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Introduction
This guide contains everything you need to know about thread
rolling and screw machine products.
You will learn about topics such as:
What is thread rolling?
Thread rolling processes
Advantages and disadvantages of thread rolling
Common defects
Types of thread rolling machines
And much more…
Chapter 1: What is Thread Rolling?
Thread rolling is a type of threading process which involves
deforming a metal stock by rolling it through dies. This process
forms external threads along the surface of the metal stock.
Internal threads can be formed using the same principle,
specifically termed thread forming. In contrast with other
widely used threading processes such as thread cutting, thread
rolling is not a subtractive process. This means it does not
remove metal from the stock. The rolled threaded fasteners offer
advantages such as stronger threads, precise final dimensions,
good surface finish, and a lower coefficient of friction.
Screw machine products are threaded machine elements such as
bolts, nuts, and screws. Threaded machine elements can be
grouped according to their function. Bolts, nuts, and screws are
structural components called fasteners. Threaded fasteners can
also be integrated to the part making threaded fittings.
Threaded fasteners create non-permanent joints which can be
loosened or dismantled through mechanical action. Power screws
and lead screws are considered mechanisms or mechanical drives.
These machine elements control movement and transmit power to
other machine parts.
Screw Thread Forms
Screw threads can be classified according to their form.
V-Thread: These are triangular threads with flanks that
typically form 60° with each other. The crests and roots
are sharp, but in some cases, as a small flat portion due to
limitations in fabrication.
American National Thread: Formerly known as the United
States Standard Screw Thread, the American National Thread is
a more standardized version of the V-thread which has specific
dimensions to the flatness of the crests and roots of the
threads. This form replaced the V-thread for general use.
British Whitworth Thread: This was the British
counterpart of the American National Thread.
Unified Thread: This thread form replaced the American
National Thread along with thread standards from Canada and
Britain. This was developed to allow interchangeability of
parts. Unified threads still have the V-shape profile but with
rounded or flat crests and roots. The Unified Thread Standard
(UTS) consists of series, namely, Unified Fine (UNF), Unified
Coarse (UNC), Unified Extra Fine (UNEF), and Unified Special
(UNS).
Metric Thread: This thread form was developed to
transition from the imperial-based measurement into the metric
system. This was brought by the ISO, displacing the UTS thread
form.
Square Thread: Square threads are special-purpose
threads used for power transmission. Theoretically, they are
the ideal thread for mechanisms and drive applications due to
the perpendicularity of the load-bearing faces or flanks with
the axis. However, this form is not practical due to
manufacturing limitations.
Acme Thread: This thread form is a modification of the
square thread. The acme thread is characterized as having a
trapezoidal form with a narrower root than its crest. Acme
threads are stronger and easier to machine than square
threads.
Buttress Thread: In this thread form, one flank is
perpendicular or with a slight angle with the axis while the
other has a 45° angle. This thread form is designed to
transmit high loads in one direction.
Knuckle Thread: Knuckle threads have highly rounded
crests and roots with a flank angle of 30°. The rounded
profile allows debris to be shifted to not interfere with the
meshing of the threads.
Chapter 2: Overview of Threading Processes
The processes of generating threads are generally classified
into three methods: subtractive, deformative, and additive.
These differ on how the thread is shaped or formed. Subtractive
processes are generally known as cutting processes. These
processes are summarized below:
Tapping: Tapping is a thread machining process for
producing internal threads. This is done using a tap which is
a cylindrical or conical cutting tool. The tap has multiple
cutting edges similar to an external thread. The internal
thread is generated by rotating the tap while axially moving
it deeper into the bore of the metal stock.
Die Threading: This process is used to produce external
threads. Its method of applying force and cutting action is
similar to tapping. A die is used to cut the metal stock with
multiple cutting points similar to an internal thread.
Different die designs exist which can be solid or
self-opening.
Single-point Cutting: Single-point cutting is done in a
lathe machine where the metal stock is held and rotated. The
cutting tool is mounted on a carriage fed linearly by a lead
screw. This process can produce both internal and external
threads. This process is slower than tapping or die-cutting.
Its advantage is that only one cutting tool is required to
produce different threads.
