Several thread processing methods commonly used in CNC machining center

Thread processing is one of the most important applications in CNC machining center. The quality and efficiency of thread processing will directly affect the machining quality of parts and the production efficiency of machining center.

With the improvement of CNC machining center performance and cutting tool, the thread processing method is also improving, and the precision and efficiency of thread processing are also gradually improved. In order to make the technicians choose the thread processing method reasonably, improve the production efficiency and avoid quality accidents, the following several thread processing methods commonly used in CNC machining center are summarized as follows:

Tap processing

Classification and characteristics of tap processing

The thread hole processing with tap is the most common method, which is mainly suitable for the screw holes with small diameter (d < 30) and low accuracy of hole position.
In the 1980s, the screw holes were all made of flexible tapping method, that is, the flexible tapping clamp was used to hold the tap, and the tapping chuck could be used to compensate the feed error caused by the asynchronous speed of the axial feed and the spindle, and ensure the correct pitch. The flexible tapping chuck has complex structure, high cost, easy to damage and low processing efficiency. In recent years, the performance of CNC machining center has been gradually improved, and the rigid tapping function has become the basic configuration of CNC machining center.
Therefore, rigid tapping is the main method of thread processing.
That is, the screw tap is clamped with the rigid spring collet, and the spindle feed speed is consistent with the spindle speed controlled by the machine tool.
Spring chuck is simple in structure, cheap in price and widely used in comparison with flexible tapping chuck. Besides holding tap, it can also hold up cutter such as end milling cutter and drill bit, which can reduce the cost of cutting tool. At the same time, the rigid tapping can be used for high-speed cutting, improving the efficiency of machining center and reducing the manufacturing cost.

Determination of bottom hole of thread before tapping

The processing of the bottom hole of thread has a great influence on the life of tap and the quality of thread processing. In general, the diameter of the thread bottom hole bit is selected to be close to the upper limit of the tolerance of the diameter of the bottom hole.
For example, the bottom hole diameter of the M8 threaded hole is Ф 6.7 + 0.27mm, the diameter of the selected bit is Ф 6.9mm。 In this way, the machining allowance of tap can be reduced, the load of tap can be reduced and the service life of tap can be improved.

Selection of tap

When selecting tap, first, the corresponding tap must be selected according to the processed material. The cutter company shall produce different types of taps according to the different processing materials. Special attention shall be paid to the selection.
Because the tap is very sensitive to the processed material compared with the milling cutter and boring tool. For example, the processing of aluminum parts with tap iron can easily cause thread dropping, thread breaking or even tap breaking, which will lead to the scrap of the workpiece. Secondly, the difference between the through tap and blind tap should be noticed. The leading end of the through hole tap is long, and the chip is the front chip. The front end of blind hole is short, and the chip is the rear chip. Blind hole is processed by through hole tap, which can not guarantee the thread processing depth. Moreover, if flexible tapping chuck is used, attention should be paid to the diameter of the tap handle and the width of the square, which should be the same as that of the tapping chuck; The diameter of the tap handle for rigid tapping shall be the same as that of spring jacket. In a word, only reasonable selection of tap can ensure the smooth processing.

NC programming of tap machining

The programming of tap processing is simple. Now the machining center generally solidified tapping subprogram, just need to assign the parameters. But it should be noted that the different NC system and the format of subprogram are different, and the meaning of some parameters is different.
For example, Siemens 840C control system, which is programmed in the form of g84 x_ Y_ R2_  R3_ R4_ R5_ R6_ R7_ R8_ R9_ R10_ R13_。 Only these 12 parameters need to be assigned to the program.

