The thread is a profiled notch that runs continuously helically (i.e. as a helical line ) in a cylindrical inner or outer wall. This continuous depression is called a thread on a screw or in a nut . It is a modification of the inclined plane , with a translation of a circumferential force into a larger longitudinal force, e.g. B. in screw presses , wine presses and jacks.

Components with external threads (screws) and those with internal threads (nuts) always form matching pairs. For example , manufacturing tolerances recorded in standards ensure that, despite separate production, component pairs with the same nominal data always function.

In some cases, the mating thread is created when first joining. Examples are self-tapping screws or self-tapping nuts , sheet metal screws , wood screws and chipboard screws .

## Manufacturing

Threads can be generated without cutting ( forming ) or cutting ( thread cutting ) . The non-cutting processes are common in mass production and are preferred because they combine technical advantages (smooth surface, increased strength, unbroken material fiber) with high efficiency (no loss due to chips to be disposed of). Threads are cut primarily in parts that have already been machined (e.g. turned parts , milled parts ).

Die M8 for the production of external threads
• machining: The thread profile is of the form fair cutting tool in a cutting dies or die stock from the material worked out. Machine thread cutting is mainly done by screw turning , screw milling or screw grinding . So-called thread whirling (variant of whirling ) produces the same high level of accuracy as thread grinding , but is much faster. Several turning tools circle the blank eccentrically and cut out short "comma chips". The threads are finished one after the other as the blank rotates slowly in the same direction.
• non-cutting: The blank has a thread flank diameter. The tool presses in the profile and displaces the material from the thread root into the thread tips. The bolt is located between two or three driven, profiled thread rolls made of high-speed steels .

Set of three M5 taps for the production of internal threads
• in holes pre-drilled with core diameter with a screw tap by hand or machine screw drilling .
• machine with thread formers .
• Cutting or self-tapping screws and thread-forming screws to DIN 7500, the required nut thread press when screwed into pre-drilled holes themselves. Self drilling cut into thin or soft materials, the hole itself.
• Circular milling : On a milling machine , a circle and a feed in the direction of the axis of the hole are driven in a hole with a special thread milling cutter. The thread milling cutter is smaller than the bore and simultaneously rotates around itself. The milling machine is controlled in such a way that the thread turn results from the superimposed movements of the circle and the feed. The advantage of the method is a significantly increased productivity because the back of the thread milling cutter, which does not cut, is free and a gap is created between it and the wall of the bore. As a result, a cooling lubricant flow can remove the chips much better. The coolant also improves the surface quality. In addition, different diameters can be produced with a single milling cutter. Tapping is limited to a certain diameter per tool.
Table for core holes
thread pitch core diameter Core hole drill
M3 0.5 2.39 2.5
M4 0.7 3.14 3.3
M5 0.8 4.02 4.2
M6 1 4.77 5
M8 1.25 6.47 6.8
M10 1.5 8.16 8.5
M12 1.75 9.85 10.2
M16 2 13.55 14th
M20 2.5 16.93 17.5
Formula for core hole drilling
Core hole drill = thread diameter - pitch

Various threads including Metric, USC, USF, BSW

A threaded rod has no tool attacks and is just a rod with an external thread. Threaded rods are set in concrete, for example, in order to subsequently fasten an object to the concrete base. Longer pushing and pulling work can also be carried out with rotating threaded rods.

### Direction of rotation of the thread

When right-hand thread or right-rising threaded run when viewed in the adjacent image to the thread flanks from left to right (top). The threads run into one another by turning clockwise .

The right-hand thread is preferred for ergonomic reasons. Most people are right-handed and can therefore apply more torque when turning clockwise than when turning counterclockwise. Due to the friction in the threads, tightening the thread requires a greater torque than loosening the thread. The right-hand thread accommodates these two circumstances.

Turnbuckle with right-hand (left in the picture) and left-hand thread (right in the picture)

In the case of a left-hand thread or a left-hand thread , the thread flanks run from right to left (top) when viewed as in the adjacent picture. Left-hand threads are identified in production drawings and in logistics with the letters LH (for L eft H and), e.g. E.g .: M16-LH. The screw head of slotted screws with counterclockwise threads is marked with a transverse groove in the watchmaking trade - a cross appears on the screw head.

