§ 14. FASTENING AND BALANCE OF THE GRINDING WHEEL.

Grinding wheel balancing on the machine. Balancing machines.

The grinding wheel is fixed on the flanges. In this case, it is necessary to ensure that the fit of the circle on the neck of the flanges is free, but without much pitching. Permissible gap between the circle and the neck of the flange is 0.1 - 0.3 mm. The circle should fit snugly against the ends of the flanges. Both flanges are fastened with screws. Cardboard spacers are placed at the ends of the circle.

Before installing the flanges with a circle fixed between them on the machine spindle, the assembled assembly must be balanced, i.e., ensure that the common center of gravity of the circle and the flanges is on the axis of rotation.

Balance circles on a special machine in special device. In the process of balancing, the imbalance of the circle is revealed and eliminated.

Since the circles rotate at a very high speed and have big sizes, then in case of imbalance, a large centrifugal force arises, tending to deflect the circle from the axis of rotation.

The imbalance of the circle increases the roughness of the machined surface - it turns out to be crushed, snags appear, in addition, the bearings of the machine spindle wear out quickly. The main reasons for the imbalance of the circle are as follows:

a) eccentric location of the hole relative to outer surface;

b) eccentric installation of the circle on the machine spindle or in flanges;

in) irregular shape outer surface;

d) uneven density of the material of the circle.

Before balancing, the circle is cleaned of dirt and sawdust and carefully inspected. Circles with cracks do not balance, but reject.

If the diameter of the hole is larger than the diameter of the mandrel, then the surface of the hole can be increased with a special mass, and the thickness of the mass with a hole diameter of more than 50 mm is up to 5 mm. The extension is carried out by specially trained workers.

Circles are balanced in several steps. First, a new circle is balanced on a cylindrical mandrel placed on supports balancing machine. Machines come with supports in the form of rollers, prisms and disks (Fig. 28).

Rice. 28. Balancing machines for balancing grinding wheels:

a - with supports in the form of rollers: 1 and 3 - rollers, 2 - grinding wheel on a mandrel; b - with supports in the form of disks: 1 and 3 - disks, 2 - grinding wheel on a mandrel

In the flange that secures the circle on the mandrel, there is an annular groove in which three weights move. If a circle mounted on a mandrel is put on supports and turned, then if the mass is unbalanced in some part of the circle, it will turn so that this mass will be in the lower vertical position. In practice, one weight is fixed in the highest position, and the other two next to it. If after that the circle is pushed, it will rotate until the weights are at the bottom. By moving the weights along the annular groove, the unbalanced mass is balanced, i.e., they achieve such a position that, with any turn through a small angle, the circle does not return to its original position.

A balanced circle is mounted on the spindle of the grinding machine and corrected - they give the circle the correct geometric shape, after which the initial balancing is violated, since when the wheel is dressed, an uneven layer of abrasive is removed. Then the wheel is removed from the machine and re-balanced carefully. After the second balancing, the circle is finally corrected and put into operation. At high requirements to surface roughness, balancing is performed more than twice. The installation of the balancing machine (see Fig. 28) is carefully checked for level.

In the process of grinding, the diameter of the circle gradually decreases and its balance may be disturbed, therefore, if the diameter of the circle decreases by 50-60 mm, it must be re-balanced.

Perform wheel balancing on the machine itself.

Recently, automated balancing devices have been used.

In accordance with the requirements of the standard, all grinding wheels are tested for strength at special machines, providing a circumferential rotation speed 1.5 times higher than the working one.

Especially carefully check the strength of high-speed circles. The time of their trial rotation on test machines is 3 minutes for a circle with a diameter of up to 90 mm, and 10 minutes with a diameter of 475 mm.

Careful balancing of the grinding wheels guarantees a long and safe operation machine and high quality produced blanks. Adjustment of the equipment takes place together with the mandrel on which the wheel is attached. The procedure involves the use of specialized equipment, which has many modifications. These can be adjusting machines, installations, machine tools, stands and other mechanisms that differ in the principle of operation.

The Importance of Balancing

The productivity and safety of work depends on how accurately the working tool is debugged.

Qualitative balancing should take place only on the last technical control device operated by qualified personnel.

Why is balancing necessary?

Working with an unbalanced machine, the master risks injury due to a break in the grinding wheel. This mishandling of the device may result in damage to the eyes and skin of the worker. Bursting, the red-hot part crumbles on small particles, which fly apart from high speed in different directions.

The importance of balancing grinding wheels also lies in the quality of the produced workpieces. When the balance is disturbed, edges, waves, and other irregularities are formed on the treated surface. This fact significantly reduces productivity.

The operation of the device occurs at a high speed of rotation of the circle. When using a non-debugged abrasive wheel, centrifugal forces arise, which provoke significant vibrations. Such a violation of technology leads to consequences for the tool:

• increased wear of the grinding machine, primarily bearings;
• the consumption of the abrasive component increases;
• service life is reduced.

