Performing grinding work on the machines
.3. The main types of grinding work
Grinding machines can perform grinding of external and internal cylindrical, conical and shaped surfaces, grinding of flat surfaces, gears, threads and other grinding work with the use of appropriate wheels and fixtures .
Grinding external cylindrical surfaces The cylindrical surfaces are ground on cylindrical grinding machines using the longitudinal and cross feed method or the in-depth method. Such surfaces can also be ground on centerless machines when working by the method of longitudinal feed or grinding to a stop.
In Fig. 73, а, Cylindrical surface grinding on a centerless machine by the longitudinal feed method is shown. Cylindrical part 1 along the feed rollers 2 и 3 is fed to the rotating grinding wheel 4 and leading 5 it is gripped by the grinding wheels and fed into the grinding station, where it is supported by the workpiece stop (knife) 6. The ground workpiece is fed from the working area to the output guideways 7.
In one pass, the grinding wheel removes a layer of metal with a thickness of 0.02-0.3 mm (per workpiece diameter).
End stop grinding is used for machining stepped workpieces.
Internal cylindrical surfaces Internal cylindrical grinding, planetary grinding and centreless grinding machines.
Internal grinding is mainly used in machining precise holes in hardened workpieces, as well as in cases when for any reason it is impossible to use other, more productive methods of precise machining of holes, such as diamond boring, honing, etc.
There are two main varieties of internal grinding: 1) grinding a hole in a rotating workpiece and 2) grinding a hole in a stationary workpiece.
The first method is used for grinding holes in small workpieces, mainly representing bodies of rotation, such as holes in gears, rings of ball and roller bearings, and the second. when grinding holes in workpieces for housing parts, which is inconvenient or impossible to fix in the machine chuck.
In both cases, there is a longitudinal feed of the grinding wheel along the axis of the ground hole; in the first case, by the movement of the spindle head, in the second. by the movement of the table.
The most important difference between internal grinding and external cylindrical grinding is that the grinding wheel is small in diameter. In internal grinding, the grinding wheel diameter is normally 0.7 to 0.9 of the grinding workpiece diameter.
In conventional spindle head designs, the peripheral speed of the wheel when grinding small diameter holes mostly does not exceed 10 m/s and increases with increasing head size in accordance with the diameter of the holes they are grinding, up to 30 m/s for hole diameters over 30 mm. The relatively low rigidity of the spindle grinding wheel limits the value of cutting depth (cross feed), which amounts (depending on the diameter of the grinding hole) for preliminary grinding of steel and cast iron 0,5-0,02 mm and for finishing grinding. 0,002-0,01 mm for one double stroke. Smaller cross feed values are used for bore diameters smaller than 40 mm and for large hole length to diameter ratios. In all cases, internal grinding is carried out with longitudinal feed. The value of longitudinal feed is, as in the case of external cylindrical grinding, 0,4-0,8 of the cylinder width. for preliminary grinding and 0,25-0,4 of the cylinder width. for finish grinding; smaller values are used when the ratio of the bore length to diameter equals three.
For internal cylindrical hole machining, the workpiece is clamped in the machine chuck and rotary motion is applied to it (Fig. 73, б). A straight, flat grinding wheel has a rotary motion and two feeds: longitudinal along the axis and transverse (radial) after each pass. The wheel must have an overrun at its height.
Large parts, which it is inconvenient or impossible to clamp in chucks, are installed on the table of planetary grinding machine, which spindle rotates not only around its axis but also around the axis of the processed hole (see grinding of external and internal parts). Fig. 73, б); in addition the grinding wheel receives a longitudinal feed along the axis.
Ring holes (Fig. 73, в) is ground on a centerless grinding machine. Workpiece 1 is introduced into the working area and positioned between the supporting 8, clamping 9 and driven 10 with steel rollers in a common housing 11. Roller 9, rotating on the lever axis presses the workpiece against the leading and supporting rollers with a specified force. The master roller, which receives motion from the drive, rotates the workpiece at a speed of 40-60 m/min. The axial thrust, resulting from the fact that the axis of the driving roller is rotated by about half a degree, presses the workpiece against the end stop and grinds it with the wheel fixed on the spindle 12. Grinding is enabled by the friction force generated between the master roller and the workpiece.
Centerless grinding offers very high grinding accuracy.
Grinding of external Tapered surfaces is produced on cylindrical grinding machines by turning: 1) table to the angle of taper of the processed part (fig. 73, г); 2) headstocks (Fig. 73, д73 73 Tapered inner surfaces are ground on centreless machines by swivelling the headstock and tool headstock (c) and (3) together with a short workpiece clamped in a jaw chuck.
