As a common tool for processing internal thread, tap can be divided into spiral groove tap, edge angle tap, straight groove tap and pipe thread tap according to the shape, hand tap and machine tap according to the use environment, metric tap, American tap and British tap according to the specification. Are you familiar with them?
01 Tap classification
(1) Cutting tap
1) Straight groove tap: used for the processing of through hole and blind hole. Iron chips exist in the tap groove, and the processed thread quality is not high. It is more often used for the processing of short chip materials, such as gray cast iron.
2) Spiral groove tap: used for blind hole processing with hole depth less than or equal to 3D, iron chips are discharged along the spiral groove, and the thread surface quality is high.
10~20 ° spiral angle tap can process thread depth less than or equal to 2D;
28~40 ° spiral angle tap can process thread depth less than or equal to 3D;
50 ° spiral angle tap can process thread depth less than or equal to 3.5D (special working condition 4D).
In some cases (hard materials, large pitch, etc.), in order to obtain better tooth tip strength, spiral groove tap will be used to process through hole.
3) Screw tap: usually only used for through-hole, with a length-diameter ratio of up to 3D~3.5D, iron chips discharged downward, low cutting torque, and high surface quality of the processed thread, also known as edge angle tap or tip tap.
During cutting, it is necessary to ensure that all cutting parts are penetrated, otherwise tooth collapse will occur.

(2) Extrusion tap
It can be used for the processing of through holes and blind holes. It can only be used for the processing of plastic materials.
Its main features:
1) Use the plastic deformation of the workpiece to process the thread;
2) The tap has large sectional area, high strength and is not easy to break;
3) The cutting speed can be higher than the cutting tap, and the productivity can also be improved accordingly;
4) Due to the cold extrusion process, the mechanical properties of the processed thread surface are improved, the surface roughness is high, and the thread strength, wear resistance and corrosion resistance are improved;
5) Chip free processing.
Its disadvantages are:
1) It can only be used for processing plastic materials;
2) High manufacturing cost.
There are two structural forms:
1) The oil-free extrusion tap is only used for the working condition of blind hole vertical addition;
2) The extruding tap with oil groove is applicable to all working conditions, but generally the small diameter tap is not designed with oil groove due to manufacturing difficulties.

02 Structural parameters of tap

(1) Overall dimension
1) Total length: pay attention to some working conditions requiring special lengthening
2) Slot length: up
3) Shank: At present, the common standards for shank include DIN (371/374/376), ANSI, JIS, ISO, etc. Pay attention to the matching relationship with the tapping tool handle when selecting
(2) Threaded part
1) Accuracy: selected according to the specific thread standard. The metric thread ISO1/2/3 level is equivalent to the national standard H1/2/3 level, but the manufacturer's internal control standard should be paid attention to.
2) Cutting tap: the cutting part of the tap has formed a fixed mode. Generally, the longer the cutting tap is, the better the life of the tap is.

3) Corrected teeth: play the role of auxiliary and correction, especially under the unstable working condition of tapping system, the more correct teeth, the greater the tapping resistance.
(3) Chip chute
1. Groove type: it affects the formation and discharge of iron filings, which is usually the internal secret of each manufacturer.
2. Front angle and back angle: the tap becomes sharp when it increases, which can significantly reduce the cutting resistance, but the strength and stability of the tooth tip decreases, and the back angle is the back angle of grinding.
3. Number of grooves: the number of grooves increases with the number of cutting edges, which can effectively improve the tap life; However, it will compress the chip removal space, which is unfavorable for chip removal.
03 Material and coating of tap
(1) Material of tap
1) Tool steel: mostly used for hand cutting tap, which is not common at present.
2) Cobalt-free high-speed steel: currently widely used as tap material, such as M2 (W6Mo5Cr4V2, 6542), M3, etc., with the mark code of HSS.
3) Cobalt-containing high-speed steel: currently widely used as tap material, such as M35, M42, etc., with the mark code of HSS-E.
4) Powder metallurgy high speed steel: used as high performance tap material, its performance is greatly improved compared with the above two, and the naming method of each manufacturer is also different, and the marking code is HSS-E-PM.
5) Cemented carbide material: usually choose superfine particles with good toughness, mainly used for manufacturing straight groove tap and processing short chip materials, such as gray cast iron, high silicon aluminum, etc.
Taps are highly dependent on materials. Selecting a good material can further optimize the structural parameters of the tap, making it suitable for efficient and more demanding working conditions, and at the same time having a higher service life. At present, the major tap manufacturers have their own material factories or material formulations. At the same time, due to the problem of cobalt resources and prices, new cobalt-free high-performance high-speed steel has also been introduced.
(2) Coating of tap
1) Steam oxidation: the tap is put into high temperature steam to form an oxide film on its surface, which has good adsorption on the coolant, can reduce friction, and prevent adhesion between the tap and the material to be cut. It is suitable for processing mild steel.
2) Nitriding treatment: the surface of the tap is nitrided to form a surface hardening layer, which is suitable for processing cast iron, cast aluminum and other materials with great wear on the tool.
3) Steam+nitriding: combine the advantages of the above two.
4) TiN: golden yellow coating, with good coating hardness and lubricity, and good coating adhesion, suitable for processing most materials.
5) TiCN: blue-gray coating, hardness about 3000HV, heat resistance up to 400 ° C.
6) TiN+TiCN: dark yellow coating, with excellent coating hardness and lubricity, suitable for processing most materials.
7) TiAlN: blue-gray coating, hardness 3300HV, heat resistance up to 900 ° C, can be used for high-speed machining.
8) CrN: silver-gray coating with excellent lubrication performance, mainly used for processing non-ferrous metals.
The coating of tap has a very obvious impact on the performance of tap, but at present, the manufacturer and coating manufacturer cooperate to study the special coating separately.
04 Factors affecting tapping

