helpful for the reader in the operation of the actual grinding machine.

1.Factors related to grinding wheel affecting the surface roughness of grinding 

    • Mainly the grain size, hardness and the dressing of the grinding wheel

The finer the grain size of the grinding wheel, the more abrasive grains per unit area of the grinding wheel, the finer the scratch on the grinding surface, and the smaller the surface roughness value. However, if the grain size is too fine, the grinding wheel is blocked easily, and the surface roughness value is increased, and at the same time, it ripples and burns easily.

  • The hardness of the grinding wheel refers to the difficulty of the abrasive grains falling off the grinding wheel after being subjected to the grinding force. The grinding wheel is too hard, the abrasive grains cannot fall off after being worn, and makes the workpiece surface subjected to strong friction and extrusion, increasing the plastic deformation, and the surface roughness value is increased, and the burn is easily caused. The grinding wheel is too soft, and the abrasive grains are easy to fall off, weakening the grinding action, and increase the surface roughness value, therefore, choose the right hardness of the grinding wheel.
  • The dressing quality of the grinding wheel is closely related to the dressing tools and the longitudinal feed of the roller dresser. The dressing of the grinding wheel is to remove the passivated abrasive grains from the outer layer of the grinding wheel with a diamond so that the abrasive cutting edge is sharp and the surface roughness value of the grinding surface is reduced. In addition, the smaller the longitudinal feed amount of the roller dresser, the more the cutting micro-blade on the finished grinding wheel, and the smaller the surface roughness value is obtained.

Sharpening is a process of sharpening the abrasive grains of a superhard abrasive wheel. In this process, remove the binding agent between the abrasive grains and the blunt abrasive grains, forming the sharp cutting edge. Sharpening should remove the tiny material from the pores on the surface of the grinding wheel to prevent the grinding force from acting on the grinding wheel from increasing. If the grinding force on the grinding wheel increases, it will cause vibration and surface burn of the parts.

Without proper sharpening, even the best grinding wheels are not likely to achieve high quality and dimensional consistency of the machined parts. In fact, it is very important to dress the grinding wheel well in order to achieve high grinding performance.

Truing can be said to be part of the preparatory work of the grinding wheel, which is carried out simultaneously with the sharpening of the conventional grinding wheel. For superabrasive grinding wheels( diamond and CBN grinding wheels), the two processes are carried out separately, and first truing the grinding wheel. In the grinding with superabrasive grinding wheels, a truing tool or roller dresser is used for truing. Vitrified bond dressing rods are used for sharpening.

Before the truing and sharpening of grinding wheels, it is important to ensure that the spindle bearing is at a certain temperature ( normal grinding state of the grinding wheel). This will avoid damage to the geometry of the part and the abnormal wear of the grinding wheel and dressing tool. Care should be taken with the dressing tool because it is usually made of hard, wear-resistant but brittle diamond material and is very sensitive to minor cracks and crushing caused by slight impact.

Since the diamond dresser itself is a tool, it needs to be very sharp. Using the blunt dressing tool to dress the surface of the wheel will make the wheel blunt. In order to keep high quality and sharp diamond dressing tools, it is necessary to rotate the single point or a tapered tip dressing tool at 1/8 turn at regular intervals. The number of rotations can be determined according to the dressing situation, and according to experience, it is rotated at least once one day. For chisel heads and shaped dressing tools, it usually rotates 180° before they are blunt.

The most cylindrical grinding machine put the parts and wheels on the horizontal line. The highest point of the cylindrical of the parts and the highest point of the cylindrical of the grinding wheel are called the part/grinding wheel contact point, and the diamond dressing tool should dress the grinding wheel as close as possible to the contact point of the part/grinding wheel. For the grinding wheel for the internal grinding machine, it is more important for a diamond dressing tool that near the highest point of the cylindrical of the grinding wheel for dressing(ie. the part/wheel contact point).

In order to shorten the dressing time, there seems to be a temptation to choose the large depth of repair. This is an extremely wrong idea. Choose the most appropriate dressing depth for the grinding wheel. Choosing too large repair depth will cause high cutting temperature, and shorten the dresser life, and remove the useful wheel layer. The final result is damage to both the dresser and the grinding wheel. The optimum dressing amount is that after several dressing, it can restore the geometric shape of the grinding wheel and just produce a good grinding edge.

With the single point dressing tool, contact the director of the diameter of the grinding wheel with a 10-15o axial inclination angle. This will cause the single point dressing tool to produce a sharpening effect when it rotates periodically. A dressing tool with multiple points of contact does not need to tilt this angle. Instead, use the entire end face of the dressing tool to make contact with the surface of the grinding wheel.

The lateral movement speed is the speed at the dressing tool passing through the surface of the grinding wheel. It plays a key role in the surface roughness and metal removal rate required for machined parts. The lateral movement speed is too slow, which will block the grinding wheel and damage the surface roughness and metal removal rate of the part. Too slow lateral movement speed can also cause the wheel to vibrate the burn the surface of the part.

Uniform and fast lateral movement speed can dress the surface of the grinding wheel, and improve the grinding performance of the grinding wheel, and increase the grinding efficiency and reducing the surface roughness of the parts.

The super abrasive grinding wheel should be truing and sharpening before grinding. Following the experience in this paper, will help your grinding wheel with the best grinding effect.

2.Factors related to workpiece material affecting the surface roughness of grinding

There are factors related to workpiece material including hardness, plasticity, and thermal conductivity of the material.

The hardness, plasticity, and thermal conductivity of the workpiece material have a significant effect on surface roughness. Soft materials such as aluminum and copper alloys will block the grinding wheel and are more difficult to grind. The heat-resistant alloy with large plasticity and poor thermal conductivity tends to cause the grain to collapse early, resulting in an increase on the roughness of the ground surface.