Outer conical surfaces are also machined by tucking the grinding wheel onto the cone, in which case improvement of the quality of grinding parts is achieved by longitudinal oscillatory motion of the wheel. Internal taper surfaces are ground on centreless machines by turning the body of the workpiece using a fixed headstock 12 (fig. 73, в73 73 On internal cylindrical grinding machines, by turning the headstock with a workpiece mounted at a fixed angle into a fixed taper angle.
Tapered surfaces Finish on cylindrical and surface grinding machines. The shape grinding also includes machining of complex shaped guideways on bed-type workpieces (Fig. 73). 74, а).
Grinding gear wheels is carried out on gear-grinding machines by the method of running-in or profile copying, using shaped grinding wheels.
Grinding the teeth with two disk wheels (fig. 74, б), which are installed so that their ends facing the spindles coincide with the sides of the teeth. The lateral profiles of the teeth are machined with complex motions of the gear wheel and grinding wheels.
In the method of profile copying (Fig. 74, в) The teeth are ground with a shaped wheel, which has the shape of a depression between the teeth. Both side profiles of the wheel teeth are ground simultaneously with this grinding wheel.
Finishing of the threads is produced on thread grinding machines with single or multi-strand profile grinding wheels. There are external and internal grinding. External grinding of threads using a sinker wheel (Fig. 74, г) part 1 set between the centers of the machine. Grinding wheel 2, The workpiece spindle mounted on the headstock spindle is driven by a separate gear and moves longitudinally one step per revolution of the workpiece.
Flat surfaces are ground on surface grinding machines. Large workpieces are fixed on the machine table with the help of stops, slats and other devices, and small pieces. with the help of electromagnetic plates.
planes are ground on the periphery or face of the grinding wheel.
a fig. 75 different surface grinding circuits with the periphery of a wheel are shown. Grinding in this way is possible with a reciprocating motion of the machine table with the workpiece (Fig. 75,а). The grinding wheel rotates and travels in a transverse motion for each double travel on the table, and a radial feed to move it to the grinding depth.
Grinding with the periphery of the wheel can also be carried out by fixing the workpiece to be machined on a circular rotating table (Fig. 75, б). The grinding wheel rotates and reciprocates parallel to the surface to be ground. Grinding of planes with the wheel face is performed both at translational motion of the workpiece and its rotation.
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Machining on grinding machines
Grinding Grinding is the process of machining workpiece surfaces with grinding wheels. Grinding is often used as a finishing operation to achieve high dimensional accuracy and a good surface finish.
You can grind flat, cylindrical, conical and various shaped surfaces of parts made of both soft and hard (including hardened) metals and alloys.
Different types of grinding are used, such as external circular grinding, internal circular grinding, flat grinding, centerless external grinding, etc.
Consider the elements of cutting mode of the most common type of machining surfaces of the parts. external circular grinding in the centers by the method of longitudinal feed. This type of grinding is characterized by: 1) cutting speed; 2) feed; 3) depth of cut; 4) machine time.
Cutting speed Vк in external cylindrical grinding is the peripheral speed of the grinding wheel. Cutting speed is expressed in meters per second; it is selected within the range of 30-50 m/s, and in some cases higher. In external cylindrical grinding, the workpiece is also rotated; its rotation speed is 15-50 m/min.
Feed S in external cylindrical grinding is the amount of movement of the workpiece per revolution along its axis (longitudinal feed); the feed is expressed in millimeters per revolution of the workpiece.
Cutting depth t (cross feed) is the thickness of the metal layer removed by the grinding wheel in one pass (Fig. 69, а).
Machining time Тм in external cylindrical grinding is the time spent directly on the process of cutting the metal with the grinding wheel in one pass. Machine time is defined on the basis of length of lengthwise motion of the table, allowance on the side of the part diameter, number of revolutions of the part, feed rate and machining accuracy.
When calculating the motor power for the rotation of grinding wheel and workpiece, the cutting force, rotation speed of wheel and workpiece are taken into account.
Outside circular grinding in centers is carried out by longitudinal and transverse feed, as well as by the plunge cut method.
When grinding with the longitudinal feed method (Fig. 69, а) the grinding wheel carries out the main rotary motion Vк, and the workpiece is rotating around its axis Vд and translational motion of the longitudinal feed Spr along the axis. grinding depth t is set by the cross feed of the grinding wheel.
In crossfeed grinding, or plunge-feed grinding, the grinding wheel receives the main rotary motion Vк around the axis and the transverse motion of the feed Spop, and the workpiece is only subject to a rotary motion Vд. This method is used for grinding workpieces with a short length of the work surface, which is completely covered by the width of the grinding wheel.
