The surface of the outer circle is the main surface of the shaft part. Therefore, it is necessary to reasonably formulate the machining process specification of the shaft part. First, it is necessary to understand the various processing methods and processing schemes of the outer circle surface. This chapter mainly introduces several commonly used cylindrical machining methods and commonly used cylindrical machining programs.
First, turning the outer surface of the circle
According to the manufacturing precision of the blank and the final processing requirements of the workpiece, the cylindrical turning can be generally classified into roughing, semi-finishing, finishing, and fine turning.
The purpose of the roughing car is to cut away rough crust and most of the excess. After processing, the workpiece size precision IT11~IT13, surface roughness Ra50~12.5 μm.
Semi-precision car size accuracy up to IT8 ~ IT10, surface roughness Ra6.3 ~ 3.2 μ m. Semi-finishing machines can be used as final machining for medium-precision surfaces, as well as pre-machining for grinding or finishing.
The dimensional accuracy of the finished car can reach IT7~IT8, and the surface roughness Ra1.6~0.8 μm.
The dimensional precision of the fine car can reach IT6~IT7, and the surface roughness Ra0.4~0.025 μm. Fine cars are particularly suitable for non-ferrous metal processing. Non-ferrous metals are generally not suitable for grinding, so fine cars are often used instead of grinding.
Second, the outer surface of the grinding
Grinding is one of the main methods for the finishing of cylindrical surfaces. It can process both hardened and unquenched surfaces.
According to different workpiece positioning methods during grinding, cylindrical grinding can be divided into two categories: center grinding and centerless grinding.
(I) Center grinding
Center grinding is ordinary cylindrical grinding. The ground workpiece is positioned by the center hole and processed on a cylindrical grinding machine or a universal cylindrical grinding machine. After grinding, the dimensional accuracy of the workpiece can reach IT6~IT8, and the surface roughness Ra0.8~0.1 μm. According to the feed method, it is divided into longitudinal feed grinding method and transverse feed grinding method.1 . Longitudinal feed grinding (longitudinal grinding)
As shown in Fig. 6-2, the grinding wheel rotates at a high speed, the workpiece is mounted on the front and rear tips, and the workpiece rotates and reciprocates longitudinally along with the table.
2 . Infeed grinding method (cut-in method)
As shown in Figure 6-3, this grinding method does not have longitudinal feed motion. When the workpiece rotates, the grinding wheel performs continuous infeed movement at a slow speed. Its high productivity, suitable for mass production, but also for forming grinding. However, the horizontal grinding force is large, the grinding temperature is high, and the machine tool and the workpiece are required to have sufficient rigidity, so that it is suitable for grinding short and thick and rigid workpieces; the machining accuracy is lower than the longitudinal grinding method.
(B) Centerless grinding
Centerless grinding is a high-throughput finishing method that uses the outer circle to be ground itself as a reference. At present, there are mainly methods for centerless grinding: penetration method and cut-in method.
As shown in Figure 6-4, the principle of the outer circle through the grinding method.
The workpiece is located between the grinding wheel and the guide wheel, and the lower surface is supported by the support plate. The axis of the grinding wheel is placed horizontally and the axis of the guide wheel is tilted by a small λ angle. In this way, the peripheral speed of the guide wheel
The guide can be decomposed into the workpiece that drives the workpiece rotation and the vertical component that axially feeds the workpiece.
Figure 6-5 shows the principle of plunge grinding. The guide wheel 3 drives the workpiece 2 to rotate and presses against the grinding wheel 1 . During processing, the workpiece and the guide wheel and the support plate are fed horizontally to the grinding wheel. After the grinding is finished, the guide wheel retreats to remove the workpiece. The axis of the guide wheel is parallel to or intersects with the axis of the grinding wheel at a small angle (0.5~1 o ). This angle can make the workpiece and the fence 4 (restricting the axial position of the workpiece) stick well.
