How to select milling tools when machining threaded holes?

Thread is the most common method of connecting mechanical parts, and thread hole processing is often at the end of the whole production process. Once the processing is unqualified, it will lead to the scrapping of components or more troublesome reprocessing. Therefore, it puts forward higher requirements for the safety of the process. There are all kinds of cutting tools for machining threaded holes. The common ones are thread turning tool, tap, extrusion tap, thread milling cutter and so on. How to select the correct machining tool? The choice of cutting tools is actually the choice of machining methods. The cutting tools used in each machining method are different. For the processing of threaded holes, there are several common ways: tapping, turning, extrusion forming and thread milling. Let’s first understand the advantages, disadvantages and use limitations of various process methods. In actual production, we can analyze which tool should be used for processing from the perspective of technology and economy according to the characteristics of these processing methods.

1. Tapping teeth

Tapping is a widely used method in thread hole machining. It can determine the forming of the thread with the help of the geometry of the tool, so there is no need for a special machine tool during processing. It can be used on ordinary machine tools, special machines for production lines and machining centers. The tapping process is that the tap first rotates forward for cutting, reverses when it reaches the bottom of the thread, leaves the workpiece, cuts in a very narrow space and discharges the chips. For different processing conditions and different processing materials, the types of taps selected are also different. Tap tapping is often used in small diameter and mass production.

2. Turning

Turning thread is turning with indexable blade. For triangular thread commonly used in production, the shape of the cutting part of the thread turning tool should be consistent with the axial section of the thread. When turning, the turning tool must move a lead longitudinally (single head thread, lead = pitch) every time the workpiece rotates, so as to process the correct thread. There are three common methods for turning triangular thread:

A. Straight thread. When turning the thread, after the trial cutting and checking that the workpiece and pitch meet the requirements, feed radially perpendicular to the workpiece axis and repeat for many times until the thread is turned. This turning method is more accurate in tooth shape. Because the two edges of the turning tool cut at the same time and the chip removal is not smooth, the force is large, the turning tool is easy to wear, and the chip will scratch the thread surface.

B. Oblique threading method. When the screw pitch of the workpiece is greater than 3mm, the oblique feed method is generally used to machine the thread. The oblique feed method is that the turning tool feeds radially along the tooth side of the thread and makes axial feed at the same time. After many times of tool walking, the thread is processed. Finally, the straight feed method is used to eat the tool to ensure the accuracy of the tooth angle of the thread.

C. Left and right cutting method. In an ordinary lathe, this method uses the scale of the horizontal carriage to control the vertical feed of the thread turning tool, and the scale of the small carriage to control the micro feed on the left and right of the turning tool. When the thread is close to cutting, the nut or thread gauge shall be used to check whether the thread size and machining accuracy are qualified. This method is easy to operate, so it is widely used.

Turning thread is generally used for holes with large diameter, and the workpiece can be firmly clamped on the lathe for rotary machining.

3. Extrusion processing

Extrusion processing belongs to chip free processing. The processing process is the same as tapping. The extrusion tap is screwed into the pre drilled hole to extrude the material in the axial and radial directions, so as to form a unique tooth thread profile. Thread extrusion is suitable for materials with good plastic deformation, and the material range is relatively small. Generally, the fracture elongation of the material is required to be greater than 7%, and the maximum tensile strength is less than 1300n / mm. It is most widely used in aluminum alloy processing.

4. Thread milling

The process of thread milling is shown in the figure below. Generally, the thread milling cutter descends to the bottom of the threaded hole, approaches the workpiece by means of screw interpolation, rotates 360 degrees along the threaded hole, rises a pitch in the z-axis direction, and then leaves the workpiece. The torque of thread milling cutter is small, which increases the safety of the process. It also has a wide range of applicability, can process a variety of different materials, and in the case of the same pitch, the use of a tool can process threads with various thread diameters or tolerance ranges. The disadvantages are: the machine tool is required to be a three coordinate CNC machine tool. In addition, compared with the tap, its processing efficiency is relatively low and the tool cost is relatively high, so it is suitable for the processing of large-diameter threaded holes in small batch production.

