(1) The working range of the machine tool can be expanded Since there are many kinds of workpieces, the types and number of machine tools are limited, and different fixtures can be used to realize one machine and multiple functions and improve the utilization of the machine tool.
(2) The workpiece quality can be stabilized After the fixture is used, the mutual positions of the workpiece surfaces are ensured by the fixture, and the machining accuracy achieved by the squaring is higher, and the positioning precision and the machining accuracy of the same batch can be basically the same. Therefore, workpiece interchangeability is high. (3) Improve productivity and reduce costs. The use of clamps generally simplifies the mounting of the workpiece, which in turn reduces the amount of auxiliary time required to install the workpiece. At the same time, fixtures can be used to stabilize the workpiece, increase the rigidity of the workpiece, increase the amount of cutting, reduce maneuvering time, and increase productivity.(4) Improve working conditions. Installing fixtures with fixtures is convenient, labor-saving, and safe. It not only improves the working conditions, but also lowers the technical level of workers.
1. Install the workpiece with a four-jaw chuckThe four-jaw chuck is shown in Figure 1a. Its four claws move independently by 4 screws. Its characteristic is that it can clamp non-revolving bodies with complex shapes such as square, rectangular, etc., and has a large clamping force. Because it can not be automatically centered after being clamped, the clamping efficiency is low. When setting up, it must be calibrated with a dash plate or a dial indicator so that the center of rotation of the workpiece is aligned with the center of the lathe spindle, as shown in Figure 1b. Find the schematic of the outer circle.
Figure 1 Four-jaw chuck clamping workpiece
2. Install the workpiece with the top
For coaxial workpieces with relatively high requirements for coaxiality and requiring turret machining, the workpiece is often mounted with double tips. As shown in Fig. 2 , the front top of the shaft is a common top, which is mounted in the spindle hole and rotates with the spindle. To live in the tail frame sleeve. The workpiece is centered between the front and rear centers using the center hole and rotates with the spindle through the dial and clamp.
Figure 2 Installing the workpiece with the top
With the top of the installation of the workpiece should pay attention to:
1) The supporting screw on the clamp cannot be supported too tightly to prevent deformation of the workpiece.
2) Since the torque is transmitted by the clamp, the cutting amount of the turned workpiece is small.
3) When drilling the center holes at both ends, use a turning tool to flatten the end face and use the center drill to drill the center hole.
When installing the dial and workpiece, first wipe the internal thread of the dial and the external thread of the spindle end, screw the dial on the spindle, and then install one end of the shaft on the clamp. Finally, install the workpiece in the middle of the double tip.
3. Install the workpiece with the mandrel
When the inner hole is used as the reference, and the coaxiality of the axis of the outer circle and the axis of the inner hole can be guaranteed, the mandrel is used for positioning. The cylindrical hole is used to position the commonly used cylindrical mandrel and small taper mandrel; Workpiece positioning of threaded holes, spline holes, commonly used corresponding cone spindle, screw spindle and spline spindle.
Figure 3 Positioning on a cylindrical mandrel
The cylindrical mandrel is centered on the outside of the cylindrical surface and the end face is pressed to clamp the workpiece, as shown in FIG. 3 . The mandrel and the workpiece hole are generally fitted with clearances of H7/h6 and H7/g6 so that the workpiece can be easily put on the mandrel. However, due to the large gap between the general, only about 0.02mm coaxiality can be guaranteed. In order to eliminate gaps and improve the positioning accuracy of the mandrel, the mandrel can be made into a cone, but the taper of the cone is very small, otherwise the workpiece will be skewed on the mandrel (see Figure 4a). The commonly used taper is C=1/1000~1/5000. When positioning, the workpiece is wedged tightly on the mandrel and the hole will be elastically deformed after the wedge (Fig. 4b), so that the workpiece will not tilt.
Figure 4 Cone mandrel mounting workpiece contact conditions
The advantage of the small taper spindle is that the workpiece is driven by the frictional force generated by the wedge. No other clamping device is required, and the centering accuracy is as high as 0.005 to 0.01 mm. The disadvantage is that the workpiece cannot be positioned in the axial direction.
