A tap is a cutting tool used to machine various small and medium-sized internal threads. It has a simple structure, is easy to use, and can be operated manually or on a machine tool, making it widely used in production. Having trouble with tapping? Today, we'll share some tips to help you better understand tapping!
What is Tapping? Tapping is the process of using a tap to cut internal threads inside a hole in a workpiece.
Image 1) Factors determining tap performance include: workpiece material, cutting speed, cutting edge material, tool holder, tap type, hole size, tap shank, cutting fluid, and hole depth.
2) Pitch: The axial distance between two corresponding points on the pitch diameter line of two adjacent threads.
Image 3) Lead: The axial distance between corresponding points on two adjacent threads on the same helix. Represented by the symbol S.
Image 4) Nominal Thread Diameter: Except for pipe threads, which use the inner diameter of the pipe (in inches) as the nominal diameter, the nominal diameter of other threads is the major diameter of the thread (in metric units).
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5) Thread Pitch Diameter: The pitch diameter is the most important because it controls the fit and strength of all threaded assemblies. The pitch diameter is located on the pitch line, where the tooth width matches the width of the adjacent tooth groove.
Image Thread Naming
Imperial Threads: Imperial threads use imperial units for thread dimensions and were developed by the United States, the United Kingdom, and Canada according to a unified system.
Image Metric Threads: Developed according to the ISO (International Organization for Standardization) system, metric threads are the global standard.
Image Designing High-Performance Tapping Machining
1) Perfect Application
Factors to consider in the tapping process include: workpiece design, tap design, and application. The goal is to reduce cutting forces while maximizing tap strength.
2) Balancing Options: All aspects of the application must be considered.
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3) Key Points of Tap Design
1) For softer, sticky materials that form long chips: Simple tap structure, large rake angle and hook angle, large clearance angle and clearance, free cutting, prone to chipping, relatively fragile overall tap, large chip space.
2) For hard materials: Taps have a heavy-duty structure, small rake angle and hook angle, small back angle and clearance angle, high cutting pressure, robust cutting edge design to reduce chipping, large cross-section, and limited chip space.
4) Factors to consider in tap design: tap flute shape, tool material, and surface hardening treatment. These design features must be balanced to provide proper cutting, chip control, lubrication, and torsional strength.
The cutting must stop midway and reverse out of the hole, while the cutting remains in the flute. This presents a major challenge to tapping and tap design in metalworking.
Image: Tap shape
1) Types of tap cutting surfaces
Image: ① Correctly select a positive hook tap
Image: ② Correctly select a small or negative hook angle tap
Image: 2) Tap cutting cone
Image: Image: Each additional cutting bevel tooth exponentially increases tap life. Tests show that adding half a cutting tooth doubles tool life. Unlike other cutting tools, the chip load of a tap can only be changed by varying the number of chip flutes and the length of the cutting cone.
3) Reverse Taper: Similar to all other cutting tools, taps also have a slight reverse taper.
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4) Thread Scraper
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Advantages of thread scraper include: lighter cutting speed, less heat buildup, less material wrapped around the tap (less adhering chips, less built-up edge), the ability to use higher tapping speeds, and compensation for plastic deformation of the workpiece material.
Disadvantages of thread scraper include: brittle cutting edge, prone to chipping; insufficient rigidity of the spindle and clamping (including floating tool holders) can lead to poorer thread quality; very small chips may become embedded during reverse rotation, causing chipping of the cutting edge.
5) Tap Tolerances
Each tap has a specific pitch diameter.
Taps marked with H or D tolerances (mainly American taps): H/D tolerance indicates the thread size of the tap. The letter indicates whether the tap size is larger or larger than the basic pitch diameter ("H" = Imperial, "D" = Metric) or smaller than the basic pitch diameter ("L" = Imperial, "DU" = Metric). The actual tap size number is related to the basic pitch diameter, such as: H2, D3, L1, or DU2. Each tap has a specific pitch diameter size!
Taps are often marked with a thread grade: general HP tap series, indicating that the tap is the correct size for the component fit grade, 3B grade taps are suitable for 2B grade components, and taps marked with "X" indicate a larger tolerance, used for precision taps, electroplated or heat-treated parts, or for materials close to (elastic memory).
Electroplated tapping threads: For internal threads, you must choose a tap with a larger tolerance grade. A larger pitch diameter will result in a slightly larger thread size, but the increase in size after electroplating will bring the thread size back to the specified value.
Tap Cutting Processes
1) Hole Types and Chip Management
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2) Extrusion Cutting, Spiral Fluid Taps: Best suited for blind and deep hole machining, recommended for materials that produce sticky chips, ideal for interrupted cutting.
