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What is a thread

What is a thread?

Thread refers to the spiral shaped continuous convex part with specific section made on the surface of cylindrical or conical parent body. Protrusion refers to the solid part on both sides of the thread, also known as tooth. Threads are divided into cylindrical threads and conical threads according to their parent shape; It can be divided into external thread and internal thread according to its position in the parent body, and can be divided into triangular thread, rectangular thread, trapezoidal thread, serrated thread and other special shape threads according to its section shape (tooth shape).
In machining, The thread is on a cylindrical shaft (or inner hole surface) it is cut with a cutter or grinding wheel. At this time, the workpiece rotates, and the cutter moves a certain distance along the axial direction of the workpiece. The trace cut by the cutter on the workpiece is the thread. The thread formed on the outer circular surface is called the external thread. The thread formed on the inner hole surface is called the internal thread. The basis of the thread is the spiral line on the surface of the circular shaft. Generally, if the section of the thread is triangular, then It is called triangular thread; The section is trapezoidal, which is called trapezoidal thread; The section is serrated, which is called serrated thread; The section is square, which is called square thread; The section is circular arc, which is called circular arc thread, etc.

Main geometric parameters of cylindrical thread

  • ① Outer diameter (major diameter), the imaginary cylinder diameter coincident with the top of external thread or the bottom of internal thread. The nominal diameter of thread is the major diameter.
  • ② Inner diameter (small diameter), the imaginary cylinder diameter coincident with the bottom of external thread or the top of internal thread.
  • ③ Pitch diameter, the bus passes through the imaginary cylinder diameter with the same width of the protrusion and groove on the tooth.
  • ④ Pitch, the axial distance between two corresponding points on the pitch line of adjacent teeth.
  • ⑤ The axial distance between two corresponding points on the pitch diameter line of adjacent teeth on the same helix.
  • ⑥ Profile angle, the included angle between two adjacent tooth sides on the thread profile.
  • ⑦ The angle between the tangent of the helix on the pitch diameter cylinder and the plane perpendicular to the thread axis.
  • ⑧ Working height, the distance of the overlapping part on the two-phase matched thread profile in the direction perpendicular to the thread axis, etc. The nominal diameter of the thread is the outer diameter, except that the pipe thread takes the inner diameter of the pipe as the nominal diameter. The thread has been standardized, including metric system (metric system) and British system. The international standard adopts metric system, and China also adopts metric system.

Except that the pipe thread takes the pipe inner diameter as the nominal diameter, the other threads take the outer diameter as the nominal diameter. The thread pair whose thread rising angle is less than the friction angle does not loosen under the action of axial force, which is called self-locking, and its transmission efficiency is low.
In cylindrical thread, triangular thread has good self-locking performance. It is divided into coarse teeth and fine teeth. Generally, coarse threads are used for connection. Fine teeth have small pitch, small rising angle and better self-locking performance. They are often used in small parts, thin-walled tubes, vibration or variable load connection, and fine-tuning devices.
The tooth form of conical thread is triangular, which mainly depends on the deformation of the tooth to ensure the tightness of the thread pair. It is mostly used for pipe fittings.

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Elements of thread

Thread includes five elements: tooth profile, nominal diameter, number of lines, pitch (or lead) and direction of rotation.
1. Tooth profile
In the section area passing through the thread axis, the contour shape of the thread is called the tooth profile. There are triangle, trapezoid, sawtooth, arc and rectangle tooth types.
Comparison of thread profile:
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2. Diameter
The thread has major diameter (D, d), pitch diameter (D2, D2) and small diameter (D1, D1). The nominal diameter is used to represent the thread, and the nominal diameter is the diameter representing the thread size.
The nominal diameter of ordinary thread is the major diameter.
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External thread (left), internal thread (right)

3. Number of lines
The thread formed along one helix is called single thread, and the thread formed by two or more helix equally spaced along the axial direction is called multi thread.
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Single thread (left), double thread (right)

4. Pitch and lead
Pitch (P) is the axial distance between two corresponding points on the pitch diameter line of two adjacent teeth.
Lead (PH) is the axial distance between two adjacent teeth on the same helix on the middiameter line.
In case of single thread, lead = pitch; Lead = pitch for multi thread × Number of lines.
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5. Rotation direction
The thread screwed in when rotating clockwise is called right-hand thread;
The thread screwed in when rotating counterclockwise is called left-hand thread.
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Left hand thread, right hand thread

