The most comprehensive solution to grinding machine processing problems in history

1, Factors affecting the surface roughness of grinding machines and their improvement measures
(1) Factors related to grinding wheels. Mainly the particle size, hardness, and dressing of the grinding wheel.
The finer the particle size of the grinding wheel, the more abrasive particles there are per unit area of the grinding wheel, the finer the scratches on the grinding surface, and the smaller the surface roughness value. However, if the particle size is too fine, the grinding wheel is prone to clogging, resulting in an increase in surface roughness, as well as the generation of ripples and causing burns.
The hardness of a grinding wheel refers to the difficulty of the abrasive particles falling off the wheel after being subjected to grinding force. The grinding wheel is too hard, and the abrasive particles cannot fall off after wear, causing strong friction and compression on the surface of the workpiece, increasing plastic deformation, increasing surface roughness, and also easily causing burns; If the grinding wheel is too soft, the abrasive particles are prone to falling off, the grinding effect is weakened, and the surface roughness value will also increase. Therefore, it is necessary to choose the appropriate hardness of the grinding wheel.
The quality of grinding wheel dressing is closely related to the dressing tools used and the longitudinal feed rate of the dressing grinding wheel. The dressing of the grinding wheel is to use diamond to remove the passivated abrasive particles on the outer layer of the grinding wheel, making the cutting edge of the abrasive particles sharp and reducing the surface roughness value of the grinding surface. In addition, the smaller the longitudinal feed rate for dressing the grinding wheel, the more cutting micro edges on the dressing wheel, and the better the contour, thereby obtaining a smaller surface roughness value.
(2) Factors related to workpiece material. Including the hardness, plasticity, thermal conductivity, etc. of the material.
The hardness, plasticity, and thermal conductivity of workpiece materials have a significant impact on surface roughness. Soft materials such as aluminum and copper alloys are prone to clogging the grinding wheel, making it difficult to grind. Heat resistant alloys with high plasticity and poor thermal conductivity are prone to early collapse of sand particles, leading to an increase in surface roughness during grinding.
(3) Factors related to processing conditions. Including grinding amount, cooling conditions, precision and vibration resistance of the process system, etc.
The grinding parameters include grinding wheel speed, workpiece speed, grinding depth, and longitudinal feed rate. By increasing the speed of the grinding wheel, the propagation speed of plastic deformation of the surface metal may not keep up with the grinding speed, and the material may not have enough time to deform, resulting in a decrease in the surface roughness value of the grinding surface. The speed of the workpiece increases, the plastic deformation increases, and the surface roughness value increases. The greater the grinding depth and longitudinal feed rate, the greater the plastic deformation, thereby increasing the surface roughness value. When grinding with grinding wheels, the temperature is high, and the effect of heat is dominant, so the role of cutting fluid is very important. The use of cutting fluid can lower the temperature of the grinding area, reduce burns, wash away falling sand particles and chips, and avoid scratching the workpiece, thereby reducing the surface roughness value. But appropriate cooling methods and cutting fluids must be selected.
In addition, for external cylindrical grinding machines, internal cylindrical grinding machines, and surface grinding machines, the spindle accuracy of the grinding wheel, the accuracy and stability of the feed system, the stiffness and vibration resistance of the entire machine tool are closely related to surface roughness.
The above factors affecting surface roughness have been analyzed. How to reduce the surface roughness value of the processed surface, in addition to taking measures from the above aspects, can also be improved from the processing methods, such as grinding, honing, ultra precision machining, polishing, etc.
2, Dressing techniques for grinding wheels
Sharpening is a process of sharpening the abrasive grains of a superhard grinding wheel. In this process, it is necessary to remove the binder between the abrasive particles and the dull grinding wheel abrasive particles, so that the grinding wheel abrasive particles with strong grinding performance protrude from the binder and form sharp cutting edges. Sharpening must also remove small materials from the pores on the surface of the grinding wheel to prevent an increase in grinding force acting on the wheel. This increase in grinding force on the wheel will cause vibration and cause surface burns on the parts.
Without proper sharpening, even the best grinding wheel cannot achieve high quality and dimensional consistency of processed parts. In fact, when you invest in high-quality grinding wheels, it is very important to trim them well in order to achieve high grinding performance.
Shaping can be said to be a part of the preparation work for grinding wheels, which is carried out simultaneously with the sharpening of ordinary grinding wheels. For superhard abrasive grinding wheels, the two processes are carried out separately, and the grinding wheel is first shaped. In the use of superhard abrasive grinding wheels, shaping is carried out using shaping tools or rollers, and sharpening is often done using a ceramic bonding rod. After shaping is completed, the grinding wheel is sharpened.
