Several key factors affecting the quality of blow molded parts

1. Elements 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 grains 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 easy to block, which increases the surface roughness value, and at the same time, it is easy to cause ripples and burns.

The hardness of the grinding wheel refers to the difficulty of the abrasive particles falling off the grinding wheel after being subjected to grinding force. The grinding wheel is too hard, and the abrasive grains cannot fall off after being worn, so that the surface of the workpiece is subjected to intense conflict and kneading, plastic deformation is added, and the surface roughness value increases, and at the same time, it is easy to cause burns; the grinding wheel is too soft, and the abrasive grains are easy to Drop, the grinding effect is weakened, and the surface roughness value will also be increased, so choose the appropriate grinding wheel hardness.

The dressing quality of the grinding wheel is closely related to the dressing tools used and the longitudinal feed 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, so that the cutting edge of the abrasive particles is sharp and the surface roughness value of the grinding surface is reduced. In addition, the smaller the longitudinal feed of the dressing grinding wheel, the more cutting micro-edges on the dressing grinding wheel, the better the contour, and the smaller the surface roughness value.

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2. Elements related to workpiece material

Including the hardness, plasticity, thermal conductivity, etc. of the material.

The hardness, plasticity and thermal conductivity of the workpiece material have a significant impact on the surface roughness. Soft materials such as aluminum and copper alloys are easy to block the grinding wheel and are more difficult to grind. Heat-resistant alloys with large plasticity and poor thermal conductivity are easy to cause the sand grains to collapse in the early stage, resulting in an increase in the surface roughness value of grinding.

3. Factors related to the processing conditions of blow molding manufacturers

Including grinding amount, cooling conditions, precision and vibration resistance of process system, etc.

Grinding amount includes grinding wheel speed, workpiece speed, grinding depth and longitudinal feed. If the grinding wheel speed is advanced, the propagation speed of the plastic deformation of the surface metal may not keep up with the grinding speed, and the material will not be deformed in time, and then the surface roughness value of the grinding surface will decrease. The workpiece speed is increased, the plastic deformation is increased, and the surface roughness value is increased. The greater the grinding depth and the longitudinal feed, the greater the plastic deformation, and then the surface roughness value is increased. The temperature of the grinding wheel is high, and the effect of heat is dominant, so the effect of cutting fluid is very important. The choice of cutting fluid can reduce the temperature of the grinding zone, reduce burns, wash off the falling sand and chips, avoid scratching the workpiece, and then reduce the surface roughness value. But it is necessary to choose the appropriate cooling method and cutting fluid.

In addition, for cylindrical grinders, internal grinders and surface grinders, the spindle accuracy of the machine tool grinding wheel, the accuracy and stability of the feed system, the stiffness and vibration resistance of the entire machine tool, etc., are closely related to the surface roughness.

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2. Dressing skills of grinding wheel

Sharpening is a process of sharpening the abrasive grains of superabrasive grinding wheels. In this process, it is necessary to form a sharp cutting edge in addition to the bonding agent between the abrasive grains and the dull grinding wheel abrasive grains, so that the grinding wheel abrasive grains with strong grinding function can be well bonded. Sharpening also needs to remove tiny materials from the pores on the surface of the grinding wheel to avoid the increase of the grinding force on the grinding wheel, which will cause vibration and cause burns on the surface of the parts.

Without proper sharpening, even a good grinding wheel cannot achieve the high quality and dimensional consistency of machined parts. In fact, when you invest in high-quality grinding wheels, it is important to dress them well in order to achieve high grinding performance.

Shaping can be said to be a part of the preparatory work of the grinding wheel, which is carried out together with the dressing of the general grinding wheel. Regarding the superabrasive grinding wheel, the two processes are carried out separately, and the grinding wheel is reshaped at the beginning. In the use of superabrasive grinding wheel grinding, shaping is performed with shaping tools or rollers, and sharpening is often performed with a vitrified dressing rod. After shaping is completed, the grinding wheel is sharpened.