Structural classification of threads

Threads are divided into triangular threads, rectangular threads, trapezoidal threads and serrated threads according to their section shape (tooth profile). Triangular threads are mainly used for connection (see thread connection). Rectangular, trapezoidal and serrated threads are mainly used for transmission. The threads distributed on the outer surface of the mother body are called external threads, and those on the inner surface of the mother body are called internal threads. The threads formed on the cylindrical mother body are called cylindrical threads, and the threads formed on the conical mother body are called conical threads. The threads are divided into left-hand and right-hand threads according to the direction of the helix, generally right-hand threads. Threads can be divided into single line and multi line, and most of the threads used for connection are single line; When used for transmission, it requires fast lifting or high efficiency. Double line or multi line is adopted, but generally no more than 4 lines.

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Thread direction
Triangular threads are mainly used for connection, while rectangular, trapezoidal and serrated threads are mainly used for transmission; According to the helix direction, it is divided into left-hand thread and right-hand thread, generally right-hand thread; According to the number of helix, it can be divided into single thread, double thread and multi thread thread; The connection is mostly single wire, and the transmission is double wire or multi wire; According to the size of teeth, it can be divided into coarse thread and fine thread. According to different application occasions and functions, it can be divided into fastening thread, pipe thread, transmission thread, special thread, etc.
In cylindrical thread, triangular thread has good self-locking performance. It is divided into coarse teeth and fine teeth. Generally, coarse threads are used for connection. Fine teeth have small pitch, small rising angle and better self-locking performance. They are often used in small parts, thin-walled tubes, vibration or variable load connection and fine-tuning devices. Pipe thread is used for tight connection of pipe fittings. Rectangular thread has high efficiency, but it is often replaced by trapezoidal thread because it is not easy to grind and it is difficult to screw in and center the internal and external threads. The working edge of serrated thread is close to the rectangular straight edge, which is mostly used to bear unidirectional axial force.
The tooth form of conical thread is triangular, which mainly depends on the deformation of the tooth to ensure the tightness of the thread pair. It is mostly used for pipe fittings.
According to the tightness, it can be divided into sealed thread and non sealed thread.

Types of threads

  • According to the tooth type, it can be divided into triangular, trapezoidal, rectangular, sawtooth and arc threads;
  • According to the thread direction, it can be divided into left rotation and right rotation;
  • According to the number of spiral lines, it can be divided into single line and multi line;
  • According to the shape of thread matrix, it is divided into cylinder and cone.

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Marking of threads

(1) Common thread
Ordinary threads are most widely used. Threads on threaded fasteners (bolts, studs, screws, nuts and other parts) are generally ordinary threads.
Common thread is divided into coarse common thread and fine common thread. Fine tooth common thread is mostly used in precision parts and thin-wall parts.
In the marking of thread, the pitch of fine tooth ordinary thread must be marked, while the pitch of coarse tooth ordinary thread is generally not marked.
The marking of common threads consists of five parts:

For example: Feature code Size Code Tolerance zone code Code of Screwing Length Rotation direction code
M 10X1 7H L LH

1) Feature code
M (for common thread)
2) Size code
Size Codes: nominal diameter × Pitch (the lead and pitch of multi thread shall be indicated, and the pitch of single thread coarse thread ordinary thread shall not be indicated).
“M10” refers to single line coarse thread ordinary thread with nominal diameter of 10 mm and pitch of 1.5 mm.
“M10 × 1 “refers to single thread fine thread ordinary thread with nominal diameter of 10mm and pitch of 1mm.
2) Tolerance zone code for common threads
It consists of tolerance class (number) and basic deviation (external thread is represented by lowercase letters and internal thread is represented by uppercase letters), such as 5g6g, 6G, 6H and 7H.
When the codes of pitch diameter tolerance zone and top diameter tolerance zone of thread are different, they shall be indicated respectively, such as M10-5g and 6G
When the middle diameter and top diameter tolerance zone codes are the same, only one code shall be noted, such as M10 × 1-7H
When the tolerance zone code is 6G or 6H (nominal diameter ≥ 1.6mm), it is not marked.
3) Screwing length of ordinary thread
There are three types: long, medium and short, which are represented by codes L, N and S respectively.
M10-5g6g-S is a thread with short screw in length
M10-7H-L is a thread with long screw in length
When the thread is of medium screwing length, the code n is not marked.
In case of special needs, the value of screwing length can be indicated.
M20×2-5g6g-40
4) Rotation direction code
Mark LH for left rotation and not for right rotation.
M10-7H-L-LH is a left-hand thread
M10-7H-L is a right-hand thread
(2) Pipe thread
Pipe thread is generally used in the connection of pipelines (water pipe, oil pipe, gas pipe, etc.). The marking of pipe thread shall be marked with a guiding method, and the leader shall point to the major diameter of the thread.
Marking of pipe threads:
It is composed of thread feature code, size code and direction of rotation. The size code is not the size of the major diameter of the thread, but the size of the drift diameter (Imperial System) of the pipe. The rotation direction not indicated in the mark is right rotation.
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Examples of markings for 55° non-sealed pipe threads.

