Tap & Tapping: Characteristic, Common Problems And Solutions

Tap is a tool for processing medium and small-sized internal threads.

It has a groove along the axial direction, also known as screw tap.

The utility model has the advantages of simple structure and convenient use. It can be operated manually or on a machine tool.

It is widely used in production.

For the small-size internal thread, the thread dimension is almost the only machining tool.

Tapping is a difficult process because the tap is almost embedded in the workpiece for cutting. The processing load of each tooth is larger than that of other tools, and the tap is very large along the thread and the contact surface with the workpiece.

When cutting the thread, it must contain and remove the chips.

The tap is divided into straight groove tap, spiral groove tap and screw tip tap (leading tap) according to its shape.

(1) Characteristics of tap

1. Straight groove tap

As shown in fig. 5-82, the straight groove tap is generally used for processing carbon steel, alloy steel and non-ferrous metal.

It is characterized by the strongest versatility, through-hole or through-hole, non-ferrous metal or ferrous metal can be processed, and the price is also the cheapest.

It has good edge strength, easy grinding, large cutting torque and poor chip breaking and chip removal ability;

The cutting cone can have 2, 4 and 6 teeth.

The short cone is used for the through hole and the long cone is used for the through hole.

As long as the bottom hole is deep enough, the longer cutting cone should be selected as far as possible, so that more teeth share the cutting load and have longer service life.

straight groove tap

Fig. 5-82 straight groove tap

2. Spiral groove tap

As shown in Fig. 5-83, it is a screw tap with spiral groove, which is more suitable for processing through-hole threads, and the chips are discharged backward during processing.

Due to the helix angle, the actual cutting rake angle of the tap will increase with the increase of the helix angle.

For machining ferrous metal taps, the spiral angle shall be selected as a small value, generally about 30 ° to ensure the strength of spiral teeth;

For processing non-ferrous metal taps, the large value of helix angle can be about 45 ° and the cutting is sharper.

Its characteristic is that the screw tap with spiral groove is of spiral type.

When tapping, the screw tap can easily discharge the chips out of the hole with the rising and rotating function of its spiral groove, so as to avoid the chips remaining or blocking in the groove, resulting in the tap breaking and edge cracking.

Therefore, it can increase the service life of the tap and process the threads with the highest precision.

The cutting speed can be increased by 30% ~ 50% compared with the straight groove tap. Generally, one tap can be used to complete the processing of threaded holes.

Fig. 5-83 spiral groove tap

3. Screw tap

As shown in Fig. 5-84, it is a screw tap with good processing effect for non-ferrous metals, stainless steel and ferrous metals.

The screw tap shall be used preferentially for through-hole threads.

Its characteristic is that the core size of the screw tap is designed to be large, the strength is good, and it can withstand large cutting force.

Because the front edge groove part has a special gun bore edge groove design, it is easy to remove chips, with small torque and stable precision, making the tap more durable.

Because the screw tap has the function of rotating and discharging chips, it can not only keep the groove clean so as to reduce the resistance during cutting, but also avoid the damage of the tap caused by chip blockage.

Therefore, the screw tap can cut high-precision threads faster than the ordinary hand tap. When machining threads, the chips are discharged forward.

Fig. 5-84 screw tap

4. Extrusion tap

As shown in fig. 5-85, the extrusion tap is mainly suitable for processing high plasticity materials, such as copper, aluminum alloy, low carbon steel, lead containing steel, austenitic stainless steel and other workpieces.

When choosing to use an extrusion tap, the machinability of the material should be evaluated first, and then whether the material is suitable for processing with an extrusion tap should be determined according to the tap diameter and pitch.

For extrusion formed threads, the smaller the diameter and pitch, the wider the range of materials that can be processed, while the extrusion Taps with large diameter and pitch are only suitable for processing very soft materials.

Its characteristics can be summarized as follows:

(1) Extrusion Taps have higher strength than cutting taps.