3.Factors related to processing conditions affecting the surface roughness of grinding

There are factors related to processing conditions including grinding amount, cooling conditions, technology system accuracy, and vibration resistance.

3.1.1 Grinding parameters

Grinding parameters refer to grinding wheel speed, workpiece speed, grinding depth, and longitudinal feed. Increasing the speed of the grinding wheel may make the propagation speed of the plastic deformation of the surface metal not follow the grinding speed, and the material may not be deformed so that the surface roughness of the grinding surface is lowered.

The workpiece speed increases, the plastic deformation increases, and the surface roughness increases. The larger the grinding depth and the longitudinal feed amount, the larger the plastic deformation, thereby increasing the surface roughness value. The grinding wheel has high temperature and the heat is dominant, so it is very important for the role of the cutting fluid. The use of cutting fluid can reduce the temperature of the grinding zone, reduce burns, wash away sand and chips, so as not to scratch the workpiece, thus reducing the surface roughness value. However, the right cooling methods and cutting fluids must be selected.

3.1.2 Keep cooling

Proper use of coolant can speed up the dressing and improve finishing efficiency. According to the experience, a 3/8” diameter coolant is used and can remove a large amount of heat from the dresser and extend the dresser life. When a diamond dressing tool passing through the grinding wheel, install a coolant nozzle to fill the entire surface of the wheel or continuously fill the coolant to the diamond dressing tool. When the dressing tool contacts the grinding wheel and starts to dress, it must not be immediately withdrawn from the coolant. Otherwise, the diamond dressing tool will crack or crush under extreme cold and hot temperature changes.

The filter is used for high precision filtration of the coolant to avoid multiple cycles of dirt or chips in the coolant. The contaminated coolant can quickly wear the wheel and increase the times of dressing.

In the dressing of the grinding wheel, it is essential to effectively reduce the vibration, avoid leaving dressing scratches on the surface of the grinding wheel, colliding and damaging the dressing tool. This means that the balance of the grinding wheel is determined by the structural features of the grinding wheel itself. The uneven density and the geometry of the entire grinding wheel will affect the inherent balance of the grinding wheel. Therefore, it is also important to choose a high-quality grinding wheel.

If it is a high-quality grinding wheel, the proper installation can keep a good balance of the grinding wheel. According to the manufacturer’s instructions, the grinding wheel is marked with an upward arrow indicating the light end orientation of the grinding wheel after rough balancing. The user can follow the arrows to pay attention to the roller dresser to achieve dynamic balance. The coolant is evenly filled and sometimes helps the wheel to keep balance.

In order to further avoid vibration, it is necessary to ensure that the dressing tool is firmly clamped on the holder and maintain a minimum amount of overhang to guarantee sufficient rigidity of the dressing tool. If the diamond tool is not firmly clamped, it will cause vibration, noise, ripples on the surface of the part, strain the surface of the part and damage the dressing tool.

3.2.2 Geometric accuracy of the grinding machine

Cylindrical grinders, internal grinders, and surface grinders, the spindle precision of the machine tool grinding wheel, the accuracy and stability of the feed system, the rigidity and vibration resistance of the whole machine tool are closely related to the surface roughness.

The geometric accuracy of the grinding machine refers to the motion accuracy and mutual positional accuracy of each component without being subjected to load. It is impossible to make the machine tool absolutely accurate, and there is always an inevitable more or less error. This kind of error will be reflected in the workpiece to varying degrees during the machining, which will affect the machining accuracy of the workpiece. Usually, there are radial runout and axial sway of the main shaft, straightness of movement of moving parts such as worktable, mutual position error of working parts and transmission error.

The large error of the radial runout and axial sway of the grinding wheel spindle and the movement of the grinding head frame not only affect the surface roughness of the workpiece after grinding but also cause roundness and end face runout of the workpiece, resulting in uneven spark during the grinding process.

Technology system accuracy

3.3.2 The rigidity of the grinding machine

It refers to the ability of the parts to resist deformation when subjected to external forces(grinding forces). That is, in the case of the same grinding force, the smaller the deformation of the part, the larger the rigidity. Conversely, the larger the deformation of the part, the smaller the rigidity is. The magnitude of these deformations destroys the static geometric accuracy of the grinder and will cause the machining error of the workpiece. Therefore, the machine tool with good rigidity has high processing precision.

3.3.3 Thermal deformation

The heat source inside the grinding machine is unevenly distributed, and the heat in each part is different.

The external heat source has different effects on various parts of the machine tool. The thermal expansion coefficient of the parts is different due to different materials, which causes the slight deformation for different parts of the machine tool, making the original geometric accuracy of the machine tool decrease, affecting the machining accuracy of the workpiece. Therefore, the precision grinding machine is preferably installed in the constant temperature room to prevent temperature changes from affecting the accuracy of the machine tool and the workpiece.

3.3.4 The part crawl of grinding machine

When the moving parts such as the grinding wheel frame of the grinder table are used for micro-period feeding or low-speed continuous movement, the movement is uneven, which is commonly called crawling. When there is a phenomenon in the grinding machine, the feeding is uneven, affecting the surface roughness of the workpiece.

3.3.5 The vibration of the grinding machine

There is vibration for the grinding machine, causing periodic changes in the relative position of the grinding wheel and the workpiece, causing vibration marks on the surface of the workpiece, which seriously affects the processing quality and precision.

Conclusion  of affecting the surface roughness of grinding:

In order to improve the accuracy of the workpiece after grinding, in addition to trying to eliminate the effect of the above factors, pay attention to the reasonable selection of the positioning reference during the machining of the workpiece, clamping method, selection and correct repairing of the grinding wheel, reasonable selection of the grinding amount and the processing method.

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