It is important to ensure that the spindle bearings are at a certain temperature (such as the usual grinding state of the grinding wheel) before the grinding wheel, including shaping and sharpening, is repaired. This can avoid damaging the geometric shape of the parts and abnormal wear of the grinding wheel and dressing tools. Tools used for repair must be handled with care, as they are generally made of hard, wear-resistant but brittle diamond materials and are highly sensitive to small cracks and fractures caused by slight collisions and stress.
As the diamond shaper itself is a cutting tool, it needs to be kept very sharp. Using a dull dressing tool to trim the surface of the grinding wheel will make it dull. In order to maintain a high-quality and sharp diamond dressing tool, it is necessary to rotate a single point or pointed dressing tool in 1/8 of a circle at regular intervals. The number of rotations can be determined based on the repair situation, and based on experience, it should be rotated at least once a day. For chisel heads and shaping tools, they generally need to be rotated 180 ° before they become dull.
Most cylindrical grinders place the parts and grinding wheel on a horizontal line. The highest point of the outer circle of the part and the highest point of the outer circle of the grinding wheel are called the contact point of the part/grinding wheel. Diamond dressing tools should trim the grinding wheel as close as possible to the contact point of the part/grinding wheel. For the grinding wheel used in the internal grinding machine, it is more important to use the diamond dressing tool to approach the highest point of the outer circle of the grinding wheel (i.e. the contact point between the part and the grinding wheel during hole grinding) for dressing.
Take micro adjustments
There seems to be a temptation to choose a larger depth of trimming in order to reduce the time required for trimming. This is an extremely wrong idea. The grinding wheel must be trimmed at the most suitable depth. Choosing a too large trimming depth can result in high cutting temperature, reduce the service life of the trimmer, and even cut off useful grinding wheel layers. The ultimate result is to damage both the trimmer and the grinding wheel, which is counterproductive. The optimal amount of trimming is based on the principle that after several rounds of trimming, the geometric shape of the grinding wheel can be restored and a good grinding edge can be precisely produced. When using a single point dressing tool, the diameter direction of the grinding wheel should be in contact with the axis at an inclination angle of 10-15o. This will create a sharpening effect when the single point trimming tool rotates regularly. A multi-point contact trimming tool does not need to be tilted at this angle. Change to using the entire end face of the trimming tool to make contact with the surface of the grinding wheel.
The lateral movement speed is the speed at which the dressing tool passes through the surface of the grinding wheel during the dressing process. It plays a crucial role in the surface roughness and metal cutting rate required for machining parts. If the lateral movement speed is too slow, it will block the grinding wheel, damage the surface roughness and metal cutting rate of the parts. Too slow lateral movement speed can also cause grinding wheel vibration and burn the surface of parts. A uniform and fast lateral movement speed can trim the surface of the grinding wheel, improve its grinding performance, increase grinding efficiency, and reduce the surface roughness of the parts.
Maintain cooling
Proper use of coolant can accelerate repair speed and improve repair efficiency. Based on experience, selecting a 3/8-inch diameter coolant can remove a large amount of heat from the trimmer and extend its service life. When the diamond dressing tool passes through the grinding wheel, install a coolant nozzle that covers the entire surface of the grinding wheel or continuously adds coolant to the diamond dressing tool. When the repair tool comes into contact with the grinding wheel and begins to repair, never allow the repair tool to be removed from the coolant. Otherwise, it will cause cracks or fractures in the diamond dressing tool under extreme temperature changes of cold and hot.
The use of filters for high-precision filtration of coolant can prevent multiple cycles of dirt or chips in the coolant. The contaminated coolant can cause the grinding wheel to wear out quickly and increase the frequency of grinding wheel repair. Dry dressing of the grinding wheel is only carried out during dry grinding (in which case the cooling of the diamond dressing tool can be interrupted). Before finishing work every day, turn off the coolant and let the sand wheel run empty for a few minutes. This can prevent the grinding wheel from breaking.