  • G: Thread characteristic designator for non-sealed pipe threads.
  • G3/4: Single-threaded right-handed cylindrical female thread with a size designation of 3/4.
  • G3/4A or G3/4B: single threaded right-handed cylindrical male thread with a size designation of 3/4, with A and B in the marking being the tolerance class of the thread median.
  • LH in G3/4LH and G3/4A-LH indicates a left-hand thread, and the thread pair consisting of both is marked with the male thread designation only.

Examples of markings for 55° sealed pipe threads.

  • Rp3/4LH: A single-line, left-handed, cylindrical female thread with a size designation of 3/4.
  • Rc3/4: single-line, right-handed, tapered female thread with a size designation of 3/4.
  • Rp/R13/4 LH and Rc/R23/4: female and male threads screwed together to form a threaded pair.
  • Rp : thread characteristic designation for sealed cylindrical female threads.
  • Rc : Thread feature designator for sealing conical female threads.
  • R1: Characteristic designator for conical male threads mating with cylindrical female threads.
  • R2: feature code for conical male threads with conical female threads.

(3) Trapezoidal and serrated threads
Trapezoidal threads and serrated threads are commonly used on screws that transmit motion and power. Trapezoidal threads work with forces on both sides of the thread, while serrated threads work with forces on one side.
The markings for trapezoidal and serrated threads are similar to those for common threads.
Example of marking for trapezoidal threads.
Tr40×7LH-7e, trapezoidal thread (thread feature code Tr), nominal diameter φ40, single thread, pitch 7, left-hand rotation, mid-diameter tolerance band code 7e; medium rotation length. Note: Only the middle diameter tolerance zone designation is marked, and there are only two types of thread lengths (designation N and L).
When the thread is multi-threaded, the markings are
Tr40×14 (P7)-7e, where “14” is the lead and “7” is the pitch, and the thread is double threaded.
For trapezoidal threads, the sub-threads are marked as Tr40×7-7H/7c, with the tolerance zone of the inner thread in front and the tolerance zone of the outer thread in the back, separated by a “/”.

Thread machining

Tapping and threading

Tapping
The processing method of machining internal thread with tap in the hole is called tapping.
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The formula for calculating the diameter of the bottom hole: Consider the plasticity of the material according to its size.
For steel parts and metal materials with large plasticity: D hole = D – P

  • D hole: diameter of threaded bottom hole drill bit
  • D: Large diameter of internal thread
  • P: Pitch

Example: We want to tap M10 thread on a steel part to calculate what is the diameter of the bottom hole?
According to the formula D hole = D-P = 10-1.5 = 8.5 mm
For cast iron and less plastic materials: D hole = D-(1.05~1.1)P
Example: What is the diameter of the bottom hole when we want to tap M10 thread on cast iron?
According to the formula D hole = D-(1.05-1.1)×1.5=8.35~8.42mm
Bushing threads
The method of cutting external threads on a round bar or pipe with a plate is called socket threading.
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Calculation formula for round rod diameter: d rod = d – 0.13P

  • d rod: diameter of round rod before setting thread, mm
  • d: large diameter of thread, mm
  • p:pitch, mm

Example: We want to make a M10 screw, what is the diameter of the round rod used?
Should be based on the formula d rod = d – 0.13P =10-0.13×1.5=9.8mm

Thread turning

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External thread and internal thread

Table of common British thread standards abroad and common thread standards in China
Code, name and standard number of British thread commonly used abroad

