§ 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 unit must be balanced, i.e., ensure that the common center of gravity of the circle and the 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 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 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.

A balanced wheel is mounted on a spindle grinding machine and rule - give the circle the right 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 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.

If grinding wheels operating at high circumferential speeds are unbalanced, vibration may occur, which accelerates the wear of the spindle and machine bearings, there is a danger of breaking the circle, the quality of processing deteriorates, the consumption of the circle increases, etc. In this regard, all circles with a diameter of more than 125 mm and 8 mm high must be balanced before mounting on the machine. Due to their relatively low height, the wheels are only statically balanced.

Wheel balancing can be performed on balancing devices before they are installed on the machine or by a special mechanism without removal from the machine. More often, the circles are balanced on the simplest devices, which differ from each other mainly in the nature of the supports for installing the mandrel with the circle put on.

Supports can be prisms, disks and rollers. These supports are equally good for balancing if they are parallel and strictly horizontal. The simplest balancing device is shown in Fig. eighteen.

a - with support knives; b - with two pairs of rotating disks

To detect static imbalance, the circle, together with the flanges, is mounted on a balancing frame and mounted on the supports of the device so that it can freely rotate about the axis of rotation. If the wheel is not statically balanced, it will set with the heavy part down. Unbalance is usually eliminated by adding a counterweight on the side of the "light" place. This is achieved by moving special balancing weights (“crackers”) placed in flanges or in special devices and devices. A balanced circle must remain stationary in any position. The magnitude of the load, which, being located on the periphery, moves the center of gravity to the axis of rotation of the circle, i.e., brings the circle into indifferent equilibrium, serves as a measure of static imbalance.

Unbalance limit values ​​are determined by GOST 3060-55 depending on the height of the circle, outer diameter and unbalance classes, which determine the accuracy of the manufacture of the circle. For each of these classes, depending on the diameter and height of the circle, the maximum allowable unbalance values ​​are established. For circles of the 1st class, the value of the imbalance, as a rule, does not exceed 0.1%, for circles of the 4th class - 0.3%.

In table. 4 shows the limit values ​​for unbalance for circles up to 25 mm high.

Grinding wheel balancing before mounting on the machine

Balancing of grinding wheels before their installation on the machine. If the center of gravity grinding wheel coincides with the axis of its rotation, then such a circle 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 balancing accuracy of 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, to receive 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 the grinding machine during its operation without removing the wheel. To do this, use special balancing mechanisms to automatically balance the circle.

Our company offers devices for static balancing of grinding wheels and their mandrel for various faceplate sizes.

Attachments with parallel knives PB-01.000 and PB-02.000

These balancing devices are used for balancing grinding wheels and cutters.

Disk balancing device PBD-01.000

Why is it necessary to balance grinding wheels and cutters?

During operation, the grinding wheel reaches high speeds. Even the slightest imbalance in it leads to the appearance of significant centrifugal forces. And this, in turn, negatively affects the operation of the entire machine and the quality of products.

There are several reasons for the appearance of imbalance in the circle. Firstly, this is the uneven distribution of mass in the body of the circle; secondly, the eccentricity of the circle in relation to the axis of the machine spindle (improper fit in the flanges), imbalance of the flanges; thirdly, the irregular geometric shape.

Grinding wheel balancing devices with parallel blades help to find the place of imbalance. Parallel knives or prisms are the simplest device, the principle of which is static balancing. The main requirement is that they must be fixed on a rigid base strictly parallel and horizontally. The essence of static balancing is as follows: when the mass distribution changes, the center of mass is brought to the axis of rotation. Unbalance correction must be carried out at the largest radius due to the fact that the influence of the mass of the corrected metal on the balance of the circle increases the more, the greater the distance from the axis.

Balancing is performed in 5 stages:

• rough balancing;
• fine balancing;
• choice of location and size of working balancing weights;
• installation and fastening of working balancing weights;
• balancing quality control.

Balancing high-speed cutters for woodworking machines

In addition to balancing grinding wheels and milling cutters, these devices are used for balancing typesetting high-speed cutters of woodworking machines.

