CNC Machining: The Method of Precision CNC Machining to Improve the Quality of Thread Machining

Threading is one of the important applications of CNC machining centers. The quality and efficiency of thread machiningdirectly affect the quality of precision CNC machined parts and the production efficiency of machining centers.

With the improvement of CNC machining center performance and tool performance, threading methods are constantly improving, and the precision and efficiency of threading are also gradually improving. In order to enable technicians to reasonably select thread machiningmethods in the process of precision CNC machining, improve the production efficiency of precision CNC machining, and avoid quality accidents, several thread machining methods commonly used in CNC machining centers are summarized:

Tap machining

Classification and characteristics of CNC machining of faucets

Tapping is the most commonly used method for machining threaded holes by CNC, and it is mainly suitable for threaded holes with small diameter (d<30) and low hole position accuracy.

The flexible tapping chuck has complex structure, high cost, easy damage, and low CNC machining 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 threading.

CNC machining

The tap is clamped with a rigid spring chuck, allowing the spindle to enter the spindle, and the rotational speed is controlled by the machine tool for consistency.

Compared with the flexible tapping chuck, the spring chuck has the advantages of simple structure, low price and wide application range. In addition to clamping taps, it can also clamp end mills, drills and other tools, which can reduce the cost of cutting tools. At the same time, the use of rigid tapping can achieve high-speed cutting, improve the efficiency of the machining center, and reduce the cost of manufacturing precision CNC machining.

Influence of technological factors on the quality of precision CNC machining

Thread machiningfactors are the basic factors that affect the quality of threads. Mastering the factors of thread machiningtechnology is of great significance to improve the quality of thread processing.
 

Proper selection of tool material and tool geometry

During the cutting process, the tool is subjected to high pressure, high temperature and high friction. Therefore, the tool must have high hardness, high heat resistance, good wear resistance and sufficient toughness. Strong and tough materials have poor hardness and wear resistance, and vice versa. CNC turning tools are commonly used high-speed steel, carbide, ceramics and superhard materials. Choosing the appropriate tool according to the material of the workpiece can improve the quality of precision CNC machining parts, improve the production efficiency of CNC machining, and reduce costs.

For threading tools, in addition to selecting the appropriate material, the reasonable selection of the tool geometry angle and the shape of the tip transition edge is also very important to improve the quality of precision CNC machined parts. The geometric angles of turning tools include main declination angle, auxiliary declination angle, cutter head angle, blade inclination angle, rake angle, convex angle and auxiliary convex angle.

The main angle of the thread turning tool is the angle of the tool head. The size of the tool head angle directly determines the thread profile. For a general triangular thread turning tool, the blade angle is generally 10°, and the tip angle is 59°16'. The calculation of the tool nose radius formula r=0.144 P (where P is the pitch).

The slope angle has the greatest influence on the surface roughness of precision CNC machined parts. The smaller the first and second declination angles, the lower the surface quality of the CNC machined thread, and the larger the tool nose radius. The larger the rake and relief angles of the tool, the sharper the tool, the higher the thread surface quality and the lower the strength.

Thread surface roughness

(1) Each tooth of the tap is too thick. Excessive thickness of each tooth will affect the surface quality of the machined thread. Usually, the thickness of each tooth is too thick if it exceeds 0.2mm. If it is to process through-hole threads, the length of the cutting cone can be appropriately increased to allow more cutter teeth to participate in cutting. The adjustment of the cutting cone length for machiningblind hole threads is limited, and often only measures can be taken to increase the number of grooves to make more cutters.

The teeth participate in cutting. Practice has proved that the M24~M42 taps are very effective to increase from 4 grooves to 6 grooves, and the number of grooves needs to be appropriately increased compared to M42 taps. By adjusting the length of the taper and appropriately increasing the number of grooves, these two measures control the cutting thickness of each tooth within 0.02-0.2mm to ensure the thread surface quality of the precision CNC machined parts that meet the requirements.

(2) The tap is broken. In the thread tapping process, the occurrence of tap chipping is a relatively common phenomenon. It not only affects the surface quality and thread accuracy of the internal thread of precision CNC machined parts, but can even cause the tap to be twisted and broken, so it is very important to solve the problem of chipping. The reasons for tooth chipping and the technical measures that should be taken are analyzed below.

Thread root deformation

The deformation of the thread bottom is entirely caused by the wear of the tip of the tap. This phenomenon occurs on the large-sized fine teeth, steel parts and blind hole threads. The preventive measures are as follows:

(1) Under the premise of ensuring the precision requirements of the thread of the precision CNC machined parts and the sufficient use limit, the outer diameter of the tap is appropriately reduced to achieve the purpose of increasing the strength without the tip of the tooth being too sharp.

(2) Implement backing on the outer diameter of the tap, and the backing amount can be roughly the same as that of the middle diameter, so as to reduce the area where the tooth top is worn.

(3) Appropriately increase the number of tap grooves, reduce the chip load per tooth, reduce the surface roughness value of the thread surface of precision CNC machined parts, and reduce the friction force on the tooth tip. For example: the tap used for 2 blind holes of M33×1.5 has been re-selected in terms of structural parameters.

The original design outer diameter is 33.102-33.052mm, the outer diameter has no shovel back, 4 grooves, and the blade width is 9mm. : The outer diameter is 33.10～33.03mm, the shovel width along the outer diameter is 0.02mm, 6 grooves, and the blade width is 6mm. After several years of use, the quality of the thread of the precision CNC machined parts is guaranteed. The phenomenon of tooth tip overwelding damage is eliminated, and the service life of the tap is increased by more than 4 times.