Tim Pinchback, Author at ��ϴ��¼�

Understanding Metric Fastener Threads

Tim Pinchback — Sun, 11 Apr 2021 01:28:12 +0000

Fastener threads are essential components in precision engineering and manufacturing. Be it on a bolt, screw, stud, or nut, these threads ensure compatibility and strength in a slew of applications, from automotive to aerospace industries and more.��We’ve already defined some basic fastener thread terminology and explained the nomenclatures of Unified Inch Series threads in a previous article if you’re interested in that. Otherwise, this article will solely focus on the basics of metric fastener threads.��

Disclaimer: At Wilson Garner, we mainly manufacture externally threaded fasteners like bolts, screws, and studs, so that’s where our focus is in this article. You can learn more about internal fastener threads here if you’d like.

A Quick Thread Terminology Debrief

If you’re not already familiar with some of the basic thread terms, here’s a brief overview of the most important ones you’ll need to know to understand the rest of what’s covered here.

Crest: The top of the thread
Root: The bottom of the thread
Flank: Connects the crest and the root
Pitch: The distance measured parallel to the thread axis between corresponding points on adjacent threads. So, pick a spot on the thread and measure the distance to the same spot on the next thread.
Major Diameter: The diameter of the fastener as measured at the crests of the threads
Minor Diameter: The diameter of the fastener as measured at the roots of the threads
Pitch Diameter: The diameter of a theoretical cylinder passing through the threads in such a manner that the distance between the crests and roots is equal.

Get more fastener terms — not just related to threads — defined here.��

How to Speak Thread – Metric Version

Now, let’s start with an example that will give you a look into how we express metric threads.��

The photo above is of a metric bolt with M14 x 2.00 6g threads. Here’s a breakdown of what that means.

The “M14”: Nominal Major Diameter

The “M14” tells us that the nominal major diameter of the threads is 14 millimeters. Remember, major diameter is the diameter as measured at the widest part of the threads, and “nominal” means “basically.” In fact, the spec on this part actually calls out a major diameter of between 13.682–13.962 millimeters but nobody wants to say “M13.962,” so we just say “M14” and move on with our lives.

The “2.00”: Thread Pitch

The “2.00” refers to the thread pitch, which, again, is the distance from one point on a thread to the same point on the next thread. Here, the “2.00” tells us that the thread pitch is 2.00 millimeters. If you look at the photo, you can see this to be true, as there are 2 millimeters between the crest of one thread and the next.��

Notice that this is a major difference from Unified Inch Series nomenclature, where the second number calls out threads per inch instead of thread pitch.��

A Note on Fine vs. Coarse Threads for Metric Fasteners

Similar to Unified Inch threads, metric fastener threads can have fine and coarse pitches. In our example above, M14 x 2.00 is a coarse thread. The fine pitch would be M14 x 1.50.��

However, in contrast to Unified Inch threads where coarse and fine pitches are fairly standard, metric fine pitches can vary depending on the specification. For the most part, coarse threads are assumed unless specified otherwise. In some specifications, the pitch isn’t even called out in the product description.��

The “6g”: Thread Fit

The “6g” refers to the metric version of thread fit. 6g is the short-form expression for the external thread fit tolerance class 6g6g, which is quite common. In our article about Unified Inch threads, we talk about the 2A and 3A thread fit. In metric, 6g is roughly comparable for 2A in the Inch system.

The left number/letter combination specifies the pitch diameter tolerance and allowance, while the right number/letter combination specifies the major diameter tolerance and allowance. The numbers specify tolerance, while the letters specify allowance.

What Are Tolerance and Allowance?

Thread fits are developed using tolerances and allowances.

A tolerance is a specified amount by which dimensions are permitted to vary for manufacturing convenience. It’s the difference between the maximum and minimum permitted limits for a given dimension.��
An allowance is an intentional clearance between mating threads. Allowances are applied to external threads, so the major, pitch, and minor diameter maximums are less than the basic size by the amount of allowance.��

Metric Thread Tolerance and Allowance Examples

If you’re a visual learner, here are some diagrams and explanations to further understand tolerance and allowance.

To start, let’s look at an example of a very popular thread, M10 x 1.50 6g6g:

Remember, the leftmost number/letter specifies the pitch diameter requirements, while the rightmost number/letter specifies the major diameter requirements. So, in this case, both the pitch diameter and major diameter have a tolerance of 6 and an allowance of g.��

Here’s what this translates to in actual numbers:

Pitch diameter: For M10 x 1.50 6g, 6 is 0.132mm tolerance, and g is 0.032mm allowance.
Major diameter: For M10 x 1.50 6g, 6 is 0.236mm tolerance, and g is 0.032mm allowance.

Notice that the major diameter has more tolerance than the pitch diameter, while the allowance is the same for both.