Mark code Name (or purpose) Country and Standard number Remarks
B.S.W. Standard Wyeth coarse thread series, general purpose cylindrical thread British Standard BS84 Inch thread with 55 ° profile angle
B.S.F. Standard Wyeth fine thread series, general purpose cylindrical thread
Whit. S Additional Wyeth optional series, general purpose cylindrical threads
Whit Non standard thread of Wyeth profile
UN Same thread of constant pitch series American Standard ANSI B1.1 Internal and external threads of standard profile (flat or randomly rounded base)
UNC Same thread of coarse thread series
UNF Same thread of fine thread series
UNEF The same thread of superfine tooth series
UNS  Same thread of special series
UNR The same thread of constant pitch series with circular tooth base Unr, unrc, unrf of circular tooth base
UNRC The same thread of arc base coarse thread series
UNRF Circular tooth base fine thread series the same thread Unref and unrs are only used for external thread and
UNREF The same thread of ultra-fine teeth with arc base
UNRS Special series of the same thread with circular tooth base There is no internal thread
NPT  Taper pipe threads for general use American Standard ANSI B1.20.1 British pipe threads with 60 ° profile angle
NPSC Straight pipe thread for pipe joint
NPTR Taper pipe thread for guide rod connection
NPSM Straight pipe thread for mechanical connection
NPSL Straight pipe thread for lock nut
NPSH Straight pipe thread for hose connection
NPTF Dry seal standard taper pipe thread American Standard ANSI B1.20.3 Type I
PTF-SAE SHORT Dry seal short taper pipe thread Type II
NPSF Straight pipe internal thread for dry sealing standard fuel oil Type III
NPS1 Dry sealing standard straight pipe internal thread Type IV
ACME  Trapezoidal thread for general purpose American Standard ANSI B1.5 Inch Drive thread with 29 ° profile angle
  • ① Dimensions and tolerances all diameter and pitch combinations other than the standard series calculated using the same formula as the standard series.
  • ② China’s 60 ° conical pipe thread GB/T 12716-1991 is equivalent.
  • ③ Acme threads include general-purpose and centering trapezoidal threads, of which the performance of general-purpose is similar to that of trapezoidal threads specified in Chinese standard GB/T 5796-1986.

Standard for threads

British Standard

  • B.S.W. Standard Whitworth coarse thread series, general purpose cylindrical threads   
  • BS84 Fractional threads with a 55° tooth angle
  • B.S.F. Standard Whitworth fine thread series, general purpose cylindrical threads
  • S additional Whitworth optional series, general purpose cylindrical threads
  • Whit’s non-standard threads

American Standard

  • UN Constant Pitch series of the same thread   
  • ANSIB1.1 Female and male threads of standard thread type (with flat or randomly rounded bottom)
  • UNC Same thread for coarse thread series
  • UNF Same thread for fine thread series
  • UNEF Same thread for extra fine thread series
  • UNS Same thread for special series
  • UNR Same thread for constant pitch series with circular threads UNR, UNRC, UNRF, UNRS with circular threads
  • UNREF, UNRS for external threads only and no internal threads
  • UNRC same thread for coarse thread series with circular thread base
  • UNRF same thread for fine thread series with circular bottom
  • UNREF Series of extra fine threads with circular bottom
  • UNRS Same thread for special series with circular bottom
  • NPT General purpose tapered pipe thread  
  • ANSIB1.20.1 Inch pipe threads with a 60° tooth angle
  • NPSC Straight pipe thread for pipe joints
  • NPTR Tapered pipe thread for guide rod connection
  • NPSM Straight pipe thread for mechanical connection
  • NPSL Straight pipe thread for lock nut
  • NPSH Straight pipe thread for hose connection
  • NPTF Dry Seal Standard Tapered Pipe Thread 
  • PTF-SAE SHORT Dry Seal Short Tapered Pipe Thread Type II
  • NPSF Dry Seal Standard Straight Female Pipe Thread for Fuel Type III
  • NPS1 Dry Seal Standard Straight Female Pipe Thread for General Use Type IV
  • ACME Trapezoidal thread for general purpose 
  • ANSIB1.5 Inch drive thread with a 29° tooth angle

