Rolling screw threads in metal is the most cost and quality
effective method of producing threaded parts. Three basic thread production
technologies exist throughout the metal working world. Axial tools produce one
thread with each revolution and have no length limitation other than the
potential stroke length of the machine the tool is mounted on. The
tool design utilizes a roll that requires a soft lead to start the metal
deformation leading to a finished thread form. Depending on the roll lead the
part will exhibit 1.5 or more unfinished thread forms at the end of the part.
The harshness or softening of the lead has a direct impact on overall roll
life. Despite the unfinished thread characteristic, it is possible to gauge to
a shoulder using an undercut behind the thread body to clear away the
unfinished thread portion.
Fette thread rolls are designed with different lead
designations labeled .6K, 1K, 2K and 3K.
0.6K lead roll leaves 1.5 unfinished threads.
1k lead leaves 2 unfinished threads.
2K lead leaves 3 unfinished threads
3K lead leaves 4 unfinished threads.
Both 1K and 2K lead rolls are considered standard and priced
accordingly. A .6K and 3K lead roll set are considered “special” and
are premium priced.
Is there any advantage to running lengthy leads? OF
COURSE! A 2K lead can add 20% to roll life in many materials. A 3K lead
is frequently used in the oil patch industry and the additional unfinished
threads have no impact on the performance of sucker rods as an example. Cost
and availability of rolls often become the driving factor in tool selection as
well as part design. Applying the best roll selection can impact on overall
cost per thread but part design is frequently the driving force behind roll
When thread design commands very short threads or little to
no relief, axial thread rolling is not the best option even though axial
rolling is always the lowest cost tooling option.
The creation of screw threads
began several centuries ago, as a cutting technology. It was first used in wood
applications and as cutting materials improved, metal applications. With the
advent of more powerful machinery, rolling threads on various metals became
possible. The ductility of most metals lead to the discovery and development of
thread rolling technology. Each technology had benefits that were strongly
influential in their direct application.
Cut threads require lower
horsepower to be successful and could be used in materials with little or no
ductility. Most threads up to 2″ diameter are easily rolled on most manual
and CNC machines, both horizontal and vertical. Mechanical requirements are
easily calculated. A one-inch length of a cut thread can be completed in many
materials in 10 seconds. Difficult materials require more passes and more cycle
time. Rolled threads can be completed at a rate of 1 inch per second of cycle
time. Difficult materials pose little if any additional cycle time
requirements. Cut threads create sharp, bright finishes and are without
question prettier than rolled threads.
Rolled threads produce
radiused root and crest of the thread and exhibit at least a 40% increase in
tensile strength over a cut thread. Rolling a thread redistributes the grain of
the material while its cut thread cousin cuts across the grain. Cut threads
will always win a beauty contest over rolled threads but will lose in overall
quality comparison in all roll-able materials. In many of the more difficult
materials to cut, rolling can increase tensile strength by as much as 300%. Cut
threads in corrosive materials will always corrode at a more rapid rate than a
rolled thread with burnished flanks. Many applications that specify rolled
threads only are taking advantage of tensile improvement and corrosion
Roll cost as a ratio of parts
produced versus insert cost per parts produced, rolling will always win the
overall tool cost comparison. All thread classes can be cut or rolled. If your
part cost can be improved by shorter cycle time and improved tensile strength,
thread rolling may be your single opportunity to garner bid work in a job shop
atmosphere. Challenge us to help you find the best cost and quality to improve
your winning bid ratio.