Introduction: Taps are cutting tools used to create internal threads of various medium and small sizes. They have a simple structure and are easy to use, capable of being operated by hand or on a machine. They are widely used in production.
What are the relevant knowledge points about tapping? This article provides a comprehensive summary.
What is tapping?
Tapping is the process of cutting internal threads in a hole of a workpiece with a tap.
(1) The factors that determine the performance of taps include:
– Workpiece material
– Cutting speed
– Cutting edge material
– Tool holder
– Tap form
– Hole size
– Tapping tool holder
– Cutting fluid
– Hole depth
(2) Pitch: The axial distance between two corresponding points on the mid-diameter line of adjacent threads on a screw thread.
(3) Lead: The axial distance between two corresponding points on adjacent threads of the same helix. It is denoted by the symbol S.
(4) Nominal diameter of a screw thread: Except for pipe threads where the nominal diameter is the inner diameter of the pipe (in inches), the nominal diameter of other screw threads is the major diameter of the thread (in metric units).
(5) Thread pitch diameter: The pitch diameter is the most crucial as it governs the fit and strength of all threaded assemblies. It lies on the pitch line, where the width of the tooth is equal to that of the adjacent tooth space.
The naming of threads
British standard thread: The British standard thread is a type of thread whose dimensions are marked in the British system. It was developed by the United States, the United Kingdom and Canada based on a unified system.
Metric thread: Developed based on the ISO (International Organization for Standardization) system, it is the global standard for metric threads.
Design high-performance tapping processing
(1) Perfect application
The factors to be considered in the tapping process include: workpiece design, tap design, and application. The objective is to minimize the cutting force while maximizing the strength of the tap.
(2) Balance various options: All aspects of the application must be taken into account.
(3) Key Points of Tap Design
1) For softer and more viscous materials that tend to form long chips:
– The tap structure is simple.
– The rake angle and hook angle are relatively large.
– The relief angle and clearance angle are relatively large.
– Free cutting.
– Prone to chipping.
– The tap as a whole is relatively fragile.
– The chip space is large.
2) For hard materials:
– The taps have a heavy-duty structure.
– The rake angle and hook angle are small.
– The clearance angle and relief angle are small.
– The cutting pressure is relatively high.
– The cutting edge design is robust to reduce chipping.
– The cross-sectional area is large.
– The chip space is limited.
(4) Factors to be considered in tap design: tap flute type, tool material, surface strengthening treatment. These design features must be balanced to provide appropriate cutting, chip control, lubrication and torsional strength. The tap must be stopped and reversed out of the hole during the cutting process, while the cutting remains in the groove. This poses the greatest challenge to tapping and tap design in metalworking.
The shape of a screw tap
(1) The types of cutting surfaces of taps
① Correctly select the right-hand helical flute tap.
② Correctly select a smaller-sized or negative rake angle tap.
(2) Tap cutting cone
Each additional cutting cone tooth will exponentially extend the service life of the tap. Tests show that for every half additional cutting thread, the tool life will double. Unlike other cutting tools, the chip load of a tap can only be changed by altering the number of chip flutes and the length of the cutting cone.
(3) Taper: Like all other cutting tools, taps also have a slight taper.
(4) Threaded shovel back
The advantages of the threaded shovel back include:
– Smooth cutting with less heat accumulation
– Less material adheres to the tap (less chip welding and built-up edge)
– Higher tapping speed can be adopted
– Compensates for the plastic deformation of the workpiece material
The disadvantages of the threaded shovel back are:
– The cutting edge becomes brittle and prone to chipping.
– Insufficient rigidity of the spindle and clamping (including floating tool holders) can cause poor thread quality.
– Very fine chips during reverse rotation may embed and cause the cutting edge to chip.
(5) Tap Tolerance
Each tap has a dedicated pitch diameter size.
Taps marked with H or D tolerances (mainly American taps)
– H/D tolerances indicate the thread size of the tap.
– The letters indicate whether the tap size is larger or higher 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 dedicated pitch diameter size!
Taps are often marked with thread grades.
– The universal HP tap series
– Indicate that the tap is the correct size for the mating part grade
– 3B grade taps are suitable for 2B grade parts
– Taps marked with “X” indicate a larger tolerance and are used for precision taps, electroplated or heat-treated parts, or for materials close to (elastic memory).
For the tapping of electroplated threads:
– For internal threads, you must select a tap with a larger tolerance grade.
– The larger pitch diameter will make the thread size slightly larger.
– The increase in size after electroplating will bring the thread size back to the specified value.