Chinese common thread standard

  • Common thread Common thread Basic thread type GB/T192 -1981 Equivalent to ISO68 
  • Common thread Diameter and pitch series GB/T193 -1981 Equivalent to ISO261 
  • Common thread Basic size GB/T196 -1981 Equivalent to ISO724 
  • Common thread Tolerance and fit GB/T197 -1981 Equivalent to ISO965/1 
  • Common thread Deviation table GB/T2516 -1981 Equivalent to ISO965/3 
  • Selection series of common threads for commodity fasteners JB/T7912 -1999 Equivalent to ISO262 
  • GB/T9145 -1988 Equivalent to ISO965/2 for medium precision common threads for commodity fasteners 
  • Optical threads Special fine threads for optical instruments ZBN30006-1988 
  • Eyepiece threads for optical instruments JB/T8204-1995 
  • Short threads for optical instruments JB/T5450-1991 
  • Tight-fitting threads Transition-fitting threads GB/T1167-1996 
  • Overfitting thread GB/T1181-1998 
  • Small thread Small thread type GB/T1505 4.1-1994 Equivalent to ISO1501 
  • Small thread diameter and pitch series GB/T1505 4.2-1994 
  • Small thread basic size GB/T1505 4.3-1994 
  • Small thread tolerance GB/T15054.4-1994 
  • Small thread limit size GB/T15054.5-1994 
  • MJ thread MJ thread base thread type GJB/T3.1 -1982 ISO5855 
  • MJ thread Bolt and nut thread size and tolerance GJB/T3.2 -1982 
  • Dimensions and tolerances of the threads of MJ threaded pipeline parts GJB/T3.3 -1985 
  • MJ threaded structure size and tolerance GJB/T3.4 -1985
  • MJ thread calculation formula GJB/T3.5 -1985 
  • MJ thread head and tail GJB52-1985
  • Trapezoidal thread Trapezoidal thread Thread type GB/T5796 .1-1986 Equivalent to ISO2901 
  • Trapezoidal thread Diameter and pitch series GB/T5796 .2-1986 Equivalent to ISO2902 
  • Trapezoidal thread Basic size GB/T5796 .3-1986 Equivalent to ISO2904 
  • Trapezoidal thread Tolerance GB/T5796 .4-1986 Equivalent to ISO2903 
  • Trapezoidal thread Limit size GB/T12359-1990 
  • Trapezoidal thread screw and nut technical conditions for machine tools JB/T2886-1992 
  • Trapezoidal threads for forged steel valves with short teeth JB/TQ374-1985 
  • Serrated thread Serrated (3 °, 30 °) thread tooth type GB/T13576.1-1992 
  • Serrated (3 °, 30 °) thread diameter and pitch series GB/T13576.2-1992 
  • Serrated (3 °, 30 °) thread basic size GB / T13576.3-1992 
  • Serrated (3 °, 30 °) thread tolerance GB / T13576.4-1992 
  • Hydraulic press 45 ° serrated thread tooth type and basic size JB2076-1984 
  • GB/T7306 -1987 Equivalent to ISO7/1 
  • Non-threaded threads GB/T7307 -1987 Equivalent to ISO228/1 
  • 60 ° conical pipe thread GB/T12716-1991 
  • Metric tapered thread GB/T1415-1992 
  • Common thread size series for pipeline screw-in end GB/T1414-1978 
  • Special thread for gas cylinder GB/T8335-1998 
  • General reference thread terminology GB/T1479 1-1993 Equivalent to ISO5408 

Standard thread labeling methods specified in the national standards

The first letter represents the thread code in the standard thread labeling method specified in the national standard, for example: M for common thread, G for non-threaded pipe thread, R for threaded pipe thread, Tr for trapezoidal thread, etc. The second number indicates the nominal diameter of the thread, which is also the large diameter of the thread. It indicates the maximum diameter of the thread in millimeters. The symbols that follow are pitch, rotation, mid-diameter tolerance designation, top diameter tolerance designation, and spin length designation.

Purpose and application of thread

Machining of threads

Dies

Method of machining threads directly from a die.