For centerless grinding, the following conditions must be met:
1 . Since the guide wheel is inclined at an angle of λ, in order to ensure smooth cutting, the guide wheel and the workpiece must maintain line contact. Therefore, the guide wheel surface should be trimmed into a hyperbolic revolving body shape.
2 . The friction coefficient of the guide wheel material should be greater than the friction coefficient of the grinding wheel material; the grinding wheel and the guide wheel rotate in the same direction, and the speed of the grinding wheel should be greater than the speed of the guide wheel; the tilt direction of the support plate should help the workpiece to cling to the guide wheel.
3 . In order to ensure the roundness of the workpiece, the center of the workpiece should be higher than the center line of the grinding wheel and guide wheel. The higher value H relates to the diameter of the workpiece. When the workpiece diameter d work = 8 ~ 30mm, H ≈ d work / 3; when d work = 30 ~ 70mm, H ≈ d work / 4.
4. The guide wheel is tilted by a λ angle. As shown in Figure 6-4, when the guide wheel rotates at speed v, it can be decomposed into:
v = = v · cos λ ; v longitudinal = v · sin λ
When rough grinding, λ takes 3 ° ~ 6 °; when grinding, λ takes 1 ° ~ 3 °.
When centerless grinding, the workpiece size accuracy can reach IT6-IT7, surface roughness Ra0.8-0.2um.
(III) Quality Analysis of Cylindrical Grinding
In the grinding process, due to a variety of factors, the parts surface is prone to various defects. The common defects and solutions are as follows:1 . Polygons There is an equidistant straight line trace along the generatrix on the surface of the part, and its depth is less than 0.5 μm, as shown in Figure 6-6.
The reason is mainly due to the periodic vibration of the grinding wheel and the workpiece. Such as the grinding wheel or the motor is not balanced; the bearing rigidity is poor or the clearance is too large; the center hole of the workpiece is in poor contact with the tip; the wear of the grinding wheel is not uniform. Measures to eliminate vibrations, such as carefully balancing the grinding wheel and the motor; improving contact between the center hole and the tip; timely dressing of the grinding wheel; adjusting the bearing clearance, etc.
2 . The surface of the workpiece after spiral grinding exhibits a deep spiral mark, and the pitch of the marks is equal to the longitudinal feed of each workpiece. As shown in Figure 6-7.
The main cause is the contour damage of the grinding wheel micro-blade or the local contact between the grinding wheel and the workpiece. Such as the wheel bus and the workpiece bus is not parallel; headstock, tailstock rigidity is not equal; wheel spindle rigidity is poor. Elimination measures, correct the grinding wheel, maintain the high edge of the micro edge, adjust the clearance of the bearing, maintain the position accuracy of the spindle, grind both sides of the grinding wheel into a shoulder or round shape so that both ends of the grinding wheel do not participate in the cutting; Appropriate, at the same time, there should be an unloading device; so that the rails are lubricated as low pressure oil.
3 . Pulling (Scratches or Scratches) Common surface pick-ups are shown in Figure 6-8.
The main reason is that the self-sharpness of the abrasive grain is too strong; the cutting fluid is not clean; the grinding debris on the grinding wheel cover falls between the grinding wheel and the workpiece. Eliminate the measure of pulling the hair, choose the grinding wheel with slightly higher hardness; The dressing of the grinding wheel is cleaned with the cutting fluid and the brush; Filter the cutting fluid; Clean up the wear debris etc. on the grinding wheel cover.
4 . Burns can be divided into spiral burns and point burns, as shown in Figure 6-9.The cause of burns is mainly due to the effect of high temperature grinding, so that the surface microstructure of the workpiece changes, so that the surface hardness of the workpiece changes significantly. Measures to eliminate burns, reduce the hardness of the grinding wheel, reduce the depth of grinding, properly increase the rotational speed of the workpiece, reduce the contact area between the grinding wheel and the workpiece, correct the grinding wheel in time, and perform adequate cooling.
