CNC thread machining is a process that uses programmed instructions to control CNC machines, utilizing specialized tools (such as taps and thread mills) to create internal or external threads on workpieces. Its core lies in precisely controlling the relative movement between the tool and the workpiece to form spiral thread profiles that meet specifications, ensuring dimensional accuracy and fitting performance of the threads.
What are the main methods of thread machining?
CNC thread machining commonly uses two methods, each suitable for different scenarios:
- Tapping with a tap
- Principle: A tap (a tool with a complete thread profile) is driven by the spindle to rotate while feeding axially, directly cutting threads into the workpiece.
- Applicable scenarios: Internal thread machining (e.g., threaded holes), especially for small-diameter threads (usually ≤ M12), or scenarios pursuing efficiency in mass production.
- Advantages: Simple operation and convenient programming, suitable for small to medium-diameter threads.
- Thread milling
- Principle: A thread mill (single-tooth or multi-tooth) makes circular motion around the workpiece axis while feeding along the thread lead. Through the combination of the tool’s rotation and axial movement, the thread profile is gradually milled out.
- Applicable scenarios: Large-diameter internal/external threads (e.g., M16 and above), precision threads (e.g., tolerance grades 6H/6g), or workpieces made of hard materials (e.g., stainless steel, titanium alloys).
- Advantages: High machining accuracy, long tool life, and the ability to machine threads of different rotation directions with a single program.
What are the steps for tapping with a tap?
Taking a CNC milling machine/ machining center as an example, the programming and operation steps for tapping with a tap are as follows:
- Select tools and parameters
- Choose a corresponding tap (machine tap) according to the thread specification (e.g., M8×1.25), which needs to match the floating function of the tool holder to compensate for feed errors.
- Set parameters: Spindle speed (RPM) = (cutting speed × 1000) / (π × tap diameter); feed rate (mm/min) = spindle speed × pitch (e.g., the feed rate for M8×1.25 = speed × 1.25).
- Core programming instructions
G54 G90 G21 ; Call workpiece coordinate system, absolute programming, metric units
T1 M6 ; Call tap tool
S500 M3 ; Spindle forward rotation, speed 500 RPM
G00 X10 Y10 Z5 ; Rapid movement to above the thread starting point
G84 R2 Z-20 F625 ; Tapping cycle: R point (safe height 2mm), thread depth 20mm, feed rate = 500×1.25 = 625mm/min
G80 ; Cancel cycle
M30 ; Program end
- Use G84 (right-hand thread tapping cycle) or G74 (left-hand thread tapping cycle) to define the tapping process. Example code:
- Key point: The feed rate must strictly match the spindle speed and pitch; otherwise, thread disorder will occur.
- Execute machining
- After starting the program, the machine automatically completes: rapid movement to the thread starting point → spindle rotation and descent to the R point → tapping to the depth at the set feed rate → spindle reversal and withdrawal → return to the safe height.
What are the steps for thread milling?
Taking external thread machining as an example, the steps for thread milling are as follows:
- Select tools and parameters
- Choose a thread mill (with a diameter slightly smaller than the thread minor diameter; e.g., a φ8mm mill can be used for an M10 external thread) and set the cutting speed (e.g., 100-150m/min, adjusted according to the material).
- Core programming logic
G54 G90 G21 ; Coordinate system setting
T2 M6 ; Call thread mill
S3000 M3 ; Spindle speed 3000 RPM
G00 X0 Y0 Z5 ; Move to above the workpiece center
G01 Z-2 F200 ; Lower the tool to the thread starting depth
G02 X5 Y0 I5 J0 F300 ; The tool makes circular motion along the thread pitch diameter (radius 5mm), and the Z-axis feeds 1mm (pitch) per circle at the same time
G01 Z-22 ; Axial feed to the full thread length (20mm)
G00 Z5 ; Retract the tool
M30 ; Program end
- Thread milling requires controlling three movements: the circular motion of the tool around the workpiece (radius = thread pitch diameter / 2), axial feed (moving one pitch per circle), and tool rotation for cutting.
- Example code (simplified version):
- Key point: Ensure accurate thread lead through the linkage of circular interpolation (G02/G03) and axial feed.
Key precautions for thread machining
- Avoid thread disorder: For tapping with a tap, the feed rate and speed must strictly match the pitch; for thread milling, the radius of the circular motion must be exactly equal to half of the thread pitch diameter.
- Cooling and lubrication: Cutting fluid (e.g., emulsion) must be used when machining steel workpieces; kerosene can be used for aluminum alloys to prevent tool overheating and wear.
- Thread inspection: After machining, check with thread gauges (go/no-go gauges) or use a coordinate measuring machine to detect parameters such as pitch diameter and thread angle.
- Tool clamping: Taps need to use floating tool holders to reduce tap breakage caused by machine axial errors; thread mills need to be rigidly clamped to ensure the accuracy of circular motion.
How to choose between the two machining methods?
- For small-diameter, mass-produced internal threads → prioritize tapping with a tap (high efficiency);
- For large-diameter, precision, or hard-material threads → prioritize thread milling (high accuracy and low tool cost).
Summary
CNC thread machining realizes automated thread machining by programming to control the linkage of tool rotation and feed. Tapping with a tap is suitable for small to medium-diameter threads and is easy to operate; thread milling is suitable for large-diameter or precision threads and has higher flexibility. Mastering the programming logic and parameter settings of both is the key to ensuring thread quality.