The depth method of grinding is characterized by the fact that the grinding wheel is generally set to the full grinding depth t and receives the main rotational motion Vк, and the workpiece is rotated around its axis Vд and longitudinal feed Spr along the workpiece axis. Short and stiff shafts are generally ground in this way.
Internal cylindrical grinding (Fig. 69, б) is used when machining relatively short workpieces clamped in cam chucks. The grinding wheel carries out the main rotary movement Vк and longitudinal feed movement Sprworkpiecepiece. only rotational motion about the axis Vд. Cutting depth t is set by the cross feed of the wheel.
Flat grinding is performed by the outer part (periphery) and the face of the grinding wheel.
When grinding with the wheel periphery (Fig. 69, в) the grinding wheel performs the main rotary movement Vк and cross feed Spop, and the workpiece being ground on the machine table. reciprocating longitudinal feed motion Sapr. Cutting depth t is set by vertical feed of the wheel. The table with the workpiece can be rotated horizontally (carousel principle) and the circle can be radially moved relative to the table.
When grinding with the face of the wheel (fig. 69, г) the workpiece performs the same motion as in grinding with the periphery of the wheel, and the grinding wheel performs the main rotary motion Vк around the vertical axis. The depth of cut is set by means of vertical feed along the wheel axis.
Centerless external grinding (Fig. 69, д) is that the cylindrical workpiece 1, supported by the stop 2, is passed by a longitudinal feed between the grinding 3 and the leading 4 circles. The grinding wheel carries out the chip removal process and the leading wheel, or feeder, located at a certain angle to the axis of the grinding wheel, provides rotation of the part and its longitudinal feed. As a result of rotating the axis of the driving wheel, its circumferential speed Vwk is decomposed into two components. rotation speed of the workpiece Vд and feed rate Vvc = – sin. where – the coefficient of slip of the workpiece on the grinding wheel ( = 0,94-0,98).
Angle is usually taken as 1-5°; the greater the angle , the greater the longitudinal feed, and vice versa.
To ensure better contact with the workpiece, the leading wheel is not cylindrical, but concave (hyperboloidal rotation form).
If the axis of the driving wheel is set parallel to the axis of the grinding wheel, then Vs = 0 and there is no axial feed of the workpiece. This is used when grinding workpieces with projections.
Each of the considered types of grinding has its own specific form of grinding wheel and machine design.
Grinding machines by design and technological features (type of work performed) are divided into cylindrical grinding, internal grinding, surface grinding, specialized, sharpening and finishing.
How Surface Grinding Works. Part 1?
A subgroup of circular grinding machines includes machines for circular grinding in centers, centerless, semi-automatic and automatic machines; a subgroup of internal grinding machines includes plain, planetary, centerless, semi-automatic and automatic machines; to the subgroup of surface grinding. longitudinal and carousel, working by periphery and end of the wheel, semi-automatic and automatic machines; to the specialized include gear, thread, copy grinding, for grinding of slotted shafts, balls, etc.; to the group of surface grinding. longitudinal grinding. д.Universal grinding machines for sharpening various tools and special machines for sharpening tools of particular type; to sharpening machines. universal for sharpening various tools and special machines for sharpening tools of particular type. Finishing machines on the application of abrasive tools are subdivided into working with grinding wheel, powder and polishing pastes. There are several models of each type of grinding machine.
In Fig. 70 Here is a general view of a universal cylindrical grinding machine. Main parts and units of the machine are: machine frame 7, table bottom 6, table top 5, wheel headstock 2, headstock 1, tailstock 4.
The top part of the table can be rotated by an angle to the grinding wheel spindle axis for the machining of flat cones. Workpieces with a large taper angle are ground with the headstock rotated by the specified value.
The angular positioning of the wheel is recommended for simultaneous grinding of the shaft journal and face. With this technological scheme, the face of the workpiece is ground by the periphery of the wheel, which reduces the contact of the wheel with the workpiece, ensuring better surface finish and eliminating the possibility of burns.
In Fig. 71 is a simplified kinematic diagram of the universal surface grinding machine of model ЗБ722. The machine has a number of kinematic circuits, the main of which. a circuit of grinding wheel rotation, circuit manual and automatic vertical feed of wheelhead, circuit manual cross travel of wheelhead, circuit control of hydraulic table gear and hydraulic gear box of grinding wheelhead, etc.
SUPERTEC CNC CENTERLESS GRINDER STC-2410 CNC
The most simple is the chain of rotation of the grinding wheel. Spindle of wheel gets its rotation from the flange motor AO62-4, power of 10 kW, 1460 rpm through a needle clutch.