Chasing: This process uses a tool called a thread
chaser which is several single-point cutting tools fitted
together. The chaser is typically mounted on the carriage of a
lathe which is indexed gradually to cut the thread.
Milling: In this process, single or multiple rotary
cutting tools are used to thread the stock. Aside from
rotating the cutting tool and indexing it axially as seen in
tapping and die-threading, the cutting tool is also revolved
along the circumference of the threaded surface. Thread
milling can generate both internal and external threads.
Grinding: Instead of cutting the stock, this process
uses abrasive tools to remove metal. This is usually done in
conjunction with other threading processes. Thread grinding is
done to produce precision threads and threads with a good
finish.
Deformation processes generate threads by working the metal
stock to form its shape. This classification includes
rolling and casting:
Rolling: As mentioned earlier, thread rolling is an
external threading process that shapes the stock by passing it
through roller dies. The roller dies have external thread-like
rollers which contact and deform the surface of the stock.
Thread rolling is generally faster than thread cutting since
the shaping process only requires few passes.
Casting: This process involves pouring or injecting the
molten metal into a die or mold. The die contains the negative
shape of the threaded part. This process requires secondary
machining processes to produce accurate threads. This process
is not suitable for making fine threads.
Finally, additive processes are methods of producing threads
by gradually adding or depositing materials. These are
extensively being used for producing plastic parts.
Advancements in technology further extend the process for
producing metal machine elements. To produce quality
threads, it is used together with secondary processes such
as grinding and lapping.
Some of the additive processes are stereolithography,
selective laser sintering, and fused filament fabrication:
Stereolithography: This is one of the most widely used
3D printing processes for producing plastic parts. This
process involves a bath of plastic resin which is cured by a
focused beam of light.
Selective Laser Sintering: This process uses a laser
beam to sinter powdered material. Plastic is a common material
used for the process, but the technology is now gaining ground
in producing metal parts.
Fused Filament Fabrication: In this process, a
continuous filament of material is melted and extruded to form
the desired shape of the part.
Leading Manufacturers and Suppliers
Chapter 3: Advantages and Disadvantages of Thread Rolling
There are many advantages and disadvantages to using a rolled
screw machine product. The main advantage of thread rolling is
the stronger surface and dimensional accuracy of the product.
However, since this process operates on the principle of metal
deformation, it is limited to soft metals and requires more
expensive tooling.
Enumerated below are the advantages of using rolled screws and
bolts:
High thread strength: Thread rolling is usually done in
relatively low temperatures making it a cold working process.
Cold working is known to produce parts with higher strengths
without the need for secondary heat treatment. This makes
rolling suitable for threading materials that do not respond
to heat treatment. Rolled threads are 10 to 20 percent
stronger than cut or ground threads.
Good surface finishes: Thread rolling inherently
imparts smooth and burnished threads without the need for
secondary polishing processes. The high compressive forces
that deform the metal remove any unevenness on the surface of
the thread. Rolled surfaces have a surface roughness of
approximately 8 to 24 microinches Ra while cut threads
typically have 64 to 125 microinches Ra. Rolled threads are
also free of tears, chatter marks, cutting marks, and burrs.
Precision threading: Since the dies used in thread
rolling are mirror images of the threads to be generated and
there is no material being removed from the stock, the process
can produce parts with high precision and accuracy over long
runs. This is true provided that the dies are accurate and
made with sufficient hardness.
Lower coefficient of friction: Good surface finish
leads to a lower coefficient of friction. A lower coefficient
of friction provides more uniform and consistent tightening of
nuts and bolts or better power transmission for lead screws.
Lower production lead times: Thread rolling is
generally faster than thread cutting. Rolling speeds depend on
the type of material, thread profile, size and capacity of the
machine, and method of feeding the metal stock. For
reciprocating dies, thread rolling can produce 30 to 40 parts
per minute with stock diameters ranging from 5/8 to 1 1/8. For
cylindrical dies, 10 to 30 parts per minute for sizes ranging
from 1 to 1 ½ inch.
Lower cost brought by the efficient use of material:
Since thread rolling is a deformative process, no amount of
material is removed throughout the process. This leads to
better energy utilization since there is no need to collect
and recycle wasted materials.
