Precision Techniques for Internal Thread Cutting in Workpiece Holes
Taps are tools designed for creating various internal threads in medium and small sizes. They have a straightforward structure and are easy to use. Taps can be operated manually or with machine tools, making them widely utilized in production processes. If you’re having trouble with tapping during machining, today, ANEBON will share some tips to help you gain a better understanding of the tapping process!
1. What is tapping?
Tapping is to use a tap to cut an internal thread inside the hole of a workpiece.
(1) Factors that determine tap performance include:
Workpiece material
Cutting speed
Cutting edge material
Tool handle
Tap type
Hole size
Tapping handle
Cutting fluid
Hole depth
(2) Pitch: The axial distance between two points on the mid-diameter line of two adjacent teeth on the thread.
(3) Lead: The axial distance between the corresponding points of two adjacent teeth on the same helical line. It is represented by the code S.
(4) Nominal diameter of thread: 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).
(5) Pitch diameter of thread: The pitch diameter is crucial because it determines the fit and strength of all threaded assemblies. It resides on the pitch line, where the width of the tooth matches the width of the adjacent groove.
2. Naming of threads
British threads: British thread sizes are designated in British units and were developed through a collaborative effort by the United States, the United Kingdom, and Canada based on a unified system.
Metric thread: Developed according to the ISO (International Organization for Standardization) system, it is the global standard for metric threads.
3. Designing high-performance tapping processes
(1) Perfect application
The factors to consider during the tapping process include workpiece design, tap design, and application. The aim is to minimize cutting forces while enhancing tap strength.
(2) Balancing various options: All aspects of the application must be considered
(3) Key points of tap design
1) For softer, sticky materials that produce long chips, the tap features a simple structure with a large rake angle, hook angle, back angle, and clearance. This design allows for easy chip breaking; however, the tap is generally fragile and has a larger chip space.
2) For harder materials, the tap is constructed with a heavy-duty design that includes a small rake angle and hook angle, as well as minimal back angle. It requires high cutting pressure and has a strong cutting edge to minimize chipping. The cross-section is large, but the chip space is limited.
(4) When designing taps, several factors should be considered, including the type of tap groove, the material of the tool, and any surface strengthening treatments. These design elements must be balanced to ensure effective cutting, efficient chip control, adequate lubrication, and sufficient torsional strength.
One of the significant challenges in tap design and metalworking is that the cutting process must be stopped and reversed midway while the tap remains in the groove.
4. Tap shape
(1) Type of tap cutting surface
① Correctly select the tap forward hook tap
② Correctly select a small tap or a negative hook angle tap
(2) Tap cutting cone
The lifespan of a tap increases significantly with each additional cutting bevel. Tests indicate that the tool’s life doubles with every extra half-tooth thread. Unlike other tools, the chip load of a tap can only be adjusted by modifying the number of chip flutes or the length of the cutting bevel.
(3) Back taper: Like all other tools, taps also have a slight back taper.
(4) Thread backing
The advantages of thread back relief are:
Efficient cutting reduces heat buildup and prevents material from wrapping around the tap, minimizing chip adhesion and built-up edges. Increased tapping speeds can be utilized to counteract the plastic deformation of the workpiece material.
The disadvantages of thread back relief are:
The cutting edge can become brittle and prone to breaking. If the spindle and clamping, including the floating tool holder, lack sufficient rigidity, the thread quality will deteriorate. Additionally, very small chips may become embedded in the cutting edge, leading to breakage when reversing the tool.
(5) Tap tolerance
Each tap has a dedicated pitch diameter size.
Taps marked with H or D tolerance (primarily American taps) indicate the thread size of the tap. The letter denotes whether the tap size is larger or higher than the basic pitch diameter (“H” for English sizes and “D” for Metric sizes) or smaller than the basic pitch diameter (“L” for English sizes and “DU” for Metric sizes). The actual tap size number is linked to the basic pitch diameter, such as H2, D3, L1, or DU2. Each tap has a specific pitch diameter size assigned to it.
Taps are commonly labeled with thread grades to indicate their specifications. The HP series of general-purpose taps signifies that the tap is the correct size for the CNC parts it will fit. Grade 3B taps are appropriate for use with grade 2B parts. Taps marked with an “X” have larger tolerances and are designed for precision tapping, electroplated or heat-treated parts, or materials that exhibit close (elastic memory) properties.
For internal threads that are electroplated, it’s essential to choose a tap with a larger tolerance grade. This larger pitch diameter results in a slightly oversized thread size. However, the increase in size after electroplating will bring the thread back to the specified dimensions.
5. Cutting treatment of taps
(1) Type of hole and chip treatment
(2) Extrusion cutting and spiral chip groove taps are particularly well-suited for processing blind holes and deep holes. Materials that tend to produce sticky chips are ideal for intermittent cutting. The core of spiral chip groove taps is quite thin, making it the most fragile part of their design. To prevent breakage, the cutting speed should be reduced by 30% to 40% compared to that of straight chip groove taps.