Mechanisms of the chain of manual and automatic vertical feed of the wheelhead perform the following basic movements: a) at manual feed from the handwheel А motion is transmitted through the gears 1 и 2 and cam clutch Б tapered pair 3 и 4 and then on to the nut 5, which is connected to the propeller 6b) in the case of the hydraulic automatic feed, when the grinding head cross-feed is reversed, oil is supplied to one or the other cavity of the feed mechanism plunger, thus moving the plunger rail 7. Rack 7 through the gear 8 turns the crank В, which through a connecting rod Г and lever system turns the ratchet 9, rigidly connected to the hand wheel А. The motion is then transmitted through the above described chain to the screw 6.
For manual cross-feeding of the wheelhead, rotation from the handwheel Д through a worm gear 10 и 11 transmitted to the rack and pinion 12.
Automatic cross feed of wheelhead by hydraulic feed regulator is carried out by means of crank И, which switches appropriate plunger in hydraulic cylinder of wheelhead and feeds each stroke of table.
Grinding work on machines
The choice of this or that type of grinding of parts to a large extent depends on the shape of the machined surface. There are three basic types of machining. flat grinding, circular external grinding and circular internal grinding of workpieces.
Surface grinding is the simplest type of surface finish. It is the most reliable method of creating high-precision planes. The part is placed on a magnetic table. With vertical and horizontal translational movement of the table and vertical movement of the tool, it is possible to grind workpiece face surfaces to about 10 µm accuracy. The disadvantage of this method is the inability to machine round and cylindrical surfaces.
External circular grinding of workpieces is used for machining of external surfaces in the rotation of the workpiece in a center or chuck. There are two types of such grinding. with longitudinal feed and plunge grinding. The first method is used when the length of the workpiece is greater than the height of the wheel. Here you need the progressive movement of the abrasive to go over the entire surface, with both the wheel and the workpiece in motion. In plunge grinding, workpiece length is less than or equal to the height of the wheel. No progressive feed is required because the workpiece can be ground in one run.
Internal grinding is used for machining internal surfaces of cylindrical, conical or shaped parts with a straight form. The method has several varieties, but the principle is similar to external circular grinding.
Laboratory work 1 workpiece machining on grinding machines
PURPOSE OF WORK: To get acquainted with the main grinding circuits and types of work performed on grinding machines. Learn the equipment, tools and fixtures used. Learn the methods of selecting machining modes.
Grinding is used for finishing and finishing treatment of high-precision parts, as well as for roughing castings, forgings and rolled products, sharpening cutting tools, etc. Grinding. is the process of machining a workpiece or component with abrasive wheels. The abrasive grains of the wheel are micro-cutters that cut the finest chips from the surface of the workpiece. As the grinding wheel works, the abrasive grit chips and blunts. The cutting force on the grit gradually increases and the grit breaks away from the bond. When the blunted grit is uniformly torn out over the entire surface of the wheel, self-sharpening occurs, i.e.е. New sharp grains from the lower layers replace the lost grains in the cutting process. As an abrasive wheel wears, its topography and geometry change. To restore the cutting ability and the correct geometric shape, you periodically straighten the ring.
The process of surface shaping is carried out due to the combination of rotational and translational movements of the wheel and workpiece. The various combinations of these motions can be reduced to one of the most common machining patterns shown in Figure 1.
Figure 1 – Machining diagrams on grinding machines
The main motion in all the schemes is rotary motion of the wheel. Scheme “a” is implemented when processing the surface by the periphery of the wheel on surface grinding machines. It is characterized by reciprocating motion of the workpiece (longitudinal feed), providing removal of metal layer along the entire length of the workpiece, and discontinuous translational motion (cross feed) of the wheel, providing surface treatment across the width of the workpiece. Transverse motion is performed at the end of the longitudinal advance of the workpiece.
Scheme “b” is carried out when machining the outer surfaces of rotation on cylindrical grinding machines. Shaping is carried out due to rotary and translational movements of the workpiece. Scheme “c” is realized when processing internal surfaces of rotation on internal grinding machines. According to the method of shaping it is similar to the scheme “b”. The required diameter of the machined surface in both cases is achieved by moving the wheel axis of rotation in the radial direction. Grinding of outer surfaces and rolls without protrusions is performed on centerless grinding machines (diagram “d”). The workpiece 3 is mounted on the knife 2 and is driven by the driving wheel 4 at a speed slightly higher than the speed of the working wheel 1. The longitudinal movement of the workpiece along the axis is achieved by inclining the grinding wheel axis at an angle (1° to 7°) to the workpiece axis. Such machines are easy to fit into automatic mowing lines and offer high productivity.
Circular grinding machines (figure 2) are used for circular cylindrical and conical external surfaces. The machine consists of the following main components: machine frame 1, table 2, headstock 3 with gearbox, grinding headstock 4, tailstock drive 5 and table drive 6. Grinding machines are subdivided into simple, universal and plunge types.