Thus, the efficiency of the machine directly depends on the debugged position of the grinding wheel.

Causes of the imbalance of the circle

For quiet and precise work on the machine, the abrasive device must be balanced so that the center of gravity and rotation are on the same axis.

The main reasons for the occurrence of deviations in the balance:

• eccentricity (degree of deviation) of the outer face relative to the mounting hole;
• violation in the geometry of the circle;
• uneven impregnation with cutting fluid (coolant);
• deviation in the parallelism of the ends of the circle;
• eccentricity that occurs when the wheel fits, due to one-sided sampling of the gap between the hole and the clamping washer;
• incorrect installation of the circle on the installation;
• uneven density of the mass of the workpiece.

How to balance a grinding wheel

For quiet and precise work on the machine, the wheel must be balanced so that the center of gravity and rotation are on the same axis.

Grinding wheel balancing is possible in many variations. Main methods: static and dynamic.

The advantage of dynamic devices is high accuracy. However, they are not used as often as static ones, due to the high cost of such equipment and high complexity in work, which requires specialized qualifications of the master.

More economical and easier to operate, static fixtures are used in production much more often.

During the manipulations, various balancing devices and additional details are used. It is also possible to set up the equipment without removing it from the grinding machine.

Below is a list of methods for adjusting the abrasive element.

1. The moment of imbalance between the mounting diameter of the clamping washer and the hole of the circle can be reduced by 2 times. For this, an elastic centering sleeve is used. It looks like a ring with petals. The part is represented by different stiffness indicators, which are selected in accordance with the level of overcoming the mass of the circle with a small margin.
2. Static balancing on equipment with two horizontal parallel rulers or cylindrical rollers. Adjusting device of this type - the most popular. Its design assumes the presence of two strictly horizontal planes of the working rulers. On which a mandrel with a circle is installed. The method has a drawback: balancing the grinding wheels damages the surface of the rulers. Dents form on them, which interferes with the accuracy of work. Eliminating this negative feature requires regular grinding and leveling.
3. A balancing device with rotating rollers (or discs) has the same principle, however, it does not have the above disadvantages. Wherein common feature reducing the balancing accuracy is a large friction torque.
4. To avoid the influence of the friction moment on the balancing, a rational design of fixtures for the static method has been created. It is based on the air cushion principle.
5. Sanding disc balancing is also possible on a balancing scale. The work is based on a static method. The equipment has a limitation: it is designed for circles whose diameter is in the range from 200 to 600mm.
6. The highest quality balancing of the grinding wheel occurs directly on the machine. For these purposes, automatic devices of various modifications are used. The method has a number of advantages:

• There is no need to remove the disk, which, together with the installation, requires a significant amount of time.
• The grinding machine is balanced during operation, which allows you to constantly monitor its balance.
• Gives the most efficient and accurate results.

When choosing a balancing method for grinding equipment, one should proceed from the requirements of accuracy, time and financial resources.

The grinding wheel is fixed on the flanges. In this case, it is necessary to ensure that the fit of the circle on the neck of the flanges is free, but without much pitching. Permissible gap between the circle and the neck of the flange is 0.1 - 0.3 mm. The circle should fit snugly against the ends of the flanges. Both flanges are fastened with screws. Cardboard spacers are placed at the ends of the circle.

Before installing the flanges with a circle fixed between them on the machine spindle, the assembled unit must be balanced, i.e., ensure that the common center of gravity of the circle and flanges is on the axis of rotation.

The circles are balanced on a special machine in a special device. In the process of balancing, the imbalance of the circle is revealed and eliminated.

Since the circles rotate at a very high speed and are large, when unbalanced, a large centrifugal force arises, tending to deviate the circle from the axis of rotation.

The imbalance of the circle increases the roughness of the machined surface - it turns out to be crushed, snags appear, in addition, the bearings of the machine spindle wear out quickly. The main reasons for the imbalance of the circle are as follows:

a) eccentric location of the hole relative to the outer surface;

b) eccentric installation of the circle on the machine spindle or in flanges;

c) irregular shape of the outer surface;

d) uneven density of the material of the circle.

Before balancing, the circle is cleaned of dirt and sawdust and carefully inspected. Circles with cracks do not balance, but reject.

If the diameter of the hole is greater than the diameter of the mandrel, then the surface of the hole can be increased with a special mass, and the thickness of the mass with a hole diameter of more than 50 mm is up to 5 mm. The extension is carried out by specially trained workers.

Circles are balanced in several steps. First, a new circle is balanced on a cylindrical mandrel placed on the supports of the balancing machine. Machines come with supports in the form of rollers, prisms and disks (Fig. 28).