Changing the Metric Thread Tolerance

Now, a second example, where we have a M10 x 1.50 4g6g thread:

As the numbers decrease, the amount of tolerance decreases. With a 4g6g thread fit, we have decreased the amount of tolerance for the pitch diameter and left everything else the same.��

Pitch diameter: For M10 x 1.50 4g, 4 is 0.085mm tolerance, and g is 0.032mm allowance.
Major diameter: For M10 x 1.50 6g, 6 is 0.236mm tolerance, and g is 0.032mm allowance.

Changing the Metric Thread Allowance

Another example. This time, we’re changing the letters for a M10 x 1.50 6e6e thread:

As letters decrease (think alphabet backwards), the amount of allowance increases. So, in the above example, the tolerances for major diameter and pitch diameter are the same as 6g6g, while the amount of allowance for both pitch and major diameters has increased.��

Pitch diameter: For M10 x 1.50 6e, 6 is 0.132mm tolerance, and e is 0.067mm allowance.
Major diameter: For M10 x 1.50 6g, 6 is 0.236mm tolerance, and g is 0.067mm allowance.

In contrast, as allowance letters increase, the amount of allowance decreases, with the “h” designation having no allowance.

Side-by-Side Comparison

For our final example, let’s look at two metric thread fit classes side by side.��

The diagram above shows the entire mated class of fit for 6g6g and 4g6g external threads with a class 6H internal metric thread. (Internal threads are expressed with capital letters. Just like Unified Inch threads, the internal metric threads generally have no allowance.)

Because of the tighter tolerance, 6H/4g6g is a tighter fit than 6H/6g6g.��

Have Questions? Talk to Our Fastener Manufacturing Experts

Hopefully, this article has been a helpful introduction to metric fastener threads. If you’d like more information, we recommend taking a look at the following standards:

You can also reach out to our team to get your questions answered. We have over 70 years of specialty bolt, screw, and stud manufacturing experience. If we don’t have a confident answer for you, we can direct you to someone who will.��

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What is Torque-Tension Testing?

Tim Pinchback — Sun, 11 Apr 2021 01:24:12 +0000

Torque-tension testing is an aid to fastener and process engineers in developing installation torque specifications for industrial assembly operations. Fastener manufacturers also use these tests in product development. Torque-tension tests are used for free-spinning as well as locking products. This post will discuss basic tests for free-spinning applications.

Basics of the Bolted Joint

Before we can talk about torque-tension testing, we should take a second and define torque, tension, and the basics of the bolted joint. To put it simply, a bolted joint is two or more things held together with a bolt or screw. For the purposes of our discussion, we’re going to be specifically referring to a bolted joint as an assembly consisting of three metal plates with existing holes being joined together by a bolt, nut, and washer. See Figure 1 above. The holes are not tapped and therefore are not interacting with the threads. Also, in this example, we’ll be tightening the nut, not the bolt.

In order to tighten the nut, you turn it. The force required to rotate the nut is referred to as torque. As the nut is tightened, everything in the joint squeezes together as the nut moves up the threads. The force of this squeeze is referred to as tension (can also be referred to as clamp load.) As you tighten further and further, the bolt itself will begin to stretch. When the steel in the bolt is stretched, it tries to spring back to its original shape. So as the bolt stretches, it adds even more tension to the load as it tries to regain its original shape. If you were to keep tightening, the bolt would eventually become deformed permanently. The force at which a bolt becomes permanently deformed is called a bolt’s yield strength.

Torque-Tension Testing Basics

A torque-tension test is the measurement of the input torque required for a bolted joint to achieve a specified tension. In other words, how tight do I need to tighten my bolt in order for the joint to reach a certain tension? A typical test set-up is shown in Figure 2. It consists of a test bolt, test washer, and test nut loosely fitted in a test fixture. The test fixture contains a load cell that can measure the amount of tension in the joint.

The nut is slowly tightened until a preset amount of tension is reached. As the joint is tightened, this action stretches the bolt, creating a clamp load on the joint (in this case the test fixture). The amount of torque needed to rotate the nut to the desired tension is measured. These tests are always run below the material’s yield strength, so no permanent deformation of the bolt occurs.

Friction

The most important factor affecting the relationship between torque and tension is friction. This makes sense: a nut with a smooth surface and lubrication will turn easier than one with a rough surface and no lubrication. There are several factors that can affect the amount of friction in a bolted joint. These include:

�󲹲��ٱ�Ա��material and grade – The type of material used and how hard it is will impact friction.
Class of fit for mating threads – Thread systems with a tighter fit will have more friction than systems with a looser fit.
Bearing surface properties and area – The amount of surface area and the roughness of the bearing surface will affect friction.
Coatings – Different fastener coatings will have a large impact on friction.
Lubricants – Different lubricants will also have a large impact on friction.