Figure 2 – General view of a cylindrical grinding machine
Universal machines have a swiveling front and rear headstock. Each headstock can be rotated by a defined angle around its vertical axis and clamped for subsequent operation. Simple machines are equipped with non-rotating headstocks. Plunge-type machines have no longitudinal feed of the table but grind along the whole length of the workpiece with wide abrasive wheel with cross feed.
Hydraulic devices are widely used for feed units of cylindrical grinding machines. The table is reciprocated by means of a hydraulic cylinder and piston. They are controlled by devices which switch the table in the end positions. Hydraulic mechanisms are also used for intermittent feed of the wheelhead. They enable infinitely variable feed control.
Circular feed Skr of the workpiece is provided by a special electric motor. Here is used the stepless speed regulation of the motor by changing the electrical resistance.
The grinding wheel is driven by the V-belt drive. After the wheel has worn down and the diameter of the wheel is reduced, another pair of pulleys is used.
Grinding in centers is most common. To improve the accuracy of machining, the centers are set stationary. Circular feed of the workpiece is ensured due to the driving device (guide and collar) driven by the rotating faceplate. It is possible to cantilever the workpieces in cam chucks.
Speed Vto the rotary motion of the wheel provides the cutting speed.
Basic (technological) time, min, required for circular grinding according to the scheme shown in Figure 1, b:
where L. table stroke length, mm; i. number of strokes.
For manufacturing of grinding wheels Electrocorundum, silicon carbide and cubic boron nitride are the most commonly used carbides.
The backbone of lapis aluminosilicate is crystalline aluminium oxide. Depending on the content of Al2O3 and the structure of the crystals differ electrocorundum normal (13A, 14A, 15A), white (22A, 23A, 24A) and monocorund (43A, 44A). The latter have the highest strength and provide high grinding performance.
Silicon carbide comes in two varieties: green (63C) and black (54C, 55C). It is more brittle than fused alumina. Due to the sharp edges resulting from crushing, it provides a very high productivity.
Cubic boron nitride (elbor) is second only to diamond in hardness and has twice the heat resistance. CBN bits are used for finishing grinding, sharpening and finishing tools.
The bonding base of the wheel can be ceramic, Bakelite or Vulcanite. Bond strength determines the maximum permissible cutting speed (usually not more than 50 m/s), as well as the hardness of the grinding wheel. For processing very hard materials, diamond grinding wheels are used, which are obtained by placing diamond powder on metal or plastic disc. As grinding is carried out at high speeds (over 30 m/s), the cutting zone must be supplied with cutting fluid (coolant).
An important characteristic of a grinding wheel is grit size. According to the grit abrasives are divided into three groups: grinding grain (00 6), grinding powders (2, 3) and micropowders (M40 M5). The grit number refers to the average grit size in hundredths of a millimeter or micrometers (for micropowders).
Abrasive material is selected depending on the physical and mechanical properties of the workpiece, the purpose of grinding (rough or finish) and economic considerations. In most cases, normal and white alumina are used when machining hardened and unhardened steels. Monocorundum is used for machining of high-temperature alloys. When grinding cast irons and some non-ferrous alloys, black silicon carbide grinding wheels are used and when grinding titanium alloys, green silicon carbide grinding wheels are used. The harder the wheel, the harder the material is to be machined (table 1).
Features of the cutting process when grinding
Abrasive tools, unlike razor blades, have many micro blades that are located in a chaotic pattern. A single grit of a grinding wheel can be located at some distance from the machined surface, slide over the machined surface (sliding grit), penetrate the machined surface to a small depth and deform the workpiece material only plastically (deforming grit), penetrate into the machined surface to a depth sufficient for chip removal (cutting grit). Compared to blade machining, grinding is characterized by increased cutting resistance, since sliding grains create additional friction, deforming grains create additional elastic and plastic deformation, and the cutting teeth have suboptimal cutting angles. In addition, the cutting force on a single grain is greater, but since microchips are removed, the total cutting force is low. Due to the additional friction and strain, the temperature in the cutting zone is much higher than with blade cutting, therefore, structural transformations of metal in the cutting zone (burning) are possible. Chips burn in the air as a shower of sparks, which requires additional fire and sanitary measures.
Works performed on cylindrical grinding machines
Circular grinding machines are used for grinding of external cylindrical, conical, face and shaped surfaces.
Grinding. processing with high-speed abrasive wheel. Abrasives (high hardness grains with sharp edges) may be in free form (powders) or may be bonded (cemented) in the form of circles, bars, segments.
