A comprehensive analysis of tap classification, parameters, selection, and factors affecting tapping.

I. Classification of Taps

(I) Cutting Taps

Cutting taps machine threads through cutting action. Based on differences in flute shape and structure, they are suitable for various applications. Specific classifications are as follows:

Straight Flute Taps:Highly versatile, suitable for through holes and blind holes. Chips are retained directly in the flute, resulting in moderate chip removal and a medium thread surface quality. Primarily used for machining short-chip materials such as gray cast iron.

Helical Flute Taps:Designed specifically for blind holes, suitable for hole depths ≤3D (D is the thread diameter). Chips are discharged along the helical flute direction, effectively preventing chip blockage and resulting in high thread surface quality. The helix angle affects the hole depth; for example, a 50° helix angle tap can machine holes 3.5D-4D deep.

Tip Taps (Pre-tip Taps):Only suitable for through holes. Their design allows chips to be discharged towards the bottom of the hole, resulting in low cutting torque and excellent thread quality. When using them, ensure the cutting part of the tap completely penetrates the workpiece to guarantee smooth chip removal and thread integrity. Image

 

(II) Extrusion Taps

Extrusion taps do not rely on cutting action; instead, they form threads by extruding the workpiece material to induce plastic deformation. This is a chipless machining method. Its core characteristics are as follows:

Applicable Scope: Only suitable for ductile materials, not brittle materials.

Advantages and Disadvantages: Advantages include high thread strength, low surface roughness, and stable quality; disadvantages include higher manufacturing costs and strict requirements on the toughness of the workpiece material.

Structural Types: Divided into oil-free and oil-grooved types. The oil-free type is suitable only for blind-hole machining, while the oil-grooved type is a general-purpose type ideal for both through and blind holes.

II. Structural Parameters of Taps

(I) External Dimensions

Key external dimensions of taps include total length, flute length, and shank square. The shank square dimension must strictly match tool holder standards, such as DIN (German standard) and ANSI (American standard), to ensure stable clamping and precise transmission. Image

(II) Thread Section

Precision Grade: Follows international or national standards, such as ISO 1/2/3 corresponding to national standard H1/2/3. Higher-precision grades result in smaller thread dimensional tolerances, making them suitable for scenarios with stringent precision requirements.

Cutting Cone and Alignment Teeth: The length of the cutting cone is positively correlated with tap life—a longer cutting cone results in a more uniform cutting load distribution and generally a longer lifespan. More aligned teeth improve thread alignment but increase resistance during tapping.

 

Image (III) Chip Grooves

The groove shape, rake angle, and clearance angle of the chip grooves directly affect the tap’s sharpness, cutting strength, and chip removal efficiency: a larger rake angle results in lighter cutting, and a larger clearance angle provides greater wear resistance. More grooves increase overall tap rigidity and lifespan but compress the chip-removal space; a balance must be struck based on the material being processed and chip-removal requirements.

III. Tap Materials and Coatings

(I) Materials

Tap material properties, from lowest to highest, are: ordinary tool steel, high-speed steel (HSS), cobalt-containing high-speed steel, powder metallurgy high-speed steel, and cemented carbide. Performance improvements are mainly reflected in wear resistance, heat resistance, and rigidity:

High-speed steel and cobalt-containing high-speed steel: Suitable for machining most general-purpose materials. Cobalt-containing materials can improve high-temperature hardness, making them suitable for difficult-to-machine materials.

Powder metallurgy high-speed steel: Has a more uniform microstructure, and its strength and toughness are superior to ordinary high-speed steel, making it suitable for high-precision, high-load machining.

Cemented carbide: Mainly used for machining short-chip materials such as cast iron and high-silicon aluminum. It has extremely high hardness but poor toughness, so appropriate machining parameters are required to avoid chipping.

(II) Coatings

Coatings can significantly optimize tap performance. Different coatings are suitable for different scenarios, as follows:

Basic coating: Steam oxidation coating is suitable for machining mild steel, improving lubrication; nitriding coating is suitable for cast iron and cast aluminum materials, enhancing wear resistance. Mainstream coatings: TiN (titanium nitride), TiCN (titanium carbonitride), TiAlN (titanium aluminum nitride), etc., can respectively improve the wear resistance, lubrication, and high-temperature resistance of taps, adapting to various materials and high-speed machining scenarios.

Specialized coatings, such as LMT’s IQ coating, with optimized formulas for specific machining conditions, further improve tap lifespan and machining stability.

IV. Key Factors Affecting Tapping Quality and Efficiency

(I) Machining Equipment

Machine Tool Type: Vertical machine tools offer better tapping accuracy and stability than horizontal machine tools, reducing the risk of chip accumulation and tap misalignment.

Tool Holder Selection: Synchronous tapping tool holders are preferred for their high rigidity and precise transmission; flexible tool holders can be used to compensate for deviations when workpiece clamping is unstable or the machining allowance is uneven.

Coolant: During tapping, the coolant should prioritize lubrication performance to reduce friction between the tap and workpiece, while also assisting in chip removal to prevent thread scoring and tap wear.

(II) Workpiece Conditions

Material Hardness: The workpiece’s material hardness should be uniform, ideally below HRC 42. Excessive hardness will significantly increase the tap load, reduce lifespan, and may even cause tap breakage.

Boot Hole Quality: The dimensional accuracy of the boot hole must meet machining requirements. The whole wall must be smooth, burr-free, and crack-free. Boot hole quality directly determines thread accuracy and surface quality, making it a crucial pre-treatment step before tapping.

(III) Machining Parameters

Spin Rotation Speed: Adjustments should be made flexibly based on workpiece material, tap material, cooling conditions, and other factors. When material hardness is uneven, cooling is insufficient, or blind holes are being machined, the spindle speed should be reduced to prevent tap overheating or breakage.

Feed Rate: For rigid tapping, set the feed rate to 1 pitch per revolution to ensure accurate thread lead. For flexible tapping, the feed rate can be appropriately reduced, typically to 0.95-0.98 times the pitch per revolution to compensate for tool holder flexibility deviation.

V. Key Points for Tap Selection

(I) Accuracy Grade Selection Adjustments should be made based on the workpiece material and tapping conditions. For example, when machining brittle materials such as cast iron, a 6HX grade tap is preferable to a higher-precision 6H grade tap to avoid thread-edge chipping and improve machining stability.

(II) Precautions Regarding the Accuracy of Japanese Taps

Japanese brand taps, such as OSG, use the OH/RH accuracy system, which is not directly equivalent to the international ISO accuracy standard. Accuracy conversion is necessary when selecting taps; they cannot be directly substituted to avoid substandard thread accuracy. (III) Core Selection Factors: Selecting a tap requires careful consideration of six fundamental factors to ensure it matches the machining requirements:

Thread Type: Clearly define the type and specifications of the thread to be machined (tolerance/imperial, US, etc.);

Hole Type: Differentiate between through holes and blind holes, and select straight flute, spiral flute, or pointed taps accordingly;

Workpiece Material and Hardness: Select cutting/forming taps and their corresponding materials and coatings based on the material’s plasticity and hardness;

Thread Depth: Select a tap with an appropriate helix angle based on the hole depth (e.g., ≤3D or deeper);

Accuracy Requirements: Determine the thread accuracy grade based on the product drawings and match the corresponding tap accuracy;

Tap Shape Standards: Ensure that the tap shank square, overall length, and other dimensions are compatible with the machine tool holder and machining space.