Coatings

Coating (sometimes referred to as finish or plating) on steel fasteners provides a number of desirable properties, including corrosion resistance. Different coatings also change the friction (lubricity) from that of a plain part. The coefficient of friction is a number that is calculated for each different coating. It can be calculated from torque-tension test results and part geometry. Coated test bolts are set up as in Figure 2. Torque measurements are made at a specified clamp load value, generally around 75% of the yield strength. This procedure is repeated a specified number of times, and the coefficient of friction is calculated from the results. The ability to determine a coefficient of friction for each coating is quite useful. This helps coating manufacturers develop and maintain products with consistent friction properties. End-user manufacturing companies also use the coefficient of friction to help determine installation torque values.

Where We Come In

��ϴ��¼� Company does not actually perform any torque-tension testing. However, we have been supplying test bolts,��nuts,��and washers for over 35 years. We manufacture all of our test bolts, and we also stock test washers and test nuts. Many large OEMs have their own specifications for torque-tension tests that must be performed by suppliers. We stock parts for General Motors,��Ford,��Chrysler, and John Deere tests, as well as parts for tests based on ISO 16047. If there’s anything we might be able to help you with, by all means,��let us know.

This article was meant as an introduction to the subject, so if you’d like more information, we recommend taking a look at the or individual OEM standards.

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Core Hardness and Surface Hardness in Fasteners

Tim Pinchback — Sun, 11 Apr 2021 01:10:35 +0000

The hardness of steel is a measure of the steel’s ability to resist deformation, such as abrasion and indentation, when a load is applied to the material. In this article, we will look at hardness for externally threaded screws, bolts, and studs made from alloy or carbon steel. Most fastener standards and specifications have two mechanical property requirements for hardness: core hardness and surface hardness. In general, fasteners made from carbon and alloy steel become stronger as they get harder. Specifically, as hardness increases, tensile strength also increases. (You can get more info about Tensile Strength here). Because of this relationship between hardness and strength, hardness is a critical factor for fasteners. This is why many fastener standards require hardness to be verified before parts can be considered compliant.

Core Hardness

Rockwell Hardness Tester used for Core Hardness and Surface Hardness Testing

Let’s talk about core hardness first. Core hardness refers to the hardness of the material throughout the inside of the part. Core hardness testing is performed on a cross-section of the interior of the part. The part must be cut to expose the interior, so core hardness tests are destructive tests. Core hardness measurements are a quick and comprehensive way to get an accurate hardness measurement throughout the diameter of the part.

As an illustration, we will use an example part with a decent amount of material: an M16 x 1.50 Property Class (PC) 10.9 Hex Head Cap Screw. To perform the test, the lab technician first prepares the sample parts by cutting a cross-section at a minimum distance of one diameter from the end of the part. The technician then places each sample in a Rockwell hardness tester. A specified load is applied to a diamond indenter, which penetrates the sample at mid-radius. The machine measures the indentation and calculates the hardness. The machine makes multiple indentations on each sample to ensure consistent hardness throughout the part.

Diamond Indenter used in Rockwell Core Hardness Testing for fasteners

For a Property Class 10.9 part, we use measurements on the Rockwell C Scale (RC).�� The lab technician compares the results to specification limits to determine conformance. For our example, the sample readings were 36-37 RC. The spec sets the limits at 32-39 RC. As you can see, the readings fell within the spec limits, and the parts conform. If you want to learn more about the testing procedure, we recommend ��Ի��.

Core Hardness and Tensile Strength

As stated above, for the materials we use, there is a good correlation between core hardness and tensile strength. Because of this correlation, most standards base their core hardness requirements on the desired tensile strength of the parts. The standards tend to set the minimum core hardness at a value that will produce a tensile strength above the desired minimum tensile strength of the part. Likewise, maximum core hardness is set at a value that will produce a tensile strength just below the maximum acceptable tensile strength of the parts. Why have an upper limit on the hardness of a part? Some steels can become brittle when a certain hardness is exceeded, so some standards limit the hardness to below the threshold where brittleness is a concern.

Surface Hardness and Carburization

Cut-Off Bolt showing indent for Rockwell Surface Hardness Test

Next, let’s take a look at surface hardness. During the heat treat process, a thin layer of carburized material can develop on the surface of the part. This carburized material is harder and more brittle than the material in the rest of the part. Most specifications set a maximum surface hardness value to ensure that the surface is not brittle. Testing for surface hardness is quick and non-destructive. The lab technician performs the test on either the unthreaded shank or the head of the fastener. A sample part is placed on a Rockwell machine, and a small load is applied to the indenter. The indenter penetrates the surface and makes small indentations on the surface of the part. The machine measures the depth of the indentations and calculates the hardness.

As you can see from the picture, on our example M16 part, the indentations for a surface hardness test are much smaller than the indentations for a core hardness test. The machine makes multiple indentations on each part. The machine measures the depth of the indentations and calculates the surface hardness. The lab technician compares the results of the test to the specification maximum and determines conformance.