It is important to ensure that the spindle bearing is at a certain temperature (as in normal grinding wheel grinding conditions) before the grinding wheel includes reshaping and sharpening. This avoids damage to the geometry of the part and abnormal wear of the grinding wheel and dressing tools. It is necessary to handle with care about dressing tools, because it is generally made of hard, wear-resistant but very brittle diamond material, and is very sensitive to small cracks and breakage caused by slight collision and force.

Because the diamond shaper itself is a tool, it needs to be very sharp. Dressing the wheel surface with a blow molding dull dressing tool will dull the wheel. In order to maintain a high quality and sharp diamond dressing tool, it is necessary to rotate the single point or tapered tip dressing tool 1/8 of a turn at regular intervals. The number of rotations can be determined according to the trimming situation, and it is rotated once a day according to the experience. For chisel heads and shaped trimming tools, it is generally necessary to make a 180° rotation before they are blunted.

Most cylindrical grinders place the part and the wheel on a level 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 touch point of the part/grinding wheel, and the diamond dressing tool should dress the grinding wheel as close as possible to the touch point of the part/grinding wheel. Regarding the grinding wheel for internal grinding machine, it is more important to dress the diamond dressing tool close to the highest point of the outer circle of the grinding wheel (that is, the contact point of the part/grinding wheel when grinding the hole).

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Adopt micro trimming

To reduce trimming time, there always seems to be a temptation to choose a larger trimming depth. This is a wrong idea. It is necessary to select a suitable dressing depth to dress the grinding wheel. Choosing too large a dressing depth will cause high cutting temperatures, reduce the service life of the dresser, and remove the useful grinding wheel layer. The end result is damage to both the dresser and the grinding wheel, plucking the seedlings to encourage growth. The amount of dressing is that after several repairs, it can not only restore the geometric shape of the grinding wheel, but also produce a good grinding edge. When using a single-point dressing tool, the diameter direction of the grinding wheel should be touched at an inclination angle of 10-15o of the axis line. This will make the single point trimming tool produce a sharpening effect when it is timed to roll. Retouching tools with multi-touch don't need to skew this point of view. Instead, touch the entire end face of the dressing tool to the surface of the grinding wheel.

The lateral movement speed is the speed at which the dressing tool passes over the surface of the grinding wheel during dressing. It has a critical effect on the surface roughness and metal removal rate required by the machined parts. Too slow lateral movement will block the grinding wheel, jeopardizing the surface roughness and metal removal rate of the part. Too slow lateral movement speed can also cause the grinding wheel to vibrate and burn the surface of the part. The uniform and fast lateral movement speed can trim the surface of the grinding wheel, advance the grinding function of the grinding wheel, increase the grinding power, and reduce the surface roughness of the parts.

keep cool

Appropriate use of coolant can accelerate dressing speed and forward dressing power. Based on experience, choosing a 3/8 inch diameter coolant can remove a lot of heat from the dresser and extend the life of the dresser. When the diamond dressing tool passes through the grinding wheel, install a coolant nozzle to cover the entire surface of the grinding wheel or continuously add coolant to the diamond dressing tool. When the dressing tool touches the grinding wheel and begins to dress, the dressing tool must not be allowed to withdraw from the coolant. Otherwise, the diamond dressing tool will crack or break under the change of cold and hot temperature.

The high-precision filtration of the coolant with a filter avoids the multiple circulation of dirt or chips in the coolant. Contaminated coolant can wear out the grinding wheel quickly, increasing the number of dressing times for the grinding wheel. Dry dressing is only used for grinding wheels when dry grinding (as long as the cooling of the diamond dressing tool can be stopped in this situation). Allow the wheel to idle for a few minutes after turning off the coolant before leaving get off work each day. This prevents the grinding wheel from breaking.

Oscillation is the enemy of trimming

In grinding wheel dressing, it is very important to effectively reduce the vibration and avoid leaving dressing marks, collision and damage to the dressing tools on the surface of the grinding wheel. 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 geometric shape of the entire grinding wheel will affect the inherent balance of a grinding wheel. Therefore, it is also very important to choose a high-quality grinding wheel. Assuming a high-quality grinding wheel, just the right setup can keep the wheel well balanced. According to the factory instructions of the grinding wheel manufacturer, the grinding wheel is marked with an upward arrow, which indicates the light end position of the grinding wheel after rough balancing. The user can then follow the arrows to pay attention to dressing the grinding wheel so that it reaches dynamic balance. Evenly filling the coolant can sometimes help the grinding wheel to maintain balance.

