After fitting Heidenhain linear scales to four traditional lathes at TRW, tests confirmed that these machine tools were now ready to meet the demanding needs of TRW’s ± 0.001" requirements for their Ford SUV parts.
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 their Navigator and Expedition SUVs, their traditional lathes were just not accurate enough for the task, according to TRW insiders. The solution was to upgrade the lathes with Heidenhain linear encoders and tackle the job.
Accuracies of ± 0.001"
The tie rod ends on the Ford rear toe-links feature a ball stud that needs to be round requiring high accuracies and repeatability. “We needed to maintain accuracies of ± 0.001" for this job and this was really pushing the limits of what a standard lathe is capable of holding,” explained Stuart Lockhart, Manufacturing Engineering Supervisor at TRW Automotive Linkage and Suspension Systems Division 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.”
That is when the conversation began with Elliott Matsuura Canada, an established organization serving the high tech metalcutting machining industries with sales and service. “We wanted to not only increase the accuracy of the lathes, but to also take the thermal growth of the ballscrew out of the machines’ equation,” explained Jim Earl, Senior CNC Service Engineer at Elliott. “I have 28 years experience in my job and know that upgrades to Heidenhain linear scales would fix them right up by serving as a reliable direct measurement source as they compensate well for many machine imperfections.”
Earl explained 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) that 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. A properly fitted linear scale measures the actual, final movement, keeping the cutter where it needs to be.
TRW had been using their four 30-horsepower CNC turning center lathes (with 12-station turrets for tooling) regularly for about five years before this rear toe-link job came into play. And while there had been much success with the various part jobs off these lathes, there had been constant adjustments necessary by the operators and common issues with scrap.
According to Lockhart, these problems were especially noticeable when they got into the daily warm-up of the lathes and the spindles would inevitably grow as the machine heated up. The ballscrews would change with the heat which affected accuracy. And if the operator needed to go on break and allowed the machine to cool down, that would change the whole process again. Operator adjustments were common and much scrap inevitable. And with the new Ford rear toe-link job, the stakes and requirements were now going to be even higher. “The thermal expansion of the ballscrew was a problem,” said Lockhart, “and we had to find a way to solve it.” But the methods and costs to do so needed be evaluated and rationalized.
Capability studies were done and it was determined that it was worth proceeding. “We decided it was well worth the cost in order to get the extra accuracy out of our lathes, as well as to reduce the amount of scrap that we would potentially generate,” said Lockhart. “It was also very important to us to achieve process capability of 1.67PpK (Process Performance Index) or greater, ensuring we had a stable process.” Lockhart authorized the scale retrofits to ensure the success of the Ford project.
Improved Process Capability
TRW’s four lathes were being tooled at Elliott’s facility in Oakville, Ontario, Canada. 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 the LC 493 absolute encoders on TRW’s lathes because of the proven reliability of their products,” explained Earl. The LC 400 series scales are a sealed, slimline variety that provides 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,” said Earl. “The scales are reliable and the service we get from Heidenhain is just great.”
Lockhart supported Earl’s recommendation by saying “we use these scales on other machines in our TRW plants with much success, so we were confident with this choice.”
The upgrade process at Elliott took only a few days. Testing following the installations showed the improvement that TRW was looking for.
“The results were just what we hoped for,” said Lockhart. Process capability studies performed at Elliott and again when the machines were installed at TRW showed the process capability had improved to the point where they exceeded TRW’s minimum requirement of 1.67 PpK. “This was unattainable without the Heidenhain linear scales,” said Lockhart. The Ford job went on to be a success.
“These upgrades also ended up making these lathes even easier for the operators to use as the need for constant adjusting had been eliminated,” said Earl. “Thanks to the linear upgrades, there are no more concerns about warm up or cool down of the machines. They are accurate and ready to go at turn on.”
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