Third, the outer surface of the precision machining
With the development of science and technology, the requirements for workpiece and machining accuracy and surface quality are also increasing. Therefore, precision machining is often performed after the outer surface is finished. Cylindrical surface precision machining methods are commonly used in high-precision grinding, ultra-precision machining, grinding and rolling processing.
High-precision grinding
The essence of high-precision grinding lies in the action of grinding wheel abrasive grains. The abrasive grains of the finely-trimmed grinding wheel form many micro-edges that can participate in grinding at the same time. As shown in Fig. 6-10a,b, these micro-edges have a good level of height, the number of cutting edges participating in grinding is greatly increased, and fine chips can be cut from the workpiece to form a surface with a small roughness value. As the grinding process continues, the sharp micro-edges become progressively passivated as shown in Figure 6-10c. The passivated abrasive grains can also serve as a polisher, further reducing the roughness.
(B) Superfinishing
The use of a fine-grained abrasive stone to apply a small amount of pressure to the workpiece, the oil stone to reciprocate and move slowly along the workpiece axis to achieve a trace of grinding a finishing method.
Figure 6-11 shows the processing principle diagram. There are three kinds of movement in the processing: workpiece low-speed rotary motion 1; grinding head axial feed motion 2; grinding head high-speed reciprocating vibration 3. If the axial feed motion of the grinding head is not considered for the moment, the trajectory of the abrasive grain passing on the workpiece surface is sinusoidal, as shown in Fig. 6-11b.
There are roughly four stages of superfinishing:
1 . At the beginning of the strong cutting stage, due to the rough surface of the workpiece, a small number of convex peaks came in contact with the whetstone and the pressure per unit area was very large, destroying the oil film, so the cutting action was strong.
2 . Normal cutting stage When a few convex peaks are flattened, the contact area increases and the pressure per unit area decreases, resulting in weakening of the cutting action and entering the normal cutting stage.
3 . In the weak cutting stage, the contact area is further increased, the pressure per unit area is smaller, the cutting effect is weak, and the fine chips form oxides and are embedded in the voids of the whetstone, so that the whetstone produces a smooth surface and has a friction polishing effect.
4 . Automatically stop the grinding of the workpiece in the cutting stage, the pressure on the unit area is very small, the liquid friction oil film is formed between the workpiece and the oil stone, no longer in contact, the cutting action stops.
The superficially finished workpiece surface roughness Ra0.08-0.01 μm. However, since the machining allowance is small (less than 0.01mm), only the surface peaks of the workpiece can be removed, and the improvement of machining accuracy is not significant.
(c) Grinding
With grinding tools and abrasives, a precision machining method that grinds a very thin layer off the surface of the workpiece is called grinding.
Grinding laps are made of materials that are softer than the workpiece material (such as cast iron, copper, babbitt, hardwood, etc.). During grinding, part of the abrasive particles are suspended between the workpiece and the lap. Part of the granules is embedded in the surface of the lap. Using the relative movement between the workpiece and the lap, the abrasive particles should be cut off a thin layer of metal. Roughness peaks. The general grinding margin is 0.01 -0.02mm. In addition to obtaining high dimensional accuracy and a small surface roughness value, the grinding can also improve the surface shape accuracy of the workpiece, but does not improve mutual position accuracy.
When the two parts require a good fit, the use of the mutual grinding of the parts is an effective method. Such as the valve and the valve seat in the internal combustion engine, the oil pump in the nozzle and so on.
(4) Rolling processing
Rolling is a processing method that uses a rolling tool to apply pressure to a metal workpiece to plastically deform it, thereby reducing the surface roughness of the workpiece and enhancing the surface properties. It is a kind of chipless processing.
Figure 6-12 shows the rolling process. Rolling processing has the following features:
1 . The surface roughness of the workpiece before rolling is not more than Ra5 μm. The surface is required to be clean with a margin of 0.02-0.03mm.
