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 is attending wire 2014 this week to demonstrate how a new test they have developed with the University of Sheffield will allow the steel supplier to compare different steel chemistries, and prove how they 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 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 machinability-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 for under cyclic loading before failing. Tata Steel has worked with its supply chain partners to develop a suite of clean gear steels that 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’s business development manager for Speciality Steels, Andrew Woods, said: “Weight and space are at a premium on any vehicle, and the design of gear steels is a particularly complex challenge due to the duty cycle these parts endure. But by working closely with our customers, we are developing products which address the differing needs of weight saving, fatigue life, strength and machinability.
“This new test offers our customers a further means of saving time, money and effort, by allowing them to understand how any given material will act in service, something that would be difficult and costly for them to do themselves.”
Dr Tom Slatter, lecturer in Mechanical Engineering at the University of Sheffield said: “Collaborating with Tata Steel has allowed us to apply our machining science expertise in producing a test that has real industrial impact.”