Tapping Explained: Type, Selection Principle, and Tapping Skills

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

It has simple structure and convenient use.

It can be operated manually or on machine tools.

It is widely used in production.

Can’t do tapping in the processing process?

Today I’ll share some tips to you to let you know more about tapping!

What is tapping?

Tapping is to cut the internal thread inside the hole of the workpiece with a tap.

1) Factors that determine tap performance include:

Workpiece material, cutting speed, cutting edge material, tool holder, tap form, hole size, tapping tool holder, cutting fluid and hole depth.

2) Pitch:

The axial distance between two adjacent teeth on the thread corresponding to two points on the pitch diameter line.


3) Lead:

The axial distance between the corresponding points of two adjacent teeth on the same helix. Denoted by codes.


4) Nominal diameter of thread:

Except that the pipe thread takes the inner diameter (inch unit) of the through pipe as the nominal diameter, the nominal diameter of other threads takes the major diameter of the thread as the nominal diameter (metric unit).

5) Pitch diameter of thread:

Pitch diameter is the most important because it controls the fit and strength of all thread assemblies.

The pitch diameter is on the pitch line, and the tooth width at this position is consistent with the adjacent tooth slot width.

Pitch diameter of thread

Designation of threads

English thread: English thread is the thread dimension marked in English. It is developed by the United States, Britain and Canada according to a unified system.

Designation of threads

Metric thread: developed according to ISO (International Organization for Standardization) system, it is the global standard of metric thread.

Metric thread

Design high-performance tapping

1) Perfect application

The factors to be considered in the tapping process are: workpiece design, tap design and application.

The goal is to reduce the cutting force and maximize the tap strength.

2) Balance options: all aspects of the application must be considered

Balance options

3) Key points of tap design

1) For the softer viscous material forming long chips: the tap has simple structure, large front angle and hook angle, large rear angle and void avoidance, free cutting, easy edge collapse, fragile tap as a whole and large chip holding space.

2) For hard materials: the tap has heavy-duty structure, small front angle and hook angle, small shovel back and back angle, high cutting pressure, strong edge design, reducing edge collapse, large cross-section and limited chip holding space.

4) Factors to be considered in tap design: tap groove type, tool material and surface strengthening treatment.

These design features must be balanced to provide proper cutting, chip control, lubrication and torsional strength.

The cutting must be stopped in the middle of the cutting and the hole must be reversed, while the cutting remains in the groove.

This brings the biggest challenge to the tapping and tap design in metal processing.

Key points of tap design

Shape of tap

1) Type of tap cutting surface

① Correct selection of positive hook tap

Correctly select the tap with small or negative hook angle

② Correctly select the tap with small or negative hook angle

Correctly select the tap with small or negative hook angle

2) Cutting tap

Cutting tap

The service life of the tap will be extended exponentially with each additional cutting tap tooth. The test shows that the service life of the tool will be doubled for every half of the cutting tooth thread.

Unlike other tools, the chip load of tap can only change with the number of chip removal slots and the length of the cutting tap.

3) Inverted tap:

Similar to all other tools, the tap is also slightly inverted.

Inverted tap

4) Thread shovel back

Advantages: less heat accumulation on the tap and less plastic chip on the back of the workpiece, which can compensate for the accumulation of chip on the thread.

The disadvantages of the thread shovel back are: the cutting edge becomes brittle and easy to collapse;

The insufficient rigidity of the main shaft and clamping (including floating tool handle) will worsen the thread;

Very small chips may be embedded during reverse rotation to collapse the cutting edge.

5) Tap tolerance

Each tap has a special pitch diameter.

Taps marked with H or D tolerance (mainly American taps): H / D tolerance indicates the thread size of the tap.

The letter indicates whether the tap size is greater than or higher than the basic pitch diameter (“H” = imperial, “D” = Metric), or lower than the basic pitch diameter (“L” = imperial, “DU” = Metric).

The actual tap size is related to the basic pitch diameter, such as H2, D3, L1, or DU2. Each tap has a special pitch diameter!

Taps are often marked with thread grade: General HP tap series, indicating that the tap is the correct size according to the fitting grade of the part, grade 3B tap is suitable for grade 2B parts, and tap grade marked “X” indicates that its tolerance is large, which is used for precision taps, electroplated or heat-treated parts, or materials close to (elastic memory).

Electroplated tapping thread: for internal thread, you must select the tap with larger tolerance grade.

