screw is a component or a cylindrical mechanic operator used to attach temporarily some pieces with others. The screw has an helicoidal groove cut in the surface of a cylinder.


Screw features

- Outer diameter of the head: in the metric system is expressed in mm and in the English system (Whitworth), in inchs.

- Tipo de rosca: (métrica, Whitworth, UNC, UNF, etc.)

- Thread type: (metrical, Whitworth, UNC, UNF, etc.)

- Thread's walkway: distance between two following crests, in the metric system is expressed in mm and in the English system in the number of threads per inch.


- Direction of the thread (left or right): almost all screws have right-hand thread, but some machinery axises have left-hand thread.

- Constituent material and mechanical resistance: with few exceptions, the greater part of the screws are made of diferent quality and mechanical resistance steels, for wood brass-made screws are the most used.




Metric thread features



The metric thread is based in the International System (IS) and it si one of the most used threads in the assembly of mechanical pieces. The move that it has in the vertexes of the coupling between the screw and the nut allows it to be greased. The structural information for this thread is the following:



- The section of the thread is an equilateral triangle whose angle is of 60 grades.

- The bottom of the thread is rounded and the crest is slightly truncated.

- The side of the triangle is equal to the walkway.

- The walkway is the distance between two counterpart items. Example: between following crests.

- Its outer diameter and the forward move are measured in mm. The forward move is the distance that the screw advances in axial direction in a complete turn.

- Naming: M24 x 2. M means metric thread, 24 means the value for the outer diameter in mm and 2 means the walkway in mm.




Whitworth thread features


The first person who created a normalized thread class was the English engineer Joseph Withworth, around 1841.

The Withworth thread system is still used for old machinery reparation and has a thicker thread than the metric thread.

The Whirtworth system, shortened BSW, was a British standard (BS 84:1956) and the standard British thin thread was presented in 1908 because the Whirtworth thread was too thick for some uses.

The angle of the thread is of 55 grades instead of 60 grades of the metric thread. The depth and the thickness of tue thread vary with the diameter of the screw (the thicker the bolt, the thicker the thread)

In this thread system, the walkway is expresed for the number of threads per inch, and the diameter is expresed in inchs. (Example 1/4", 5/16")




Standard UNF American thread Features


The United States have their own thread system, generally named the standard unified UNF thread wire, which is commonly used in Canada as well, and in most of the countries around the world.

Machine screws are described as 0-80, 2-56, 3-48, 4-40, 5-40, 6-32, 8-32, 10-32, 10-24, etc. until size 16. Sizes 1/4" diameter and larger are marked as 1/4" - 20, 1/4" - 28, etc. The first number is a diameter, the second one is the number of thread wires per inch.

Most of thread wire sizes are availabe in UNC (unified thick thread wire, for example 1/4 " - 20) or UNF (example: 1/4 " - 28 UNF or UNEF).




SCREW TYPES:


Screw term is generally used generically, there are many varieties of materials, types and sizes. A first clasification might be the following:

  • - Mid splinter screws
  • - Borderline screws
  • - Wooden screws also known as silly screws
  • - Dodecahedraon thread screws
  • - Threaded ribs of 1m length





                  Screw heads

Design of screw heads responds, generally, to two needs: on the one hand, to get a suitable support surface for the tightening tool so necessary stregth can be reached without the head breakes or deforms. On the other hand, security needs imply (even in official regulations) that certain devices requieres especial tools for opening, which means that the screw (if it is the means chosen to close) can not be unrolled by a conventional screwdriver (for example, Phillips), complicating this way that not allowed personnel open it.



In this way, there are heads of different shapes: hexagonal, rounded, cylindrical, hazelnut-shape; combined with diferent tightening systems: hexagonal or squared for adjustable spanner, hexagonal whole for Allen key, slot and Phillips for screwdriver, etc.





With new  electric and pneumatic screwdrivers  use of autoscrew crews is very common as much in wooden as metal since it is a fast screwing system. In metal pieces its use is more limited because the tighten-torque applied is low and exposed to be loosen during the operation of the machine.



Which are the screws to be used in different kinds of materials and heads?


      -  WOODEN SCREW (WOODEN LAG SCREW)

Size and quality of wooden screws is regulated by DIN-97 Rule. Its thread is of ¾ of the length of the pin and thanks to the fact that this kind of screw narrows at the end, it is able to pave the way as it is inserted to make autoscrewing easier, because it is not necesary to do a previous hole.