(4) Straight chip flute taps
It is recommended to use the strongest taps for materials that break easily, such as brass, cast iron, or hardened steel. These taps typically require a coolant or gas to help flush the chips into the chip flute. They also come in various cutting taper forms.
– Taper (Form A) “A” – Primary taper
– Plug (Forms B & D) “B/D” – Medium taper
– (Form C) “C” – Semi-flat bottom or modified flat bottom
– (Form E) “E” – Flat bottom
(5) Extrusion tap: Its processing characteristic is that no chips are generated in either through holes or blind holes.
(6) Comparison between cutting taps and extrusion taps
(7) Effect of bottom hole size on extruded thread
6. Coating of taps
(1) Advantages of coating
- Surface treatment enhances the appearance of HSS taps without altering their size.
- It extends the lifespan of the taps by providing high wear resistance, reducing friction and power consumption, and minimizing chipping and breakage. Additionally, it slightly improves surface hardness.
- This treatment improves the surface quality and dimensional accuracy of screw holes. It keeps the cutting edge sharp, lubricates to reduce load and abrasion, and minimizes built-up edge.
(2) What is built-up edge?
The residue of prototype CNC workpiece material welded or bitten on the cutting edge.
(3) Traditional surface treatment
(4) Thin film coating
7. Tips for successful tapping
(1) Determine the thread percentage
The drill bit hole size determines the percentage of thread base diameter to thread height. The larger the drill diameter you use, the smaller the thread height ratio you achieve.
(2) Selection of bottom hole size
Generally, 65% to 70% of the thread height is preferred!
A thread with 83% height is only 2% stronger than a thread with 65% height, but the tapping torque is more than twice that of a thread with 83% height.
(3) Frequently asked questions
① Causes of the top cutting
Manual Tapping: If the manual feed is not well-coordinated, it may be either too fast or too slow.
Machine Tapping: Improper programming of the asynchronous tapping cycle can lead to inconsistencies.
Lead Screw Machine: Wear on the screw or a loose screw adjustment nut may cause backlash.
Cam Feed Machine: An incorrect or worn cam profile can result in operational issues.
Pneumatic or Hydraulic Machine: High or low pressure can be difficult to control.
Feed Machine: Improper gear adjustment or wear can also lead to backlash.
② Solve the problem of top-cutting
For the most accurate threading, the feed rate must synchronize with the spindle speed. The feed rate and spindle rotation should align with the thread pitch.
(6) Advantages of synchronous tapping on custom aluminum CNC machines include consistent hole-to-hole size control and the elimination of the need for a top cutting tap when necessary.
(7) Selection of tool handle
For asynchronous tapping, CNC machines use drill feed with fixed tapping cycles, employing cam, gear, pneumatic, or hydraulic feed mechanisms.
For synchronous tapping, specific attention is required when encountering over/undersized threads.
(8) Toolholder maintenance
Maintaining tool holders properly is essential for producing high-quality threads and maximizing the lifespan of taps. It’s important to keep internal mechanisms free of chips and debris. Regular lubrication is necessary to ensure smooth movement of parts and to prevent rust. Additionally, it’s advisable to test tool holders regularly, particularly when using water-soluble coolant, in order to troubleshoot any issues.
(1) Too large threads
CNC settings for tapping
When tapping on a CNC machine without a rigid tapping cycle, set the feed to 95% to 98% of the tap’s backstroke. Use a tool holder that has only an extension or a telescopic tool holder with a compression lock.
When tapping on a CNC machine with a rigid tapping cycle, set the tap lead to 100% of the feed. Use either an integral tool holder or a synchronous tool holder.
If top cutting causes the end stop of the thread gauge to exceed its limits, reprogram the machine. Follow the “non-rigid” procedure and consider using a quick-change joint. A minimum “float” is allowed.
(2) Chip entanglement
Change tap type → Straight groove → Smaller helix angle
Shorten cutting cone
Change rake angle shape
Increase the number of grooves
Change speed
Small hook shape
If it is rigid tapping, increase the pecking cycle
Consider extrusion tap
(3) Lubrication selection
The purpose of lubrication during tapping is to reduce friction. As a result, lubrication is typically preferred over coolant when tapping. However, if coolant is used, it should contain EP (extra high pressure) or HP (high pressure) additives.
The tap operates at a fixed large feed rate determined by its pitch, while the drilling feed rate can be adjusted to manage the load effectively.
(4) Coolant application
9. Basics of tap selection
Before selecting a tap, we need to understand:
The type of hole, through hole, blind hole or deep hole The minimum drilling depth The minimum thread depth required Whether the workpiece material to be tapped with an extrusion tap is considered
If you want to know more or inquiry, please feel free to contact info@anebon.com
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