Below are the disadvantages of thread rolling. It can be
seen that these mostly affect the manufacturer rather than
the end-user. Ultimately, these contribute to the product
cost making rolled threaded products, in some cases, more
expensive than products produced from other processes.
Not practical for hard materials: Thread rolling is
mostly done on malleable metals. Even though it is possible,
thread rolling is not done for metals above 40 Rockwell C.
Beyond this hardness level, thread grinding is more practical.
Rolling hard materials significantly decreases tool life.
More expensive tooling: Dies used for rolling must be
hard and precise. Any deformity of the die will result in poor
dimensional accuracy of the threads. Because of the required
hardness, precisely fabricating the dies are difficult.
Stock diameter must be precise: There must be the right
amount of material that will flow to be displaced and raised
above the original surface. The diameter of the stock must be
calculated and verified by trial especially when producing
accurate threads. To get the right diameter, the stock may
require a preliminary turning process.
Chapter 4: Factors to Consider in Thread Rolling
Like any other machining process, there are several factors to
consider ensuring optimum operating conditions and product
quality. Listed below are some of the most significant variables
affecting thread rolling.
Material Requirements: A known disadvantage of thread
rolling is its incompatibility with hard materials. The
materials to be rolled must have a hardness not greater than
HRC 40. Materials that can be rolled are low-carbon steels,
mild steels, stainless steels, copper alloys, and often,
aluminum. Moreover, the material must have the right degree of
ductility. The recommended range is 12 to 20% elongation
factor.
Stock Diameter: The correct stock diameter is almost
the same as the pitch diameter of the screw or bolts. Usually,
the space or cavity between the threads and below the pitch
line is the same as the volume of the thread above the pitch
line. Some adjustments for tolerances may be needed to attain
the desired crest formation, especially if secondary processes
such as coating, or plating are needed to be done.
Chamfer Angle: Chamfer is the tapered conical surface
at the start of a thread. Before rolling, the edge at one end
of the stock must be machined to have a chamfer. A correct
chamfer angle must be set to properly shape the thread at the
end. The recommended chamfer angle is 30° for most cases.
Feeding: There are three basic techniques for feeding
the stock into the dies: radial infeed, tangential feed, and
through feed. In radial infeed, the dies move radially towards
the axis of the stock. For tangential feed, the pitch of the
stock approaches the rollers from its side making square,
tangential contact. Lastly, through feed involves a
cylindrical die that mates against the stock causing it to
move axially.
Thread Rolling Speeds: Thread rolling speeds depend on
the mechanical and power limitations of the machine, the
thread diameter, and the material and hardness of the metal
stock. Rolling speeds can range from 30 to 100 m/min. Low
rolling speeds are required for hard materials while high
speeds are for soft and ductile materials.
Coolant and Lubricant: Coolants or cutting fluids are
extensively used in thread cutting, but these are also
necessary for thread rolling. Deforming the metal also
generates heat which can compromise both the dies and stock.
Moreover, coolants can act also as lubricants to reduce the
friction between the dies and stock.
Chapter 5: Common Defects
Though the thread rolling process offers higher precision than
others, upsets and irregularities in the operation will
inevitably create defects. Most of the defects arise from the
out-of-tolerance stock dimensions, worn-out or misaligned
rollers, and improper stock feeding. Below are the most common
defects seen in thread rolling.
Truncated Thread Crest: This defect is described by a
non-fully formed crest or an excessively truncated crest. One
reason could be an undersized stock where there is
insufficient material to flow and create the crests. This is
fixed by gradually increasing the size of the stock. If the
pitch diameter is oversized, then the more probable root cause
is a loose threading head which is solved by sizing-in. If
not, then the defect is possibly caused by too much hardness
of the material. Thus, it is then necessary to change into a
softer material.
Flaking: Flaking or slivers causes unusual roughness on
the surface of the threads. This is usually caused by the
incompatibility of the material for rolling. The root causes
can be excessive lead and sulfur content, inconsistent grain
structure, and sometimes cold working before rolling. If the
material being used already has a good rollability, then other
possible causes can be mismatched rollers or dies, rough
roller surface, overfilling, or slow rolling speeds.
Drunken Threads: This defect is seen as wavy or uneven
thread crests. This is a result of mismatched dies, misaligned
feeding of the stock, or poor die construction. The best
solution is to check the condition of the rolls and their
bushings.