They have a length of ¾ of the pin and can be compounded of mild steel, stainless, brass, copper, bronze, aluminium and can be galvanized, nickel-plated,etc.

Normally they are  cordlees screwdriver o manual screwdrivers. 


          - AUTOSCREWINGS AND AUTODRILLING FOR METALIC SHEETS AND HARD WOODENS

These kinds of screws can pave the way when being fixed.

Autoscrewings for metal have a cutting thread all along their length. Their shape is sharp, pointed. This kind of screws are used in car industry.

Autodrillings have a point like a bit, what avoids having to do guide drills to install them.



          - CYLINDRICAL THREAD SCREWS FOR METAL UNIONS

For unite metal pieces there are triangular thread screws which can be screwed in a blind hole or in a nut with washer through a clearance hole.

There are variations from some system to others depending on the thread system (metric thread of normal or thin walkway, Whitworth of normal or thin walkway, American thread) and on the type of head of these screws (hexagonal type DIN 933 and DIN 931, Allen type DIN 912, hazelnut-shaped, cylindrical head DIN 84, Torx head).




                   - Screw heads types 
 

  • Flat head: It is used basically when it is necessary to leave the head at surface level, for example in capentry.

  • Oval head:  The shape of the head permits it to sink in the surface and let stand above just the rounded upper part. Thanks to that, it is easier to take them out and they have better look than falt headed ones. 

  • Normally they are used to affix metal components just like tools or knocker sheets.


  • Rounded head: It is used for similar functions than oval head, but in no-hazelnuted holes. Normally it is used to affix too thin pieces to permit the screw to sink in them; also to unite parts that need washers. 

  • SLOTS







Slots can be of groove, palette, quadrant, L-shape, etc. It should be pointed out that for each model there is a specific tool or screwdriver.
 

The slot is an area in which palette and screwdriver will be attached. It is posible that a screw has no slot, as cup screws; in this case the screwdriver would have a slot. Existe la posibilidad que nos encontremos con un tornillo que no tenga ranura como en el caso de los tornillos de copa, en este caso la ranura la lleva el destornillador.
 

It is very important not to damage the slot, so handling must be careful and only with the suitable tool, the palette of the screwdriver do not have to be too big or too small, because if it too big the screw head can be damaged by the walls of the slot depth and if it is too small the screw slot could be deformed.




Trapezoidal flat head                

  • Phillips Head:  It has crossed slot, to minimise the chance of the screwdriver slipping.

               

              

  • Allen-kind head: with hexagonal hole, to affix an Allen key or a T-shaped screwdriver:

               




                 


  • Torx head: with a star-shaped hole in the head of Torx exclusive design to affixe an Allen wrench or T-shaped screwdriver:


                                           


                        


Apart from screwdrivers showed previously, there are many other kinds of screwdrivers and each one has its own function to make work easier, confortable and safe.

For example:

Socket  screwdrivers:



1000V screwdrivers :

These screwdrivers are suitable for powered works thanks to its insulation. Resistant do not vary and its use is exactly equall than conventional screwdrivers.

For example, 1000V screwdrivers of EGA Master are subjected to a dielectric test of 10000V (ten times its intensity). Screwdrivers are also subjected to adhesion, dielectric penetration, impact and flame propagation tests, accorfing to the European IEC 900 resp. DIN EN 60900 for bigger security.
 

Mini screwdrivers (Microtronic):

These screwdrivers are suitable for electronic devices. The upper head is designed to permit an effective support in the three microtechnical posotions. Furthermore, this electronic head is rotatory to optimize the rotation movement.



In addition, heads are of different colors to identify easily each size, this way, work will be optimized as the tool will be chosen instantly.


ESD screwdrivers:

ESD miniscrewdrivers are suitable for electronic devices that could have static electricity. These screwdrivers make this energy derive in a controlled way and it is secure for the user and the electronic components.



ESD screwdrivers guarantee a safe and confortable work thanks to its dissipater handle, which is ergnomic, soft and confortable for use. Ribs are manufactured in Cr-V steel of high quality. Points offer a high accuracy mechanic and long durability, with a cromated matt finish.



Torque screwdriver:

Suitable for those works in which it must be applied a specific and accurate torque to the screws.