As a rule, grinding is a finishing operation providing high dimensional accuracy (up to 0,002 mm) and the required surface roughness (Ja = 0,15h-1,2µm), is used for machining external and internal cylindrical and cone, flat and curvilinear surfaces for all metals and alloys.
Grinding is also used for roughing work (e.g. in cleaning castings), for sharpening cutting tools. Most grinding work is carried out using a rapidly rotating abrasive wheel.
Circumferential speed of the wheel is 8-50 m/s and higher, depending on the hardness of the processed material (the harder the material, the lower the speed), the presence or absence of cooling and other conditions.
Fig. 7.25 shows a schematic diagram of a universal center cylindrical grinding machine for external grinding. Table 4 rests on the bed rails 6 and carries a front 1 and rear 3 headstock. The workpiece to be machined is located between the centers of the front and rear headstock. It is driven in rotary motion by the motor of the headstock. The rotating workpiece, together with the table, is capable of reciprocating-
Fig. 7.25. Circular grinding machine: 1. headstock; 2. wheelhead; 3. tailstock; 4. table; 5. cams; 6. frame; 7- hand wheel;
The rotary motion of the workpiece along the entire length of the wheel headstock 2.
The stroke of the table is limited by the cams 5, which are controlled by the lever 8 the table travel direction is switched; the table is moved by means of a hydraulic drive. The grinding headstock is mounted on a cross slide and by means of the handwheel 7 has a transverse movement to set the grinding depth. For processing of cones the upper part of the table is made rotary.
Technique and technology of grinding on cylindrical grinding machines
The machine is prepared for operation in the following sequence:
- Check in the adjustment mode: lubrication system operation; wheel mounting and direction of rotation; longitudinal feed of the dressing device; circumferential wheel speed and workpiece rotation frequency; cycle and feed values in accordance with the adjustment chart; cooling system operation;
- Adjust the machine assemblies: set and align the center, set the front and rear headstocks in the axial position, check the correctness of workpiece clamping and lunette adjustment, align the relative position (axial and radial) of the wheel and workpiece;
- 0 idle; set the straightener’s longitudinal travel speed and pre- straighten the wheel; balance the wheel; adjust the loading and unloading device;
- 0 perform test grinding: adjust the position of the grinding headstock; process a trial batch of workpieces, turn off the machine and measure the finished parts; if necessary, adjust the settings and grind the second trial batch of workpieces; adjust the measuring instrument by the reference part;
- 0 the machine is tested in automatic cycle with the required productivity and accuracy.
Grinding modes. All operations circular external grinding operations on the intensity of stock removal are divided into roughing (roughing), preliminary, final and fine grinding.
Roughing (rough) grinding involves machining without a preliminary turning operation with removal of an increased (1 mm or more) diameter allowance. It is expedient to perform this operation in the modes of force and speed grinding with the grinding wheel speed of 50-60 m/s. Contrary to turning, rough grinding is more precise (8th to 9th quality) and has a lower roughness (Ra = 2,5-5 μm), does not require subsequent pre-grinding. Its application is reasonable if there are precise workpieces or workpieces which are badly machined with the blading tools.
Preliminary grinding is usually performed after turning with an increased wheel speed of 40-60 m/s. Grinding is performed before heat treatment to create base surfaces or as an intermediate operation to prepare the surface for final machining. Preliminary grinding achieves the accuracy corresponding to 6. to the 9th quality level, and the surface roughness Rd = 1,2-2,5 µm.
Final grinding allows to obtain surfaces with accuracy corresponding to 5-6th quality grades, and roughness Д, = 0,2- 1,2 micron. This operation is most often carried out with a wheel speed of 35-40 m/s.
Fine grinding is mainly used for achieving the surface roughness R^ = 0,025-0,1 µm. It requires very good preconditioning, since the removed allowance exceeds 0,05-0,1 mm per diameter. Fine grinding is only possible with high-precision machines and special grinding wheels. It is only economically justified for application in individual and small series production.
When setting up the machine for grinding of tapered surfaces, it is necessary to take into account the conicity. the ratio between the diameters of the large and small base of the cone and its length: k= (D. d)/l. Taper is expressed in dimensionless values. On cylindrical grinding machines tapered surfaces are ground by rotation of the front headstock table or by rotation of the grinding wheel headstock.
Grinding of surfaces with a small taper (taper angle 12-14 °) does not require complex adjustment: the machine table is set parallel to the axis of the grinding spindle and rotated by half the angle at the top of the cone of the workpiece (angle of inclination a, fig. 10.8). Feed movement is given either to the workpiece or to the workpiece 2 (fig. 10.8, a, b), or on the grinding wheel 1 (fig. 10.8, в).