Curved Pitch Line: This is viewed as the tapering of
the threads towards the ends of the threaded segments of the
bolt or screw. The curvature can be concave or convex. Its
root causes are inconsistent stock diameter, misaligned stock
relative to the roller, wearing of rollers, or too much
deformation of the material causing it to flow towards the end
of the stock.
Out-of-tolerance Helix Angle: This can be a result of a
variety of causes such as unsynchronized rollers, imperfect
rollers, incorrect feeding of the stock, or screw jacking.
This can be solved by correctly timing and aligning the
rollers, proper stock feeding, and optimizing the rolling
speed.
Poor Finish: Poor finish is a result of factors such as
worn-out dies, high material hardness, oversized stock
diameter, or presence of contaminants in the coolant supply.
Cupped End: A cupped end appears as a concave-end
caused by forcing the metal to flow over an insufficient
chamfer. This is more evident on softer metals. The defect is
solved by properly chamfering the stock, usually about
30°.
Chapter 6: Types of Thread Rolling Machines
Thread rolling is a straightforward process where a metal bar is
initially cut to length and forged to make the bolt or screw
head. Afterward, it is machined to attain the right stock
diameter and chamfer on one end. It is then fed into the
threading machine where it passes through dies which form the
shape of the thread. After rolling, the stock is then sent to
secondary processes such as plating, anodizing, and coating.
This process summary is true to any type of thread rolling.
There are different types of thread rolling machines that vary
according to the type of die used. Thread rolling machines can
be a flat-die, planetary, or cylindrical-die type.
Flat-die Type: This type of thread rolling machine
consists of two rectangular dies where one is stationary while
the other is reciprocating. The reciprocating die moves
parallel to the stationary die. The surface of the dies
contains ridges representing the profile of the thread to be
produced. These ridges are inclined at an angle equal to the
helix angle of the thread. The distance between the crests of
the dies is equal to the minor diameter of the thread.
The threads are formed typically in one passage only. The
length of the die allows the stock to be rolled around six
to eight times. The stock is inserted on one end, either
manually or automatically. The dies roll the stock
tangentially which carries it up to the opposite end by
friction.
Segment or Planetary Type: A planetary type operates by
rolling the stock through one stationary and one moving
surface. However, this machine uses rotating motion instead of
translation. This type involves stationary curved dies and a
central rotating die. One or more stationary dies can be
matched with a single rotating die. A stationary die rolls one
stock at a time.
Similar to the flat-die type, the planetary machine has a
finite rolling surface that forms the thread through one
passage. The stock is inserted on one end of the curved die.
The rotating die then rotates a full arc of the curved die
revolving the stock until ejected on the opposite end.
Cylindrical-die Type: Cylindrical dies or rollers are
regarded as dies with infinite work surfaces. These machines
usually operate through the combination of radial and through
feeding. Unlike the flat-die and planetary types, the
cylindrical-die type deforms the metal through multiple passes
as it rolls. Cylindrical-die type machines can be further
divided into two major categories: two-die and three-die
machines.
Two-die: This type of threading machine has two
parallel rollers wherein one or both can move radially to
accept and penetrate the stock. The stock is positioned
with a slight offset from the plane of the centerline of
the dies to prevent it from rising out. A smooth roller
support or rest bar is located in the middle to hold the
stock as it is being threaded.
Three-die: This machine has three rollers
positioned 120° from each other. Typically, all
rollers can move radially wherein the position of the
stock is maintained at the center during penetration.
Compared with two-die machines, three-die types have
better force balance but are more difficult and complex to
adjust.
Conclusion:
Thread rolling is a type of threading process which involves
deforming a metal stock by rolling it through dies to form
external threads along its surface. Internal threads can be
formed using the same principle, specifically termed thread
forming.
The processes of generating threads are generally classified
into three methods: subtractive, deformative, and additive.
These differ on how the thread is shaped or formed.
The main advantage of thread rolling is the stronger surface
and dimensional accuracy of the product. However, the process
is limited to soft metals and requires more expensive tooling.
There are different types of thread rolling machines that vary
according to the type of die used. Thread rolling machines can
be a flat-die, planetary, or cylindrical-die type.
Leading Manufacturers and Suppliers
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