For our M16 example, we use measurements on the Rockwell 30N scale (R30N).�� Of all the measurements taken, the highest hardness result was measured at 57 R30N. The specification maximum was 58.6 R30N. Therefore, the parts are in conformance to requirements. For more information on specific test requirements, see ��Ի��

Decarburization

Surface hardness can also be affected by another condition that can occur during the heat treatment process: decarburization. Decarburization can produce a thin soft surface layer that may reduce thread strength. Luckily, we have tests that can detect when decarburization is present. Some procurement specifications specifically require fasteners to be tested for decarburization. If you want to learn more about this testing, we recommend Class 2 optical methods.

This concludes our discussion on fastener hardness. Thanks for reading. For more information on fastener materials, check out our article on material hardenability. Here is a teaser: Hardness and hardenability are not the same things.

Need a Special Fastener?

��ϴ��¼� produces small-quantity specialty bolts, screws, and studs you can’t find anywhere else. ��ϴ��¼� Us and see what we can do for you!

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What is Hardenability? Different Steel Grades and Hardenability

Tim Pinchback — Sun, 11 Apr 2021 01:00:50 +0000

Hardenability of steel is the ability of the steel to achieve a hardness value at a particular depth beneath the surface.��(Click here for more information on core hardness and surface hardness in fasteners). The chemical composition of a steel provides the foundation for hardenability. The iron content in steel is around 98%. Carbon (C) is the primary hardening element. Other alloying elements such as manganese (Mn), molybdenum (Mo), chromium (Cr), and nickel (Ni) are often added in small amounts to increase hardenability. The methods of heat treatment, quenching, and tempering of steel develop the material’s metallurgical structure and mechanical properties, including hardness. A measure of steel’s hardenability aids fastener engineers in selecting materials that will produce parts with the desired surface and core hardness.

Chemical Composition Among Different Steel Grades

Figure 1

We should begin with some examples of different steel grades and their chemical composition, just to give an idea of the different compositions of different grades. We’ll use three grades that we at ��ϴ��¼� use frequently: 1541, 4037, and 4140. The chemical composition numbers below were taken directly from the steel certifications provided by our steel supplier. See Figure 1.

Hardenability Testing

As stated above, different chemical composition has an effect on hardenability. In order to quantify the differences in hardenability, engineers developed a standard reference test for hardenability, the Jominy end quench test per . This test provides data for the changes in hardness along the length of a 1″ diameter x 4″ long round test bar. The test is run as follows: A test bar is heated to 1600 degrees F, then quickly hung vertically in a fixture. The lower end is quenched using a room temperature water spray. After cooling, a narrow flat is ground along the surface of the 4″ length. Rockwell C hardness measurements are made on the flat surface at 1/16″ distance increments from the quenched end. The hardness results at each distance location are plotted on a graph, creating what is commonly called a Hardenability Curve.

Hardenability Curves

Figure 2 below shows Hardenability Curve results for three grades of steel, 1541, 4037, and 4140, using Jominy test data provided by our steel suppliers with their shipment to our company. The material at the quenched end has the fastest cooling rate and the highest hardness. Moving away from the quenched end, the cooling rate slows, and the hardness decreases. As you can clearly see from the two figures, different chemical compositions can have a major effect on the hardenability of different grades.

Figure 2

Steel Grade Numbering

Before we conclude, let’s take a look at the AISI/SAE nomenclature for carbon and alloy steel grade chemical compositions. The designation system consists of four numbers (XXXX). The left two digits represent the type of material, beginning with 10 (plain carbon steel) and progressing up through many alloy combinations to 98 (nickel-chromium-molybdenum steel). . The right two digits represent the amount of carbon (C) in the material, expressed as a percentage by weight. This is a nominal percentage, meaning that specifications allow a range of carbon content.

Here’s an example: Grade 1038
Material Type – 10XX – Non-modified plain carbon steel.
Carbon content – XX38 – SAE carbon specification range is 0.35% minimum to 0.42% maximum (the nominal value 38 just splits the difference between the high and low limits).

Here’s another example: Grade 8640
Material Type – 86XX – Nickel-Chromium-Molybdenum Steel
Carbon Content – XX40 – SAE carbon specification range is 0.38% minimum to 0.43% maximum.

And with that, we’ve come to the end of our discussion on hardenability. If you would like to learn more about hardenability for carbon and alloy steel grades, we recommend as a good reference. And click here for a primer on core and surface hardness in fasteners.

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The Difference Between a Bolt and a Screw, According to One Trusted Agency

Tim Pinchback — Sun, 11 Apr 2021 00:50:38 +0000

As with so many things in the fastener world, the answer to “What’s the difference between a bolt and a screw?” can best be answered by saying, “It depends on who you ask.”

Most people have an idea in their head of what a bolt is and what a screw is. However, there are such a wide variety of both bolts and screws that it can be difficult to look at a particular fastener and know which one it is. This article should provide some guidance.

