As a common tool for processing internal threads, taps can be divided into spiral flute taps, blade inclination taps, straight flute taps and pipe thread taps according to their shapes. According to the use environment, they can be divided into hand taps and machine taps. Divided into metric, American, and British taps. Are you familiar with them all?
01 Tap classification
(1) Cutting tap
1) Straight flute taps: used for processing through holes and blind holes. Iron filings exist in the tap grooves, and the thread quality of the processed threads is not high. It is more commonly used for processing short chip materials, such as gray cast iron.
2) Spiral flute tap: used for blind hole machining with a hole depth less than or equal to 3D, iron filings are discharged along the spiral flute, and the thread surface quality is high.
10~20° helix angle tap can process thread depth less than or equal to 2D;
28~40° helix angle tap can process thread depth less than or equal to 3D;
The 50° helix angle tap can process the thread depth less than or equal to 3.5D (4D under special conditions).
In some cases (hard materials, large pitches, etc.), in order to obtain better tooth tip strength, spiral fluted taps are used to process through holes.
3) Spiral point taps: usually only used for through holes, the ratio of length to diameter can reach 3D~3.5D, iron filings are discharged downward, the cutting torque is small, and the surface quality of the processed thread is high. It is also called the edge tap or Tip tap.
When cutting, it is necessary to ensure that all cutting parts are penetrated, otherwise tooth chipping will occur.
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(2) Extrusion tap
It can be used for the processing of through holes and blind holes. The tooth profile is formed through plastic deformation of the material, and it can only be used for processing plastic materials.
Its main features:
1) Use the plastic deformation of the workpiece to process the thread;
2) The tap has a large cross-sectional area, high strength, and is not easy to break;
3) The cutting speed is higher than that of cutting taps, and the productivity is also improved accordingly;
4) Because it is cold extrusion processing, 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) No chip processing.
The disadvantages are:
1) It can only be used for processing plastic materials;
2) High manufacturing cost.
There are two structural forms:
1) Extrusion taps without oil grooves are only used for blind hole vertical machining;
2) Extrusion taps with oil grooves are suitable for all working conditions, but usually small diameter taps are not designed with oil grooves due to the difficulty of manufacturing.
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02 The structural parameters of the tap
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(1) Dimensions
1) Total length: some working conditions that require special lengthening need to be paid attention to
2) Slot length: pass on
3) Shank side: At present, the common shank side standards include DIN(371/374/376), ANSI, JIS, ISO, etc. Please pay attention to the matching relationship with the tapping tool holder when selecting
(2) Threaded part
1) Accuracy: It is selected by the specific thread standard. The metric thread ISO1/2/3 is equivalent to the national standard H1/2/3, but the manufacturer’s internal control standards should be paid attention to.
2) Cutting cone: The cutting part of the tap has formed a partial fixed pattern. Generally, the longer the cutting cone, the better the life of the tap.
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3) Correction teeth: play an auxiliary and correction role, especially in the unstable working conditions of the tapping system, the more correction teeth, the greater the tapping resistance.
(3) Chip flute
1. Groove shape: It affects the forming and discharge of iron filings, which is usually an internal secret of each manufacturer.
2. Rake angle and relief angle: The tap becomes sharp when it increases, which can significantly reduce the cutting resistance, but the strength and stability of the tooth tip decrease, and the relief angle is the relief angle.
3. Number of grooves: increasing the number of grooves and increasing the number of cutting edges can effectively increase the life of the tap; but it will compress the chip removal space, which is unfavorable for chip removal.
03 Material and coating of tap
(1) The material of the tap
1) Tool steel: It is mostly used for hand-use incisor taps, which are not common anymore.
2) Cobalt-free high-speed steel: At present, it is widely used as tap material, such as M2 (W6Mo5Cr4V2, 6542), M3, etc., and the mark code is HSS.
3) Cobalt-containing high-speed steel: At present, it is widely used as tap material, such as M35, M42, etc., and the marking code is HSS-E.
4) Powder metallurgy high-speed steel: used as a high-performance tap material, the performance is greatly improved compared with the above two, and the naming methods of each manufacturer are also different, and the marking code is HSS-E-PM.
5) Cemented carbide materials: usually select ultra-fine particles, good toughness grades, mainly used to make straight-groove taps processing short chip materials, such as gray cast iron, high silicon aluminum, etc.
The taps are highly dependent on materials. The selection of good materials can further optimize the structural parameters of the taps, making them suitable for efficient and more demanding working conditions, and at the same time have a higher life. At present, major tap manufacturers have their own material factories or material formulas. At the same time, due to cobalt resources and price issues, new cobalt-free high-performance high-speed steels have also come out.
