Tool selection in internal thread processing

1. Machine tool problems
Check whether the machine tool is operating normally, whether the spindle jumps too much, whether the machine spindle and the bottom hole are coaxial, whether the processing program is correct, and so on.
2. Workpiece material
Check whether the material strength of the workpiece is too high, whether the material quality is stable, whether there are pores, residual ballast, etc.
3. Threaded bottom hole diameter and hole depth
Check whether the diameter of the threaded bottom hole is correct. If the bottom hole diameter is too small, the root of the tap will contact the workpiece during cutting, which may easily cause the tap to break. The diameter of the threaded bottom hole is marked in the tap sample, or the formula (bottom hole diameter = thread diameter-pitch) can be used to obtain the bottom hole diameter. For extrusion taps, the diameter of the threaded bottom hole is different from that of the cutting tap. You can also calculate the approximate bottom hole diameter according to the formula (bottom hole diameter = thread diameter-pitch / 2).
For blind holes, the depth of the bottom hole also needs to be considered. Since the front end of the tap has several cutting teeth, the diameter of these cutting teeth is relatively small and cannot be regarded as an effective thread. Therefore, the depth of the bottom hole must also consider the depth of the cutting teeth and the size of the sharp corner of the front end of the tap. In production, there have also been instances where the bottom hole is not deep enough, and the tip of the tap hits the bottom of the hole, causing the tap to break.
4. Is the correct tap type selected?
As mentioned earlier, for different processing conditions, 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 a through hole, select a straight flute tap, and the chips are discharged downward, and in the case of a blind hole, select a spiral tap and the chips are discharged upward. For short chip materials, such as cast iron, iron chips are chips that can be contained in the chip flute, so both through holes and blind holes can be processed with straight flute taps. In another case, the downward chips formed by the left-hand tap are separated. This type of tap is suitable for situations where the workpiece and the tooling are relatively close, and the chip removal space is insufficient.
In production, we often see that spiral groove taps are used in the processing of through holes, which is an incorrect method of use. There are three reasons: First, spiral flute taps are upward chip removal. In order to achieve this effect, the structure of the tap itself is more complicated, the rigidity is not good, and the chip transmission stroke is long, and it is easy to jam during the spiral flute transmission. Circumstances, causing chipping or breaking. Second, the number of cutting teeth in front of the two types of taps is not the same. Spiral flute taps generally have 2-3 cutting teeth, while straight flute taps have 3-5 cutting teeth. The life of the tap is proportional to the number of cutting teeth. Third, spiral fluted taps are more expensive than straight fluted taps, which is not economical.
On the other hand, for cutting taps and processing different materials, we should choose taps with different geometries. There are various angles on the tap, such as rake angle, relief angle, guide angle, blade inclination, etc. These angles are designed according to the characteristics of different materials. For example, steel and cast iron, due to the relatively long chipping of steel, The rake angle of the tap is designed to be larger, while the iron filings of cast iron are generally debris, and the rake angle is relatively small, even 0° rake angle (see the figure below). Tool companies will give different recommended taps for different workpiece materials. For taps of common materials such as steel, aluminum alloy, cast iron, and stainless steel, they may also be distinguished by different color circles on 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, the linear speed of high-speed steel taper and cemented carbide tap is very different under the same conditions. This speed has a certain range. The linear speed of high-speed steel taper is generally within 20m/min (the feed of the tap is fixed, that is Pitch), too fast or too slow will cause the tap to fail. Choosing appropriate cutting parameters can achieve a relatively high tool life while ensuring production efficiency.
6. Cooling and lubrication
As we mentioned earlier, the tap is cut in a very narrow space and discharges the chips. A lot of heat is generated during the machining process. Therefore, cooling and lubrication are very important. For materials with greater toughness, the concentration of the coolant can be increased, or oil-based coolant can be used.