GRINDING The grinding process, which is a basic shaping method, has been used since ancient times for sharpening bones and making weapons, sharpening swords, and making many cutting and piercing tools. By some manufacturers, the grinding process is not considered a machining method, but is listed among the surface improvement processes (honing, lapping, etc.). This is because the chip removed in the process is less. However, the place of grinding in the manufacturing sector is too important to ignore.
Grinding is a machining process with a hard, angular, abrasive grain brought into a certain geometry or in free form, and tools that are completely formed from these grains. Grinding is one of the basic material processing steps. Small inserts on the grains create chips. During chip removal, the abrasive stone rotates around its own axis at very high speeds. The cutting process takes place thanks to the friction between the abrasive stone and the workpiece. Abrasive stone is obtained by bonding abrasive particles together with various binders. The main purposes of the grinding process are: Machining of workpieces within the specified precision tolerance range. Elimination of distortions after heat treatment. Chip removal from hardened surfaces. In this regard, with the grinding process, much higher quality surfaces are obtained compared to the turning and milling processes. Grinding, which is usually the last step in manufacturing, is also preferred to obtain quality and bright surfaces. GRINDING MACHINES While people used to process tools with abrasive stones, later on with the development of grinding machines, a large gap in this field was filled. Today, jet engines, high-pressure automobile engines and similar parts requiring precision are processed on grinding machines. It is not possible to machine parts that require precision without these machines. And this degree of precision is extraordinary today. When James Watt invented the steam engine (1777), one millimeter was considered sufficient.
After the 2nd World War, the desired measurement accuracy in war vehicles increased gradually, and today the precision of 0.00018 mm of the fuel injection nozzle of jet aircraft is obtained on grinding machines. TYPES OF GRINDING MACHINES 1 Plane (Surface) Grinding Machines 2 Cylindrical Grinding Machines 3 Centerless Grinding Machines 4 CNC Grinding Machines 1 PLANE (SURFACE) GRINDING MACHINES These are the machines used for grinding the flat surfaces of the workpieces. They are divided into two groups as follows; Horizontal Spindle Plane Grinding Machines Vertical Spindle Plane Grinding Machines Horizontal Spindle Plane Grinding Machines This type of grinding machine also has the stone spindle in horizontal position. In this case, the grinding tool, which is the cutting tool, cuts with its surroundings. It moves back and forth to the right-left of the workbench table to which the workpiece is attached and to the body of the workbench. The amount of sawdust (pass) for cutting is given by the up and down movement of the stone head. The up and down movement sensitivity of the stone head is 0.01 mm. Bench table works fully or semi-automatically with hydraulic system. Small sized and sensitive parts are grinded with horizontal spindle plane grinding machines. In addition, straight channels and similar grooves are also ground in these benches.
Horizontal Spindle Plane Grinding VERTICAL SPINDLE PLANE GRINDING MACHINES In this type of grinding machines, the stone spindle is in the vertical (vertical) position. As such, the grinding tool, which is the cutting tool, cuts with the forehead surfaces. Although they are especially used for grinding large, voluminous and heavy workpieces, there are also types used for grinding circular-section workpieces. As can be seen in the pictures below, the table top in large-size vertical spindle grinding machines is linear (right-left and forward-backward), just like in horizontal spindle grinding machines.
However, in small-sized ones, the movement of the worktop is circular. Since large-sized and wide-surfaced workpieces are grinded on vertical spindle grinding machines, the grinding wheels used in large-sized ones are made with fragmented stones. In this type of workbenches, the passes given are more than the ones that cut from the periphery. Although they provide significant contributions in terms of machining time, the surface qualities obtained are coarser. Since the friction surface of the stone is large, especially thin workpieces heat up more and get warped. Therefore, when grinding thin parts, it is necessary to narrow the friction surface of the stone by sharpening. If cup stones are used, the stone is conical inwardly sharpened to make it easier to cut and to reduce the contact surface. Thus, excessive heat generation is prevented. However, as the stone becomes easier to cut, it wears out more quickly.
CYLINDRICAL GRINDING MACHINES It is a machine that grinds the outer surfaces of cylindrical workpieces as cylindrical or conical. It is used to grind cylindrical or conical surfaces such as sleeves, pins, shafts. Hole grinding is also done with such machines. Hole grinding is done with the help of special hole grinding apparatus. In cylindrical grinding machines, workpieces are grinded in three different ways: between the chuck, between the mirror center and between the two centers. Workpieces that are too long are also supported by bearings.
The main cylindrical grinding operations are classified as cylindrical external grinding, conical external grinding, hole grinding, plunge grinding, profile grinding, crank and cam grinding and gear grinding.
Crank and Cam Grinding, crank and cam grinding machines are grinding machines used in industry for special purposes. Cam and crankshafts, especially used in the automotive industry, are grinded on these benches so that they can be used more precisely. These machines are fully or semi-automatic according to their usage areas. It works with hydraulic system. The most important part of gear wheels, which are gear grinding, power and motion transmission elements, is the diameter of the section circle. Because gear
impellers work by contacting each other on this diameter. Precise machining of these parts increases the working efficiency of the gears and provides a more comfortable transmission of power and movements. Gear grinding methods are generally of two types: Form (Shape) grinding Linear (Rounding) grinding Gear grinding with the rolling method is performed by attaching a single form stone or two bowl stones to the workbench. While the gear wheel moves back and forth under the stone, it also makes an oscillating (rolling) movement. The oscillating movement of the gear wheel is carried out by the bench table in accordance with the tooth profile. Internal surfaces are grinded in a way that Hole (Internal) Grinding is the opposite of Cylindrical grinding. Stone geometries are cylindrical, conical, etc. can be supplied in accordance with the hole geometry to be ground in shapes.