One Government Agency’s Opinion

Here’s a publication from the US Customs and Border Protection (CBP) titled from July 2012. This is a short document that lays out a procedure to determine the identity of an item based on visual inspection and design criteria. The technical content and illustrations are drawn from the and the . These are excellent references, and we highly recommend them to those who might need more detailed information.

The CBP publication lays out four primary criteria to identify an externally threaded fastener as a bolt or a screw. If the identity cannot be determined by the primary criteria, the document provides nine supplementary criteria. For the purposes of this article, we will focus on the four primary criteria in some depth and give a brief summary of the nine supplementary criteria.

Bolt vs. Screw: Definitions

Let’s begin with a couple of definitions provided by the CBP which are quoted from ANSI-ASME B18.2.1:

Bolt: A bolt is an externally threaded fastener designed for insertion through the holes in assembled parts, and is normally intended to be tightened or released by torquing a nut.
Screw: A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head.

A bolt is meant to be used with a nut, and it is tightened by torquing the nut. A screw is designed to be used in either a preformed or threaded hole, or it is capable of forming its own threaded hole. A screw is designed to be tightened by torquing the head.��

Bolt vs. Screw: Four Primary Criteria

The four primary criteria are based on the root of the two definitions above. Each of the four criteria essentially attempts to confirm a key element of one definition or the other. If a part in question clearly satisfies any of the four criteria, it can be identified without any further examination.

These parts cannot be turned by the head and can only be tightened by turning a nut. Therefore, they are bolts.

Criterion 1 — Bolt

If the fastener in question has a head or other design feature that prevents turning during assembly and which can be tightened only by turning a nut, then the fastener is a bolt. This criterion is a process of elimination. Screws are designed to be tightened by turning the head, so if you can’t turn the head, the part must be a bolt.

These parts are bolts because they must be used with a nut to perform their intended service.

Criterion Two — Bolt

If an externally threaded fastener has an intended function that requires it to be assembled with a nut, then the fastener is a bolt. This criterion obviously requires knowledge of the intended function of the fastener in question.

These parts are screws because their threads will not work with a nut.

Criterion Three — Screw

If the fastener in question has a thread form that prohibits it from being assembled with a nut, then the part is a screw. This is another process of elimination situation. If the threads of a part prevent the part from being used with a nut, then the part cannot be a bolt and must be a screw.

These parts are screws because they are designed to be turned by the head into a tapped hole.

Criterion Four — Screw

If the fastener is designed to be torqued by its head into a tapped or preformed hole, then the fastener is a screw. Again, knowing the intended function of a part can be tremendously helpful. Any part designed to be torqued by the head into a tapped hole is a screw.

Bolt vs. Screw: Nine Supplementary Criteria

If you cannot determine that a part meets any of the four primary criteria, there are nine supplementary criteria that you can examine. A fastener that satisfies five of the nine supplementary criteria can be classified as a screw.

In general terms, screws are more tightly toleranced than bolts. If you have access to the blueprint or spec for the part in question, you can compare the tolerances to those of industry-standard tolerances for bolts and screws to determine which is closer to the part in question.

Point – The configuration at the end of the shank has a chamfer or other specially prepared point at its end to facilitate entry into the hole and engagement with the internal thread.
Bearing Surface – The under-head bearing surface should be smooth and flat to reduce friction or scoring when tightening the head.
Under-Head Fillet Radius – The angle of the junction of the head with the body should be controlled to ensure proper seating of the head.
Head Angularity – The squareness of the underhead bearing surface with the shank should be controlled to ensure proper seating.
Thread Concentricity – Threads should be concentric with the body axis.
Thread Length – Measured as the axial length of the full form threads in addition to up to two incomplete threads at the start of the thread, the thread length should be sufficient to fully engage with the tapped hole.
Body – The body, or unthreaded part of the shank, should be controlled in size and roundness to fit into tightly toleranced holes.
Shank Straightness – The shank should not have a bow or camber that would prevent it from fitting into tightly toleranced holes.
Length – The overall length should be controlled to prevent the screw from bottoming out in the hole.

A Small Disclaimer

One final thought before we wrap up: All of the criteria listed above are taken from a publication from the US Customs and Border Protection. We like it because it is reasonably clear and concise. For customs applications, this is a great document.��

However, there are plenty of other opinions out there regarding the difference between a bolt and a screw, and the criteria that defines each fastener. So, don’t take this one as definitive if you’ve got a document more specific to your situation that is telling you something different.

Questions? ��ϴ��¼� Us

At ��ϴ��¼�, we’re happy to help answer any questions you may have about fasteners — whether that’s the difference between a bolt and a screw, what in the world “proof load” means, or what our custom manufacturing capabilities are. Give us a call or reach out to us online and we’ll get back to you as soon as possible.

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Surface Finishes for Steel Bolts and Screws

Tim Pinchback — Sun, 11 Apr 2021 00:21:15 +0000

Surface finishes are often applied to plain steel fasteners in order to improve performance. When it comes to steel bolts and screws, there are dozens of different coatings and platings to choose from, each with different properties and costs.