We just mentioned that tap is the most widely used tool for machining small-diameter threaded holes, and tapping is a more complex machining process, so there are more problems in the machining process. Common problems of tap include fracture, broken edge, wear, etc. The fracture is mainly along the whole cross-section of the tap. The performance of edge collapse is that the cutting edge collapses, and the wear of the tap refers to that a part of the cutting edge of the tap and die is worn off after a short service time, making the tooth size smaller and unusable. The taps that fail in these three ways are far from reaching the normal service life. After these problems appear in tap machining, we can focus on the following aspects for analysis.

1. Machine tool problem

Check whether the machine tool operates normally, whether the runout of the spindle is too large, whether the spindle of the machine tool is coaxial with the bottom hole, whether the processing program is correct, etc.

2. Workpiece material

Check whether the material strength of the workpiece is too high, whether the material quality is stable, and whether there are pores, residues, etc.

3. Diameter and depth of threaded bottom hole

Check whether the diameter of the thread bottom hole is correct. If the diameter of the bottom hole is too small, the root of the tap contacts the workpiece during cutting, which is easy to cause the tap to break. The diameter of the threaded bottom hole is marked in the tap sample, or the bottom hole diameter can also be obtained by using the formula (bottom hole diameter = thread diameter – pitch). For extrusion tap, the diameter of thread bottom hole is different from that of cutting tap. The approximate bottom hole diameter can also be calculated according to the formula (bottom hole diameter = thread diameter – pitch / 2).

For blind holes, the depth of bottom hole shall also be considered. Because there are several cutting teeth at the front end of the tap, the diameter of these cutting teeth is relatively small and can not be regarded as an effective thread. Therefore, the depth of the bottom hole should also consider the depth of the cutting teeth and the size of the sharp corner at the front end of the tap. In production, there have also been instances where the bottom hole is not deep enough and the front end of the tap touches the bottom of the hole, causing the tap to break.

4. Is the correct tap type selected

As mentioned earlier, for different processing conditions and different processing materials, the types of taps selected are also different. Firstly, for the two different machining conditions of through hole and blind hole, the types of cutting taps are different. For materials with long chips, such as steel, in the case of through hole, select straight groove tap, and the chips will be discharged downward; in the case of blind hole, select Spiral Tap, and the chips will be discharged upward. For short chip materials, such as cast iron, iron chips are chips and can be contained in the chip removal groove. Therefore, both through holes and blind holes can be processed with straight groove tap. In another case, the downward chips formed by the left-hand tap are separated. This tap is suitable for occasions where the workpiece is close to the tooling and there is not enough space for chip removal. mash welder

In production, we often see that spiral groove tap is used in the processing of through hole, which is an incorrect method. There are three reasons: first, the spiral groove tap discharges chips upward. In order to achieve this effect, the structure of the tap itself is relatively complex, the rigidity is not good, and the chip transmission stroke is long. It is easy to get stuck in the spiral groove transmission process, causing edge collapse or fracture. Second, the number of cutting teeth in front of the two taps is not the same. Spiral groove taps generally have 2-3 cutting teeth, while straight groove taps have 3-5 cutting teeth. The service life of tap is directly proportional to the number of cutting teeth. Third, the price of spiral groove tap is more expensive than that of straight groove tap, and the economy is not good.

On the other hand, for cutting taps and processing different materials, we should choose taps with different groove types. There are various angles on the tap, such as front angle, rear angle, guide angle, edge inclination angle, etc. the design of these angles is based on the characteristics of different materials. For example, for steel and cast iron, the front angle of the tap is designed to be larger due to the long chip of steel, while the chip of cast iron is generally debris, and the front angle is relatively small, even 0 ° front angle. The tool company will give different recommended taps for different workpiece materials. For taps of common materials such as steel parts, aluminum alloy, cast iron and stainless steel, different color circles may be used to distinguish the handle.

5. Cutting parameters

Cutting parameters are very important. Different parameters should be selected for different types of taps, different processing conditions and different workpiece materials. For example, under the same conditions, the offline speed of high-speed steel wire tap and cemented carbide tap is very different. This speed has a certain range. The linear speed of high-speed steel wire tap is generally within 20m / min (the feed of tap is fixed, that is, pitch). Too fast or too slow will lead to tap failure. Selecting appropriate cutting parameters can ensure production efficiency and achieve relatively high tool life at the same time.

6. Cooling and lubrication

As we mentioned earlier, the tap cuts in a very narrow space and discharges the chips. A lot of heat will be generated in the machining process. Therefore, cooling and lubrication are very important. For materials with high toughness, the concentration of coolant can be increased or oily coolant can be used.