Attachments, commonly known as fixtures, are devices used to support and secure workpieces during machining operations. Their application has significant technical and economic benefits. The key roles of fixtures can be summarized as follows:
1. **Expanding the Machine Tool's Working Range**
Since there are various types of workpieces and machine tools are limited in number and variety, different fixtures can be used to enable a single machine tool to perform multiple functions. This improves the overall utilization of the machine.
2. **Stabilizing Workpiece Quality**
Once a fixture is used, it ensures the relative positions of the workpiece surfaces, resulting in higher machining accuracy. The positioning precision and machining consistency across the same batch of parts are maintained, leading to high interchangeability.
3. **Improving Productivity and Reducing Costs**
Fixtures simplify the process of securing the workpiece, reducing the time required for setup and auxiliary operations. They also stabilize the workpiece, increase its rigidity, allow for larger cutting amounts, reduce manual effort, and ultimately boost productivity.
4. **Enhancing Working Conditions**
Using fixtures makes the installation process more convenient, labor-saving, and safer. It not only improves the working environment but also lowers the skill requirements for operators.
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**1. Mounting the Workpiece with a Four-Jaw Chuck**
The four-jaw chuck, as shown in Figure 1a, allows each of its four jaws to move independently via four screws. It is ideal for clamping non-rotational workpieces with complex shapes such as square or rectangular parts, and it provides a strong clamping force. However, since it cannot automatically center the workpiece, the clamping efficiency is relatively low. Before use, the workpiece must be aligned using a dial indicator or a test plate so that its rotational center matches the lathe spindle’s center, as illustrated in Figure 1b.
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**2. Mounting the Workpiece with a Center**
For coaxial workpieces that require precise alignment and turret machining, the workpiece is often mounted between two centers. As shown in Figure 2, the front center is typically a standard center mounted in the spindle, while the rear center is located in the tailstock sleeve. The workpiece is centered using its center hole and rotates with the spindle through the dial and clamping mechanism.
When using a center, keep the following in mind:
- Avoid tightening the support screw too much to prevent workpiece deformation.
- Due to torque transmission through the clamp, the cutting amount should be small.
- When drilling center holes, first flatten the end face with a turning tool and then drill the center hole with a center drill.
To install the dial and workpiece, clean the internal thread of the dial and the external thread of the spindle, screw the dial onto the spindle, and then mount one end of the shaft on the center. Finally, place the workpiece in the middle between the two centers.
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**3. Mounting the Workpiece with a Mandrel**
When the inner hole is used as a reference and the coaxiality between the outer and inner holes needs to be ensured, a mandrel is used for positioning. Common types include cylindrical mandrels, small taper mandrels, and specialized ones like conical, threaded, or spline mandrels.
A cylindrical mandrel is inserted into the workpiece’s outer surface and clamped using the end face. The clearance fit between the mandrel and the workpiece is usually H7/h6 or H7/g6, allowing easy mounting. However, this large gap may result in about 0.02mm of coaxiality. To improve accuracy, the mandrel can be made tapered (C=1/1000–1/5000), which eliminates gaps and prevents tilting by causing elastic deformation when wedged into place.
Tapered mandrels offer high centering accuracy (up to 0.005–0.01 mm) without additional clamping devices, but they do not provide axial positioning. For smaller diameters, a taper of 1:1000–1:2000 is suitable. For larger diameters, a cylindrical mandrel with a compression nut is preferred for greater clamping force, though with lower centering accuracy.
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**4. Using a Center Frame and Tool Holder**
When the ratio of workpiece length to diameter exceeds 25 (L/d > 25), the workpiece becomes less rigid. During turning, bending forces, vibration, and thermal expansion can affect the roundness and surface finish. In such cases, a center frame or tool holder is used to support the workpiece.
**1) Supporting a Long Shaft with a Center Frame**
As shown in Figure 5, a center frame is used to increase the rigidity of long shafts. A groove must be machined in the middle of the blank before mounting. The contact area should have good surface finish and lubrication. The workpiece axis should be adjusted to align with the spindle center.
If the groove is difficult to machine, a transition sleeve can be used to bring the supporting claws into contact with the outer surface of the sleeve, as shown in Figure 6.
**2) Supporting a Long Shaft with a Tool Holder**
For shafts unsuitable for U-turns, a tool holder is used to increase rigidity. The holder is mounted on the saddle, with two or three supporting claws that follow the cutting tool to reduce vibration and improve shape accuracy.
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**5. Mounting the Workpiece with a Faceplate, Jaw Plate, Pressure Plate, and Bolt**
Irregularly shaped workpieces that cannot be clamped with a three-jaw or four-jaw chuck can be mounted on a faceplate. The faceplate is a large disk mounted on the lathe spindle with many grooves for bolts. The workpiece can be directly fastened with bolts, or an angle iron can be attached to the faceplate for better support.
For example, when machining the end face and inner bore of a bearing seat, a curved plate is used to hold the workpiece. To prevent imbalance during rotation, balance weights are added on the opposite side.
Careful alignment is essential to ensure the workpiece is properly positioned on the faceplate.
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