Screw’s thread; Structure & Type of Thread, the basic info you should know.

It’s the basic information that if we don’t pay attention, which may be cause of failing the work being done or the project being produced. So, we should start with this basic information as “Thread” and a screw has the details for the structure as follow:

  1. Major Diameter: The length of the diameter measured from the edge of the crest on one side to the edge of the crest on the other side. Sometimes we can use a Vernier to measure. For example, if a screw is specified to 6 mm, but measured with a Vernier. Getting the size of 5.9 mm. It is considered round to 6 mm. or sometimes we can call it M6.
  2. Minor Diameter: The length of the diameter by measuring the base or the root of the screw on both sides of the screw.
  3. Pitch: The distance between the first crest and the next crest.
  4. Pitch Diameter: The diameter of the screw thread, approximately halfway between the major and minor diameters.
  5. Crest: The end of the outer end or the end of the spiral edge. 
  6. Root: The groove formed at the base of the thread. 
  7. Helix Angle: The angle of the inclination of the thread or the tooth of the thread. 
  8. Thread Angle (Include Angle): Internal inclination angle of thread look like V-shape. 
  9. Depth of Thread: The depth of the threaded groove starts from the crest’s top to the root of the thread. 
  10. Number of thread: The frequency of the number of threads in 1 inch, which is measured in TPI (Thread per Inch). 

Continued issue from thread’s structure information, the next issue we must know is the scientific measurement unit system because these concerns to the thread’s design, the number of threads per unit, and usability. So, a measurement which has been used in the world has 2 units concerned as follow:

  1. Imperial System: It is the oldest measurement system in the world still in use today which is often called the “English system” it is called the originator and specification which comes from England and the measurement system has a length unit related to the screw is Inch. As the Inch Measurement is used to determine, it’s resulting in affecting the screw design of the screw including the number of threads / frequency of the thread on the specified length.  
  2. Metric System:  this is a measurement system that occurred during the French Revolution which was invented from brainstorming by scientists all over the world to find measurements that prevent errors and reduce the confusion from the Imperial system’s measurement system, and finally to facilitate international trade. Many countries have agreed to the definition of this new unit of measure “Metric System” and later this system has developed more accurate measurements. Until this system became the SI system, which has 7 basic measures and 2 additional units.  For length measurements from this system were calculated with 10 parts and call as millimeters, centimeters, meters, and kilometers. 

From the units of both measurement systems, there are many types of screw threads and the main types well-known about 3-4 types as follows:

 1.Metric Thread (M-Thread): or sometimes known as threading millimeters which is a universal screw that is used throughout the world the characteristic of the thread is the total angle at the top of the spiral 60 degrees (both at the base and the top of the thread). The size of the thread is a symbol of the length of the diameter of the top (crest) in millimeters, followed by the pitch in millimeters. For example, M12 x 1.75 is the diameter of 12 millimeters x the pitch distance is 1.75 millimeters. There are 2 types of threads under this thread: 

1.1 ordinary metric thread and 1.2 ISO metric thread. Overall, there are almost no differences between these two threads. But there is a starting point in the mechanical calculation’s formula to be used in the design. 

 2. Whitworth Thread: or sometimes referred to as the British Thread, since the originator of this type is British which design of the top of thread and thread grooves is rounded and having a degree of angle at the top of the thread at 55 Degrees and the symbolic size is the number of threads per inch. There are 2 types of threads under this thread: 1) BSW (British Standard Whitworth), which is a coarse thread with a certain thread spacing. 2) BSF (British Standard Fine), which is threaded type. More closely with the frequency of the threads. 

 3. American Thread / Inch Thread: It is a type of triangle screw with a total angle of 60 degrees and the thread is designed under Imperial measurement unit. Inch length is used to determine the number of threads. Therefore, it is often used to count the number of threads per inch as the symbol for the screw size. There are 3 types of threads under this type: 

  1. NC = National Coarse Thread 
  2. NF = National Fine Thread 
  3. NEF = National Extra Fine Thread
Conversion Table

Type of Screws

Machine Screws: this type is a screw or bolt is fully thread and flat point which is often driven into the tapped hole or nut for attachment the part and material. Available for many applications with a variety of driven head.

