Blog

How to choose a tap?

Taps, as a commonly used tool for processing internal threads, can be divided into spiral groove taps, blade angle taps, straight groove taps, and pipe thread taps according to their shapes. They can also be divided into manual taps and machine taps according to their usage environment, and metric, American, and British taps according to their specifications. Tap is also a mainstream machining tool used during tapping. So how to choose a tap? Today, I will share with you a guide for selecting taps to help you choose the right one.

Tap classification

1. Cutting tap

1. 1 Straight groove tap: used for processing through holes and blind holes. Iron chips exist in the tap groove, and the quality of the processed threads is not high. It is more commonly used for processing short chip materials such as gray cast iron.

1. 2 Spiral groove tap: used for blind hole machining 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 threads with a depth of no more than 2D;

28 °~40 ° spiral angle tap can process thread depths less than or equal to 3D;

The 50 ° spiral angle tap can process threads with a depth of less than or equal to 3.5D (special working condition 4D).

Sometimes (for hard materials, large tooth spacing, etc.), in order to achieve better tooth tip strength, spiral groove taps are used to machine through holes.

1.3. Screw tip tap: It is usually only used for through holes, with a length to diameter ratio of up to 3D~3.5D. The iron chips are discharged downwards, the cutting torque is small, and the surface quality of the machined thread is high. It is also known as a rake angle tap or a tip tap.

When cutting, it is necessary to ensure that all cutting parts are penetrated, otherwise tooth breakage may occur.

2 Squeezing taps

Can be used for processing through holes and blind holes, forming tooth profiles through material plastic deformation, and can only be used for processing plastic materials.

Its main features are:

1. Using the plastic deformation of the workpiece to process threads;

2. The cross-sectional area of the tap is large, the strength is high, and it is not easy to break;

3. The cutting speed can be higher than that of a cutting tap, and the productivity is correspondingly improved;

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 enhanced;

5. No chip processing.

Its shortcomings are:

1. Can only be used for processing plastic materials;

2. The manufacturing cost is high.

There are two structural forms:

1. The oil-free groove extrusion tap is only used for the working condition of blind hole vertical addition;

2. Oil groove extrusion taps are suitable for all working conditions, but small diameter taps are usually not designed with oil grooves due to manufacturing difficulties

Structural parameters of tap

1. External dimensions

1. Overall length: Attention should be paid to certain working conditions that require special lengthening.

2. Slot length: Connected.

3. Handle: Currently, common handle standards include DIN (371/374/376), ANSI, JIS, ISO, etc. When selecting, attention should be paid to the matching relationship with the tapping handle.

2 threaded parts

1. Accuracy: Selected based on specific thread standards, metric thread ISO 1/2/3 level is equivalent to national standard H1/2/3 level, but attention should be paid to the manufacturer's internal control standards.

2. Cutting cone: The cutting part of a tap has formed a partially fixed pattern, and usually the longer the cutting cone, the better the lifespan of the tap.

3. Calibration teeth: They play an auxiliary and calibration role, especially in unstable working conditions of the tapping system. The more calibration teeth there are, the greater the tapping resistance.

3 chip slots

1. Groove type: affects the formation and discharge of iron chips, and is usually confidential within each manufacturer.

2. Front angle and back angle: As the size increases, the tap becomes sharper, which can significantly reduce cutting resistance, but the strength and stability of the tooth tip decrease, and the back angle, which is the scraping and grinding back angle.

3. Number of slots: Increasing the number of slots and cutting edges can effectively improve the lifespan of the tap. But it will compress the chip removal space, which is not conducive to chip removal.

The material of the tap

1. Tool steel: commonly used for hand cutting taps, it is no longer common.

2. Cobalt free high-speed steel: currently widely used as tap material, such as M2 (W6Mo5Cr4V2, 6542), M3, etc., marked with the code HSS.

3. Cobalt containing high-speed steel: currently widely used as tap material, such as M35, M42, etc., with the marking code HSS-E.

4. Powder metallurgy high-speed steel: used as a high-performance tap material, its performance is greatly improved compared to the two, and the naming method of each manufacturer is also different, with the marking code HSS-E-PM.

5. Hard alloy materials: usually selected with ultra-fine particles and good toughness, mainly used for manufacturing short chip materials for straight groove tap processing, such as gray cast iron, high silicon aluminum, etc.

Taps are highly dependent on materials, and selecting good materials can further optimize the structural parameters of the tap, making it suitable for efficient and more demanding working conditions, while also having a higher lifespan. At present, major tap manufacturers have their own material factories or formulas, and due to issues with cobalt resources and prices, new cobalt free high-performance high-speed steels have also emerged.

Coating of tap

1. Steam oxidation: The tap is placed in high-temperature steam to form an oxide film on its surface, which has good adsorption properties for coolant and can reduce friction while preventing adhesion between the tap and the material being cut. It is suitable for processing soft steel.