For the purposes of this article, we’ll present a basic introduction to five types of finishes that our customers specify to us far more than any others. All of these finishes are good options and do the job they were designed to perform. The following list is ordered from our highest to lowest volume:

Zinc Phosphate & Oil Coating (Phos & Oil)
Zinc Electroplate (Zinc)
Cadmium Electroplate (Cad)
Zinc Non-Electrolytically Applied Coating (Zinc Flake)
Zinc-Nickel Plating (Zn-Ni)

Importance of Surface Finishes for Steel Bolts and Screws

Surface finishes are applied to steel bolts and screws for a number of reasons, but the primary one is to improve corrosion resistance. Corrosion resistance is a material’s ability to withstand damage caused by oxidation or other chemical reactions.

Each finish offers a different level of corrosion resistance. Typically, the higher the corrosion resistance of a finish, the more it costs.

Key Considerations: Cost & Corrosion Resistance

The bolt and screw finishes discussed in this article all have different costs and corrosion resistance. As always, cost is influenced by volume. Therefore, the costs provided in this article are relative and can vary. They are our best estimates based on our experience over the last year.

Corrosion resistance is measured using a standard neutral salt spray test (NSS), which is specified in certain quality standards like or . Parts are exposed to a salt spray inside of a test chamber for a specified test time in hours. The parts are then visually examined for the appearance of corrosion. If parts are free from corrosion after the specified test time, the parts are said to have passed the test. Higher hours specify higher corrosion resistance.

For our five finishes, we have presented the relative costs along with the specified minimum hours of corrosion resistance in Table 1.

Comparison of Common Surface Coatings
Coating	Relative Cost	Corrosion Resistance (NSS Hours)
Phos & Oil	1	2-72
Zinc	2	72-384
Zi-Ni	6	1000
Cad	9	72-168
Zinc Flake	12	240-1000

Table 1 – Comparison of Common Surface Coatings

5 Steel Bolt and Screw Surface Finishes

Let’s take a look at the properties and benefits of each of these five finishes. There are multiple standards and OEM specifications for each type of finish. We will reference the ones that our customers most often require.

Zinc Phosphate & Oil Coating (Phos & Oil)

Zinc phosphate and oil is a liquid bath chemical conversion process followed by the application of protective oil. The coating is thin and black. This is the lowest cost finish option of the finishes we are examining.

This coating provides corrosion protection suitable for handling, packing, storing, and general use in everyday dry ambient conditions. 82% of the specifications we receive from customers are one of the following specs (see Table 2).

Common Phos & Oil Specifications
Specification	Color	Corrosion Resistance (NSS Hours)
GMW3179 – Oil	Black	24
GMW3179 – Oil with Corrosion Preventative	Black	72
MIL-DTL-16232 – Oil	Black	2
MIL-DTL-16232 – Oil with Corrosion Preventative	Black	72

Table 2 – Common Phos & Oil Specifications

Phosphate and oil coatings slightly increase the lubricity compared to plain steel parts. In general, there is no coefficient of friction (COF) requirement. does have a topcoat with friction control option with a COF of 0.13 ±0.03.

Screws and bolts finished with phos & oil have a very low risk for hydrogen embrittlement. Hardened parts must be processed per the above specifications to detect and eliminate hydrogen. In over 40 years, our company has never seen any hydrogen embrittlement in our phos & oil coated products.

Hydrogen embrittlement can be more of an issue with other types of finishes, as we’ll see below.

Zinc Electroplate (Zinc)

Zinc electroplate is the cathodic deposition of metallic zinc onto the steel fastener surface from an ionic water solution driven by the application of electrical current. The basic finish is gray in color. However, the color can be changed by the application of chromate coatings or passivation methods, so many colors are available.

Zinc electroplate is versatile. You can apply a range of zinc thicknesses on parts that increase the corrosion resistance. Thinner coatings are suitable for dry indoor conditions, thicker layers for wet outdoor and saline exposure.

As you can see in Table 1, zinc costs twice as much as phos & oil, but it provides more corrosion protection. 62% of the zinc specifications we get from customers, the most by a wide margin, are . The exact call out is ASTM B633 type II SC2 with yellow dichromate, shown in the picture above. SC2 is the moderate thickness class 8 µm (0.0003″).

Let’s compare this specification to others with the same thickness for corrosion resistance (see Table 3).

Common Zinc Electroplate Specifications
Specification	Color	Corrosion Resistance (NSS Hours)
ASTM B633-19 Type II SC2	Yellow	96
ASTM F1941-17 FE/ZN 8C	Yellow	120
GMW3044 9U72/0	Clear	72
GMW3044 8K240/120	Clear	240
Ford 347U Passivate	Clear	200

Table 3 – Common Zinc Electroplate Specifications

Zinc electroplate has a moderately rough surface, which decreases the lubricity compared to plain steel bolts and screws. Historically, zinc specifications have not had requirements for lubricity. The companies that make chemicals for zinc electroplate have developed sealers and lubricants that improve lubricity and corrosion resistance.

