Story About Screws

Story About Screws

What is “NEJI” (Screw) ?

It has been known for centuries that when you are trying to move a heavy load, it is better to move it up by using a slope, or with rollers. Moreover, it is easier to climb a gentle slope than a steep slope. Slopes allow a heavy load to be moved vertical with smaller amount of force than simply lifting it.

So, why does a slope have such an advantage on lifting heavy load? In order to lift an object vertically, we have to have the sufficient force to overcome gravity. According to the vector space (geometry), however, when an object is on a slope, mathematically, the gravity force on the object will be divided by the vertical and the parallel force of the slope. The vertical force is equal to gravity based on the action-reaction law (Newton’s Third Law). Therefore, when you move an object on a slope, you just need to have the force that is equal to the parallel force (of the slope). When the parallel force is on an object on a slope, the object will slide down the slope easily if the surface of the slope is extremely smooth. However, because surfaces of common materials are not perfectly smooth, there is always friction on an object and a slope, which means that there are always forces acting against the parallel force.

According to the vector space, the rate of the force that is parallel to the slope and the actual force of gravity is equal to the rate of the right triangle’s height and its slope (while the base is on a flat surface, and the slope is toward the right angle).

Thus, the smaller the angle of a slope, the less force is needed. That is, to move an object along a slope, it needs more force than the counteracting force that is parallel with the slope. With this principle, we are able to lift an object with a force that is much less than the force of gravity.

However, the distance to move an object is far greater than the distance to move the object vertically. Although, the smaller the angle of the slope, the less force is needed to move an object up, but it also needs more distance to move it to the same height as a steeper slope. Thus, for example, while you are in a hurry to reach to the top of a mountain, it is easier to climb a gentle-sloped main path rather than to climb a steep-sloped side path.

A screw and a wedge are based on this principle of a slope. A screw, based on the slope principle, is basically a wedge put around a column. Another way to look at it is right triangle wrapped around a column. Where the larger side of two sides not the hypotenuse is vertical to the column axis, the hypotenuse becomes a “helix” on the column. A thread of a screw is basically a wedge along this “helix”.

The groove of a screw thread is called the “root,” and the crest of a screw thread is called the “ridge.” A screw’s stem is called the “shank.” A nut is a thing that carves a groove inside the hole where the crest and the root of a thread fits. Generally, bolts and nuts work in tandem like screws. When a screw is turned one circumference of its shank, the distance the thread ridge moves toward the axis is called the “lead.” It is the “pitch” that is the distance from a point on one thread ridge to a corresponding point on the next thread ridge measured parallel to the axis.

Screws have the same movement as a slope. In brief, the moving along a slope corresponds to the rotation of the screw, and the lifting corresponds to the movement of the shaft. Therefore, screws have a large potential energy along its axis of direction when screws have been given only a little energy to rotate. Screws have been described as a rolled slope on a round stick, and as the slope angle changes to a smaller angle, the pitch changes to a smaller pitch. Since the slope function has more power as the slope angle gets smaller and smaller, the screw fastening power is stronger the more the pitch gets smaller and smaller. But, if the pitch is too small and if the screw is strongly fastened, it will cause eroding of the thread or shearing off of the screw. Thus, the pitch has to be made properly in accordance with the method, material and size.

The JIS is a standard of various fasteners. However, it does not mean that all of those fasteners have been produced by and stocked in fastener companies; in contrast, there have been produced fasteners that are not standardized in large quantities. If those purchased quantities are several hundred thousand pieces or more, they are very cheap. But, if the quantity is small, the cost of the fastener is very high. For example, high tension bolts of boron steel for construction, indented hex bolts, and tapping screws for the automotive industry are produced in very large quantities though they are not standardized. On the other hand, bolts for airplanes are very expensive because they are produced in small quantities by American standards and plated with special coating (Cadmium coating). Common machine screws are usually a course pitch except a thread precision of 6g (for female screws) and 6H (for male screws) for some cases, and a fine pitch for automotive screws. The thread forms of screws for plastics are not standardized in some cases.