Assembly and preparation of grinding wheels. Before mounting on the machine, the wheels should be checked for compliance with their hardness and grit values ​​specified in technological map. Each circle must be carefully inspected and checked by light tapping with a wooden mallet for cracks in it (the sound should be clear, without rattling).

Circles 1 (Fig. 9.16) should be assembled on the mandrel 2 according to the grinding machine setup drawing. Landing a circle on the mandrel should be easy, without the use of force to avoid breaking it. The gap between the seat, the mandrel and the hole diameter of the circle should be 0.3 ... 0.5 mm; the deviation from the perpendicularity of the ends of the circle to its axis should not exceed 0.15 mm (at the periphery of the circle with a diameter of 500 ... 600 mm), which is achieved by turning the ends of the circle, maintaining dimensions A, B, C and D. cardboard oiled pads 4 up to 1 mm thick; when fixing circles on the faceplate using flanges, it is necessary that the latter are precisely centered. To avoid distortion of the flanges and the destruction of the circle when assembling it on the faceplate, nuts 3 should be tightened alternately (through 180° from opposite sides).

Test of endurance. Before working on the machine, the wheels are tested for strength by trial rotation at high speed. For this issue special machines, providing wheel speeds 1.5 times higher than the operational speed. Tests should be carried out with a delay in time at a given test speed. The test mode is automated. Management is carried out from the remote control. The circle is tested according to a given program - acceleration, holding at a test speed and braking to a complete stop. The speed is infinitely adjustable. Test benches are equipped with lifting and installation devices. The circle is put on the flanges, while it is necessary to pay attention to the fact that the gap between the inner hole of the circle and the mounting diameters of the flanges is uniform around the entire circumference.

Balancing circles. In order to achieve high precision and vibration-free grinding of workpieces, the wheels and faceplate assembly must be balanced. When working with an unbalanced circle, the machined surface turns out to be faceted, wavy, and the spindle bearings wear out quickly. The reasons for the imbalance of the circle may be the uneven distribution of mass in the body of the circle, the eccentric arrangement mounting hole in relation to the outer surface of the circle, non-parallelism and non-perpendicularity of the ends, uneven impregnation of the circle with coolant, incorrect installation - non-concentric installation of the circle, etc.

With an unbalanced circle, centrifugal force arises, which causes vibrations. With high-speed grinding, the risk of wheel breakage from cutting forces decreases, but the risk of breakage from centrifugal force increases.

The circle is balanced outside the grinding machine on balancing stands. A circle mounted on a mandrel is mounted on supports - cylindrical rollers or disks (Fig. 9.17). Both devices (Fig. 9.17, a, b) have general disadvantage- a large friction torque, which reduces the balancing accuracy. The use of the "air cushion" principle made it possible to create a rational design of the device for static balancing (Fig. 9.18). The advantage of the "air cushion" device is that the mandrel with the circle easily rotates under the influence of a small moment of forces. To bring a mandrel with a circle mounted on cylindrical rollers out of rest, a moment is required that is 7 times greater, and with disks, 40 times greater.

On the spindle of the machine, the circles are fixed with the help of a faceplate, in the end grooves of which weights are placed - segments for balancing the circles. Balancing is done by changing the position of three weights in the annular undercut of the grinding wheel flange. An unbalanced circle with a heavier part will turn down. By moving the weights in the faceplate, they ensure that the circle in any position on the supports remains motionless. As the wheel wears out, its balancing may be disturbed due to the uneven distribution of mass in the body of the circle, so it is advisable to re-balance the wheel. To do this, it is recommended to rotate the wheel at operating speed for 1-2 minutes, turning off the cooling, so that the liquid does not accumulate in the pores of the lower part of the wheel and does not disturb the balance.

In more critical cases, balancing weights are used. With careful manufacturing of balance parts, the balancing accuracy can be brought to a residual center of gravity displacement of 5 µm. The average balancing time is 15...20 min. Balancing scales are designed for balancing grinding wheels with a diameter of 200 to 600 mm. Devices for balancing wheels directly on the grinding machine are also used.