(2) Coating of tap
1) Steam oxidation: the tap is placed in high-temperature water vapor to form an oxide film on the surface, which has good adsorption to the coolant, can reduce friction, and prevent the tap and the material being cut from sticking. Suitable for processing mild steel.
2) Nitriding treatment: The surface of the tap is nitrided to form a surface hardened layer, which is suitable for processing cast iron, cast aluminum and other materials that have a lot of tool wear.
3) Steam + Nitriding: Combining 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 is 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 processing.
8) CrN: Silver-gray coating, excellent lubricating performance, mainly used for processing non-ferrous metals.
The coating of the tap has a very obvious effect on the performance of the tap, but at present, most manufacturers and coating manufacturers cooperate to study special coatings separately.
04 Factors affecting tapping
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(1) Tapping equipment
1) Machine tools: It can be divided into two processing methods: vertical and horizontal. For tapping, vertical is better than horizontal. When horizontal processing is externally cooled, it is necessary to consider whether the cooling is sufficient.
2) Tapping tool holder: It is recommended to use a special tapping tool holder for tapping. The machine tool is rigid and stable. Synchronous tapping tool holders are preferred. On the contrary, flexible tapping tool holders with axial/radial compensation should be used as much as possible. . Except for small diameter taps (<M8), use square drive as much as possible.
3) Cooling conditions: For tapping, especially for extrusion taps, the requirement for coolant is lubrication>cooling; in actual use, it can be adjusted according to the conditions of the machine tool (when using emulsion, the recommended concentration is greater than 10%).
(2) Workpiece
1) The material and hardness of the workpiece: the hardness of the workpiece material should be uniform. Generally, it is not recommended to use a tap to process workpieces exceeding HRC42.
2) Tapping bottom hole: bottom hole structure, choose a suitable drill bit; bottom hole size accuracy; bottom hole wall quality.
(3) Processing parameters
1) Rotation speed: The basis of the speed setting is the type of tap, material, processed material and hardness, and the quality of the tapping equipment.
It is usually selected according to the parameters given by the tap manufacturer, and the speed must be reduced under the following conditions:
-Poor machine rigidity; large tap jump; insufficient cooling;
-Uneven material or hardness of the tapping area, such as solder joints;
-The tap is lengthened, or an extension rod is used;
-Lying plus, cold outside;
-Manual operation, such as bench drills, radial drills, etc.;
2) Feed: rigid tapping, feed=1 pitch/revolution.
When flexible tapping, and the tool holder compensation variable is sufficient:
Feed = (0.95-0.98) pitch/revolution.
05 A few tips for tap selection
(1) Tolerance of taps with different precision grades
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Selection basis: can not only select and determine the accuracy level of the tap based on the accuracy level of the machined thread
1) The material and hardness of the workpiece to be processed;
2) Tapping equipment (such as machine tool conditions, clamping tool holders, cooling rings, etc.);
3) The accuracy and manufacturing error of the tap itself.
For example: when processing 6H threads, when processing steel parts, you can choose 6H precision taps; when processing gray cast iron, because the pitch diameter of the tap wears faster and the expansion of the screw hole is small, it is better to choose 6HX precision Tap, the life will be better.
Note on the accuracy of Japanese taps:
1) Cutting tap OSG uses the OH precision system, which is different from the ISO standard. The OH precision system forces the entire tolerance zone width to start from the lowest limit, and each 0.02mm is regarded as a precision level, named OH1, OH2, OH3, etc.;
2) The extrusion tap OSG uses the RH precision system. The RH precision system forces the entire tolerance zone width to start from the lowest limit, with each 0.0127mm as a precision level, named RH1, RH2, RH3, etc.
Therefore, when using ISO precision taps to replace OH precision taps, 6H cannot be simply considered to be approximately equal to OH3 or OH4. It needs to be determined by conversion or based on the actual situation of the customer.
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(2) External dimensions of the tap
1) DIN, ANSI, ISO, JIS, etc. are currently the most widely used;
2) It is possible to choose the appropriate total length, blade length and shank size according to different processing requirements or existing conditions of customers;
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3) Interference during processing;
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(3) 6 basic elements for tap selection
1) The type of thread to be processed, metric, imperial, American, etc.;
2) Type of threaded bottom hole, through hole or blind hole;
3) Material and hardness of the workpiece to be processed;
4) The depth of the complete thread of the workpiece and the depth of the bottom hole;
5) The required accuracy of the workpiece thread;
6) Appearance standard of tap (special requirements need to be marked out)