Hex Bolt: Some time was called “Hex Cap Screws or Hex Tap Bolt” and they are driven with the wrench. 

The application for this screw type is in machinery and construction which can be used with a nut or tapped hole for attachment. For the full thread are known as tap bolts.

Socket Screws: they are designed for machinery or auto motive’s application which has the internal hex socket and was driven with the socket wrench or Allen key. The longer lengths may have a more smooth shank and the term screw typically refer to a type of threaded screw which its head diameter is normally 1.5 times or more than of the screw shank’s diameter.

Carriage Bolt: one of the screws in which its application is in machinery or construction and know as a “Coach Bolt”. This screw type has a smooth dome head with the square section underneath the head grip which as designed to fasten into the material to prevent from spinning or turning when the nut is tightened.

Grub Screw: One of the screws which are typically used in the machinery’s application and sometimes known as “set screw” which means that this type with no head and fully threaded.

Wood Screws: This screw type has 3 mains characteristic (1) the smooth shank for holding 2 pieces of material (2) partial threaded with large thread (3) Deep & Sharp thread which was designed for cutting the material and slide through the wood and tightly pull all board attachment together.

 Hex Lag Bolt: You may know as “Lag Screw” and there are some of the toughest fasteners and it was designed for heavy-duty application as its extremely sturdy fasteners are usually use to connect the framing or heavy lumber that are bearing the heavy load.

The hex head is designed to hold up the application where need a lot of torque is necessary.

Self-Drilling Screws: It’s designed for the application which needs to fasten the material without pre-drill. As there are 2 common characters: (1) its thread is designed like the sheet metal screws (Sharp & Deep thread) & (2) this screw has a self-driller cutting (TEK) point to pierce through metal. Exactly the higher number of TEK means that the drill point will larger to pierce heavier gauge metals.

Sheet Metal Screws: This screw type has a sharp point and fully threaded. It is designed to drive directly onto sheet metal or fasten one metal object with another object which needs to attach with such as wood, plastic, or any other kind of surface. Sometimes it can be used in softer material.

Self-Tapping Screws: There are used for a wide variety of applications. Tapping screws are essential for the support you to quick work completion with may or may not require the pre-drilling (depending on the material you are installing into). When you use this screw, it creates mating thread result in thread formation.

 Dry Wall Screws: This screw type is very suite to hold securely in drywall or the weak surface. 

There are 2 thread types that you have to beware of selection (1) Coarse Thread: it work perfectly for the application which involves the drywall and wood stud as the wide thread is suited for gripping into the wood and pulling the drywall attach the stud. (2) Fine Thread: it works perfectly for installation drywall to metal stud as the fine thread is self-threading with the metal (but the coarse-thread ten to chew through the metal, unable to proper traction).

Deck Screw: The common fastener used for a wide range of applications. The thread designed and the notched point at the tip support to remove the chip and cutting which allows for easy installation in wood and composite deck materials. This screw type is well suited for use in pressure-treated lumber which always uses in notably deck and outdoor woodwork.

Concrete Screws: There are 3 outstanding characters for this screw type.

  1. It’s the newest & popular concrete screw which is a very suite for installation into the concrete or in the heavy-duty construction.
  2. Having 2 levels of thread (High & Low) which aid to bite tightly or firm placement to the side of the hole to ensure that solid attachment.
  3. This screw type is very easy to use for heavy-duty construction as you only to drill and drive in the screw without any requirement of a hammer or additional anchor and it’s also very easily be removed when needed.

Fastener Class / Grade and Information

Smaller fasteners are designed to support a small amount of load, which usually doesn’t have to be a grade/class. Because of the strength and load support, will not be considered as relevant but when looking for a fastener that can support a larger application and need to specify the purpose of use especially in terms of load support. The thing that must be seriously considered is the strength.