They have the advantages of not easy to wear, long service life, low breaking rate and high production efficiency.

(2) The extrusion tap forms thread through metal flow, so it has the characteristics of high thread surface strength, smooth surface and easy to ensure machining size.

(3) Extrusion taps are self guiding and can be used on general tapping equipment.

However, because extrusion Taps require greater torque than cutting, the torque requirements for tapping equipment are greater.

The torque required for extrusion Taps is generally 1 ~ 1.5 times that of cutting taps.

(4) The cutting tool has good strength, no chip, high internal thread precision, and can tap the thread to the bottom of the blind hole;

It is suitable for processing soft materials, through holes and blind holes with good toughness and ductility.

Fig. 5-85 extrusion tap

5. Content chip tap

As shown in Fig. 5-86, the chip tap is mainly characterized by a large diameter chip holding cavity opened at the end face of the cutting part to store chips, which can greatly improve the accuracy of the processed thread and reduce the surface roughness value, and is suitable for processing blind holes and horizontal through-hole threads that are not easy to remove chips.

The content chip tap has the following characteristics:

(1) The inner chip tap is mainly used to process the internal thread with larger diameter.

At the same time, due to the limitation of its structure, the size is generally larger.

(2) The cutting speed is low, the tapping torque is large, and the cutting claw is easy to break.

The cutting speed of the inner chip tap is generally between 2 ~ 6m/min.

At the same time, due to the large diameter of the inner chip tap, a large tapping torque will be generated in the tapping process, and a large tapping torque will break the cutting claw.

(3) The thread of the guide part of the chip tap has no axial edge groove.

This structure makes the calibration part not participate in the cutting process, so there will be no “over cutting” phenomenon.

(4) Since the chips produced by the inner chip tap during tapping are mainly discharged through the bottom hole of the thread or stored in the chip holding cavity, the contact between the chips and the machined thread surface is avoided, thus avoiding the damage of the thread surface. Therefore, the thread surface processed by the inner chip tap is of high quality.

(5) The chip tap adopts a large inclination angle of the positive edge, which makes the chip always discharge forward in the process of tapping.

This form of chip removal effectively avoids the breaking of the cutting claw caused by chip sticking in the process of tool withdrawal.

Fig. 5-86 chip tap

(2) Common problems and solutions in tapping

The problems of larger internal thread, smaller internal thread, higher surface roughness value of internal thread and tap breaking during machining or tool withdrawal often occur in tap tapping, as shown in table 5-22.

The causes and solutions of these problems are listed.

Table 5-22 common problems and solutions in tapping




Too large internal thread

Incorrect tap selection

Select the correct tap according to the processing conditions

Eccentric bottom hole

Change to floating tapping / improve bottom hole

Uneven axial feed

Change to floating tapping / improve the machine tool

Tap tolerance selection error

Select the correct tap tolerance

Chip blockage

Reduce the number of tap eyebrow slots, expand the space for shoulder slots and chips: increase and deepen the bottom hole

Cutting speed too high

Reduce cutting speed

Small internal thread

Incorrect tap selection

Select the correct tap according to the processing conditions

Poor rigidity of machine tool spindle

Use floating tapping instead

Tap tolerance selection error

Select the correct tap tolerance

Internal thread surface roughness value is too high

Incorrect tap selection

Select the correct tap according to the processing conditions

Cutting speed too high

Reduce cutting speed

Insufficient cutting cone length

Increase cutting cone length

Poor chip removal

Select the correct tap according to the processing conditions

Bottom hole diameter is too small

Select the bottom hole diameter according to corresponding standards

Cold welding

Improve cooling and lubrication conditions

The tap is broken during machining or tool withdrawal

Tap touches hole bottom during tapping

Check bottom hole depth

Excessive cutting torque

Shorten tapping length, use spiral groove tap

Bottom hole too small

Select the bottom hole diameter according to corresponding standards

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