The threads run into one another by turning counterclockwise. Left-hand threads are used:

• For special cases in which the screw connection would unintentionally loosen due to the usually prevailing load, such as
• If errors can be avoided through the different directions of rotation. For example , the screw connections on gas cylinders that contain flammable gases have left-hand threads and cannot be connected in place of an inert gas cylinder .
• In the past, wheel nuts or bolts of motor vehicles (for example Opel Blitz , Fiat 1500 and 1300 , Mercedes 170V , Jaguar XK 120-150 , Daf 400 ) were partially designed on the right side of the vehicle to prevent loosening when driving with a left-hand thread. With extremely powerful vehicles such as tractors and z. B. the Porsche Carrera GT or the Ferrari Enzo Ferrari , the wheel nuts on the right side of the vehicle (central locks) are still provided with left-hand threads. These are then (in the case of the Porsche Carrera GT ) specially marked in color: left side of the vehicle = right-hand thread = red nuts, right side of the vehicle = left-hand thread = blue nuts.
• A turnbuckle , also known as a tensioning sleeve, rope or chain tensioner, requires a left-hand and a right-hand thread in order to achieve loosening or tensioning by turning it, for example to couple railroad cars, the wire for a garden fence or the shrouds of one To tension the sailboat.
• Left-hand threads (e.g. Kalashnikov ) are used on the muzzle nut screwed onto the “target-side end” of the barrel of a firearm .
2a: Trapezoidal thread with 2 turns
s = pitch
d = outer diameter
di = inner diameter

### Measurement system

Inch system : Inch thread - according to the UTS standard ( Unified Thread Standard ) in countries thatmeasurethe length in inches (e.g. USA ). Inch threads are common around the world in house installation ( Whitworth thread ), in some areas of precision engineering (for example on tripods and computer housings ) and in aviation.

### Shape of the thread flank

The following forms of thread flanks are common:

#### Thread according to DIN standards

designation Profile sketch Flank angle Identification letters Abbreviation 1) Example Nominal size
[mm]
according to standard application
Metric ISO standard thread / Pointed thread (single and multiple) 60 ° M. M 0.8 0.3-0.9 DIN 14-1 to DIN 14-4 Clock and precision engineering
M 8 2) 1 - 68 DIN 13-1 general (standard thread)
M 24 × 4 P 2 DIN 13-52
M 6 × 0.75 2)
M 8 × 1 - LH 2)
1 - 1000 DIN 13-2 to DIN 13-11 generally, if the pitch of the standard thread is too large (fine thread)
M 24 × 4 P 2 DIN 13-52
M 64 × 4 64 - 76 DIN 6630 External thread for barrel screw connections
M 30 × 2 - 4H5H 1.4-355 LH9163-1 to LH9163-4 LH9163-10 and LH9163-11 for aerospace
Metric ISO thread with transition tolerance field (previously thread for interference fit) M 10 Sn 4
M 10 Sk 6
3 - 150 DIN 13-51 for screw-in ends on studs not sealing
M 10 Sn 4 tight sealing
Metric ISO thread with large clearance M 36 12-180 DIN 2510-2 for screw-in connections with reduced shank
Metric ISO thread for tight fit MFS MFS 12 × 1.5 5 - 16 DIN 8141-1 for tight fit in cast aluminum alloys (standard and fine thread)
Metric tapered male thread Taper ratio to the axis of rotation: 1:16 M. M 30 × 2 taper 6 - 60 DIN 158-1 for screw plug and grease nipple
M 30 × 2 taper short
self-forming, tapered male thread Taper angle to the axis of rotation: 7 ° 30 ' 105 ° S. S 8 × 1 6-10 DIN 71412 for conical grease nipples; Thread similar to DIN 158-1, but flank angle 105 °
MJ thread 60 ° MJ MJ 6 × 1 - 4h6h 1.6 - 39 ISO 5855-1 and ISO 5855-2 Aerospace
MJ 6 × 1 - 4H6H
Bicycle thread 60 ° FG FG 9.5 2 - 34.8 DIN 79012 Bicycle and moped technology
1) Complete designations are contained in the relevant standards listed in the table.