Rolling

This method is used to obtain threads by plastic deformation of the workpiece with a forming and rolling die. Thread rolling is usually done on a thread rolling machine. It is suitable for mass production of external threads of standard fasteners and other threaded joints. The outer diameter of the rolled thread is generally not more than 25 mm, the length is not more than 100 mm, the thread accuracy is up to grade 2 (GB197-63), and the diameter of the blank used is roughly equal to the middle diameter of the thread being processed. However, for soft workpieces, a slotless extrusion tap can be used to cold extrude internal threads (up to a maximum diameter of 30 mm), which works similarly to tapping. The torque required for cold extrusion of internal threads is about one times larger than tapping, and the machining accuracy and surface quality are slightly higher than tapping.
The advantages of thread rolling are: the surface roughness is smaller than that of turning, milling and grinding; the surface of the thread after rolling is cold hardened to improve strength and hardness; the material utilization rate is high; the productivity is exponentially higher than that of cutting, and it is easy to automate; and the life of the rolling die is long. However, rolling thread requires the hardness of the workpiece material to be no more than HRC40; it requires high accuracy of the blank size; it requires high accuracy and hardness of the rolling die, and it is difficult to make the die; it is not suitable for rolling threads with asymmetric tooth shape.
According to the different rolling dies, thread rolling can be divided into two categories: thread rolling and thread rolling.
Two thread rolling plates with thread tooth shape are staggered by 1/2 pitch, the static plate is fixed, and the dynamic plate makes a reciprocating linear motion parallel to the static plate. When the workpiece is fed between the two plates, the moving plate advances to roll the workpiece, so that its surface plastic deformation into threads.
There are three types of thread rolling: radial thread rolling, tangential thread rolling and thread rolling with rolling head.
Radial thread rolling: 2 (or 3) threaded wheels are installed on parallel axes, and the workpiece is placed on the support between the two wheels, and the two wheels rotate at the same speed in the same direction, and one of them also makes radial feeding movement. The workpiece is driven by the thread roller to rotate and the surface is squeezed radially to form threads. A similar method can be used for some screws that do not require high precision.
Tangential rolling: Also known as planetary rolling, the rolling tool consists of a rotating central thread roller and three fixed curved thread plates. When rolling, the workpiece can be fed continuously, so the productivity is higher than that of thread rolling and radial rolling.
Rolling head thread rolling: in automatic lathe, generally used for processing short threads on the workpiece. There are 3 to 4 thread rolling wheels evenly distributed around the workpiece in the rolling head. When rolling, the workpiece rotates and the rolling head feeds axially to roll the threads out of the workpiece.

Thread cutting

This refers to the method of machining threads on a workpiece with a forming tool or abrasive.
Thread milling: Milling on a thread milling machine with a disc milling cutter or comb milling cutter. Disc milling cutters are mainly used for milling trapezoidal external threads on workpieces such as screw and worm shafts. Comb milling cutters are used for milling internal and external threads and tapered threads. Since the workpiece is milled with a multi-flute cutter and the length of the working part is larger than the length of the thread to be machined, the workpiece only needs to rotate 1.25 to 1.5 revolutions to complete the process, so the productivity is very high. The pitch accuracy of thread milling can generally reach 8 to 9, and the surface roughness is R5 to 0.63 micron. This method is suitable for batch production of threaded workpieces of general accuracy or rough machining before grinding.
In today’s technologically advanced world, machining centers have become an irreplaceable tool for manufacturers, so thread machining is increasingly done by milling, which is highly efficient and simplifies the steps for high precision, thus bringing greater benefits to companies. In order to meet this demand, many companies have emerged. We offer professional solutions for special threads.
Thread grinding: It is mainly used to process the precision threads of hardened workpieces on thread grinding machines, and is divided into two types of grinding wheels: single-line grinding wheel and multi-line grinding wheel, depending on the shape of the grinding wheel cross-section. Single-wire grinding can achieve a pitch accuracy of 5 to 6, a surface roughness of R1.25 to 0.08 micron, and easy wheel dressing. This method is suitable for grinding precision screws, threaded gauges, worm gears, small batches of threaded workpieces and shovel grinding precision hobs. Multiline grinding is divided into two types: longitudinal grinding and plunge grinding. In the longitudinal grinding method, the width of the grinding wheel is smaller than the length of the thread to be ground, and the grinding wheel can be moved longitudinally in one or several strokes to grind the thread to its final size. In the plunge grinding method, the width of the grinding wheel is greater than the length of the thread to be ground, and the grinding wheel is cut radially into the surface of the workpiece, and the workpiece can be ground in about 1.25 revolutions, so the productivity is higher, but the accuracy is slightly lower and the dressing of the grinding wheel is more complicated. The plunge grinding method is suitable for shoveling large quantities of taps and grinding certain threads for fastening.
Thread grinding: A nut or screw type thread grinding tool made of soft material such as cast iron is used to grind the machined threads on the workpiece in the forward and reverse direction to improve the pitch accuracy. Hardened internal threads are also usually ground to eliminate deformation and improve accuracy.
Tapping and threading: Tapping is to use a certain torque to screw the tap into the pre-drilled bottom hole on the workpiece to produce internal threads. Threading is to cut external threads on a bar (or tube) workpiece with a plate. The accuracy of tapping or threading depends on the accuracy of the tap or plate. Although there are many ways to process internal and external threads, the internal threads of small diameters can only be processed by taps. Tapping and threading can be done manually, or by lathe, drill press, tapping machine and threading machine.