However, before going to consider the strength divided by that grade We should understand that one of the basic information is that Fastener used in this world is divided into 2 groups as follows:-

  1. Inch Fastener Group: This is widely used in America and 2 other countries, often called the division of strength in the name of the beginning with Grade or ASTM rate. 
  2. Metric Fastener Group: For fastener groups, this is often used to refer to the classifications of strength characteristics as call with prefix “Class”.

In the following, before getting to know the grade or class, including the specification of each grade, we also need to know the word and its meaning which is related to screw strength, such as Tensile strength or yield strength. 

Tensile Strength: is one of fastener mechanical property and it is the maximum tensile load that supports the fastener before its fracture. So, tensile strength is the maximum tensile stress/strain which is applied to one unit of the area until it reaches the fracture point and its units is MPa.

 Stress/Strain: When we put less stress on the load amount per 1 working area of the screw which is mechanically referred to as “Proportional Limit and Elastic Limit”, at this point the fastener will begin to deform, but if the force applied the thread will return to its original shape.  

Yield Strength: When we increase the load force to the fastener or bolt beyond the elastic limit point, the next statement is the yield point. At this point, the stress that has a lot of loads will cause the fastener or bolt to be immediately extended, which will be resulting in permanent deformation and that means fastener or bolt is permanently stretched.

Ultimate Tensile Strength (UTS): The point is simply called the maximum load point before the breakdown of fasteners or bolts. If more loads are added up to this point, the fasteners or bolts will be stretched until they deform to “Neck”, and if more loads are added, next is the fracture or brittle will occur.

Metric Fasteners Class: The Strength Class for bolts of this group is specified by 2 number separately.

1st: the number which is in front of a dot which indicate the ratio between tensile strength and yield strength such as Bolt 8.8 means that the tensile strength can support the load force in each area 800 MPa and yield point stress is 0.8×800=640Mpa.

2nd: the number is after a dot which represent the ultimate tensile strength which is calculated by MPa/100.

Generally, Materials commonly found in fastener manufacturing, such as bolt, screws, nuts, are usually austenitic type stainless steel which has an “A” indicator on top of head’s bolt, as this material has the outstanding in corrosion resistance. There are 3 types of symbols used to identify Stainless Steel on Bolt. The symbols that are often seen are

A1: This Stainless Steel class is often seen in home decoration parts such as dowels, pin and others.

A2: Stainless steel which content 18% Chromium and 8% Nickel that we see most often and used in a variety of applications because it is a type that is resistant to corrosion to a certain extent such as automotive, construction, food industry, kitchen equipment, tableware, home appliance, dishwasher, sink, cooking utensil and etc.

A4: Stainless steel is often referred to as “Marine Grade” because this class is resistant to strong acidic conditions and can withstand chlorine, saline or citric acid. We can see that this stainless steel is in work like lab, medical, kitchens or marine.

Of course, the explanation of the letters as given above which inform you that If you find a letter indicating that A2 means what type of stainless steel, the next number, such as A2-70, number 70 means the tensile strength that can support Load per an area of 700 MPa or A2-80. Tensile Strength is 800 MPa.

Inch Fastener Grade (in North America): It is often used in unified inch threads made from Carbon and Alloy Steel and is mainly used in America. These bolts are marked on top of the bolt’s head to identify the grade which is divided into 3 grades as follows:-

Grade 2: Because it is a low tensile bolt, it means that it can handle a low load per an area. So, on the head of the bolt, there is no mark on it at the top.

Grade 5: A bolt that is often used in automotive applications that have better tensile strength than Grade 2. We can see that it is marked on the top of the bolt with 3 radial lines that are equally spaced. Grade 5 This will match Europe’s / Metric Class is 8.8.

Grade 8: Bolt is manufactured from Alloy and is stronger than Grade 5 and also its tensile strength is better than Grade 5, so this grade is used in a wide range of applications. Therefore, we will find the marks made on the bolt head with 6 radial lines. This grade is equivalent to the European class or Metric system, Class 10.9.