(1) Tapping equipment
1) Machine tool: it can be divided into vertical and horizontal machining methods. For tapping, vertical machining is better than horizontal machining. When horizontal machining is cooled externally, consider whether the cooling is sufficient.
2) Tapping tool handle: special tapping tool handle is recommended for tapping. Synchronous tapping tool handle is preferred if the machine tool is rigid and stable. On the contrary, flexible tapping tool handle with axial/radial compensation should be selected as far as possible. Except for the small diameter tap (<M8), the square drive shall be used as far as possible.
3) Cooling conditions: for tapping, especially for extrusion tap, the requirement for coolant is lubrication>cooling; In actual use, it can be prepared according to the conditions of the machine tool (when using emulsion, the recommended concentration is more than 10%).
(2) Machined parts
1) Material and hardness of the workpiece to be processed: the hardness of the workpiece material should be uniform, and it is generally not recommended to use a tap to process the workpiece exceeding HRC42.
2) Tapping bottom hole: bottom hole structure, select appropriate drill bit; Size accuracy of bottom hole; Bottom hole wall quality.
(3) Processing parameters
1) Rotational speed: the given rotational speed is based on the type of tap, material, processed material and hardness, and the advantages and disadvantages of tapping equipment.
Generally, it is selected according to the parameters given by the tap manufacturer, and the speed must be reduced under the following conditions:
-Poor rigidity of machine tool; Large tap runout; Insufficient cooling;
-Uneven material or hardness of tapping area, such as welding spot;
-The tap is lengthened, or an extension rod is used;
-Lying plus external cooling;
-Manual operation, such as bench drill, rocker drill, etc;
2) Feed: rigid tapping, feed=1 pitch/revolution.
When flexible tapping and tool shank compensation variable is sufficient:
Feed=(0.95-0.98) pitches/revolution.
05 Tips for tap selection
(1) Tolerances of taps with different accuracy levels

Selection basis: the accuracy grade of the tap cannot be selected and determined only according to the accuracy grade of the processed thread
1) The material and hardness of the workpiece to be processed;
2) Tapping equipment (such as machine tool conditions, clamping tool handle, cooling ring, etc.);
3) Tap accuracy and manufacturing error.
For example, when processing 6H thread on steel parts, 6H precision tap can be selected; When processing gray cast iron, because the pitch diameter of the tap is worn quickly and the expansion of the screw hole is small, it is better to use a 6HX precision tap, which will have a better life.
Notes on the accuracy of Japanese tap:
1) The cutting tap OSG uses the OH accuracy system, which is different from the ISO standard. The OH accuracy system forces the width of the entire tolerance zone to start from the lowest limit, every 0.02mm as a precision grade, and is named OH1, OH2, OH3, etc;
2) The extrusion tap OSG uses the RH precision system, which forces the width of the entire tolerance zone to start from the lowest limit, every 0.0127mm as a precision grade, named RH1, RH2, RH3, etc.
Therefore, when replacing the OH precision tap with the ISO precision tap, it cannot be simply considered that 6H is about equal to OH3 or OH4, which needs to be determined by conversion or according to the actual situation of the customer.

(2) External dimension of tap
1) At present, the most widely used are DIN, ANSI, ISO, JIS, etc;
2) It is allowed to select appropriate total length, blade length and shank size according to different processing requirements of customers or existing conditions;

3) Interference during processing;

(3) Six basic elements of tap selection
1) Type of thread processed, metric, British, American, etc;
2) The type of threaded bottom hole, through hole or blind hole;
3) Material and hardness of processed workpiece;
4) The depth of complete thread and bottom hole of the workpiece;
5) The required accuracy of workpiece thread;
6) Appearance standard of tap (special