A good example is the OEM specification for the Ford S437 zinc finish. The NSS requirement is 384 hours and the COF is 0.15.

One potential downside to electroplated zinc is hydrogen embrittlement. Hydrogen is produced at the surface of steel during the electroplating process. Hydrogen can penetrate into the steel and cause embrittlement in hardened high-strength parts. Plating specifications account for this possibility by including post-electroplating baking procedures that decrease the amount of hydrogen in the steel. These baking requirements can be as long as 24 hours or more, depending on the plating thickness.

For more information on baking requirements, we recommend checking out .

Cadmium Electroplate (Cad)

Cadmium electroplate is produced using the same process as zinc. The basic finish is gray. The color can be changed by the application of chromate coatings.

Cad used to be specified extensively on commercial and military fasteners. However, in recent years, cad has been virtually eliminated from commercial products because of its toxicity. It‘s also been eliminated from new design for military parts. The only cad specification we continue to get from customers is .

The exact call out is AMS-QQ-P-416 Type II CL2 (0.0003) with yellow dichromate, shown to the left. Let’s look at the corrosion resistance for (0.0003″) thickness parts (see Table 4).

Common Cadmium Electroplate Specifications
Specification	Color	Corrosion Resistance (NSS Hours)
AMS QQ-P-416 Type II CL2	Yellow	96
ASTM F1941-17 FE/CD 8C	Yellow	72

Table 4 – Common Cadmium Electroplate Specifications

Cad electroplate has a smooth surface that increases the lubricity compared to plain steel bolts and screws. There is no COF requirement in the two referenced specifications. Cad parts coated to AMS QQ-P-416 torque more consistently than zinc parts coated per the above ASTM B633.

High-strength, hardened cad-electroplated parts must also be baked after plating to reduce the risk of hydrogen embrittlement.

Cad pricing used to be competitive with that of zinc electroplating. However, the relative rarity of cad usage has caused many plating applicators to stop offering cad plating. As a result, the price of cad has increased to more than four times that of zinc.

Zinc Non-Electrolytically Applied Coating (Zinc Flake)

Zinc flake is a coating that is non-electrolytically applied to the surface of a bolt or screw. Parts are dipped into a zinc-rich liquid, removed, spun to remove excess coating, and cured using heat.

This process produces a zinc flake base coat that is gray in color. Various topcoats can be added that contain lubricants or pigments for color (typically black), if desired. A good general reference for zinc flake coating is .

As you can see below in Table 5, zinc flake coatings provide very good corrosion resistance. This high level of corrosion resistance is necessary for parts designed to be used in harsh environments.

Common Zinc Flake Specifications
Specification	Color	Corrosion Resistance (NSS Hours)
TACOM 12424710 Method 4	Black	700
GMW3359 Code A	Grey	720

Table 5 – Common Zinc Flake Specifications

For our company, the cost of zinc flake is on the high end — twelve times as much as the cost for phos & oil. However, it must be noted that our volume for zinc flake is much lower than our volume for phos & oil, and this low volume will result in a higher price. Over 90% of the zinc flake specifications we get from customers are US Army/TACOM 12424710 Method 4 black (left, top) and gray (left, bottom). Both specifications have a thickness class of 12 µm (0.0005″).

Lubricants are commonly applied to zinc flake to increase lubricity. TACOM 12424710 has a COF requirement of 0.13 ±0.03 using the ISO 16047 test method. GMW3359 requires a COF determination per ISO 16047 in the part approval process.

Since it’s not applied using the electroplating method, zinc flake coated fasteners have no risk for hydrogen embrittlement, which is always a good thing.

Zinc-Nickel Electroplate (Zn-Ni)

Historically, zinc-nickel electroplate has not been as common as the other finishes listed here. However, it’s recently gained popularity as a less toxic, more environmentally friendly alternative to cadmium. New government regulations are pushing for use of zinc-nickel, encouraging more automotive, truck, military vehicle, and engine designers to fully replace cadmium with it.

Zinc-nickel typically consists of a 8-14 μm layer of 12-16% nickel alloy next to the steel material, topped with a 0.06-0.15 μm trivalent passivate layer and a 0.5-4 μm layer of top coat. It offers great corrosion resistance (up to 10x higher than conventional zinc), wear resistance in moving parts, and thermal stress resistance in parts subject to high operating temperatures.

Because it’s applied using the electroplating method, zinc-nickel coated fasteners are at risk for hydrogen embrittlement. They must be baked after plating to reduce the risk.

Here’s a look at common zinc-nickel specifications and their corrosion resistance levels (see Table 6).