The Fastener Grade and Class in overview as the table below:-

5 Major types of Fastener Materials

Fasteners are manufactured in a wide range of materials from common steel to titanium, plastic, and other exotic materials. The material which content in the fastener, is an important factor to lead your project to success, so the material selection should be based on considerations such as – strength required, stresses, corrosive environment, weight, magnetic properties, electrical conductivity, coatings/plating required, reusability and expected life.

Here is the idea for consideration the proper fastener material to complete your project

  • Strength
  • Brittleness
  • Corrosion Resistance
  • Galvanic Corrosion Properties
  • Cost

Let’s start to understand the properties of each Fastener Materials

Steel / Carbon Steel: This is the most popular fastener material. The steel fasteners are available with or without surface treatments. The steel bolts are commonly available in the following grades:

Steel is the most common fastener material. Steel fasteners are available plain as well as with various surface treatments such as zinc plating, galvanization, and chrome plating.

Carbon steel is the most common type of steel used in fastener production. Grades 2, 5, and 8 are typically the standard for carbon-steel based screws and bolts, with alloyed carbon steel being a higher-end variation on these metals. Their mechanical strength ranges from approximately 50 ksi (kilo-pound per square inch) up to 300 ksi in a finished product. Material properties for these grades include:

Grade 2: This is the standard steel grade for hardware. The grade 2 bolts may feature a manufacturer’s mark.

Grade 2 is a standard hardware grade steel. This is the most common grade of steel fastener and is the least expensive. Except for a possible manufacturer’s mark, Grade 2 bolts have no head marking.

Grade 2: This is a low carbon category that features the least expensive, but also least durable, types of steel. Grade 2 material is highly workable, and forms the bulk of steel grade fasteners.

Grade 5: This is also known as grade F, and is used in automotive applications. The bolts made from grade 5 feature radial lined heads.

Grade 5 bolts are hardened to increase strength and are the most common bolts found in automotive applications. Grade 5 bolts have 3 evenly spaced radial lines on the head.

Grade F is roughly equivalent to Grade 5. Grade F nuts are used with Grade 5 bolts

Grade 5: Grade 5 steels are produced from unalloyed medium carbon groups, such as type 1038, and are usually work-hardened to improve their strength. This is the most common grade used in automotive applications.

Grade 8: These bolts are harder than grade 5 and are used in demanding applications such as automotive suspensions and high-stress environments.

Grade 8 / Grade G

Grade 8 bolts have been hardened more than grade 5 bolts. Thus they are stronger and are used in demanding applications such as automotive suspensions. Grade 8 bolts have 6 evenly spaced radial lines on the head.

Grade 8: These steels are typically medium carbon alloys, such as types 4037 and 4340. They are work-hardened to a high degree, making them stronger and better-suited for mechanically straining applications, like vehicle suspension systems.

Alloy Steel: The alloy steel bolts are heat-treated for extra strength. They are distinguished by their gray-black finish.

Alloy steel bolts are made from a high strength steel alloy and are further heat treated. Alloy steel bolts are typically not plated, resulting in a dull black finish. Alloy steel bolts are extremely strong but very brittle.

Alloy Steel: This is an alloy formed with high-strength carbon steel that can be thermally treated up to 300 ksi. Alloy steel has low corrosion resistance and typically benefits from the additional coating. These steels are extremely strong but can be rigid and brittle.

Stainless Steel: Stainless steel is an alloy containing chromium and low carbon steel. The stainless steel fasteners provide high corrosion resistance. The fastener made from this metal will not lose its resistance even if it is nicked or scratched at the time of installation. Stainless steel grades with special chemical formulation are used for certain fastener applications. The grades are as follows:

Stainless steel is an alloy of low carbon steel and chromium for enhanced corrosion characteristics. Stainless steel is highly corrosion resistant to the price. Because the anti-corrosive properties are inherent to the metal, it will not lose this resistance if scratched during installation or use.

