What Ensures Machining Accuracy in CNC Machines?
The machining accuracy of CNC machines ultimately depends on the intrinsic precision of the machine itself. This includes geometric accuracy, positioning accuracy, repeatability, and cutting accuracy.
1. Types of Machining Accuracy
● Geometric Accuracy
Also called static accuracy, this refers to the combined geometric shape error of key machine components after assembly.
● Positioning Accuracy
This indicates the accuracy a machine component can reach under CNC control. It reflects the maximum possible part accuracy the machine can achieve automatically. The smaller the difference between the actual and theoretical positions, the higher the precision.
● Repeat Positioning Accuracy
This is the consistency of positioning when running the same program multiple times under the same conditions. It measures how consistent the machine is in producing parts within a batch.
● Cutting Accuracy
This is a comprehensive check of both geometric and positioning accuracy under actual machining conditions.
Both mechanical and electrical systems affect the final accuracy. Mechanically, factors include spindle runout, ball screw precision, fixture accuracy, and machine rigidity. Electrically, factors include control system type (open-loop, semi-closed-loop, full-closed-loop), feedback and compensation systems, and interpolation accuracy. Thus, a full closed-loop system does not automatically mean better accuracy.
2. Introduction to the Principles Affecting Machining Accuracy
A CNC machine’s motion chain includes:
CNC Controller → Encoder → Servo Drive → Motor → Ball Screw → Moving Parts
Depending on the position of the feedback device, systems fall into:
- Full Closed-Loop Control
- Semi Closed-Loop Control
- Open-Loop Control
2.1 Full Closed-Loop Control
In this system, a position feedback device (like a linear scale or encoder) is mounted directly on moving components such as the worktable. It gives real-time feedback to the controller, which then instructs the motor to correct motion errors.
Although this improves accuracy by compensating for errors in the screw, nut, and moving components, it:
- Introduces complex tuning difficulties due to high inertia;
- Uses expensive and fragile components;
- May cause oscillation due to overcompensation;
- Is rarely used in standard CNC machines.
2.2 Semi Closed-Loop Control
Here, the feedback device is mounted at the motor or screw end, indirectly measuring the moving part’s position. Due to improved screw and sensor precision, semi closed-loop systems can now achieve very high feed accuracy. As a result, most CNC machine manufacturers use semi closed-loop systems.
3. Real-World Applications about Machining Accuracy
3.1 Full Closed-Loop Systems
These systems rely on precision feedback devices (accuracy grades: ±0.01 mm, ±0.005 mm, etc.). However, even full closed-loop control is not immune to errors.
Thermal Effects:
Position sensors are usually made from non-metallic materials with different thermal expansion coefficients. Heat during machining causes thermal deformation, reducing precision. High-end machines counter this using:
- Hollow ball screw cooling,
- Guideway lubrication,
- Constant-temperature coolant.
Installation Factors:
Ideally, sensors should be mounted close to the screw axis. But due to space constraints, they’re often mounted on the machine frame. If not properly aligned, feedback signals become inaccurate, leading to oscillation. The machine constantly overcorrects motion—causing more vibration instead of better control.
Environmental Impact:
In standard machine shop environments, dust, oil mist, and vibration reduce the reliability of optical sensors. These contaminants disrupt signal transmission, causing frequent alarms and reduced accuracy. Over time, a machine that was accurate on day one can lose performance drastically.
3.2 Semi Closed-Loop Systems
Since the sensors are installed on well-sealed areas, such as the ends of motors or lead screws, they are largely shielded from environmental interference. The main factor affecting accuracy in such systems is:
Backlash between the ball screw and nut assembly.
However, with advancements in ball screw manufacturing—especially using imported high-precision components—and through:
- Preloaded double-nut ball screw configurations
- Pre-tensioned screw installations
CNC systems using semi closed-loop control can now achieve excellent accuracy. Under standard factory conditions, their performance can rival that of fully closed-loop systems.
4. Conclusion
In theory, full closed-loop systems offer higher fundamental accuracy. But in practice, if heat control, installation precision, and shop cleanliness are not maintained, semi closed-loop systems can outperform full closed-loop systems.
Over time, contaminants and thermal variation degrade the accuracy of full closed-loop machines. Worse yet, if a sensor fails, it may trigger system alarms and halt production.
Therefore, unless operating in cleanroom-like conditions with rigorous thermal control, a well-optimized semi closed-loop system often delivers better, more reliable results in real-world machining.