2 . The shape accuracy and position accuracy after rolling are mainly determined by the previous process.
3 . The rolled workpiece material is generally a plastic material and the material structure is uniform. Iron castings are generally not suitable for rolling.
4 . Rolling process productivity is high.
Fourth, the choice of cylindrical surface processing plan
The above describes several processing methods commonly used on cylindrical surfaces and their characteristics. Some precision-required surfaces on parts require only one type of processing method to fail to meet their specified technical requirements. These surfaces must be sequentially roughed, semifinished, and finished to gradually increase their surface accuracy. The orderly combination of different processing methods is the processing plan. Table 3-14 shows the machining plan of the outer cylindrical surface.
Table 3-14 Processing Method of Outer Cylindrical Surface
No.
processing method
Economic accuracy
(tolerance level representation)
Economic roughness
Ra / um
Scope of application
1
Rough car
IT18~13
12.5~50
Suitable for various metals other than hardened steel
2
Roughing car - semi-finish car
IT11~10
3.2~6.3
3
Roughing Car - Semi-Fine Car - Finishing Car
IT7~8
0.8~1.6
4
Roughing - Semi Finishing - Finishing - Rolling (or Polishing)
IT7~8
0.25~0.2
5
Roughing - Semi Finishing - Grinding
IT7~8
0.4~0.8
Mainly used for hardened steel, but also for unhardened steel, but not for non-ferrous metals
6
Roughing Cars - Semi Finished Cars - Rough Grinding - Grinding
IT6~7
0.1~0.4
7
Roughing - Semi Finishing - Rough Grinding - Fine Grinding - Super Finishing (or Wheel Super Finishing)
IT5
0.012~0.1
(or RZ 0.1)
8
Roughing Car - Semi-Fine Car - Precision Car - Fine Car (Viamond Car)
IT6~7
0.025~0.4
Mainly used for non-ferrous metal processing
9
Roughing - Semi Finishing - Rough Grinding - Fine Grinding - Super Finishing (or Mirror Grinding)
IT5 and above
0.006~0.025
(or RZ 0.05)
Extremely accurate cylindrical machining
10
Roughing - Semi Finishing - Rough Grinding - Fine Grinding - Grinding
IT5 and above
0.006~0.1
(or RZ 0.05)
When determining the machining plan for a certain surface, the final processing method is first determined by the technical requirements (processing accuracy, surface roughness, etc.) of the machining surface, and then the processing method of the previous process is determined according to the characteristics of this processing method, and so on. However, due to the fact that there are several methods for obtaining the same precision and surface roughness, the actual selection should be fully considered in conjunction with the structure, shape, size, material and heat treatment requirements of the parts.
In Table 3-14, the two kinds of machining programs with the serial number 3 (roughing car - semi-finishing car - finishing car) and serial number 5 (roughing car - semi-finishing car - grinding) can reach the same accuracy level. However, when the machined surface needs to be hardened, the final machining method can only be used for grinding. If the machined surface is not hardened, both machining options can be used. If the part material is non-ferrous metal, it is generally not suitable for grinding.
Another example is the serial number 7 in Table 3-14 (rough-vehicle-semi-finishing-grinding-finishing-superfinishing) and serial number 10 (rough-semi-finishing-grinding-grinding-grinding-grinding). The same processing accuracy can also be achieved. When the precision of surface matching is relatively high, the final processing method is more suitable for grinding; when only a small surface roughness value is required, ultra-precision machining is more appropriate. However, no matter if grinding or super-finishing is used, the shape accuracy and position accuracy of the machining surface are not significantly improved. Therefore, the former process should use fine grinding so that the positional accuracy and geometrical accuracy of the machining surface have reached the technical requirements.A hydrofoil is a lifting surface, or foil, that operates in water. It can be applied on the kireboard, surfboard and SUP.
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