Rice. 28. Balancing machines for balancing grinding wheels:

a - with supports in the form of rollers: 1 and 3 - rollers, 2 - grinding wheel on a mandrel; b - with supports in the form of disks: 1 and 3 - disks, 2 - grinding wheel on a mandrel

In the flange that secures the circle on the mandrel, there is an annular groove in which three weights move. If a circle mounted on a mandrel is put on supports and rotated, then if the mass is unbalanced in some part of the circle, it will rotate so that this mass will be in the lower vertical position. In practice, one weight is fixed in the highest position, and the other two next to it. If after that the circle is pushed, it will rotate until the weights are at the bottom. By moving the weights along the annular groove, the unbalanced mass is balanced, i.e., they achieve such a position that, with any turn through a small angle, the circle does not return to its original position.

The balanced wheel is mounted on the spindle of the grinding machine and corrected - the circle is given the correct geometric shape, after which the initial balancing is violated, since an uneven layer of abrasive is removed when the wheel is dressed. Then the wheel is removed from the machine and re-balanced carefully. After the second balancing, the circle is finally corrected and put into operation. With high requirements for surface roughness, balancing is performed more than twice. The installation of the balancing machine (see Fig. 28) is carefully checked for level.

In the process of grinding, the diameter of the circle gradually decreases and its balance may be disturbed, therefore, if the diameter of the circle decreases by 50-60 mm, it must be re-balanced.

Perform wheel balancing on the machine itself.

Recently, automated balancing devices have been used.

In accordance with the requirements of the standard, all grinding wheels are tested for strength before being sent to the workshop on special machines that provide a circumferential rotation speed that is 1.5 times higher than the working one.

Especially carefully check the strength of high-speed circles. The time of their trial rotation on test machines is 3 minutes for a circle with a diameter of up to 90 mm, and 10 minutes with a diameter of 475 mm.

Grinding wheel balancing before mounting on the machine

Balancing of grinding wheels before their installation on the machine. If the center of gravity of the grinding wheel coincides with its axis of rotation, then such a wheel is balanced and can work reliably at high circumferential speeds. The imbalance of the grinding wheel causes intense wear of the machine components and mechanisms, primarily the spindle bearings; deterioration of the roughness of the treated surface; increase in the consumption of the abrasive material of the circle and the means for its editing; reduction in processing accuracy; an increase in stresses in the circle, as a result of which its destruction may occur. All grinding wheels with a diameter of more than 100 mm are subjected to balancing. Before balancing, the circle must be inspected and made sure that there are no cracks.

To perform balancing, various balancing devices are used. Due to the simplicity of design and ensuring sufficient accuracy of balancing grinding wheels, balancing devices with rollers are most widely used (Fig. 4.4).

The main parts of the fixture are: two parallel heat-treated and ground steel rollers 1, bed 3 and mounting flanges 2 with balancers.

Grinding wheel balancing is performed in the following sequence.

• 1. A circle with flanges and a mandrel is mounted on a balancing device (Fig. 4.4), making sure that the axis of the mandrel is perpendicular to the axis of the balancing rollers 1, and the circle was located between them symmetrically; balancing crackers are set to the position of mutual balancing.
• 2. With a slight push, make the circle, together with the mandrel, rotate on the rollers of the balancing device until it stops completely, while the heavier part of the circle will take the lowest position.

Balancing device with two cylindrical rollers:

• 1 - roller; 2 - flange-sleeve with balancing crackers; 3 - bed.
• 3. After stopping the circle at the very top point its periphery draws a radial chalk line. Then the circle is turned at an angle of approximately 90 ° from the chalk line, either in one direction or the other, and the circle is allowed to roll along the rollers along with the mandrel. If after each turn the circle is set so that the chalk line is in the same position, then the heavy part of the circle is correctly determined.
• 4. The balancing crackers are moved along the groove in one half of the groove circumference to the chalk line and fixed in this position. The middle cracker is set symmetrically with respect to the chalk line, and the two extreme ones are approximately at an equal distance from the middle one.
• 5. The circle is again turned on the rollers at an angle of 90 ° to the right and left sides, and if the circle returns to its original position, i.e. if the heavy part of it tends to take the lower position, then the circle is balanced by moving the extreme crackers relative to the middle one, standing under the chalky trait. A balanced circle, rotated to the right or left by 90 ° relative to the chalk line, remains motionless in this position. This indicates that the circle is balanced.
• 6. To control the correctness of balancing, the rotation of the circle relative to the chalk line at an angle of 70-90 ° should be repeated 2-3 times.

Balancing of grinding wheels on the machine.

In the process of work, the balance of the circle may change. This is mainly due to the uneven density of the wheel and the displacement of its center of gravity from the geometric axis as the wheel wears out. Re-balancing is especially necessary when performing fine work, for getting high class surface roughness and high-speed grinding.