2) Designations according to ISO 965-1

designation Profile sketch Flank angle Identification letters Abbreviation 1) Example Nominal size [mm] according to standard application Trapezoidal thread according to standards Metric ISO trapezoidal thread (single and multiple) 30 ° Tr Door 40 × 7 8-300 DIN 103-1 to 103-8 general Tr 40 × 14 P 7 flat, metric ISO trapezoidal thread (single and multiple) Door 40 × 14 DIN 380-1 and 380-2 Tr 40 × 14 P 7 Trapezoidal thread (single and multiple) with play Door 48 × 12 48 DIN 263-1 and 263-2 for rail vehicles Tr 40 × 16 P 8 40 Tr 32 × 1.5 10 - 56 DIN 6341-2 for draw-back collets rounded trapezoidal thread Door 40 × 5 26 - 80 DIN 30295-1 and DIN 30295-2 for rail vehicles Trapezoidal thread 20 ° KT KT 22 10 - 50 DIN 6063-2 for plastic containers 1) Complete designations are contained in the relevant standards listed in the table.

designation Profile sketch Flank angle Identification letters Abbreviation 1) Example Nominal size [mm] according to standard application Buttress threads according to DIN standards metric buttress thread (single and multiple) a thread flank inclined by 3 ° perpendicular to the bolt axis 30 ° S. S 48 × 8 10-640 DIN 513-1 to DIN 513-3 when absorbing unilateral forces S 40 × 14 P 7 Buttress thread 45 ° a thread flank perpendicular to the bolt axis 45 ° S. S 630 × 20 100-1250 DIN 2781 for hydraulic presses Buttress thread a thread flank inclined by 3 ° perpendicular to the bolt axis 30 ° S. S 25 x 1.5 6 - 40 DIN 20401-1 and DIN 20401-2 in mining P. 22 10 - 50 DIN 55525 for plastic containers in packaging rear thread flank inclined by 10 ° perpendicular to the bolt axis GS GS 22 KS KS 22 40 ° + 10 ° KS 22 10 - 50 DIN 6063-1 for plastic containers in packaging

Milk thread (see DIN 11851 and DIN 405) is a metric round thread with a coarse pitch to make cleaning easier.

### use

The binding definitions of the thread parameters are specified in DIN 2244 and ISO 5408, regardless of the thread type. Both norms are almost identical.

In addition, a basic distinction must be made between the nominal dimensions and the permissible limit dimensions. So has z. For example, the pitch diameter of the metric ISO standard thread M16x2 (for external and internal threads) has a nominal value of 14.701 mm, but the corresponding external thread must have a pitch diameter between 14.503 and 14.663 mm with the usual external thread tolerance zone 6g.

### Nominal diameter

largest diameter of the thread geometry.
In the case of a thread (in the following example a metric) M 20 , the number stands for a nominal diameter of 20 millimeters.

### Pitch diameter

Diameter (d 2 for external threads or D 2 for internal threads) of an imaginary, geometrically ideal circular cylinder (flank cylinder) which cuts through the thread profile in such a way that the widths of the resulting profile valleys (empty spaces) and peaks (teeth) are the same.

### core diameter

smallest diameter of the thread geometry.
The following applies to representations in drawings or in CAD models:
Nominal diameter - pitch = shown core diameter.
In fact, the dimensions differ from this due to tolerances and production, of course, whereby the following applies:
The core diameter of the screw is always smaller than the core diameter of the associated nut.
The core diameter of the nut is the diameter of the hole in which the nut thread is to be cut.

### pitch

With metric threads, the path that is covered by one revolution. In other words, the distance between two thread peaks in mm (previously referred to as the pitch ).
In the case of inch threads, on the other hand, the value of the pitch denotes the number of threads on the 1 inch route ("tpi" = threads per inch).

### Pitch angle

The pitch angle is obtained by calculating the arctangent of pitch / (pitch diameter * ). With the ISO standard thread, this angle is around 3 ° for M6 and around 2 ° for M20.${\ displaystyle \ pi}$

### division

In the case of multiple threads, the division is the distance between two thread notches.
The division is usually the slope divided by the number of gears.
Example: The designation Tr 60 P20 means trapezoidal thread with 60 mm diameter and 60/20 = 3 threads, as well as a distance of 20 mm from thread to thread.
In the case of single-start threads, the pitch is equal to the pitch.