For grinding in the centers of cones with a large angle at the apex, a special device is used (fig. 10.9, а), And for grinding in the chuck rotate the headstock by the required angle (fig. 10.9, б). Grinding of truncated cones with an angle of up to 60° of small height is carried out by plunge method when placing the workpiece in the centers with turned wheelhead (Fig. 10.9, в).
Technique and technology of grinding on surface grinding machines
Technological features of surface grinding. Flat surfaces can be ground with the periphery and face of the wheel.
Flat grinding with the wheel periphery (see, e.g. Fig. 10.3, ё) is carried out on machines with reciprocating or rotary motion of the table. On machines with rectilinear reciprocating table motion, after each longitudinal stroke the grinding wheel is moved in the grinding plane perpendicular to the table motion by a distance equal to the feed. For planar grinding by periphery of the disk are mainly used disks form PP with outer diameter 175-450 mm and height 16-40 mm; hardness and granularity of the disk select depending on the material of the workpiece.
When working on machines with rectangular table, the allowance is removed in one of the following ways:
- 0 Grinding with cross passes (Fig. 10.10, а), when the cross feed of a wheel (workpiece) along the spindle axis is performed for each stroke of the table. The grinding wheel removes a layer of metal with a thickness equal to the cutting depth and a width equal to the cross feed of the wheel in one travel of the table. After penetrating the whole machined surface the wheel is moved back to a certain depth and the next layer of metal is removed. Such passes are repeated until the allowance is completely removed;
- 0 Grinding with depth method (Fig. 10.10, б), when the grinding wheel removes most of the allowance for each stroke of the table. After each stroke of the table circle (table) moves along the axis of the spindle to 3 /4-‘/5 of its height. The remaining 0.01-0.02 mm of allowance is removed using the cross traverse method. Depth grinding is carried out at low speed of longitudinal movement of the table and is used mainly when working on high-powered grinding machines;
Grinding with a step wheel (Fig. 10.10, в), When the wheel is tucked (profiled surface) steps. The main part of the allowance is distributed between the individual steps and is removed in one pass. The last step usually removes a small layer of metal. Then finish grinding in the crosswise method.
When grinding unhardened steel, the most time is spent grinding with multiple cross passes, and the least time is spent grinding with a stepped wheel. Productivity in step grinding depends on the quality of dressing of the cutting surface of the grinding wheel. It is dressed with a tool that allows you to get the entire profile at once (shaped rollers or special discs), otherwise the increased consumption of grinding wheels and high time spent on dressing will excessively increase the cost and time of processing.
Machines operating with the periphery of a wheel can be used to grind contoured surfaces. The high machining accuracy is achieved and the workpieces are not heated up to any great extent. The latter is very important when machining workpieces subject to warping.
Flat grinding using the face of the wheel productive than with the periphery of the wheel, since more abrasive grit is used in the cutting process. The table of a surface grinding machine designed for such grinding, makes reciprocating or rotating motion. The table is circular in the latter case.
Usually the end of grinding wheel covers the whole width of the workpiece mounted on the machine table. In the process of face grinding, the following movements are carried out: rotation of the wheel, workpiece feed, cross feed of the wheel after each stroke of the table or after each turn of the table.
To improve conditions for chip removal and reduction of temperature in the face grinding zone:
- 0 segment grinding wheels are used to create an interrupted cutting surface;
- 0 reduce the contact area of the wheel with the workpiece by undercutting (reduction in diameter by making steps or a cone) its face surface or (where possible) by mounting the grinding head with a small inclination;
O use coarser and less hard grinding wheels.
Grinding modes are determined by such basic technological factors as the specified surface accuracy and roughness, the power of the main drive motor, and the durability of the grinding wheel.
Indicators of the cutting mode in flat grinding by the wheel periphery are the grinding wheel speed (circumferential), workpiece speed, transverse (parallel to the spindle axis) feed and grinding depth (in grinding by the wheel face transverse feed is usually not used).
Grinding wheel speed depends on the type (conventional or high-speed) of grinding and the capabilities of the machine. The speed of the workpiece in flat grinding coincides with the longitudinal or circular feed of the table on which it is clamped. Increasing the velocity of the blank increases machining capacity, but also reduces heating and deformation of the workpiece. Therefore, it is recommended to choose high workpiece speeds, especially for preparatory operations and large allowances. It is recommended to reduce the workpiece speed for finishing operations.
What is Grinding? Types of Grinding Machine and Operations
Increasing the cross feed increases productivity, but at the same time roughening and grinding wheel wear increases. Therefore, a lower cross feed rate is advisable for finishing operations.