Grinding margin is kept as low as possible in internal grinding operations. Because the diameter of the stone is small compared to the diameter of the ground hole, and in this respect, the chip removal capacity is reduced. The internal grinding spindle gets its movement either directly from the motor or from the belt and pulley. The properties of the stone used in the hole grinding process are different from the wheels used in the cylindrical outer surface grinding processes. The stones used in hole grinding should be softer and less textured. The vibration caused by the gap in the shaft bearings, where the stone moves from the motor, may cause the stone to break. Therefore, this type of countertops should be used by choosing a harder stone. Plunge Grinding The rotation direction of the stone and the rotation direction of the work are the same. However, the workpiece only rotates around its axis, does not move longitudinally (right-left). The stone plunges into the part up to the depth of cut. The front surface of the grinding wheel is flat. So it doesn't have any profile.
Profile Grinding Similar to plunge grinding. In this type of grinding, the stone has a special profile and this profile passes to the workpiece during grinding. The grinding wheel is pre-sharpened on the profile of the workpiece in profile grinding machines. The movements of the stone and the work are just like in plunge grinding. 3 CENTERLESS GRINDING MACHINES In centerless grinding, the workpiece is grinded by passing it between two stones on a guide rail, without binding, as shown in the picture below. While the workpiece rotates with the speed it gets from the cutting wheel, it also makes longitudinal progress like a screw with the effect of the lead stone.
Grinding stone (cutting stone) does the main grinding process. The cutting wheel starts to rotate the work as soon as it contacts the work at the time of grinding. At the same time, it presses the work to both the runner and the lead stone. The task of the propulsion stone is to reduce the rotation speed of the work by braking and to advance the work. Therefore, its speed is very low. Fine grinding is done at low speeds and coarse grinding is done at high speeds. Work Advance Movement An angle is given to the lead stone for the work progress motion. The larger this angle, the faster the work progresses. The stone is sharpened by giving the same angle to the grinding device of the moving stone. As a result of this sharpening, the surface of the lead stone becomes pitted. However, during grinding, the lead stone comes into contact with the work over its entire surface. Thanks to these two angles, the propulsion stone advances the work like a screw. According to the grinding capacity of the machine, the driving stone is made with two revolutions (20-36 rpm) in order to grind the works of all diameters with the appropriate surface quality. In this way, both rough serial grinding operations and precision and fine surface quality grinding operations are performed. When grinding thin rods up to 6 mm in diameter, the work axis is approximately
It should be adjusted to be under mm. In this position, the workpiece cannot bounce while grinding. Centerless Grinding Processes Centerless longitudinal grinding Centerless plunge grinding (profile grinding) Centerless stop grinding Centerless Longitudinal Grinding, in longitudinal centerless grinding, stepless, longitudinal cylindrical works are grinded. The work passes between two stones. Shafts, trunnions, coupling pins, rings and rings carry with this system. When choosing the feed rate to be given to the work in grinding, the type of material, the amount of sawdust, the surface quality, the type of stone, the cooling liquid and the condition of the workbench should be taken into consideration. Adjusting the side supports also requires special attention. When adjusting the side supports, it must be ensured that the work is seated on the entire surface of the cutting stone and lead stone.
Centerless Plunge Grinding (Profile Grinding) It is used for grinding many identical parts with different profiles. Stepped and profiled shafts are placed between two stones from above. 1/4-1/2º angle is given to the propulsion stone. With the help of this angle, the work progresses slowly and rests on the counter gauge. When the stop is pushed by the workpiece, the cut-off wheel is pushed back with the help of a switch and the piece falls. Centerless Grinding with Stop In this system, the progress of the work is limited by a stop. The work is carried out by the lead stone or by hand until it touches the abutment. The support is controlled by a lever system from the side where the machine is used. When the work comes into contact with the stop, it is taken back by pushing it back by the control lever. 4 CNC GRINDING MACHINES Cylindrical and plane grinding processes require high precision and surface quality. For this reason, it is very suitable for the basic philosophy of numerical control, especially in terms of technology. Unfortunately, the use of CNC in the field of grinding is a little late compared to other fields. The manufacturing companies, which successfully solved some special problems related to grinding, developed their own CNC systems by integrating numerical control with their manufacturing.
Standard control designs used in lathes and machining centers are not useful in grinding machines. For this reason, different solutions are needed in the control systems of grinding machines from other types of machines. Sometimes a high precision of up to 0.1 micron (mm) and a very large feedrate range (Feeding speeds range from 0.02 mm/min to 60 m/min.) are required. Special canned grinding cycles are also required for grinding operations. Gradual advance increase, standby, oscillation, routine, stone sharpening, etc. are examples of these cycles. Automatic cutter compensation should be done after grinding wheel and linear and curvilinear interpolation should be done without interruption. In case of any contour deviations, grinding of the grinding wheel, data entry to the program and corrections to be made later should also be done easily. Since the amount of cutter used in grinding machines is not much, the compensation process is simpler.