Chatter in machine tools is caused due to -
Self - excited vibration
Chatter in machine tools is a complex phenomenon that results in undesirable vibrations during machining operations. This vibration can lead to poor surface finish, increased tool wear, noise, and even damage to the machine tool itself. Understanding the cause of chatter is crucial for implementing effective suppression strategies.
Understanding Machine Tool Chatter
Chatter is essentially a self-sustaining vibration that occurs during machining. It is often characterized by a high-pitched noise and visible waves or patterns on the machined surface. Unlike other types of vibrations that might be caused by external forces or initial disturbances, chatter is generated and maintained by the interaction between the cutting process and the machine tool structure.
Types of Vibration in Machining
Let's look at the types of vibration mentioned in the options and their relevance to machine tool chatter:
- Transient vibration: This type of vibration occurs after a sudden disturbance and eventually dies out over time due to damping. While a disturbance might initiate a vibration, transient vibration itself is not the primary cause of persistent chatter.
- Self-excited vibration: This is a type of vibration where the energy that sustains the oscillation is drawn from the process itself, in this case, the cutting process. The vibration affects the cutting force, which in turn affects the vibration, creating a feedback loop.
- Free vibration: This occurs when a system is disturbed from its equilibrium position and allowed to vibrate without any continuous external force or energy input, except for the initial disturbance. Like transient vibration, it typically decays over time due to damping and is not the cause of sustained chatter.
- Forced vibration: This type of vibration is caused by an external, time-varying force or displacement acting on the system. Examples include vibrations caused by imbalance in rotating components, motor vibrations, or vibrations transmitted from the factory floor. While forced vibrations can exist during machining, they are usually at frequencies related to the forcing source (e.g., spindle speed) and are distinct from the self-sustaining nature of chatter.
Why Self-Excited Vibration Causes Chatter
Chatter in machine tools is predominantly caused by self-excited vibration. Here's a simplified explanation of the mechanism:
- As the cutting tool removes material, it interacts with the workpiece surface.
- Vibrations in the machine structure cause relative movement between the tool and the workpiece.
- This relative movement changes the chip thickness being cut. For example, if the tool vibrates slightly into the workpiece, the chip thickness increases; if it moves away, the thickness decreases.
- The change in chip thickness directly affects the cutting force. A thicker chip generally results in a higher cutting force, and a thinner chip results in a lower force.
- Crucially, in self-excited vibration, there is a phase difference (often around 90 degrees) between the vibration displacement and the resulting force variation. This phase difference allows the varying cutting force to feed energy back into the vibration, sustaining or even amplifying it.
- A major contributing factor to this self-excitation in machining is the "regenerative effect." The wavy surface left by a previous pass affects the chip thickness of the current pass, creating a feedback loop involving the tool's current position and the surface profile generated during the previous cut.
This positive feedback loop, where the cutting process itself feeds energy into the vibration, is characteristic of self-excited vibration and is the primary mechanism behind machine tool chatter.
Comparing Vibration Types and Chatter
| Vibration Type | Energy Source | Sustained by | Relevance to Chatter |
|---|---|---|---|
| Transient | Initial disturbance | Damping causes decay | Can initiate, but not sustain chatter |
| Self-excited | Process energy (cutting) | Positive feedback loop | Primary cause of sustained chatter |
| Free | Initial disturbance | Damping causes decay | Not a cause of sustained chatter |
| Forced | External force | External force | Can occur, but mechanism different from chatter |
Based on the analysis, the sustained and self-perpetuating nature of chatter in machine tools is a direct result of self-excited vibration.
Revision Table - Machine Tool Chatter and Vibration
| Term | Definition/Relation to Chatter |
|---|---|
| Chatter | Undesirable, self-sustaining vibration during machining. |
| Self-excited Vibration | Vibration sustained by energy from the process itself (cutting). The primary cause of chatter. |
| Regenerative Effect | Feedback mechanism in chatter where current chip thickness depends on surface profile from previous pass. |
| Forced Vibration | Vibration caused by external forces (e.g., imbalance). Distinct from chatter. |
| Transient Vibration | Temporary vibration after a disturbance, decays over time. |
Additional Information - Preventing Machine Tool Chatter
Preventing or reducing chatter is a major goal in machining. Strategies include:
- Increasing machine tool stiffness and damping.
- Selecting appropriate cutting parameters (speed, feed, depth of cut). There's often a stable "chatter-free" zone for specific parameters.
- Using tools with high damping characteristics or dynamic absorbers.
- Optimizing tool geometry.
- Using predictive software to identify stable cutting windows.
- Adjusting the spindle speed to avoid frequencies prone to resonance and self-excitation.
Important Questions from Machine Tools
Turning operation on a lathe is primarily used to create:
In a Capstan lathe, turret is mounted on -
Which of the following chucks in the lathe machine is known as Universal Chuck?
Single point thread cutting tool should ideally have:
Which of the following planers are specially designed for cutting the edges of heavy steel plates, pressure vessels and armoured plates?