The cutting depth mainly determines the machining performance and depends on the grit size of the wheel, the required surface roughness, the power of the grinding head drive motor and a number of other factors. When machining with coarse-grained wheels greater cutting depths are possible. When grinding with shallow grinding wheels with great depth, considerable wear of soft wheels and rapid shrinkage of hard wheels are observed. Roughing operations should be carried out at high speeds and depths, and at finishing operations decrease speed and depth of cut.
Flat surface grinding jigs. At
direct clamping of workpieces to the table is used, when workpieces cannot be fastened to the magnetic plate or other fixtures.
Reciprocating vise (Fig. 10.11) differs from conventional machine vises in the precision of manufacture and the possibility of canting. The fixed jaw of the vise is one piece with the base 2. Slots in the housing for the passage of the sliding jaw 3, which is moved by a screw 4. Body base has holes with threads for fastening the vice to various attachments. All vise planes are machined at an angle of 90°. Pressed cylindrical measuring pin 7 serves to measure inclined planes.
Electromagnetic plates have a device (Fig. 10.12, а), based on the principle of electromagnetic induction, which manifests itself in the following: if an iron core (Fig. 10.12, б) to put a wire and pass a direct current through it, the core will be magnetized and a steel object brought to one end of the core will be attracted to the ser-
to the probe. When the current in the winding ceases, the magnetic action of the core will also cease.
The connection of the bent horseshoe-shaped (Fig. 10.12, в) the cores form the basis of the electromagnetic plate.
а. general view; б. a diagram of the magnetic action of a current; в. horseshoe magnet
The poles of the magnets brought out on top of the plate are carefully isolated from its body by non-magnetic alloys (babbitt, zinc) so that the magnetic flux is not dispersed in the body of the plate, but goes directly to the body of the workpiece. Only magnetic metals (e.g. steel, iron, cast iron) can be attracted to the electromagnetic plate.
Electromagnetic stoves are available in various sizes and in round and rectangular forms. Only direct current is suitable for their power supply, which is why machines are equipped with AC to DC converters.
Electromagnetic plates ensure fast and safe clamping of the workpieces to be ground. To maintain the plate in operability, it must be protected from shocks and impacts and the coolant must not come into contact with the windings. Immediately after finishing work, wipe the work surface of the hob dry.
Magnetic plates Permanent magnet plates are also used on grinding machines. No special fixtures are required for these types of plates. However, their magnetic force is generally weaker than that of the electromagnetic plates.
The rectangular contact plate design and operating principle is illustrated in Fig. 10.13. The upper part is made of steel plates 1 with non-magnetic interlayers 2 between them (fig. 10.13, а). Strong permanent magnets 4 can be switched by closing them with the Jto handle on the iron plates, then on the part to be clamped. In Fig. 10.13.5 shows the position of the magnets when clamping the part 5, and Fig. 10.13, в. during removal or installation of the part. Bottom of plate 6 clamped on the machine table.
Abrasive segment wheels are used for grinding of flat surfaces. Flat grinding with large-diameter solid circles is economically unprofitable because of considerable waste, increased heat generation and possibility of their breakage during transportation. over, if a wheel breaks or fractures partially, it must be replaced and a great deal of good abrasive material is lost. These inconveniences are eliminated when using grinding wheels made of inserted abrasive segments (Fig. 10.14). If one or more of these segments break, they are easy to replace.
Insertion segments are used almost until completely worn out. Two segments can be removed with the release of one clamp. By
As they wear, the height of the segments decreases, so shims are put under them.
Grinding of thin workpieces on the magnetic table of a surface grinding machine requires the preparation of the base planes. Concavity (Fig. 10.15, а) or convexity (Fig. 10.15, б) Planes on such workpieces that are planed or milled cannot be straightened by normal plate mounting. The magnets, attracting the part, straighten it, but after removing the plate, the part takes its original shape.
Sheet metal parts are especially prone to buckling. Their bending direction is always the same, with the concavity forming on the grinding wheel side. The best way to prevent warping is to remove equal layers of metal from both sides of the plate. The plate becomes straight or slightly bent. To achieve parallelism of planes on such parts, it is necessary to grind as follows. The part is laid convex upwards and ground to obtain straightness, then rotated machined plane downwards and from it the size. Since the first surface will also be slightly convex, it is necessary to make several passes and turn the workpiece several times.
Grinding flat surfaces
Often subjected to machining flat body workpieces of different metal. The operation carried out to change the surface roughness is characterized by the following features:
- The workpiece is placed on a special table, which provides a secure fixture. Clamping can be mechanical or magnetic.
- The main rotation is transferred to the abrasive wheel, the backward motion is transferred to the workpiece or tool.
By choosing a wheel with the most suitable profile it is possible to work on the most complex shapes. Cooling water can be injected into the contact area between the tool and the workpiece.