The core of a spiral fluid tap is very thin and is the weakest part of the tap design. Therefore, the speed should be 30%~40% lower than that of a straight fluid tap to prevent breakage.
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3) Pull-out Cutting
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4) Straight Fluid Taps: The strongest taps, recommended for materials that easily break chips, such as brass and cast iron or hardened steel. Usually requires coolant or gas to flush the chips in the flutes. Available in various cutting cone forms.
– Taper (Form A) "A" – Initial taper
– Plug (Forms B & D) "B/D" – Medium taper
– (Form C) "C" – Semi-flat or modified flat bottom
– (Form E) "E" – Flat bottom
Image 5) Forming tap: Its machining characteristic is that it does not produce chips in either through holes or blind holes.
Image 6) Comparison of cutting taps and forming taps
Image 7) Influence of bottom hole size on forming threads
Image Tap coatings
1) Advantages of coatings
① Surface treatment: Improves the appearance of HSS taps without changing the tap size.
② Increased tap life: High wear resistance, reduces friction and power consumption, reduces chipping and breakage, and slightly increases surface hardness.
③ Improves the surface quality and dimensional accuracy of the threaded hole: Keeps the cutting edge sharp, has a lubricating effect, reduces load and abrasion, and minimizes built-up edge.
2) What is built-up edge?
The residue left on the cutting edge where workpiece material is welded or seized.
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3) Traditional Surface Treatment
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4) Thin Film Coating
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Secrets to Successful Tapping
1) Determining the Thread Percentage
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The drill bit size determines the percentage of the thread root diameter to the thread height. The larger the drill bit diameter you use, the smaller the achieved thread height ratio!
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2) Choosing the Pilot Hole Size
Generally, 65% to 70% of the thread height is preferred!
An 83% thread height is only 2% stronger than a 65% thread height, but the tapping torque is more than twice as high!
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3) Common Problems
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① Causes of Clipping
Manual Tapping – Uncoordinated manual feed, feed too fast or too slow
Machine Tapping – Improper programming of asynchronous tapping cycles
Lead Screw Machines – Wear or loose lead screw adjusting nut causing backlash
Cam Feed Machines – Incorrect cam profile or wear
Pneumatic or Hydraulic Machines – Uncontrollable, pressure too high or too low
Gear Feed Machines – Improper gear adjustment or wear causing backlash
② Solving Clipping Problems
For the most precise threads, 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 on CNC Machines: Thread depth control, consistent hole-to-hole dimensions, elimination of clipping, and the ability to re-tape when necessary.
Image 7) Tool Holder Selection Image Image For asynchronous tapping – CNC machine tools with drill feed and fixed tapping cycle – Cam, gear, pneumatic, or hydraulic feed mechanism For synchronous tapping – When encountering oversized/undersized threads Image 8) Tool Holder Maintenance Proper tool holder maintenance ensures high-quality thread machining and tap lifespan. Internal mechanisms should be free of chips and debris.
Frequent lubrication ensures smooth component movement and prevents rust.
Frequent testing of tool holders, especially when using water-soluble coolant.
Troubleshooting
1) Oversized Threads
Tapping CNC Settings
When tapping on a CNC machine without a rigid tapping cycle:
Program the feed rate to 95%~98% of the tap's reverse stroke.
Use a tool holder with only extension or a telescopic tool holder with compression locking.
When tapping on a CNC machine with a rigid tapping cycle:
Program the tap lead to 100% of the feed rate.
Use a solid tool holder or a synchronized tool holder.
If the cutting causes the thread gauge's stop to pass:
Reprogram, following the "non-rigid" procedure.
Consider using a quick-change adapter. Minimal "float" is possible.
Image 2) Chip entanglement
Image Change tap type
→ Straight flute
→ Smaller helix angle
Shorten the cutting cone
Change the rake angle shape
Increase the number of flutes
Change the speed
Smaller hook shape
If it is rigid tapping, increase the pecking cycle
Consider extrusion taps
3) Lubrication selection
For tapping, the purpose of lubrication is to reduce friction. Therefore, generally, tapping uses lubricant, not coolant; if coolant is used, EP (ultra-high pressure) or HP (high pressure) additives should be added.
Taps have a fixed large feed rate, controlled by the tap pitch, and the drilling feed rate can be adjusted to control the load.
Image 4) Coolant application
Image Image Tap selection basics
Before selecting a tap, we need to know:
Hole type, through hole, blind hole, or deep hole
Minimum drilling depth
Minimum required thread depth
Whether to consider using an extrusion tap
Workpiece material to be tapped