2. Nitriding treatment: The surface of the tap is nitrided to form a surface hardening layer, suitable for processing materials with high tool wear such as cast iron and cast aluminum.

3. Steam+Nitriding: Combining the advantages of both.

4. TiN: a golden coating with good coating hardness and lubricity, and good adhesion performance, suitable for processing most materials.

5. TiCN: Blue gray coating, with a hardness of approximately 3000HV and a heat resistance of up to 400 ° C.

6. TiN+TiCN: a deep yellow coating with excellent coating hardness and lubricity, suitable for processing the vast majority of materials.

7. TiAlN: Blue gray coating, hardness 3300HV, heat resistance up to 900 ° C, suitable for high-speed machining.

8. CrN: Silver gray coating with superior lubrication performance, mainly used for processing non-ferrous metals.

The coating of the tap has a significant impact on the performance of the tap, but currently, manufacturers and coating manufacturers are working together to study specialized coatings, such as LMT's IQ and Walter's THL.

Factors affecting tapping

1. Tapping equipment

   
   

1. Machine tools can be divided into two processing methods: vertical and horizontal. For tapping, vertical machining is superior to horizontal machining, and when horizontal machining is externally cooled, sufficient cooling should be considered.

2. Tapping tool holder: It is recommended to use a dedicated tapping tool holder for tapping. For machine tools with good rigidity and stability, synchronous tapping tool holders are preferred. Conversely, flexible tapping tool holders with axial/radial compensation should be used as much as possible. Except for small diameter taps (<M8), choose square drive as much as possible.

3. Cooling conditions: For tapping, especially extrusion taps, the requirement for coolant is lubrication>cooling. In actual use, it can be adjusted according to the machine conditions (when using emulsion, it is recommended to have a concentration greater than 10%).

2 processed parts

1. The material and hardness of the workpiece: The hardness of the workpiece material should be uniform, and it is generally not recommended to use a tap to process workpieces exceeding HRC42.

2. Tapping bottom hole: bottom hole structure, choose the appropriate drill bit; Bottom hole size accuracy; Quality of bottom hole wall.

3 Processing Parameters

1. Rotational speed: The basis for setting the rotational speed is the type, material, processed material, and hardness of the tap, as well as the quality of the tapping equipment.

Usually selected based on the parameters given by the tap manufacturer, the speed must be reduced under the following conditions:

-Poor rigidity of machine tools, large tap jumping, and insufficient cooling;

-Uneven material or hardness in the tapping area, such as welding points;

-The tap is lengthened or an extension rod is used;

- horizontal machining centre,cold outside;

-Manual operations, such as bench drills, rocker drills, etc.

2. Feed rate:

Rigid tapping, feed=1 pitch/turn;

Flexible tapping, with sufficient tool compensation variable, feed=(0.95~0.98) pitch/revolution.

Several tips for selecting taps:

1. Tolerance of taps with different precision levels

Selection criteria: The accuracy level of the tap cannot be selected and determined solely based on the precision level of the processed thread

The material and hardness of the processed workpiece;

Tapping equipment (such as machine conditions, clamping tool holders, cooling environment, etc.);

The precision and manufacturing error of the tap itself.

For example, when processing 6H threads on steel parts, a 6H precision tap can be used; When processing gray cast iron, due to the fast wear of the pitch diameter of the tap and the small expansion of the screw hole, it is advisable to use a tap with a precision of 6HX, which will have a better service life.

Explanation on the Accuracy of Japanese Taps:

The cutting tap OSG uses the OH precision system, which is different from the ISO standard. The OH precision system forces the entire tolerance band width to start from the lowest limit, with every 0.02mm as a precision level, named OH1, OH2, OH3, etc;

The extrusion tap OSG uses the RH precision system, which forces the entire tolerance band width to start from the lowest limit, with every 0.0127mm as a precision level, named RH1, RH2, RH3, etc.

So when replacing OH precision taps with ISO precision taps, it cannot be simply assumed that 6H is approximately equal to OH3 or OH4 grade. It needs to be determined through conversion or based on the actual situation of the customer.

2. External dimensions of tap

1. Currently, the most widely used are DIN, ANSI, ISO, and JIS;

2. Choose the appropriate total length, blade length, and shank size based on different processing requirements or existing conditions of the customer;

3. Interference during processing

 basic elements for selecting 6 taps

1.Types of thread processing, including metric, imperial, and American systems;

 2. The type of threaded bottom hole, through hole or blind hole;

3. The material and hardness of the processed workpiece;

4. The depth of the complete thread of the workpiece and the depth of the bottom hole;

5. Accuracy required for workpiece threads;

6. The appearance standard of the tap (special requirements need to be marked).