Other than general (metric) screws, tapping screws are commonly used.

Screw’s Mechanical Property

In order to use a screw properly, it is not enough to just fasten it. You need to use the proper screw and fasten with sufficient torque to hold things together properly. When you choose a screw, you need to know its other mechanical properties besides its size.

There are two ways to evaluate mechanical property. One way is to examine a bolt through a universal testing machine. The other is to cut a part off and examine if the bolt is too big. An easy inspection method is examining the hardness of the bolt. However, for the screws made with a cold header, we need to cut off an axis and examine the cross section, because the head of a bolt and the edge of a screw are hard.

The examination of a tapping screw is to screw it into a testing board that has a specific hardness, thickness, and internal diameter, then measure the torque it takes to screw the tapping screw through the opposite side of the board. Also, the edge of the screw is held firm and the torque is measured with over-screwing. When we use a tapping screw, we don’t have to worry about a hole, which it screws into, so its use has been increasing. When a steel plate is thin, we get burring on the board, which makes the edge of the exit hole of the tapping screw slightly thicker. We need to be careful with choosing an internal blank diameter. For a synthetic resin, we deal with it by fixing the pitch, an included burring angle, and a thread profile.

Screw fastening

The screw head is the key to screw fastening as it is where the torque force is transmitted into the screw. Due to the tendency to slip out, single-slotted screw drive is no longer common today. It has largely been replaced by cross-recess and hexagon socket drives, among dozens of other modified drive types. The commonality/uniqueness of the drive type is one important factor that is needed to be considered when choosing which type that best suits the application. Most screws/bolts are fastened by screwdrivers and wrenches.

It is not uncommon that screws for important positions will have their required fastening torque indicated on the engineering prints, and will have to be checked by torque wrenches after fastening.
To ensure the axial force of the screw is properly represented by the fastening torque, the friction coefficient of the screw drive must be properly controlled. Extra attention is needed at production stages and storage, and proper handling during the fastening as well as the oiling/painting processes is required as well. Air or electric powered screwdrivers are the norms nowadays, many of them are equipped with torque control.

Screw material

Low-carton steel is the most common material for screw making, followed by medium-carbon steel (SC), alloy steel (such as SCM), and stainless steel (such as SUS304). Boron steel is commonly used for building/construction screws. Brass is the ideal material for shipbuilding bolts, while aluminum bronze is the choice for screws inside refrigerating equipment and air conditioning. Aluminum alloy is sometimes employed as screw material for special applications as well.

Production Process – Bolts and Machine Screws

I. Blanks

The first step to making external threads (i.e. bolts and machine screws) is to create blanks, or non-threaded screws. This is done by hitting and upswelling the head of the cut wire. The process is called head forming, which in most cases is done by an automatic heading machine, commonly known as a “header”. During the heading process, the material (i.e. the wire) is first aligned by straightening rollers, and then cut into the required length, before it is passed to the round dice. The protruding dice then form the head by compressing the material two times.

The first step to making external threads (i.e. bolts and machine screws) is to create blanks, or non-threaded screws. This is done by hitting and upswelling the head of the cut wire. The process is called head forming, which in most cases is done by an automatic heading machine, commonly known as a “header”. During the heading process, the material (i.e. the wire) is first aligned by straightening rollers, and then cut into the required length, before it is passed to the round dice. The protruding dice then form the head by compressing the material two times.

In cases where the required head diameter is large, a thicker wire spec is needed in order to form the head successfully. After the head is formed, the non-head portion is narrowed and fine-tuned until the required screw diameter is achieved. This process was done by a multi-stage header before, but it has largely been replaced by a 2-die/3-blow method nowadays which, as its name suggests, requires only two dice. There are machines that even utilize a 2-die/2-blow mechanism. These highly productive machines are able to produce 60 blanks per minute for M20, and 300-500 blanks per minute for M6.

