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How to select milling tools for machining threaded holes?

October 19, 2022

The thread is the most common method for connecting mechanical parts, and the processing of threaded holes is often at the end of the entire production process. Once the processing is unqualified, it will lead to the scrapping of components or more troublesome reprocessing, so it puts forward higher requirements for the safety of the process. There are various tools for machining threaded holes, including thread turning tools, taps, extrusion taps, thread milling cutters, etc. How to select the correct machining tool? The selection of tools is actually the selection of processing methods. Each processing method uses different tools. For the processing of threaded holes, there are several common ways: tapping, turning, extrusion molding, and thread milling. Now let's first understand the advantages and disadvantages of various processing methods and the use restrictions. In actual production, we can analyze which tool to use from the technical and economic perspective according to the characteristics of these processing methods.

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1. Tooth tapping
Tapping is a widely used method in the processing of threaded holes. It can determine the forming of thread with the aid of the geometric shape of the cutter, so no special machine tool is required for processing, and it can be used on ordinary machine tools, production line special machines and machining centers. The tapping process is that the tap rotates forward to cut, 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 refers to turning with indexable inserts. For triangular threads commonly used in production, the shape of the cutting part of the thread turning tool should conform to the axial section of the thread. When turning, the turning tool must move a lead (single head thread, lead=pitch) longitudinally for each rotation of the workpiece to process the correct thread. There are three common methods for turning triangular threads:

A. Thread turning with straight method. When turning the thread, after the test cutting check that the workpiece and the thread pitch meet the requirements, the radial direction is perpendicular to the workpiece axis and the feed is repeated for many times until the thread is properly turned. The tooth profile of this turning method is more accurate. Because the two edges of the turning tool cut at the same time and chip removal is not smooth, the turning tool is easy to wear due to large force, and the chip will scratch the thread surface.
B. Thread turning by oblique method. When the thread pitch of the workpiece is greater than 3MM, the oblique method is generally used to turn the thread. The oblique method is that the turning tool feeds along the thread profile side in the radial direction while making the axial feed. After several times of cutting, the thread is processed. Finally, the straight method is used to eat the tool to ensure the accuracy of the thread profile angle.
C. Left and right knife entry 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 uses the scale of the small carriage to control the left and right micro feed 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 processing accuracy are qualified. This method is easy to operate, so it is widely used.

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

3. Extrusion processing
Extrusion processing belongs to chip free processing. The processing process is the same as tapping, the extruding tap is screwed into the pre drilling hole, and the material is extruded in the axial and radial direction, thus forming a unique tooth shaped thread profile. Thread extrusion is applicable to materials with good plastic deformation. The range of materials is relatively small. Generally, the fracture elongation of materials is required to be greater than 7%, and the maximum tensile strength is less than 1300N/MM. It is widely used in aluminum alloy processing.

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4. Thread milling
The process of thread milling is as shown in the figure below. The thread milling cutter generally descends to the bottom of the threaded hole, approaches the workpiece by means of spiral interpolation, rotates 360 degrees along the threaded hole, rises a pitch in the Z 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, and can process various materials. In the case of the same pitch, a tool can be used to process threads with various thread diameters or tolerance ranges. Disadvantages: 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 processing large diameter threaded holes in small batch production.
As we mentioned just now, the tap is the most widely used tool for machining small diameter threaded holes, and tapping is a relatively complex processing process, so there are many problems encountered in the processing process. Common problems of tap include fracture, chipping, wear, etc. The fracture is mainly along the whole cross section of the tap. The appearance of edge chipping is that the cutting edge is chipped off. The wear of the tap refers to that the cutting edge of the tap and die is worn off when the tap and die have not been used for a long time, making the tooth size smaller and unusable. The taps that fail in these three ways are far from reaching their normal service life. After these problems occur in tap processing, we can focus on the following aspects for analysis.

1. Machine tool problems
Check whether the machine tool operates normally, whether the spindle runout is too large, whether the machine tool spindle is coaxial with the bottom hole, and whether the processing program is correct.
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 will contact the workpiece during cutting, which is easy to cause the tap to break. The diameter of the thread bottom hole is marked in the tap sample, or the formula (bottom hole diameter=thread diameter - thread pitch) can be used to obtain the bottom hole diameter. For extrusion taps, the diameter of the thread bottom hole is different from that of the cutting tap. The approximate bottom hole diameter can also be calculated according to the formula (bottom hole diameter=thread diameter - thread pitch/2).
For blind holes, the depth of the bottom hole should also be considered. Since there are several cutting teeth at the front end of the tap, and the diameter of these cutting teeth is relatively small, they cannot be considered as effective threads, so 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 are also cases 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. Whether the correct tap type is selected
As mentioned earlier, for different processing conditions and different processing materials, the types of taps selected are also different. First of all, for the two different processing conditions of through hole and blind hole, the types of cutting taps selected are different. For materials with long chips, such as steel, in the case of through holes, select a straight slot tap to discharge chips downward, and in the case of blind holes, select a spiral tap to discharge chips upward. For short chip materials, such as cast iron, iron chips are chips that can be contained in the chip removal slot, so through holes and blind holes can be processed with straight slot taps. In another case, the chips formed by the left hand tap are separated. This tap is suitable for situations where the workpiece is close to the tooling and the chip removal space is insufficient. mash welder

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In production, we often see that it is an incorrect method to use the spiral groove tap in the processing of through holes. 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 complex, the rigidity is not good, and the chip transmission stroke is long, so it is easy to get stuck during the spiral groove transmission process, causing blade breakage or fracture. Second, the number of cutting teeth in front of the two taps is different. The spiral groove tap generally has 2-3 cutting teeth, while the straight groove tap has 3-5 cutting teeth. The life of the tap is proportional to the number of cutting teeth. Third, the spiral groove tap is more expensive than the straight groove tap, which is not economical.
On the other hand, for cutting taps, we should choose taps with different grooves for processing different materials. There are various angles on the tap, such as front angle, rear angle, guide angle, blade inclination, etc. The design of these angles is based on the characteristics of different materials. For example, for steel parts and cast iron, the front angle of the tap is larger because the chips of steel parts are longer, while the iron chips of cast iron are generally chips, and the front angle is smaller, even 0 ° front angle. The tool company will give different recommended taps for different workpiece materials. For taps processing steel, aluminum alloy, cast iron, stainless steel and other common materials, different color rings may be used to distinguish the handle.

5. Cutting parameters
Cutting parameters are very important. Different types of taps, different processing conditions, and different workpiece materials should choose different parameters. For example, under the same conditions, the linear speeds of high-speed wire taps and cemented carbide taps differ greatly. This speed has a certain range. The linear speed of high-speed wire taps is generally within 20M/MIN (the feed of the tap is fixed, that is, the 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.

6. Cooling and lubrication
As we mentioned earlier, the tap is used to cut in a very narrow space and discharge the chips, which will generate a lot of heat during the processing. 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.