Thread machining is one of the most important applications of CNC machining centers. The machining quality and efficiency of threads will directly affect the machining quality of parts and the production efficiency of machining centers.
With the improvement of the performance of cnc machining centers and the improvement of cutting tools, the method of threading is also constantly improving, and the accuracy and efficiency of threading are also gradually improving. In order to enable technicians to reasonably select threading methods in processing, improve production efficiency and avoid quality accidents, several threading methods commonly used in CNC machining centers in practice are summarized as follows:
1. Tap processing method
1.1 Classification and characteristics of tap processing
Using taps to process threaded holes is the most common processing method, and it is mainly suitable for threaded holes with small diameters (D<30) and low requirements for hole position accuracy.
In the 1980s, flexible tapping methods were adopted for threaded holes, that is, a flexible tapping chuck was used to hold the tap, and the tapping chuck could be used for axial compensation to compensate for the feed caused by the asynchronous feed of the machine tool and the spindle speed. Give the error to ensure the correct pitch. The flexible tapping chuck has a complex structure, high cost, easy damage and low processing efficiency. In recent years, the performance of CNC machining centers has gradually improved, and the rigid tapping function has become the basic configuration of CNC machining centers.
Therefore, rigid tapping has become the main method of thread processing at present.
That is, the tap is clamped with a rigid collet, and the spindle feed and spindle speed are controlled by the machine tool.
Compared with the flexible tapping chuck, the spring collet has a simple structure, low price, and a wide range of uses. In addition to holding taps, it can also hold tools such as end mills and drills, which can reduce tool costs. At the same time, the rigid tapping can be used for high-speed cutting, which improves the efficiency of the machining center and reduces the manufacturing cost.
1.2 Determination of threaded bottom hole before tapping
The processing of the bottom hole of the thread has a great influence on the life of the tap and the quality of the thread processing. Usually, the diameter of the threaded bottom hole drill is selected close to the upper limit of the threaded bottom hole diameter tolerance,
For example, the bottom hole diameter of the M8 threaded hole is Ф6.7+0.27mm, and the diameter of the selected drill bit is Ф6.9mm. In this way, the machining allowance of the tap can be reduced, the load of the tap can be reduced, and the service life of the tap can be improved.（CNC Machining Speaker Spikes）
1.3 Selection of Taps
When choosing a tap, first of all, the corresponding tap must be selected according to the material to be processed. The tool company produces different types of taps according to the different materials to be processed, and special attention should be paid to the selection.
Because taps are very sensitive to the material to be processed compared to milling cutters and boring tools. For example, using taps for processing cast iron to process aluminum parts is easy to cause thread loss, random buckles or even tap breakage, resulting in scrapped workpieces. Secondly, attention should be paid to the difference between through-hole taps and blind-hole taps. The front-end guide of through-hole taps is longer, and the chip removal is front chip removal. The front end of the blind hole guide is short, and the chip evacuation is rear chip evacuation. Blind holes are processed with through-hole taps, and the depth of threading cannot be guaranteed. Furthermore, if a flexible tapping chuck is used, it should also be noted that the diameter of the tap shank and the width of the square should be the same as that of the tapping chuck; the diameter of the tap shank for rigid tapping should be the same as the diameter of the spring jacket. In short, only a reasonable selection of taps can ensure the smooth progress of processing.
1.4 NC programming of tap machining
Programming for tap machining is relatively simple. Now the machining center generally solidifies the tapping subroutine, just assign each parameter value. However, it should be noted that different numerical control systems have different subprogram formats, and the meanings of some parameters are different.
For example, SIEMEN840C control system, its programming format is: G84 X_Y_R2_ R3_R4_R5_R6_R7_R8_R9_R10_R13_. You only need to assign these 12 parameters when programming.
2. Thread milling method
2.1 Features of thread milling
Thread milling is to use thread milling tool, three-axis linkage of machining center, that is, X, Y axis circular interpolation, Z axis linear feed milling method to process threads.
Thread milling is mainly used for the processing of large-hole threads and threaded holes of difficult-to-machine materials. It mainly has the following characteristics:
(1) The processing speed is fast, the efficiency is high, and the processing precision is high. The tool material is generally cemented carbide material, and the cutting speed is fast. The tool is manufactured with high precision, so the thread milling precision is high.