Cutting treatment of screw taps
(1) Types of holes and chip disposal
(2) Extrusion cutting, helical chip flute tap
• Most suitable for blind hole and deep hole processing
• Recommended for materials that produce adherent chips
• Very suitable for intermittent cutting
The core of the spiral flute tap is very thin and is the most vulnerable part in the design of the tap. Therefore, the speed should be 30% to 40% lower than that of the straight flute tap to avoid breakage.
(3) Draw cutting
(4) Straight chip flute tap
– The strongest tap
– Recommended for materials that break chips easily, such as brass, cast iron or hardened steel
– Usually requires coolant or gas to flush chips from the chip flute
– Can have various cutting cone forms
– Taper (Form A) “A” – Initial cone
– Plug (Forms B & D) “B/D” – Middle cone
– (Form C) “C” – Semi-flat bottom or modified flat bottom
– (Form E) “E” – Flat bottom
(5) Extrusion taps: The processing feature of this type is that no chips are produced whether in through holes or blind holes.
(6) Comparison between cutting taps and extrusion taps
(7) The Influence of Bottom Hole Size on Extruded Threads
Coating of screw taps
(1) Advantages of the coating
① Surface treatment
– Improves the appearance of HSS taps
– Does not change the size of the taps
② Longer service life of taps
– High wear resistance
– Reduced friction and power consumption
– Decreased chipping and breaking
– Slightly increased surface hardness
③ Improve the surface quality and dimensional accuracy of the screw holes
– Keep the cutting edge sharp
– Have a lubricating effect
– Reduce load and scratching
– Minimize built-up edge
(2) What is built-up edge?
The accumulation of workpiece material that is melted and welded or stuck to the cutting edge.
(3) Traditional surface treatment
(4) Thin Film Coating
The key to successful tapping processing
(1) Determine the thread percentage
The size of the drill bit for drilling determines the percentage of the thread bottom diameter to the thread height. The larger the diameter of the drill bit you use, the smaller the thread height ratio you will achieve!
(2) Selection of the bottom hole size
Generally, 65% to 70% of the thread height is preferred!
A thread with 83% height is only 2% stronger than one with 65% height, but the tapping torque is more than twice as much!
(3) Common Questions
① The reasons for the occurrence of clipping.
Manual tapping
– Manual feed is not coordinated, feed is too fast or too slow
Machine tapping
– Improper programming of asynchronous tapping cycle
Screw machine
– Backlash due to screw wear or loose adjusting nut of screw
Cam feed machine
– Incorrect or worn cam profile
Machine using pneumatic or hydraulic power
– Uncontrollable, pressure is too high or too low
Gear feed machine
– Backlash due to improper gear adjustment or gear wear
② Solve the problem of peak clipping
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 CNC machine synchronous tapping
• Thread depth control
• Consistent hole-to-hole dimensions
• Elimination of thread topping
• Retapping possible when necessary
(4) Handle Selection
For non-synchronous tapping
– CNC machines with fixed tapping cycles and feed by drilling – Cam, gear, pneumatic, or hydraulic feed mechanisms
For synchronous tapping
– When encountering oversized or undersized threads
(5) Knife Handle Maintenance
Proper maintenance of the tool holder can ensure the processing of high-quality threads and the service life of the tap.
– There should be no chips or debris inside the mechanism.
– Regular lubrication ensures the flexible movement of the components and prevents rust.
– Regularly test the tool holder, especially when using water-soluble coolant.
Troubleshooting
(1) The thread is too large.
Tapping CNC Settings
When tapping on CNC machines without rigid tapping cycles:
– Program the feed rate to 95% to 98% of the tap’s back-off distance.
– Use only extended toolholders or telescopic toolholders with compression locking.
When tapping on a CNC machine with a rigid tapping cycle:
– Program the tap lead as 100% of the feed rate.
– Use a solid shank or synchronous shank.
If the stop end of the thread gauge passes through after the top is cut off:
– Reprogram following the “non-rigid” procedure
– Consider using a quick-change adapter. There can be a minimum “float”
(2) Chip wrapping
– Change tap form: straight flute → smaller helix angle
– Shorten cutting cone
– Alter rake angle shape
– Increase number of flutes
– Alter speed
– Smaller hook
– If rigid tapping, add pecking cycle
– Consider extrusion tap
(3) Lubrication Selection
For tapping, the purpose of lubrication is to reduce friction. Therefore, generally, lubricants are used for tapping rather than coolant; if coolant is used, EP (extreme pressure) or HP (high pressure) additives should be added.
The tap has a fixed large feed rate, which is controlled by the pitch of the tap teeth. The feed rate of the drill can be adjusted to control the load.
(4) Application of Coolant
Selection Basis of Tap
Before choosing taps, we need to understand:
– Hole type: through hole, blind hole or deep hole
– Minimum drilling depth
– Required minimum thread depth
– Whether to consider using extrusion taps
– Material of the workpiece to be tapped
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