TRW has been making parts for the automotive industry for many years, but when Ford Automotive considered giving them the contract to manufacture the rear toe-links for its Navigator and Expedition SUVs, the company's traditional lathes were just not accurate enough for the task, according to TRW insiders.
The tie rod ends on the Ford rear toe-links feature a ball stud which needs to be round, requiring high accuracies and repeatability. "We needed to maintain accuracies of ±0.001 inch for this job and this was really pushing the limits of what a standard lathe is capable of holding," says Stuart Lockhart, manufacturing engineering supervisor at TRW Automotive Linkage and Suspension Systems Div. in St. Catharines, Ontario, Canada. "And because all four of our lathes here only had rotary encoders on motors connected to ballscrews for positioning, we also had backlash and thermal expansion issues that were proving unacceptable to us as well."
Jim Earl, CNC service engineer at Eliott Matsura Canada who became involved with upgrading the lathes, says that when machines rely on the rotary encoder on the motor to determine the machine positioning, the user is depending on the rest of the machine the ballscrew and ballnut to all have perfect mechanical integrity. Here, the motor is supposed to run a certain number of revolutions to move the tool to a certain distance. With high traverse speeds and high acceleration/deceleration rates, significant forces are being placed on the ballscrew, resulting in thermal expansion of the ballscrew (through friction), which changes the position of the ballnut that travels along it. In these lathes, this changed position means the cutter is in the wrong place and therefore cutting inaccurately. Alternatively, a properly fitted linear scale measures the actual, final movement, keeping the cutter where it needs to be.
Knowing this, TRW agreed with the upgrade of the four lathes. An absolute linear scale system from HEIDENHAIN was integrated on each lathe, via custom bracketry. Electrical interface to the controls were then completed and parameters set. "We used HEIDENHAIN's LC 493 absolute encoders on TRW's lathes because of the proven reliability of HEIDENHAIN products," says Earl. The LC 400 series scales are of a sealed slimline variety that provide true absolute position value without any previous traverse required. "In all my years in this business, I don't know that we've used any other manufacturer's scales," says Earl. "The scales are reliable and the service we get from HEIDENHAIN is just great."
The upgrade process at Elliott took only a few days.
"The results were just what we hoped for," says Lockhart. Process capability studies performed at Elliott, and again when the machines were installed at TRW, showed the process capability improved to the point where they exceeded TRW's minimum requirement of 1.67 PpK. "This was unattainable without the HEIDENHAIN linear scales," says Lockhart. The Ford job went on to be a success.