⑵ Milling tools have a wide range of applications. As long as the pitch is the same, whether it is a left-handed thread or a right-handed thread, one tool can be used, which is beneficial to reduce tool costs.
(3) Milling is easy for chip removal and cooling. Compared with taps, the cutting performance is better. It is especially suitable for threading of difficult-to-machine materials such as aluminum, copper, and stainless steel, especially for large parts and parts of precious materials. Guarantee the quality of thread processing and the safety of the workpiece.
⑷ Because there is no tool front guide, it is suitable for processing blind holes with short threaded bottom holes and holes without undercuts.
2.2 Classification of thread milling tools
Thread milling tools can be divided into two types, one is a machine-clamped carbide insert milling cutter, and the other is a solid carbide milling cutter. The machine-clamped tool is suitable for a wide range of applications. It can machine holes with a thread depth less than the length of the insert, or holes with a thread depth greater than the length of the insert. Solid carbide milling cutters are generally used to machine holes with a thread depth less than the tool length.（AUDIO SPIKE）
2.3 NC programming for thread milling
The programming of thread milling tools is different from the programming of other tools. If the processing program is wrong, it is easy to cause tool damage or thread processing errors. When compiling, pay attention to the following points:
⑴ First of all, the threaded bottom hole should be processed well, the small diameter hole should be processed with a drill, and the larger hole should be processed by boring to ensure the accuracy of the threaded bottom hole.
(2) When cutting in and out, the tool should use a circular arc trajectory, usually 1/2 circle for cutting in or out, and at the same time, the Z axis direction should travel 1/2 pitch to ensure the shape of the thread. The tool radius compensation value should be brought in at this time.
(3) X, Y axis arc interpolation for one cycle, the spindle should travel one pitch along the Z axis direction, otherwise, the thread will be screwed randomly.
⑷ Specific example program: the diameter of the thread milling cutter is Φ16, the threaded hole is M48×1.5, and the depth of the threaded hole is 14.
The processing program is as follows:
(The procedure for the threaded bottom hole is omitted, the hole should be a boring bottom hole)
G0 G90 G54 X0 Y0
G0 Z10 M3 S1400 M8
G0 Z-14.75 Feed to the deepest part of the thread
G01 G41 X-16 Y0 F2000 Move to the feed position and add radius compensation
G03 X24 Y0 Z-14 I20 J0 F500 Use 1/2 circle arc to cut in
G03 X24 Y0 Z0 I-24 J0 F400 Cut the entire thread
G03 X-16 Y0 Z0.75 I-20 J0 F500 Use 1/2 circle arc to cut out G01 G40 X0 Y0 Return to the center, cancel radius compensation
3. Pick-and-drop method
3.1 Characteristics of the pick-and-button method
Large threaded holes can sometimes be encountered on box-type parts. In the absence of taps and thread milling cutters, a method similar to lathe picking can be used.
Install a thread turning tool on the boring bar for thread boring.
The company once processed a batch of parts, the thread is M52x1.5, the position is 0.1mm (see Figure 1), because the position is required to be high, the threaded hole is large, the tap cannot be used for processing, and there is no thread milling cutter, after testing , the use of pick and buckle method to ensure the processing requirements.
3.2 Precautions for the pick-and-drop method
⑴ After the spindle starts, there should be a delay time to ensure that the spindle reaches the rated speed.
(2) When retracting the tool, if it is a hand-ground thread tool, since the tool cannot be sharpened symmetrically, the reverse retraction cannot be used, and the spindle must be oriented, the tool radially moves, and then retracted.
(3) The manufacture of the arbor must be accurate, especially the position of the kerf must be consistent. If they are inconsistent, multi-tool bar processing cannot be used. Otherwise, it will cause confusion.
⑷ Even if it is a very thin buckle, it should not be picked with one knife, otherwise it will cause tooth loss and poor surface roughness, at least two cuts should be made.
⑸ The processing efficiency is low, and it is only suitable for single-piece small batches, special pitch threads and no corresponding tools.
3.3 Specific example procedures
N5 G90 G54 G0 X0 Y0
N15 S100 M3 M8
N20 G04 X5 delay, make the spindle reach the rated speed
N25 G33 Z-50 K1.5 pick button
N30 M19 Spindle orientation
N35 G0 X-2 let the knife
N40 G0 Z15 Retract tool