### Flank shape

see shape of the thread flank

### Flank angle

The flank angle is measured between the mutually facing flanks of two adjacent threads.
With normal thread it is 60 °. It varies from 0 ° for the flat thread , up to 80 ° for the steel armored pipe thread .

### Distance of the thread flanks (pitch) in relation to the thread diameter

High helix threads are threads with a large pitch in relation to the diameter of the thread.
High helix threads cause a relatively large axial movement per revolution. To increase the steepness of a thread, either the thread profile is widened or a multiple thread with several parallel threads is produced. Both of these increase the pitch of the thread.

Fine threads (e.g. M6 × 0.5 mm) are threads with a low pitch
Find z. B. Use on adjusting screws of measuring devices. The low feed per revolution allows precise settings. The thread profile is reduced in proportion to the pitch.

Single-start threads are the rule, they are mainly used for fastening. In the case of multiple threads, several threads are "wound" in parallel (for example) around the screw shaft. They are also used for fastening or with a high helix thread to increase the thread stroke.

In the first case, the inside diameter of a banjo bolt can be increased and / or a thin-walled nut can be used. The thread profile is smaller, but the load is absorbed by several turns. This multi-start thread has on the whole the same pitch as a single-start standard thread with the same outside diameter.

In the second case, the thread profile is retained, but the pitch is increased. The space not required by the profile is filled by a second or further thread turn. You get "quick screw connections", a certain stroke can be reached with fewer turns or in a shorter time. Power amplification and self-locking are smaller than with the standard thread because it is inversely proportional to the number of threads.

The screw caps of canning jars are a typical application of a multi-start thread. The thin-walled cover in particular requires a small thread profile that creates space for additional threads. The higher number of threads also has the advantage that there is more than one point around the circumference where the thread begins to “grip”. There is usually no high-helix thread. The pitch is the standard pitch belonging to the large thread diameter here. It is so large that such a lock can be operated quickly. This is also aided by the fact that, as a rule, no more than one rotation between closed and open has to be provided. The self-locking is retained.

Real multi-thread quick-release fasteners are used when the thread diameter cannot be increased. They can be found on bottles for cosmetics or drinks and on high-quality filler caps. The self-locking required for reclosing can be achieved through additional measures (for example with snap connections or well-adhering surface coatings on the lid). The thread pitch and thus the self-locking is also dependent on the diameter and the width of the thread. Therefore, multi-start threads with threads that are narrow in relation to the diameter can be self-locking even without additional measures.

In gas and water installation technology, pipe threads are usually used to produce detachable pipe connections . The parameter is given in inches. This originally referred to the inner diameter or the nominal nominal diameter (DN) of moderate threaded pipes. In order to ensure the compatibility of the threads, light and heavy threaded pipes have the same outside diameter despite different wall thicknesses and different inside diameters despite nominally the same nominal width.

Due to the need to keep the outer diameter, naming the thread according to nominal size or inner diameter is confusing:

• The outside diameter of a one-inch pipe thread is not 25.4 mm, but is in the range from 32.89 to 33.25 mm.
• High pressure pipes with a 1 ″ pipe thread have a smaller inner diameter because the wall thickness is greater.

The Whitworth thread is common in Europe . Cylindrical threads are also abbreviated to BSP (British Standard Pipe).

According to the standard , a distinction is made between pipe threads that seal in the thread ( EN 10226-1, before: DIN 2999) and pipe threads that do not seal in the thread ( ISO 228). The tightness of threads that do not seal in the thread is achieved by sealing surfaces arranged outside the thread.

The most common pipe threads are sealing in the thread, they are designed as cylindrical internal threads and tapered external threads with dimensional overlap in diameter.

On the American continent, on the other hand, the US NPT ( National Pipe Thread ) is used. For NPT threads, the dimensions include the diameter coding and the number of threads on an inch.

Due to the different number of threads per inch and minor differences in diameter, BSP and NPT threads cannot be completely screwed together. This becomes noticeable when the thread cannot be screwed in at all or only a few (few) turns.

