What Is a Dead Center?
A Dead Center is a fixture attached to the tip of chucks in machine tools such as lathes to secure workpieces.
In machine tools like lathes, the workpiece is held in place by jaws attached to the tip of the chuck to rotate the workpiece. There are two types of jaws: hard centers and dead centers.
Hard centers, hardened through heat treatment, are used in the roughing stages due to their hardness, making them difficult to shape. On the other hand, dead centers are made of non-heat-treated metal, allowing for easy shaping, and are used in the finishing stages. They can create a contact surface shape tailored to the shape of the workpiece, securing even irregularly shaped workpieces like those with a star-shaped cross-section.
Uses of Dead Center
Dead Centers are used in machine tools such as lathes to process workpieces in the finishing stages or when working on workpieces with special shapes.
They are employed in various machine tools, including general lathes, milling machines, NC lathes, drill presses, machining centers, etc. In these machine tools, the workpiece is clamped between the chuck and the immersed jaws. If we liken the relationship between the chuck and the jaws to the human hand, the chuck is the palm, and the jaws act as the fingers.
While there are two types of jaws, hard centers, and dead centers, dead centers can shape jaws that fit each workpiece securely. Therefore, hard centers are used in the roughing stage, and dead centers are used in the finishing stage.
Additionally, when dealing with workpieces with special cross-sectional shapes or when conventional-shaped jaws cannot grip and hold conical-shaped workpieces effectively, customized dead centers are shaped to match the workpiece’s shape, allowing both roughing and finishing processes to be carried out using dead centers.
Principles of Dead Center
The shapes and sizes of workpieces handled by machine tools like lathes vary widely. Therefore, by replacing the jaws attached to the chuck, one machine can process workpieces of various shapes and sizes.
Jaws of a certain shape and size are provided as standard for each chuck. However, it is impossible to provide standard jaws that fit perfectly with all workpiece sizes and shapes. Therefore, dead centers that can be shaped to fit the workpiece are often used.
While hard centers, which have undergone hardening treatment, are challenging to process after hardening, dead centers, not subjected to hardening, are easy to shape. As a result, they can firmly grip the workpiece, preventing core runout and enabling higher precision processing.
Types of Dead Centers
Dead centers are classified into standard centers, small-diameter centers, high centers, wide centers, etc., based on their shape and purpose.
Standard centers are the dead centers that come with the chuck when purchased, with the lowest height becoming the standard. Small-diameter centers are dead centers designed to grip small-diameter workpieces. High centers are dead centers with greater height than others, used not only to match the material’s shape but also to prevent interference between the chuck and tools. Wide centers have a width wider than the standard, used for gripping large workpieces.
Additionally, there are custom-order dead centers that can be adjusted in height, width, length, shape, etc. Dead centers are often considered consumables since they are often cut along with the workpiece during cutting.
How to Choose Dead Center
When choosing a dead center, paying attention to the following points is crucial.
1. Material
Dead centers are generally made of iron or aluminum, and the choice depends on the material of the workpiece. Although iron is widely used today, different materials may be chosen based on specific requirements or non-ferrous metal processing.
Also, in cases where the working environment or the material being processed is highly corrosive, it is important to choose a dead center made of a corrosion-resistant material.
2. Hardness
Hardness is directly related to the durability and lifespan of the dead center. Higher hardness improves wear resistance, but it may also make the dead center brittle. Therefore, it is necessary to select the appropriate hardness according to the workpiece and cutting conditions. For example, dead centers with high hardness are suitable for high-speed cutting or processing of hard materials.
Additionally, the hardness of dead centers can be altered by heat treatment. Proper heat treatment during the manufacturing process is crucial to ensure accurate and consistent hardness.
3. Shape and Size
The shape and size of dead centers must be suitable for the shape of the target object. For instance, three-jaw chucks may be suitable for cylindrical workpieces, but for asymmetrical shapes or complex parts, special-shaped dead centers may be required.
If the dead center’s shape is symmetrical, the force during processing is evenly distributed, ensuring stable holding. It is essential to be mindful when changing the target object, as using dead centers that are too large or too small can lead to unstable holding, adversely affecting the quality of the processed item.
4. Wear Resistance
Since dead centers are subjected to friction and cutting forces, sufficient wear resistance is crucial.
In cases where high rotational speeds or strong cutting forces are involved, dead centers with low wear resistance will deteriorate quickly, resulting in a shorter lifespan. Surface treatments or coatings may be applied to improve wear resistance. Dead centers made of materials with high wear resistance withstand prolonged use, reducing the frequency of maintenance.