II. External Thread Ridges

Except for certain specialty items, most threads are made by flat-die rolling. Threads are created when a moving flat-die horizontally rolls against a fixed flat-die in designated time.
With improved reliability of the rolling machines, flat dies today can achieve high dimensional accuracy and provide great satisfaction toward screw precision.
Screw precision is commonly inspected by Go-NoGo limit gauges. Special tip formation on tapping screws can also be processed simultaneously with flat-die rolling.

Production Process – Nuts

I. Blanks
Nut blanks for standard items M10 or under are usually made from coiled round steel wire by multi-stage automatic stamping machines called “nutformers”, in a fashion similar to how bolt blanks are made. Hex-shaped steel wire is also used if one wishes to reduce the number of stamping stages. Flat wire (hoop) can be used to enable one to produce nut blanks by a regular stamping method without a transfer device. Blanks for nuts of larger diameter, such as those between M22 and M36 are usually made from steel rod by hot multi-stage stamping machines.
II. Internal Thread Ridges
Unlike external threads, ridges of internal threads are drilled by automated tappers. At the time of writing, internal threads are not able to be manufactured by rolling, but certain processes can be added to eliminate cutting scraps by using fluteless taps.

Production Process – Tapping Screws

Production of tapping screws involves carburizing, quenching, and tempering processes after the screw thread is rolled. Consecutive heat treatment is usually done by a mesh belt conveyor furnace. Washers, if coupled with tapping screws, need to be anti-carburizing and copper-plated.

Common Screw Thread Terminology (based on JIS)

Basic Profile

An imaginary cross-sectional shape of one pitch which serves as the basis for establishing the actual cross-section profile of a thread ridge. A profile that includes the axial line of the screw has been the norm. Also known as a thread form.

Lead

The distance a screw thread advances axially in one complete turn.

Pitch

The distance from a point on one thread to a corresponding point on the next thread measured parallel to the axis.

Lead Angle

On a straight thread, the lead angle is the angle made by the helix of the thread at the pitch line with a plane perpendicular to the axis. On a taper thread, the lead angle at a given axial position is the angle made by the conical spiral of the thread with the perpendicular to the axis at the pitch line.
On other types of thread, the lead angle is created by the helix on the imaginary cylinder (or cone) that touches the pitch diameter.

Major Diameter of External Thread

The largest diameter of the external thread. It is the diameter of the imaginary cylinder (or cone) that connects the crest of external thread.

Minor Diameter of External Thread

The smallest diameter of the external thread. It is the diameter of an imaginary cylinder (or cone) that connects the root of external thread.

Major Diameter of Internal Thread

The largest diameter of the internal thread. It is the diameter of an imaginary cylinder (or cone) that connects the root of internal thread.

Minor Diameter of Internal Thread

The smallest diameter of the internal thread. It is the diameter of an imaginary cylinder (or cone) that connects the crest of internal thread.

Pitch diameter

The mean diameter between the major and minor diameters. It is the diameter of an imaginary cylinder (or cone) where the width of thread ridge and the width of thread groove are equal.

Nominal diameter

The diameter that defines how the screw thread is called. In most cases, the major diameter is used as nominal diameter.

Flank

Sometimes called a thread face, it is the surface between the crest and the root. It is usually represented by straight, sloped lines when appears in a cross-section profile.

Pressure Flank

The side of the flank which directly receives the load when screwed-in.

Clearance Flank

The side of the flank which is at the opposite side of the pressure flank.

Crest

The surface joining the ridge of two adjacent flanks of a thread. In an external thread, the outmost portion is called a crest, whereas in an internal thread, the bottommost portion is called a crest.

Root

The surface joining the groove of two adjacent flanks. In an external thread, the bottommost portion is called a root, whereas in an internal thread, the outmost portion is called a root.

Flank Angle

The angle between a flank and an imaginary straight line perpendicular to the axial line of a cross-section profile.

Thread Angle

Also called thread full angle, it is the angle formed by two adjacent flanks in a cross-section profile.

Thread Overlap

The shared distance between an external and an internal thread when they concentrically fit.

Percentage of Thread Engagement

The percentage of thread overlap (H1’) relative to the basic thread overlap (H1).