• for a tapered Whitworth external pipe thread: pipe thread DIN EN 10226-R½
• for a cylindrical Whitworth internal pipe thread: pipe thread DIN EN 10226-Rp½

(A, B for the tolerance class)

Outer pipe threads are often  roughened - especially with brass parts - so that the hemp or the sealing tape for sealing holds better in the thread when screwing in and does not shift when screwing in.

An armored steel pipe thread (formerly also a PG thread) is used to screw cable laying pipes in electrical installations. Since the pipes are relatively thin-walled, the thread depth must not be very large.

Two methods of manufacturing threads in sheet metal cups

Edison threads are pressed / rolled / stamped into sheet metal as round threads and are also used for screw locks and heating elements. Common sizes are E5.5 (e.g. model construction lamps), E10 (flashlight, bicycle light), E14 (Mignon), E27 (normal), E40 (powerful metal halide and headlight lamps), whereby the number indicates the diameter in mm.

Ball screws are rolled or ground into the surface of ball screw spindles , while roller threads are ground into the surface of the roller screw spindle using special grinding processes. Ball and roller screw drives are used as drive elements in linear technology, for example for moving the support of a lathe .

## Technical drawing

Representation and dimensioning of a threaded hole as a top view in a technical drawing
Threaded hole in a sectional view with dimensions in a technical drawing

In technical drawings , threads (external threads, internal threads and threaded holes ) are represented by standardized, symbolic representations that are standardized in more detail in ISO 6410 .

The representation of the external thread (bolt thread) and internal thread (nut thread) is different in the technical drawing. The following applies to the top view:

• wide solid line as a complete circle (diameter = nominal diameter)
• narrow solid line as 3/4 circle (diameter = nominal diameter - pitch)
• wide solid line as a complete circle (diameter = nominal diameter - pitch)
• narrow solid line as a 3/4 circle (diameter = nominal diameter)

The following applies to the side view:

• outside wide solid line (distance between both lines = nominal diameter)
• inside narrow solid line (distance between both lines = nominal diameter - slope)
• The end of the thread is shown with a wide solid line
• Inner surface of the hole: wide solid line (distance between both lines = nominal diameter - pitch)
• Outer surface of the thread: narrow solid line (distance between both lines = nominal diameter)
• The end of the thread is shown with a wide solid line

If a bolt is shown in a nut thread in the drawing, the illustration of the bolt has priority (see also ISO 6410-1).

## standardization

Usually threads are used that are subject to international standardization . There are now and then manufacturers who use threads that deviate from the standard for various reasons. This can be due to safety or construction reasons or for reasons of competition, so that you definitely have to use original spare parts .

Dimensions of the common threads and general designations can be found in books of tables or in standards that are available for a fee .

Slope error
Wobble
The wobble error is the pitch error measured on a gear.
Form defects
The form error describes the deviation from the theoretically exact thread form. The theoretically exact shape of the thread can be obtained by cutting a thread at the helix angle. In almost every type of thread production, a form defect remains.
Flank surface defects
The surface of the flanks does not have the desired roughness depth (is too rough), so that the screws seize in the area of ​​the thread and can no longer be loosened (this can happen with very large threads that are exposed to very high pressure).

## Historical

The Archimedean screw has a helix. It is around 200 BC. Proven in ancient Egypt and was used for irrigation. A pairing of internal and external threads is not implemented here.

In Central Europe, threads appear as a pairing of internal and external threads in numerous illustrations of tree presses, the background of these illustrations is the biblical motif "Christ in the wine press". It is likely to have consisted of wooden threads, as they were still in use in such presses in the 19th century. The earliest of these images (frescoes and miniatures) date to the 12th century, but there are no references to other uses of such threads for the next three centuries. A wooden spindle is shown on a picture of a heavy-duty crane by Francesco di Giorgio . The picture should have been made around 1480.

Around 1800 Henry Maudslay improved the lead screw of the lathe in such a way that the separate production of external and internal threads became possible. Until then, the screw and nut pair was always a non-interchangeable unit. In the case of machines, these pairs have been marked to keep track of things. Maudslay started with the standardization of the thread.

Joseph Whitworth (1803–1887) built on this. After systematic investigations, he determined the flank angle of 55 °. Together with the pitch, there was now a reliable standard for the pairing of external and internal threads.