Common Zinc-Nickel Specifications
Specification	Color	Corrosion Resistance NSS Hours
MIL-PRF-32647 Type K16	No color specified; yellow is discouraged	1000
GMW16730	Black	1000

Table 6 – Common Zinc-Nickel Specifications

Need Help Figuring Out Screw and Bolt Finishes? ��ϴ��¼� Us.

This concludes our short discussion on surface finishes for steel bolts and screws. We tried to provide a basic lay of the land. To learn more about other coating properties, along with dimensional and gauging requirements, check out the standards referenced in this article or reach out to our team.

As a custom fastener manufacturer with over 40 years of industry experience, we know the ins and outs of common surface finishes. We’re happy to help you find the right one for your application, and we’re committed to proven manufacturing processes that ensure quality results.

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The Difference Between 1A, 2A, and 3A Threads

Tim Pinchback — Sat, 10 Apr 2021 01:35:11 +0000

When it comes to screws, bolts, and other threaded fasteners, precision and accuracy are critical. Engineers and manufacturers rely on standardized measurements and classifications to ensure seamless assembly and optimal performance.

In Part 1 of our Threads Series, we provided some terminology and explained some of the nomenclatures of Unified Inch series threads. Now, we’ll look at three particular classes of fit within that series, as we discuss the difference between 1A, 2A, and 3A threads.

What Are 1A, 2A, and 3A Fastener Threads?

The terms “1A,” “2A,” and “3A” refer to classes of fit for external Unified Inch series threads on screws, bolts, and studs. Internal threads, such as those found in nuts or tapped holes, have thread fit classes termed “1B,” “2B,” and “3B.” The A/B mating parts are designed to fit together to allow free-running assembly with no interference.

The term “thread fit” is defined as the measure of the looseness or tightness between mating threads when an externally threaded fastener is assembled into an internally threaded hole or nut. As you’ll see below, 3A/3B is a tighter fit than 2A/2B, and 2A/2B is a tighter fit than 1A/1B.

1A vs. 2A vs. 3A Threads

TL;DR — The key distinction between 1A, 2A and 3A threads lies in their thread fit characteristics and tolerance levels.

1A Threads	2A Threads	3A Threads
Very loose fit for easy assembly Same allowance (standard) as 2A Relatively high tolerance (more than 2A & 3A) Relatively low minimum diameter	Most common type of external thread in the U.S. Standard level of tolerance and allowance Relatively loose fit Suitable for generic applications with a moderate level of precision	Closer tolerance than 2A (no allowance) Tighter fit and increased precision Suitable for applications where a high degree of accuracy is essential (aerospace, automotive, precision engineering)

To understand the difference between these fastener threads, you must first understand thread fit, allowances, and tolerances. Basically, thread fits are developed using allowances and tolerances.��

An allowance is an intentional clearance between mating threads. Allowances are applied to external threads. The major, pitch, and minor diameter maximums are less than the basic size by the amount of the allowance.��

A tolerance is the difference between the maximum and minimum permitted limits for a given dimension. Tolerances are specified amounts by which dimensions are permitted to vary for manufacturing convenience.��

And here’s the main difference between the three types of bolt, stud, or screw threads:

Class 1A threads have standard allowance and lots of tolerance (loosest fit)
Class 2A threads have standard allowance and tolerance (medium fit)
Class 3A threads have no allowance and close tolerance (tight fit)

The two diagrams below should nicely illustrate allowance and tolerance.

Here’s an Example

Let’s look at a specific example. This one compares 2A and 3A threads (we don’t manufacture parts with 1A threads at ��ϴ��¼�). The difference between class 2A and 3A external threads is shown below for 5/8-18 UNF parts.��

Note that for class 2A, both the major and pitch diameter maximums are below their respective basic values by the 0.0014 inch allowance. The allowance is 30% of the class 2A tolerance.��

For class 3A, the major and pitch diameter maximums are at the basic size. Also, the pitch diameter tolerance for class 3A is 0.0035 inches, which is smaller than the class 2A tolerance of 0.0047 inches.

A Side-by-Side Comparison

Now that we’ve examined an example of external threads, we are going to look at an entire mated thread system. Keeping the 5/8-18 example, let’s look at a side-by-side comparison of the complete clearance.��

The complete clearance consists of the allowance (if any) for the external thread and tolerances for the external and internal mating threads. See Figure 6 below for a side-by-side comparison of 5/8-18 UNF 2A/2B and 3A/3B pitch diameter. The 3A/3B thread fit has no allowance and smaller tolerances than the class 2A/2B thread fit, resulting in a tighter fit.

Learn More About Bolt and Screw Threads

We’ve provided some basic facts about Unified Inch series threads here. For more information, we recommend referring to the .��

Our Threads Series continues in Part 3, where we provide a brief overview of metric series threads.

At ��ϴ��¼�, we produce small-quantity bolts, screws, and studs that are difficult to find everywhere else. View our capabilities, then contact us or request a quote to see how we can help you.

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Tim Pinchback, Author at ���ϴ��¼