When the workpiece diameter is not too large, a taper spindle (taper 1:1000 to 1:2000) can be used. The workpiece is nested tightly, and it is firmly fixed to the mandrel by friction. Tapered mandrel centering accuracy, high precision machining, loading and unloading convenient, but can not withstand excessive torque.
When the workpiece diameter is large, a cylindrical mandrel with a compression nut should be used. Its clamping force is larger, but its centering accuracy is lower than that of the tapered mandrel.
4. Use of center frame and knife holder
When the ratio of workpiece length to diameter is more than 25 times (L/d>25), the rigidity of the workpiece itself deteriorates. When turning, the workpiece is subjected to bending force and vibration due to cutting force, self weight, and centrifugal force during rotation. Severely affect the cylindricity and surface roughness, at the same time, in the cutting process, the workpiece is subject to bending deformation caused by heat elongation, turning is difficult to carry out, in severe cases will make the workpiece stuck between the top. In this case, it is necessary to use a center frame or a knife holder to support the workpiece.
1) Support the car's slender shaft with the center frame
Generally, when turning an elongated shaft, a center frame is used to increase the rigidity of the workpiece. When the workpiece can be segmented, the center frame is supported in the middle of the workpiece, as shown in FIG. 5 . Before the workpiece is mounted on the center frame, a section of the groove supporting the center frame supporting claw must be driven in the middle of the blank. The surface roughness and the cylindrical error must be small, and lubrication oil is often added at the contact point between the supporting claw and the workpiece. In order to improve the accuracy of the workpiece, the axis of the workpiece should be adjusted to be coaxial with the rotation center of the spindle of the machine before turning.
Fig. 5 Supporting turning elongated shaft with center frame
When the groove supporting the center frame of the turning support is difficult or some of the slender shafts that do not need machining are used, a transition sleeve can be used to bring the supporting claws into contact with the outer surface of the transition sleeve, as shown in FIG. 6, two of the transition sleeves. Each end is equipped with four screws. The screws are used to clamp the surface of the blank, and the axis of the sleeve is adjusted to coincide with the axis of rotation of the spindle.
c) Use with knife holder
1. Three-jaw chuck 2. Workpiece 3. Knife holder 4. Top
Figure 6 Supporting long shaft with knife holder
2) Support the car's slender shaft with a knife holder
For slender shafts that are not suitable for turning in the U-turn, they cannot be supported by the center frame, but they must be turned with the tool holder to increase the rigidity of the workpiece, as shown in FIG. 6 . With the tool holder fixed on the saddle, there are generally two supporting claws, which can follow the turning tool to offset the radial cutting force, improve the shape accuracy of the slender shaft and reduce the surface roughness, as shown in Figure 6a. Two-jaw and knife holder, because the cutting resistance of the turning tool to the workpiece is F'r, the workpiece is attached to the two supporting jaws of the rest holder, but due to the downward gravity force of the workpiece itself and accidental bending, it will be instantaneous when turning. Vibration is generated when leaving the support pawl and contacting the support pawl. Therefore, the ideal center frame requires a three-claw center frame, as shown in Figure 6b. At this time, the three claws and the turning tool are pressed against the workpiece so that the upper and lower parts and the left and right cannot be moved, and the turning is stable and vibration is hard to occur.
5. Install the workpiece with faceplate, jaw plate, pressure plate and bolt
Figure 7 Installing Parts on the Faceplate
Irregularly shaped workpieces that cannot be clamped with a three-jaw or four-jaw chuck can be clamped with a faceplate. The disk is a large disk mounted on the spindle of the lathe. Many long grooves on the disk surface are used to insert the bolts. The workpiece can be directly mounted on the disk with bolts, as shown in Figure 7. It is also possible to hold the auxiliary supporting angle iron (bending plate) firmly on the face plate with screws and the workpiece is mounted on the bending plate. Fig. 8 shows the mounting on the faceplate when machining the end face and inner bore of a bearing seat. In order to prevent vibration when the center of gravity deviates to one side during rotation, balance iron is added on the other side of the workpiece. The position of the workpiece on the faceplate must be carefully corrected.
Figure 8 Using curved plates to mount parts on the faceplate
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