For re-balancing, balancing mechanisms should be used that allow this operation to be performed directly on grinding machine during his work without removing the circle. To do this, use special balancing mechanisms to automatically balance the circle.

The state of the grinding wheel, characterized by such a distribution of masses, which during rotation causes variable loads on the supports of the machine spindle and its bending, is called circle imbalance. Unbalanced point mass of a circle call the conditional mass, the radius vector (eccentricity) of which relative to the axis mounting hole equal to the radius of the outer surface (periphery). Depending on the permissible unbalanced masses for grinding wheels on ceramic, bakelite, vulcapite and special organic bonds, according to GOST 3060-86, four classes of unbalance of grinding wheels are established, denoted by the numbers 1, 2, 3 and 4.

Grinding wheels with a diameter of 250 mm or more in accordance with GOST 2424-83 must pass the unbalance control in accordance with GOST 3060-86 before installing them on the machine.

The imbalance of the circle increases the roughness of the machined surface and leads to the appearance of crushing and burns on it, increased wear of the circle, premature failure of the spindle and other machine components.

Of particular importance is the degree of balance of the circles in connection with the increasing introduction of high-speed and ultra-high-speed grinding.

The main reasons for the imbalance of the circle are as follows:

• ? eccentric location of the hole relative to the outer surface;
• ? eccentric mounting of the wheel on machine spindles or in flanges;
• ? irregular shape of the outer surface;
• ? uneven density of the material of the circle;
• ? wheel wear during grinding.

According to GOST 3060-86, four classes of unbalance of grinding wheels are established (Table 2.18), according to which the permissible unbalanced masses are determined depending on the mass of the wheels.

Table 2.18

Permissible unbalanced masses of circles with dense and medium structures (GOST 3060-86)

Circles are balanced on special stands on mandrels (Fig. 2.3).

Rice. 2.3.

Balancing is carried out by moving the crackers after installing the mandrel with a circle on the stand. At the beginning of balancing, the crackers are set at about 120 ° so that their mass does not affect the position of the center of gravity of the circle. With a light push, the circle with the mandrel is rotated. After stopping the circle with a mandrel, make a mark at the lowest point of the circle and connect this point to the center, get the radius at which the center of gravity of the circle is supposedly located.

Rice. 2.4.

Due to friction, the set direction may not coincide with the actual position of the center of gravity, so the circle is rotated 90 ° alternately in both directions so that the marked radius takes horizontal position and then release the circle. After it stops, the lower position of the circle is again noted.

If all three marks coincide, then the position of the unbalanced mass is determined correctly. If the marks do not match, the position of the unbalanced mass is determined between the last two marks at an equal distance from each of them. Then, on the opposite side of the circle, a mark is made for the installation of weights. After that, the circle is rotated so that the found mark takes a horizontal position, and the crackers are moved to a position in which the circle does not rotate, i.e. so that the horizontal position of the previously set mark does not change.

On fig. 2.4 shown general form disk stand for balancing the circle.

The balancing accuracy, determined by the displacement of the center of gravity / 0 , is found by the formula

where p is the coefficient of rolling friction (0.01 ... 0.05); R- radius of the large disk, mm; R! - coefficient of rolling friction in ball bearings (0.001...0.005); p is the radius of the ball bearing bore, mm; y is the radius of the support neck of the balancing mandrel, mm; a - the angle formed by the straight line connecting the centers of the balancing mandrel and the large disk with the vertical.

With a = 2 ... 3 ° / o = 0.01 ... 0.02 mm.

On fig. 2.5 shows balancing weights. The balancing accuracy /o on the scales is the highest and reaches 0.005 ... 0.008 mm. By design, the scales are more complex than stands, and the balancing operation on them is longer, since it is necessary to wait for the damping of the oscillations of the arrow 5.

The circle is balanced by setting the weight 6 to the zero position and with the help of a load 2 balance the rocker without a circle. Then the scales are arrested and a mandrel is installed 7 with a circle. Release the locking device 3 , observe the deviation of the arrow 5 and when it deviates to the left, the circle is turned counterclockwise until the arrow takes up on the scale 4 middle zero position. When arrow 5 deviates to the right

Rice. 2.5.

with a circle (b):

• 1 - additional load; 2,6 - cargo; 3 - arresting device;
• 4 - scale; 5 - arrow; 7 - mandrel

the circle is rotated clockwise. Then the vertical axis of the circle is marked with risks, the circle is rotated 90 ° and the horizontal axis is marked. Moving cargo 6, lead arrow 5 to the zero position and on the scale 4 determine the imbalance, which is eliminated by moving an additional load 1.

Modern grinding machines use various devices, allowing you to balance the circle directly on the grinding machine. These devices allow you to determine and eliminate the imbalance of the circle during the entire time of its operation on the machine.