It is a common misconception that stainless steel is stronger than regular steel. In fact, due to their low carbon content, many stainless steel alloys cannot be hardened through heat treatment. Therefore, when compared to regular steel, the stainless alloys used in bolts are slightly stronger than an un-hardened (grade 2) steel but significantly weaker than hardened steel fasteners. Unless great care is taken, stainless fasteners are susceptible to seizing up during installation, a phenomenon known as galling.

Stainless steel is an alloy that combines the properties of low carbon grades with certain percentages of chromium and nickel. Its chromium component lends stainless steel a high degree of corrosion resistance that does not decrease from deformation or long-term use. However, the low carbon content prevents it from being effectively hardened, making the metal stronger than most grade 2 steels, but weaker than many hardened grade 5 and 8 varieties. The final strength of most stainless steel grades ranges from around 70 to 220 ksi, depending on the ratio of metals in the alloy. Stainless steel fasteners are also less magnetic than their standard steel counterparts. The two main categories of stainless steel fastener materials are:

  • Martensitic Stainless Steel: The martensitic group includes strong, durable stainless steels that can be further strengthened through heat treatments. They are more magnetic than other types of steel but have lower corrosion resistance.
  • Austenitic Stainless Steel: The vast majority of stainless steel fasteners are produced with metals from the austenitic family. Their high levels of chromium and nickel provide tough corrosion resistance and the ability to withstand considerable physical strain without fracturing, albeit at a higher cost than the martensitic varieties.

Most stainless steel fasteners are much less magnetic than regular steel fasteners though some grades will be slightly magnetic.

18-8 Stainless: This is a special variety of stainless steel featuring 8% nickel and 18% chromium. The 18-8 stainless steel is used for producing hardware for industrial applications, as well as marine fasteners.

18-8 refers to any stainless steel containing approximately 18% chromium and 8% nickel. This is the most common stainless designation for hardware. For information on 18-8 stainless steel material properties see our Material Grade Identification and Properties Chart.

Stainless Steel 316: This is a corrosion-resistant grade of stainless steel and is ideal for medical, marine, and saltwater applications.

A highly corrosion resistant grade of stainless steel. Ideal in salt water and chlorine environments. More expensive than 18-8.

Stainless Steel 410: This is a less corrosive magnetic stainless steel material and is harder than the 18-8 variety.

A stainless alloy that is harder than 18-8 stainless steel, but not as resistant to corrosion.

Aluminum: This material is soft, lightweight, and corrosion-resistant. Aluminum fasteners retain their corrosion resistance even if scratched or nicked during installation or regular use. Various fasteners are made from special aluminum formulations where elements such as iron, magnesium, silicon, zinc, and more are added during the process.

Aluminum is a light, soft, corrosion-resistant metal. Like stainless steel, aluminum’s corrosion resistance is inherent to the material. Therefore, scratches and nicks will not affect the corrosion resistance.

Fasteners are made from a variety of aluminum alloys, with elements such as manganese, silicon, iron, magnesium, zinc, copper, and silicon being added to increase strength and melting point.

Rivets are often made from aluminum alloys in the 5000-series, which uses magnesium as the primary alloying element.

Brass: This is an alloy of zinc and copper. It is electrically conductive as well as anti-corrosive. Custom Brass fasteners plate well and are used in many fluid exposed applications.

Brass is an alloy of primarily copper and zinc. Brass is highly corrosion resistant and electrically conductive. However, its use as a fastener is somewhat limited due to its relative softness. It is used primarily for its appearance.

The bronze used in fastener production is an alloy primarily consisting of tin and copper. With its high corrosion resistance, bronze is well-suited for aquatic applications, such as shipbuilding or underwater construction.

It shares copper’s reddish color but is relatively expensive compared to other fastener materials. Brass, an alloy of copper and zinc, is similar to bronze in its anti-corrosive and electric conductivity properties, but it has lower tensile strength and is a relatively soft metal. Part of the brass’s appeal as a fabricating material lies in its yellowish golden color.