Detailed explanation of tapping machining technology in CNC machine tools
In the field of mechanical processing, the processing accuracy of threads directly affects the assembly quality and service performance of parts. CNC machine tools have become the core equipment for modern tapping machining with their high-precision motion control capabilities and automation characteristics.
Compared with traditional manual or ordinary machine tool tapping, CNC machine tools can improve thread accuracy to ISO 5H level or above, and improve processing efficiency by 3-5 times, which is particularly suitable for mass production of complex parts. From dense thread processing of automobile engine cylinder blocks to deep hole tapping of titanium alloy structural parts in aerospace, CNC machine tools have solved the problems of unstable accuracy and low efficiency in traditional processing through rigid tapping functions, precise parameter control and intelligent strategies, and have become a key technical link in high-end equipment manufacturing.
1. Basic principles of tapping machining
(1) Rigid tapping and flexible tapping
The core of CNC tapping machining lies in the synchronous control of spindle rotation (C axis) and axial feed (Z axis), and its implementation methods are divided into two categories:
Rigid tapping: The synchronization of the spindle and feed axis is directly controlled by the CNC system, without the need for a floating chuck. The spindle encoder provides real-time feedback of the rotation speed to ensure that the feed rate per rotation is strictly equal to the pitch (V_f=N×P). This mode has high precision and is suitable for high-precision thread processing.
Flexible tapping: Use an elastic chuck to compensate for small errors between the spindle and the feed axis. It is suitable for low-precision machine tools or old equipment, but it is easy to cause thread rot due to asynchrony.
(2) Synchronous control principle:
The CNC system calculates the feed speed V_f based on the pitch P and dynamically matches it with the spindle speed N. For example, when processing M8×1.25 threads, if the spindle speed N=200 RPM, the feed speed must be: V_f=200×1.25=250mm/min
The system adjusts the servo motor in real time through the PID algorithm to ensure that the synchronization error is less than 0.5%.
2. Programming points of CNC tapping machining
(1) Common G code instructions
G84 (rigid tapping cycle):
Format: G84 X_Y_Z_R_P_Q_F_
X/Y: hole coordinates;
Z: thread depth (absolute coordinates);
R: safety height;
P: hole bottom dwell time (unit: milliseconds);
Q: step tapping depth (for deep holes);
F: feed rate (F=N×P).
Example: Processing M6×1 thread, depth 10mm, speed 300 RPM:
G84 X50 Y30 Z-10 R2 P200 F300
G74 (left-hand tapping cycle): used for processing left-hand threads, parameters are similar to G84, but the spindle is reversed.
(2) Key parameter settings
Spindle speed: The choice of spindle speed should be determined based on factors such as workpiece material, tap diameter and pitch. Generally speaking, smaller diameter taps are suitable for higher speeds, while larger diameter taps require lower speeds to ensure stability.
Pitch compensation: Some systems require the pitch value to be entered in the parameter table to ensure feed synchronization.
Tool compensation (H code): Call tool length compensation to avoid tap tool setting errors.
(3) Programming considerations
Retraction speed: Usually set to 1.5-2 times the feed speed to reduce retraction time.
Layered tapping (Q value): When processing deep holes, tapping in steps (such as 5mm each time) can improve chip removal.
Thread finishing: Set the pause at the bottom of the hole (0.2-0.5 seconds) through the P value to improve the quality of the bottom of the thread.
3. Programming example: CNC center tapping process
Processing task: Process 3 M12×1.75 through holes (hole depth 30mm) on a 45 steel workpiece using a φ12 high-speed steel tap (recommended v t=20m/min).
Process calculation:
Spindle speed N=v_t×1000/π D_t=20×1000/3.14×12≈530r/min;
Feed speed F=P×N=1.75×530=927.5mm/min
Processing procedure:
| Description |
| Program number |
| Absolute coordinate, XY plane |
| Spindle forward rotation, 530 RPM |
| Tool length compensation |
| Tapping cycle, safe height 5mm, hole depth -35mm (with 5mm overrun) |
| Process the first hole |
| Process the second hole |
| Process the third hole |
| Cancel cycle, retract to initial height |
| Program end |
4. Actual case analysis
Case 1: Deep hole tapping of automobile gearbox housing
Processing requirements: Process 20 M12×1.75 threads on the cast iron housing, with a depth of 25mm and an accuracy of IT7.
Challenge: The hole depth makes it difficult to remove chips, and cast iron abrasion wears the tap.
Solution:
Tool selection: Use TiAlN coated spiral groove taps to improve wear resistance.
Programming optimization: Use layered tapping (Q=5mm) and remove chips at each layer.
Parameter settings: speed 150 RPM, feed F=150×1.75=262.5 mm/min, internal cooling pressure 10 bar.
Result: tap life increased from 50 holes to 200 holes, thread qualification rate 99.5%.
Case 2: Stainless steel micro thread processing for medical equipment
Requirement: Process M1.2×0.25 thread on 316L stainless steel, depth 2mm, surface roughness Ra≤0.8μm.
Difficulty: The tap has a small diameter (1.2mm) and is easy to break; the material is significantly hardened.
Key measures:
Spindle accuracy calibration: Use a pneumatic spindle (speed 3000 RPM), radial runout ≤1μm.
Minimum quantity lubrication (MQL): Reduce cutting heat and avoid material hardening.
Parameter adjustment: Feed overshoot is set to 105% to compensate for machine tool backlash.
Results: Thread surface roughness Ra=0.6μm, tap life reaches 5000 holes.
5. Conclusion
Core advantages of CNC tapping machining
Precision guarantee: Through rigid synchronous control and digital compensation technology, the thread accuracy is improved by 2-3 levels compared with traditional processing, meeting the high-precision requirements of aerospace, medical equipment and other fields;
Efficiency breakthrough: High-speed tapping technology reduces the processing time of a single hole by more than 60%, adapting to mass production scenarios such as automobile manufacturing;
Process flexibility: Supports full-specification thread processing from M1-M100, compatible with complex working conditions such as through holes, blind holes, deep holes, and quickly switches processing strategies through programming.
The progress of CNC machine tool tapping machining technology is essentially a deep integration of mechanical motion control, tool materials and process parameters. With the advancement of Industry 4.0 and intelligent manufacturing, CNC tapping will move from "program-driven" to "data-driven", and achieve a dual breakthrough in processing quality and efficiency through real-time monitoring and intelligent optimization, becoming an indispensable key technology in high-end equipment manufacturing.
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