WCA July 2014

20 / 61

18 Wire & Cable ASIA – July/August 2014
Industrynews
New test to compare
gear steel machinability
AS car transmission systems become more complex, with
higher numbers of gears, there is more of a premium on the
package space required for these systems. Designers want
to specify cleaner steels to create stronger parts able to
handle high loads without additional weight or reduction in
fatigue life, while machinists require steels with excellent
machinability for ease of manufacture.
Tata Steel’s automotive team demonstrated at wire 2014
how a new test they have developed with the University of
Sheffield, UK, will allow the steel supplier to compare
different steel chemistries, and prove how it can offer the
optimum balance between performance and ease of
processing for forgers, machinists and OEMs.
Good machinability is an essential factor for manufacturers
because it allows production rates to increase while
maintaining good chip control, surface finish and
dimensional stability. Chip control refers to the form in
which the excess material is generated when machining
steel. Small chips are better than longer ones, as they are
less likely to clog machines, avoiding costly machine stops
to clear entangled swarf.
In order to properly compare the machinability of different
steel chemistries in action, Tata Steel’s Swinden Technology
Centre, in Rotherham, UK, has worked with the University of
Sheffield to develop a bespoke machinability test that
replicates features of the hobbing process – the machining
process for gear cutting which cuts the ‘teeth’ out of the
forged blanks. The test allows steels with different machina-
bility-enhancing elements to be compared, without the need
to procure expensive gear hobbing tools or access to
specialist hobbing machines.
Designing gear steels requires a careful balance to combine
ease of machinability with high fatigue strength – the length
of time a product can function under cyclic loading before
failing. Tata Steel has worked with its supply chain partners
to develop a suite of clean gear steels that it claims offer the
best of both.
Adding controlled quantities of key elements can enhance
machinability, but has a negative impact on fatigue
performance. Adding sulphur to the steel reduces the shear
stress needed to create small chips.
Other additions such as calcium can be used to encourage
the formation of protective layers on cutting tools that
reduce tool wear rates. The addition of bismuth and
tellurium contributes to reduced cutting forces and lower
tool wear, offering an improved surface finish and – again –
smaller chips.
The new test compares machinability under interrupted
cutting conditions at speeds similar to those experienced in
production hobbing operations. The University of Sheffield
designed cutting tools with geometries similar to those used
in hobbing, and is developing a rapid data-logging system
for tool force measurement.
Tata Steel – India Website :