Turning of threads

Note for thread turning: Considering the expansion of the thread, the large diameter of the external thread (nominal diameter d) should generally be 0.2~0.4mm smaller than the basic size (about 0.13P), and the width of the top of the thread should be 0.125P (P is the pitch) after the thread is turned. Thread cutting should take care to set up sufficient ascending feed section δ1 and descending retreat section δ2 at both ends to eliminate the non-standard pitch thread section at both ends due to variable speed. Similarly, the feed speed trim function and the feed pause function are not valid during thread cutting; if the feed pause button is pressed at this time, the tool will stop moving only after the threaded segment is processed. The feed amount for thread machining can be referred to the thread base diameter, which is the final feed position of the threaded tool. The small diameter of the thread is the large diameter – 1.2 times the pitch; the amount of feed should be reduced continuously, depending on the tool and the workpiece material, but the last feed should not be less than 0.1 mm; the quality of the thread can be judged by observing the thread shape after the thread is finished. However, it should be noted that for external threads, when the top of the thread is not pointed, increasing the amount of cutter entry will increase the diameter of the thread, and the amount of increase depends on the plasticity of the material. For standard threads, the thread ring gauge or plug gauge is used to measure the thread. When measuring external threads, if the “over-end” ring gauge (through gauge) is screwed in, but the “stop-end” ring gauge (stop gauge) is not screwed in, it means the threads processed meet the requirements, and the other way around, they are not qualified. When measuring the internal thread, use the thread plug gauge to measure in the same way. In addition to the thread ring gauge or plug gauge measurement, other gauges can be used to measure the threads, such as the thread micrometer to measure the thread diameter.
Thread bottom hole D drill = D – P

  • D: thread diameter
  • P: Pitch

Determine the purpose: not to squeeze the thread when tapping, so that the top of the thread and the bottom of the thread do not bite each other (this phenomenon also occurs when tapping with a tap) to ensure the effective working height, etc.!
Bottom hole depth H drilling = h effective + 0.7D

  • h: effective thread effective depth
  • Dthread large diameter

Determine the purpose: mainly when tapping with a tap without a through hole the tap cutting part has a taper angle and the end cannot cut out a complete tooth, so the drilling depth should be greater than the effective depth of the thread.
M8 has coarse and fine threads

  • 1. Coarse thread M8, pitch P=1.25, coarse thread notation: M8 (omitting pitch).
  • 2. Fine thread M8, pitch P=1, fine thread notation: M8x1.

Thread sealing

Any plane can not be completely close contact, need to leak-proof seal, the traditional method is to use rubber, asbestos, metal and other gaskets, but due to aging or corrosion will soon leak. Instead of solid gasket, anaerobic adhesive can be used to achieve close contact after curing, making the seal more durable. Metrolite Super Lubricant is a special inert material with many uses, which is used for sealing of threaded pipe joints and threaded plugs, sealing of mating surfaces of flanges, sealing of combined surfaces of mechanical cases, etc., all of which have good leak-proof effects.
Super lubricant is mainly used to reduce metal-to-metal contact. As a thread sealing compound, the product forms a contact surface between the male and female threads that protects the joint from friction and wear, while withstanding pressures of 1407 kg/cm2, even on worn, corroded or mismachined thread surfaces. It is also an excellent additive for gearboxes, forming a thin film on the internal parts. This reduces friction, gear noise and leakage. It also significantly reduces torque stresses and meets power decompression requirements. It can be used on gasket faces or as a packing supplement to prevent fluid leakage by sealing. It can be applied at temperatures up to 316°C. The product can be constructed in stainless steel, aluminum, iron, barium, fiberglass, plastic and will not be affected by acids, alkalis or common solvents.
Thread sealing application instructions: Apply Metrolite Blu-Goo Super Seal Lubricant to both female and male threads, using a brush or other application tool to apply evenly.

How to measure the threads

  1. For general standard threads, thread ring gauge or plug gauge is used to measure.
  2. Thread micrometer is used to measure the thread median. The method of measuring the middle diameter of the thread with a measuring pin is called the three-pin method.
  3. Thickness vernier calipers are used to measure the trapezoid thread diameter and worm diameter.
  4. Other parameters are measured with special gauges and instruments.

