Heavy machinery and steam turbine cylinders need high-strength connections in harsh conditions, typically using M95 or 3″–6″ threads.
The high accuracy requirement of the internal thread makes its machining challenging.
Tapping often causes poor chip removal, leading to low thread quality, inaccuracy, and defects like messy or missing teeth.
Therefore, now the main use of rotary milling processing of large diameter internal threads.
Cyclone milling principle, characteristics and process
Basic principle
As shown in Figure 1, the rotary milling movement is divided into the main movement and feed movement.
The main movement is spindle 2 rotating around axis II via universal joint shaft 4;
feeding includes circular (eccentric seat 3 rotating around axis I) and axial motions.
The axial feeding motion is the up and down motion of the transmission box 5 driven by the screw and nut.
The machine’s transmission mechanism ensures the ratio between circumferential and axial feeding motions.
The circumferential feeding motion rotates for one week, and the axial feeding motion moves one pitch of the thread to be machined.
Fig. 2 shows the cutter plate rotating on two axes, with circular feeding around axis I and slow axial feeding, forming precise helical motion.
Cyclone milling method processing thread characteristics
Adjusting the eccentric seat’s eccentricity with different block gauges allows machining threads of varying diameters.
By changing the transmission ratio of the drive mechanism or replacing the drive screw, threads of different pitches can be processed.
High processing efficiency, rotary milling thread processing using high-speed cutting, much more efficient than tap thread processing.
Low surface roughness and good surface quality of thread.
High-speed milling at 210 m/min improves thread surface quality, achieving roughness up to Ra = 1.6µm.
The milling cutter disc inserts are mounted so that their front faces are offset from the disc center by a distance h.
The milling cutter disc inserts are mounted on the cutter disc.
This structure has the following advantages.
- Forming a front angle, reducing the main cutting force;
- The formation of positive edge inclination, which is conducive to chip removal, reducing radial cutting force, reducing the letting knife (Figure 3, Figure 4)
Cyclone milling process
Clamp the workpiece with a threaded bottom hole on the machine table and align it.
Install the rotary milling attachment, then mount the milling head and block gauge to adjust the eccentric seat for the correct thread diameter (see Fig. 4).
Load the prepared machining program with the required thread parameters, then start the machine to complete the process.
Finally, the machine is started and finished.
Machining points of cyclone milling
Calculation of block gauge
Em-block gauge length; D-nominal diameter of internal thread; P-pitch.
H/4-0.2165xP (when the tooth angle is 60°) or 0.32016xP (when the tooth angle is 55°).
Sk – diameter of the cutting circle of the rotary milling cutter body (theoretical tip diameter).
TD2– Tolerance of female thread center diameter; 0.0349 – 2xtan (1°);
L2:- Length of the rotary milling cutter body to the tip of the cutting circle;
L4– Length of the flange face of the extension body;
Calculation of block gauge dimensions for machining a M140x4 internally threaded hole as an example.
D=140,P=4,Sk=112,L2 +L4=450,TD,=0.335,H/4=0.2165×4=0.866
For large thread volumes, process in multiple passes by starting with a larger block gauge and gradually reducing its size until the final dimension is reached.
CNC programming
Processing thread pitch, screw hole depth, the direction of rotation of the thread and other technologies
Requirements need to ensure that the processing program, the following SIEMENS 840D system as an example, explain the processing program.
Subroutine (Fig. 6)
L1:
N1 M41 M57 (speed range, release the Z-axis clamping)
N2 SPOS = 110 (spindle positioning direction)
N3 C00 G91 X = -R01 (from the center of the hole removed from the size of the A)
N4 C90 Z = R03 M33 M00 (into the hole, connected to the rotary milling spindle)
N5 G91 G01 X = R1 F60 (move to the center of the hole, feed)
N6 S = R07 M04 (machine spindle on)
N7 G90 G33 Z=R02 K=R09 (cyclone milling threads from the bottom of the hole)
N8 G00 Z=R10 M35 M05 (return to plane, spindle stop)
N9 M17 (end of subroutine)
Main program:
N10 G54 G60 (determine the programmable zero point)
N10 G17 (determine the machining plane)
N20 G00 xY-(center of the first hole)
N30 L1 (call subroutine for machining the first hole thread)
N40 X…Y…(center of the second hole)
N50 L1 (call subroutine for machining the second hole thread)
N60 X…Y…(center of the third hole)
N70 L1(call subroutine to process the third hole thread)
N80 M2(end of main program)
Conclusion
The previous section covered rotary milling block selection and CNC programming.
Due to machine accuracy, tool wear, and measurement errors, the calculated block gauge size may be off.
A trial cut should be done first to correct any errors before actual production.
Thread direction (left or right) can be controlled by adjusting spindle rotation in the program.
In short, proper selection of rotary milling elements enables efficient machining of large-diameter internal threads, as proven in production.