Larger pitch diameter will make the thread size slightly larger, and the increase after electroplating will make the thread size return to the specified value.

Cutting treatment of tap

1) Hole type and chip handling

Hole type and chip handling

2) Extrusion cutting, spiral chip removal slot tap:

It is most suitable for blind hole and deep hole processing.

It is recommended to use the material to produce adhesive chips, which is very suitable for intermittent cutting.

The taper core of spiral chip removal slot tap is very thin, which is the most vulnerable part of tap design.

Therefore, the speed should be 30% ~ 40% lower than that of the straight chip removal slot tap to avoid fracture.

Extrusion cutting, spiral chip removal slot tap

3) Pull out cutting

Pull out cutting

4) Straight chip removal groove tap:

The tap with the highest strength is recommended for materials that are easy to break chips, such as brass and cast iron or hardened steel.

It usually needs coolant or gas to flush the chips in the chip removal groove.

It can have a variety of cutting tap forms.

– taper (form a) “A” – initial tap

– plug (forms B & D) “B / D” – middle tap

– (Form C) “C” – semi flat bottom or modified flat bottom

– (Form E) “E” – flat bottom

Straight chip removal groove tap

5) Extrusion tap:

Its processing feature is that there is no chip in the through hole or blind hole.

Extrusion tap

6) Comparison between cutting tap and extrusion tap

7) Influence of bottom hole size on extrusion thread

Influence of bottom hole size on extrusion thread

Coating of tap

1) Advantages of coating

Surface treatment: improve the appearance of HSS tap without changing the size of tap

② Improved tap life: high wear resistance, reduced friction and power consumption, reduced blade collapse and fracture, and slightly improved surface hardness.

③ Improve the surface quality and dimensional accuracy of screw holes: the cutting edge shall be kept sharp, with lubrication effect, reduce load and scratch, and minimize chip accumulation.

2) What is a chip tumor?

The workpiece material is welded or bitten on the chip edge.

What is a chip tumor

At the beginning of cutting, chip accretion tumor is formed and expands in the whole cutting process.

When the chip deposit is serious, the cutting pressure leads to its fragmentation, and the fragments are embedded into the workpiece, resulting in poor finish and out of tolerance of the size of the workpiece.

When the chip deposit breaks up, the particles of the tool material decompose, resulting in tool loss.

3) Traditional surface treatment

Oxidation treatment:

Increase cutting speed by 5% – 10%
  • Controlled oxidation (rust!)
  • Black
  • The porosity of oxidation keeps the cutting fluid on the tool surface
  • Applicable to ferrous (iron-based) materials, not applicable to titanium, brass, aluminum, composite materials, copper, zinc or plastics

Nitriding treatment:

Increase cutting speed by 5% – 10%
  • The surface hardening of the tool forms a thin and hard protective layer
  • Moire or clear surface
  • Improve the hardness and prolong the wear life of wear-resistant materials
  • Applicable to non-ferrous and non-metallic materials
  • Be careful! The increase of hardness will increase the brittleness

Oxynitride treatment:

Increase cutting speed by 5% – 10%
  • Composite treatment has the advantages of lubrication and wear resistance
  • Good for iron-based materials, such as superalloys, titanium alloys, high-strength steel and stainless steel
  • The blackening treatment after nitriding is mainly used for the surface treatment of tap

4) Film coating

(TIN) titanium nitride coating

  • Bright gold
  • Low Rockwell hardness (RC) 80
  • It is applicable to various materials with improved performance
  • The most common film
  • Titanium and titanium alloys avoid the use of tin coated tools

(TiCN) titanium carbonitride coating

  • Blue grey
  • Medium Rockwell hardness (RC) 80
  • The lubricity is good, and the friction coefficient is smaller than that of tin coating
  • Suitable for aluminum alloy, steel, stainless steel and superalloy
  • Allow higher feed rate

(TiAIN) titanium nitride aluminide coating

  • Purple black
  • Medium Rockwell hardness (RC) 80
  • High lubricity and low friction coefficient
  • Suitable for steel, stainless steel, superalloys and plastics
  • Suitable for high speed machining
Film coating

(CRN) chromium nitride coating

  • Bright silver
  • Replace traditional chrome plating
  • High Rockwell (RC) hardness 70
  • Suitable for non-ferrous materials and titanium alloys
  • The performance and stability are better than those of chromium plating
  • Increase cutting speed by more than 25%

