Attachments, commonly known as fixtures, are essential tools used to support and secure workpieces during machining operations. Their effective application significantly enhances both the technical performance and economic efficiency of manufacturing processes. The key roles of fixtures can be summarized as follows:
1. **Expanding Machine Tool Capabilities**
Since machine tools have limited types and quantities, fixtures allow one machine to perform multiple functions. By using different fixtures, complex workpieces with various shapes can be processed, thereby improving the utilization rate of the machine tool.
2. **Ensuring Workpiece Quality and Consistency**
Fixtures ensure precise positioning of the workpiece, which leads to higher machining accuracy. This consistency in positioning and machining allows for high interchangeability among parts from the same batch.
3. **Improving Productivity and Reducing Costs**
Using fixtures simplifies the process of mounting workpieces, reducing auxiliary time. Additionally, fixtures stabilize the workpiece, increasing its rigidity, allowing for higher cutting speeds and reduced manual effort, ultimately boosting productivity.
4. **Enhancing Working Conditions**
Fixtures make the installation process more convenient, safe, and less labor-intensive. They also lower the skill requirements for operators, making the overall working environment more efficient and safer.
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**1. Mounting the Workpiece Using a Four-Jaw Chuck**
The four-jaw chuck, as shown in Figure 1a, is equipped with four independent jaws that move via screws. It is ideal for clamping non-rotational parts with irregular shapes such as square or rectangular pieces, offering strong clamping force. However, it cannot automatically center the workpiece, so careful alignment with a dial indicator or a test plate is necessary before use, as illustrated in Figure 1b.
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**2. Mounting the Workpiece Using Centers (Top)**
For workpieces requiring high coaxiality and turret machining, the double-center method is often employed. As shown in Figure 2, the front center is mounted in the spindle and rotates with it, while the rear center is placed in the tailstock. The workpiece is centered using the center hole and rotates along with the spindle.
When using centers, keep the following in mind:
- Avoid over-tightening the supporting screw to prevent workpiece deformation.
- The cutting amount should be small due to torque transmission through the clamp.
- When drilling center holes, first flatten the end face with a turning tool, then drill the center hole with a center drill.
To install the workpiece, clean the spindle and dial threads, mount the dial on the spindle, place one end of the shaft on the center, and finally position the workpiece between the two centers.
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**3. Mounting the Workpiece Using a Mandrel**
When the inner hole is used as a reference, a mandrel is typically used for positioning. Common types include cylindrical and tapered mandrels for cylindrical holes, and conical, threaded, and spline spindles for other hole types.
Cylindrical mandrels are positioned by the outer surface and clamped by pressing the end face. The fit between the mandrel and the workpiece is usually H7/h6 or H7/g6, allowing easy assembly. However, this clearance may result in a small coaxiality error of about 0.02 mm. To improve accuracy, a tapered mandrel can be used, with a very slight taper (typically 1:1000 to 1:5000) to avoid tilting. The workpiece is tightly wedged onto the mandrel, causing elastic deformation that prevents tilting.
Tapered mandrels offer high centering accuracy (0.005–0.01 mm), but they do not provide axial positioning. For larger workpieces, a cylindrical mandrel with a compression nut is preferred, though it offers lower centering accuracy.
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**4. Using a Center Rest and Tool Post**
When the length-to-diameter ratio of the workpiece exceeds 25 (L/d > 25), its rigidity decreases, leading to bending, vibration, and poor surface finish. In such cases, a center rest or tool post is used to support the workpiece.
**1) Supporting a Long Shaft with a Center Rest**
As shown in Figure 5, a center rest is used to increase the rigidity of long shafts. Before installation, a groove must be machined in the middle of the blank to accommodate the center rest's claws. Lubrication is often applied at the contact points to reduce friction.
If machining is not required, a transition sleeve can be used to bring the claws into contact with the outer surface of the sleeve.
**2) Supporting a Long Shaft with a Tool Post**
For workpieces that cannot be supported by a center rest, a tool post is used. As shown in Figure 6, the tool post is fixed on the saddle and supports the workpiece, counteracting radial cutting forces and improving shape accuracy.
A three-jaw center rest is ideal for stability, preventing vibrations caused by uneven weight distribution.
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**5. Mounting the Workpiece Using a Faceplate, Jaw Plate, Pressure Plate, and Bolts**
For irregularly shaped workpieces that cannot be clamped with a standard chuck, a faceplate is used. As shown in Figure 7, the faceplate is mounted on the lathe spindle and has grooves for bolts. The workpiece is directly secured using bolts or mounted on an angle iron attached to the faceplate.
In some cases, balance weights are added to prevent vibration when the workpiece’s center of gravity is off-center. The position of the workpiece on the faceplate must be carefully adjusted for accurate machining.
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Wuan Jianfeng Casting Co., Ltd. , https://www.hbjfcasting.com