20211224221500 46901 - What is a thread
Measurement of thread

Inspection and measurement

Inspection system

① The hardware system thread parameter automatic inspection system is based on PC architecture, mainly composed of lighting system, CCD camera and optical imaging system, image acquisition card, image processing system, mechanical inspection system and electric control system.
During the measurement, after illumination by the lighting system, the thread image of the fastener is collected by the CCD camera and optical imaging system, and then the digital image is transmitted to the computer through the image acquisition card. After processing the digital image according to the corresponding algorithm, the computer calculates the parameters such as thread angle, pitch and median diameter, and compares them with the standard to calculate the deviation.
② The software system for automatic thread parameter inspection mainly includes the system control module, image processing module, etc.
System control module: initialization; complete the initial configuration of the system image interface card and standard parameters. Image operation; capture and save the 8-bit gray image and capture the image to memory.
Image processing module: The image processing module includes image pre-processing algorithms for reducing noise and highlighting thread contour edge information, edge detection and extraction algorithms for detecting and extracting contour edge information, system calibration algorithms for measuring the actual size of digital images, and algorithms for calculating thread parameters, etc.

Measurement methods of thread

① Measurement of thread clamping angle
The thread clamping angle is also called the tooth angle. The measurement of the thread angle can be done by measuring the lateral angle, which is the angle between the side of the thread and the vertical surface of the thread axis. The approximate profile of the threaded tooth is sampled in a straight line section on both sides of the thread, and a linear least squares fit is performed on the sampled points.
② Pitch measurement
The pitch is the distance between a point on the thread and the corresponding point on the adjacent threaded tooth. The measurement must be parallel to the thread axis.
③ Measurement of the thread median
The thread median is the distance along the median line perpendicular to the axis, and the median line is an imaginary line.

Measurement results of thread

Using machine vision technology, we designed an automatic inspection system for measuring the thread parameters and measured the thread clamping angle, pitch and median diameter of the standard thread specimen. The measurement results showed that the thread parameters were within the specified error range. Due to the high efficiency and reliability of the visual inspection, it was further investigated that it could be used in production practice.
Pitch table

Metric thread nominal diameter and pitch table
Nominal diameter Pitch Nominal diameter Pitch Nominal diameter Pitch
1 0.25 12 1.75 36 4
2 0.4 14 2 39 4
2.5 0.45 16 2 42 4.5
3 0.5 18 2.5 45 4.5
3.5 0.5 20 2.5 48 5
4 0.7 22 2.5 52 5
5 0.8 24 3 56 5.5
6 1 27 3 60 5.5
8 1.25 30 3.5 64 6
10 1.5 33 3.5 68 6
Nominal diameter and pitch table of British screw thread
Nominal diameter Major diameter Narrow path Nominal diameter Major diameter Narrow path
3/16 4.63 3.7 7/8 21.96 19.3
1/4 6.2 5.1 1 25.11 22
5/16 7.78 6.5 11/8 28.25 24.7
3/8 9.36 7.9 11/4 31.42 27.9
7/16 10.93 9.3 11/2 37.73 33.5
1/2 12.5 10.5 8/15 40.85 35.8
9/16 14.08 12.1 4/13 44.02 39
5/8 15.65 13.5 8/17 47.15 41.5
3/4 18.81 16.4 2 50.32 44.7
Nominal diameter table of cylindrical pipe thread
Nominal diameter Major diameter Narrow path Nominal diameter Major diameter Narrow path
1/8 9.81 8.5 1 33.25 30.5
1/4 13.16 11.7 11/8 38.2 35.2
3/8 16.66 15.2 11/4 41.91 39.2
1/2 20.95 18.9 8/13 43.85 41.6
5/8 22.85 20.8 11/2 47.8 45.1
3/4 26.44 24.3 4/13 53.62 51
7/8 29.28 28.1 2 59.62 57
Table of nominal diameter of tapered pipe thread
Nominal diameter Major diameter Narrow path Nominal diameter Major diameter Narrow path
1/8 9.73 8.4 1 33.25 30
1/4 13.16 11.3 11/8 37.6 34.7
3/8 16.66 14.8 11/4 41.91 38.6
1/2 20.95 18.5 8/13 43.4 41.1
5/8 22.7 20.5 11/2 47.8 44.6
3/4 26.44 24 4/13 53.62 50.5
7/8 30.02 27.6 2 59.61 56.3

Source: Network Arrangement – China Thread Pipe Fitting Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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