Diamond film

  • Black
  • Medium Rockwell hardness (RB) 90
  • Recommended for high wear resistant materials
  • – Aluminum alloy
  • – Grey Cast Iron
  • – Composites
  • – Wear resistant plastic
  • The cutting speed is increased by more than 100%

Tin + CRC / C composite coating

  • Patented multi-layer coating technology
  • Smooth surface
  • Best comprehensive high performance coating
  • It is effectively used in ferrous and non-ferrous materials, including aluminum alloy and titanium alloy
  • Cutting speed increased by more than 50%

Key to the success of tapping

1) Determine thread percentage

Determine thread percentage

The drill hole size determines the percentage of thread bottom diameter and thread height.

The larger the drill diameter you use, the smaller the thread height ratio you reach!

How to determine thread percentage

2) Selection of bottom hole size

Generally, 65% to 70% of the thread height is preferred!

The thread strength of 83% height is only 2% higher than that of 65% height, but the tapping torque is more than twice!

Selection of bottom hole size
Tap/drill chart

3) Frequently asked questions

① Causes of tap top cutting

Manual tapping – the manual feed is not coordinated, and the feed is too fast or too slow.

Evidence of topping on tap

Machine tapping – asynchronous tapping cycle is not programmed properly.

Lead screw machine tool – back clearance caused by worn lead screw or loose lead screw adjusting nut.

Cam feed machine – the cam profile is incorrect or worn.

Pneumatic or hydraulic machine tools – uncontrollable, pressure too high or too low.

Gear feed machine – backlash caused by improper gear adjustment or wear.

② Solve the problem of tap top cutting

For the most precise thread, the feed should be synchronized with the spindle speed!

The feed rate and spindle rotation must match the thread pitch.

6) Advantages of synchronous tapping of CNC machine tool:

Thread depth control, consistent hole to hole size, elimination of roof cutting, and re tapping when necessary.


If the CNC machine programmed as g84 tapping cycle is operated manually, does it mean that rigid tapping can be carried out?


No! Many CNC machine tools have a fixed tapping program, and the feed of the fixed program is in the form of drilling.

Such programs cannot be synchronized with the spindle.

Be careful! The data provided by the machine tool must indicate whether the machine tool has “synchronous” or “rigid” tapping.

7) Selection of tool handle

Selection of tool handle
Floating handle

For asynchronous tapping

– CNC machine tool with fixed tapping cycle

– cam, gear, pneumatic or hydraulic feed mechanism

For synchronous tapping

– in case of oversized / undersized threads

8) Tool handle maintenance

Proper tool holder maintenance can ensure the machining of high-quality threads and the service life of taps.

  • The internal mechanism should be free of chips and debris
  • Lubricate frequently to ensure flexible movement of components and prevent rust
  • Test the tool handle frequently, especially when using water-soluble coolant


1) Excessive thread

Tapping NC setting

When tapping with CNC machine tools without rigid tapping cycle:

  • Program the feed rate to 95% ~ 98% of the tap reverse stroke
  • Use only extended tool handles or telescopic tool handles with compression locking

When tapping on CNC machine tools with rigid tapping cycle:

  • Tap lead is programmed to feed rate of 100%
  • Use an integral shank or synchronous shank

If the end of the thread gauge is passed by topping:

  • Reprogram and follow the “non rigid” procedure
  • Consider using quick change fittings. Can have minimum “float”
Excessive thread

2) Chip winding

  • Change tap form

→ straight groove

→ smaller helix angle

  • Shorten cutting tap
  • Change front corner shape
  • Increase the number of slots
  • Change speed
  • Smaller hook
  • In case of rigid tapping, increase the pecking cycle
  • Consider extrusion tap

3) Lubrication selection

For tapping, the purpose of lubrication is to reduce friction.

Therefore, generally, lubricant is used for tapping, not coolant;

If it is a coolant, add EP (ultra-high pressure) or HP (high pressure) additives.

The tap has a fixed large amount of feed, which is controlled by the tap pitch, and the drilling feed can be adjusted to the control load.

4) Coolant application

Tap selection basis

Before selecting a tap, we need to understand:

  • Type of hole, through hole, blind hole or deep hole
  • Minimum drilling depth
  • Minimum thread depth required
  • Are extrusion Taps considered?
  • Material of workpiece to be tapped
Tap selection basis
Select the most appropriate cutting cone length of the tap according to the hole style

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