Thread milling

Characteristics of thread milling

Thread milling is a milling method which uses thread milling tool, three axes linkage of machining center, namely X and Y axis arc interpolation, and z-axis straight feed.
Thread milling is mainly used for the processing of large hole thread and hard to process material. It 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 tool speed is fast. The manufacturing precision of the tool is high, so the thread precision of milling is high.
  • (2) the milling tool has a wide range of application. As long as the pitch is the same, whether it is left-hand thread or right screw thread, a tool can be used, which is conducive to reducing the tool cost.
  • (3) milling is easy to chip removal and cooling. Compared with tap, it is better to cut, especially for thread processing of hard to process materials such as aluminum, copper and stainless steel, especially for thread processing of large parts and precious materials, which can ensure the thread processing quality and safety of workpiece.
  • (4) because there is no tool front-end guide, it is suitable for machining blind holes with short bottom holes of thread and holes without tool receding groove.

Classification of thread milling tools

The thread milling tools can be divided into two types: one is machine clamp type carbide cutter, the other is integral carbide milling cutter. The machine clamp tool has a wide range of applications, which can process holes with thread depth less than the length of blade, and also holes with thread depth greater than blade length. The integral carbide milling cutter is generally used to process holes with thread depth less than the tool length.

NC programming of thread milling

The programming of thread milling tool is different from other tools. If the programming of machining program is wrong, it is easy to cause tool damage or thread processing error. The following points should be noted in the preparation:

  • (1) the bottom hole of thread shall be processed well first, and the small diameter hole shall be processed with drill bit, and for larger holes, boring shall be used to ensure the accuracy of the bottom hole of thread.
  • (2) when cutting the tool, the circular arc track shall be adopted, usually 1/2 turn for cutting or cutting out, and the z-axis direction shall travel 1/2 pitch to ensure the thread shape. The tool radius compensation value should be brought in at this time.
  • (3) the X and y-axis arc shall be inserted for one week, and the main shaft shall travel a pitch along the z-axis direction, otherwise, the thread will be disordered.
  • (4) specific example program: diameter of thread milling cutter is Φ 16. Screw hole is M48 × 1.5, the depth of the threaded hole is 14.

The processing procedure is as follows:

(the thread bottom hole procedure is omitted, and the hole shall be bored)

  • G0 G90 G54 X0 Y0;
  • G0 Z10 M3 S1400 M8;
  • G0 z-14.75 feed to the deepest thread;
  • G01 G41 x-16 Y0 F2000 move to feed position and add radius compensation;
  • G03 x24 Y0 z-14 I20 J0 f500 cut in with 1 / 2 circle arc;
  • G03 x24 Y0 Z0 I-24 J0 F400 cutting the whole thread;
  • When cutting out 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 and cancel radius compensation;
  • G0 Z10.

Pick button method

Characteristics of pick button method
Sometimes, large thread holes can be found in box parts. In the case of no tap and thread milling cutter, the method similar to lathe thread picking can be adopted.
The thread turning tool is installed on the boring bar to bore the thread.
The company has processed a number of parts, the thread is m52x1.5, the position degree is 0.1mm (see Figure 1), because the position degree requirement is high, the thread hole is large, can not use the tap for processing, and there is no thread milling cutter, after the test, the use of pick button method to ensure the processing requirements.

20210423024059 33796 - Several thread processing methods commonly used in CNC machining center

Points for attention of pick button method

  • (1) 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, because the tool can’t be grinded symmetrically, reverse retraction can’t be used. The spindle orientation must be adopted, the tool moves radially, and then retraction.
  • (3) the manufacturing of tool holder must be accurate, especially the position of tool slot must be consistent. If it is inconsistent, multi tool bar machining cannot be used. Otherwise, it will cause disorderly deduction.
  • (4) even if it’s a very thin buckle, it can’t be made by one knife, otherwise it will cause tooth loss and poor surface roughness, so it should be divided into two knives at least.
  • (5) low processing efficiency, only suitable for single small batch, special pitch thread and no corresponding tool.

Specific procedures

  • N5 G90 G54 G0 X0 Y0;
  • N10 Z15;
  • N15 S100 M3 M8;
  • N20 G04 X5 delay to make the spindle reach the rated speed;
  • N25 G33 z-50 K1.5 flip button;
  • N30 M19 spindle orientation;
  • N35 G0 X-2 let go;
  • N40 G0 z15 tool retraction.

Source: China Threaded Flange Manufacturer – Yaang Pipe Industry (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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