In order to further avoid vibration, it is necessary to ensure that the trimming tool is firmly clamped on the clamping seat and maintains a small amount of overhang to ensure that the trimming tool has sufficient rigidity. If the diamond tool is not firmly clamped, it will cause vibration, noise, ripples on the surface of the part, strain on the surface of the part and damage to the trimming tool.

A superabrasive grinding wheel must be reshaped and sharpened before starting grinding. Following the experience described in this article will help your grinding wheel to produce the grinding results.

3. The influence of the grinding machine accuracy on the machining accuracy of the workpiece

The geometric accuracy, stiffness, thermal deformation, motion stability and vibration resistance of the grinder will directly affect the machining accuracy of the workpiece.

1. The geometric accuracy of the grinder

Refers to the motion accuracy and mutual orientation accuracy of each component under the condition of not receiving a load. It is impossible to make a machine tool with certainty and accuracy, and there are always more or less errors that are inevitable. This kind of error will be reflected to the workpiece to varying degrees during the processing of the workpiece, which will affect the machining accuracy of the workpiece. Generally, there are radial runout and axial movement of the main shaft, the straightness of the movement of moving parts such as the worktable, and the misalignment and transmission errors of the work parts.

The radial runout and axial runout of the grinding wheel spindle and the movement error of the grinder headstock are large, which not only affect the surface roughness of the workpiece after grinding, but also make the workpiece roundness and end face runout, resulting in uneven sparks during the grinding process. When the worktable moves on a straight surface that is not straight, the inner and outer cylindrical grinding machines will affect the linearity of the workpiece busbar, and when the plane is ground on the surface grinder, the workpiece flatness error will be large. The center line of the grinding wheel spindle axis of the cylindrical grinder and the center line of the grinding wheel axis of the internal cylindrical grinder are not at the same height as the center line of the workpiece headstock axis. When grinding the inner and outer cones, the workpiece generatrix is a hyperbola. The center line of the grinding wheel spindle axis is not parallel to the moving direction of the worktable, which affects the flatness of the workpiece end surface after grinding. The transmission error of the grinding machine has a great influence on the machining accuracy of thread grinding and gear grinding.

2. The stiffness of the grinder

It refers to the ability of the grinding machine to resist deformation when it receives external force (grinding force). That is, under the same grinding force, the smaller the deformation of the part, the greater the stiffness. On the contrary, the large deformation of the part indicates that the rigidity of the part is small. The size of these deformations destroys the static original geometric accuracy of the grinder, which will cause the size of the machining error of the workpiece. Therefore, the rigidity of the machine tool is high, and the machining accuracy of the workpiece is high.

3. Thermal deformation

The heat source inside the grinder is unevenly distributed, and the amount of heat generated in each part is different. The influence of the external heat source on each part of the machine tool is also different, and the thermal expansion coefficient of the parts is different due to different materials, resulting in different parts of the machine tool. The slight deformation reduces the original geometric accuracy of the machine tool and affects the machining accuracy of the workpiece. Therefore, the fine grinder device is used in a constant temperature chamber to avoid the impact of temperature changes on the accuracy of the machine tool and workpiece.

4. Grinder moving parts creeping

When the moving parts such as the grinding wheel frame of the grinding machine work table are fed in a micro-periodic manner or continuously moved at a low speed, the movement is uneven, which is commonly known as creeping. When this phenomenon occurs on the grinding machine, the feed in the grinding process is uneven, which affects the surface roughness of the workpiece grinding.

5. Oscillation of the grinder

The grinding machine oscillates during the grinding process, causing periodic changes in the relative orientation between the grinding wheel and the workpiece, causing vibration lines on the surface of the workpiece, which seriously affects the processing quality and accuracy.

In order to improve the accuracy of the workpiece after grinding, in addition to trying to eliminate the influence of the above factors, it is also necessary to pay attention to the reasonable selection of the positioning datum, the clamping method, the selection and correct repair of the grinding wheel, and the reasonable selection of the grinding amount and process in the workpiece processing process. Method.