Vibration is the enemy of repair
It is crucial to effectively reduce vibration, avoid leaving repair marks, collisions, and damage to repair tools on the surface of the grinding wheel during grinding wheel repair. This means that it is also necessary to maintain the balance of the grinding wheel, which is determined by the structural characteristics of the grinding wheel itself. The uneven density and the overall geometric shape of the grinding wheel can both affect the inherent balance of a grinding wheel. Therefore, choosing a high-quality grinding wheel is also very important. If it is a high-quality grinding wheel, only proper installation is needed to maintain good balance of the grinding wheel. According to the manufacturer's factory instructions, the grinding wheel is marked with an upward arrow, which indicates the light end orientation of the grinding wheel after rough balancing. Then users can follow the arrow instructions and pay attention to adjusting the grinding wheel to achieve dynamic balance. Adding coolant evenly can sometimes help the grinding wheel maintain balance.
In order to further avoid vibration, it is necessary to ensure that the trimming tool is firmly clamped on the clamp and maintains the minimum overhang to ensure that the trimming tool has sufficient rigidity. If the diamond tool is not firmly clamped, it will cause vibration, generate noise, create ripples on the surface of the parts, strain the surface of the parts, and damage the repair tool.
A superhard abrasive wheel must be shaped and sharpened before starting grinding. Following the experience introduced in this article will help your grinding wheel produce the best grinding effect.
3, The influence of grinding machine accuracy on the machining accuracy of workpieces
The geometric accuracy, stiffness, thermal deformation, motion stability, and vibration resistance of grinding machines will directly affect the machining accuracy of workpieces.
(1) The geometric accuracy of a grinding machine refers to the motion accuracy and mutual position accuracy of each component without bearing a load. It is impossible to manufacture machine tools with absolute precision, as there are inevitably more or less errors. This type of error will be reflected to varying degrees on the workpiece during machining, affecting its machining accuracy. Generally, there are radial runout and axial displacement of the spindle, straightness of movement of moving parts such as the workbench, positional errors and transmission errors of working parts.
The radial runout and axial displacement of the grinding wheel spindle, as well as the large movement errors of the grinding machine head, 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 sparks during the grinding process. When the worktable moves on a vertical plane that is not perpendicular, it affects the straightness of the workpiece's generatrix on the inner and outer cylindrical grinding machines, resulting in a large flatness error of the workpiece when grinding the plane on the surface grinding machine. The centerline of the grinding wheel spindle shaft of an external cylindrical grinder and the centerline of the grinding wheel spindle shaft of an internal cylindrical grinder are not equal in height to the centerline of the workpiece head frame shaft. When grinding the inner and outer cones, the workpiece busbar is a hyperbolic curve. The centerline of the grinding wheel spindle axis is not parallel to the direction of movement of the worktable, which affects the flatness of the workpiece end face after grinding. The transmission error of the grinding machine has a significant impact on the machining accuracy of thread grinding and gear grinding.
(2) The stiffness of the grinding machine. It refers to the ability of a grinder's components to resist deformation when subjected to external forces (grinding forces). That is to say, under the same grinding force, the smaller the deformation of the component, the greater the stiffness. On the contrary, if the deformation of the component is large, it indicates that the stiffness of the component is small. The magnitude of these deformations disrupts the original geometric accuracy of the static state of the grinding machine and will cause the magnitude of machining errors in the workpiece. So a machine with good rigidity has high machining accuracy for the workpiece.
(3) Thermal deformation. The distribution of heat sources inside the grinding machine is uneven, and the amount of heat generated by each part during movement is also different. The influence of external heat sources on each part of the machine tool is also different. The thermal expansion coefficient of components varies due to different materials, causing different micro deformations in each part of the machine tool, resulting in a decrease in the original geometric accuracy of the machine tool and affecting the machining accuracy of the workpiece. So it is best to install a precision grinder in a constant temperature chamber to prevent temperature changes from affecting the accuracy of the machine tool and workpiece.
(4) The moving parts of the grinder crawl. The phenomenon of uneven movement of moving parts such as the grinding wheel holder on the grinding machine worktable during micro periodic feed or low-speed continuous movement is commonly referred to as crawling. When this phenomenon occurs in the grinding machine, it causes uneven feed during the grinding process, which affects the surface roughness of the workpiece during grinding.
(5) The vibration of the grinding machine. The grinding machine generates vibration during the grinding process, causing periodic changes in the relative position between the grinding wheel and the workpiece, resulting in vibration patterns on the surface of the workpiece, which seriously affects the machining quality and accuracy.
To improve the accuracy of the workpiece after grinding, in addition to striving to eliminate the influence of the above factors, it is also necessary to pay attention to the reasonable selection of positioning reference, clamping method, selection and correct repair of grinding wheel, and reasonable selection of grinding amount and process method during the workpiece processing.