Major Tool & Machine (MTM; Indianapolis) faced a challenge when it decided to rebuild its Schiess FZG6 (DSI) gantry mill in order to machine complex, high-precision large parts utilizing current absolute linear scale technology from Heidenhain Corp. (Schaumburg, IL).
The gantry mill was thought to be one of the largest machine tools in the world when it was originally commissioned in Germany in 1995. Change didn’t come easy because of the mill’s massive size, but nonetheless in 2010 the company made significant updates to the super-large gantry mill, a machine so large that MTM executives built a separate building for it in Indianapolis.
At the time of commissioning in 1995, Gunther Zimmermann was hired by MTM and came to the US from Germany to oversee the project. Having established roots in Indiana, Gunther became a US citizen and remains an important employee at MTM.
“We have many significant machine tools here, but this is a very unique machine and our largest,” CNC controls engineer Zimmermann said. “Its bed goes 40′ [12 m] into the ground, its workspace is huge, and it has both five-axis milling and turning capabilities. Handling the machining of large parts for many years, it has proven to be a true workhorse.” MTM manufactures hardware for a variety of industries, including aerospace, power generation, nuclear and commercial/industrial.
“But technical developments have come a long way in over 20 years,” said Zimmerman. So in 2014, the Schiess (DSI) machine rebuild began and included a complete retrofit of the electrical system, new CNC control/drive systems, and new position feedback with Heidenhain absolute linear scales. “Absolute technology in super long scales was not even available a few short years ago,” said Zimmerman. After more than two years of work, the 1990s-era Schiess (DSI) gantry mill is now working with all the agility and precision that 2017 has to offer.
The major steps of the Schiess (DSI) retrofit started in late 2014 with contracting FIVES Cincinnati to overhaul the electrical system and the CNC control and drive systems to Siemens 840D. It then moved to the removal of the ram, which was replaced with a design from FIVES Cincinnati, which was instrumental in the rebuild. “With that came a built-in spindle motor and C axis, including all the newest developments in drive technology,” said Zimmermann.
Staff replaced the existing milling heads/adapters with five new ones, plus one for turning mode. In the middle of all this, they re-levelled the machine and re-adjusted all the geometries. The next step was putting new feedback on the machine, in this case, new Heidenhain linear scales on six master axes to provide the latest advances in measurement technology. All devices connect to the new control system. Including the nonmachining axes, the total number of configured axes and spindles is 20.
The travels of the Schiess (DSI) machine master axes are extraordinarily long, with the two X axes being 740″ (18.8 m) each, the Y axis 377″ (9.5 m), the Z axis 98″ (2.5 m) and the two W axes 115″ (2.9 m). Zimmermann said that the original mill already had Heidenhain LB linear scales, but the new Heidenhain LC 201 absolute linear scales offered major improvements and new features. “The Heidenhain encoders get better every year,” said Zimmermann.
The entire rebuild process took about two years, with partial use of the machine during various stages. A time-lapse camera was set up to film the course of the rebuild.
The rotary table on the rebuilt machine is six meters in diameter and is capable of contouring together with all other machine axes, providing positioning accuracy of less than 10 arc sec. Four different milling heads up to 100 hp (75 kW) and with maximum spindle speeds of 2800 rpm are in place. The machine also has turning capabilities using a turning head with CAPTO toolholders; in this mode, the table can run up to 60 rpm and has a part weight capacity of 125 metric tons.
Before the rebuild, one of the most time-consuming procedures used to be referencing all the axes before being able to machine parts. Because of its size (with a 22-m travel in X and 10-m travel in Y alone), start-up took a long time. “It was typically a half an hour before the machine was ready to work,” said Zimmermann.
As a result, MTM looked at the option of going with long absolute linear scales recently introduced by Heidenhain. The company discussed the concept with area integrator Tech Tools Inc. (South Bend, IN) and decided to implement the new scales when they became available. “Using absolute scales makes axes running as a gantry system much more reliable since the CNC knows their location immediately after start-up,” Zimmerman explained.
“Major Tool wanted to save time, so we knew absolute scales would eliminate having to home-out the machine,” explained Casey Binder, vice president at Tech Tools. “Absolute scales on a machine tool allow the user to power up the machine, move it, and know exactly where it is and be ready to go. This is especially important if a power outage were to occur, which is not uncommon in Indiana in early Spring.
“Also, the challenge of retrofitting this particular machine at Major Tool was the extraordinary size and the lengths of its axes,” Binder continued. “At the same time, it was important that we ensure whatever linear scales were used would maintain very tight tolerances and accuracies. With the kind of work that they do, even a few microns is crucial. And Heidenhain really is number one in feedback, so we were confident with the recommendation.”
To make it happen, Heidenhain provided the new very long linear scales: two at 3640 mm (one standard and one mirrored), one at 10,440 mm across the head, and the two “granddaddies” at 21,040 mm each, the longest Heidenhain absolute scales to be sold to date in the US. “This is huge. The largest I’ve seen in my 17 years here,” said Binder, whose staff handled the installation.
The decision to use Heidenhain LC 201 absolute linear scales has been a success. “They have been working great,” said Zimmermann, who explained that the machine went into full-power working mode in October 2016. He also said that the main machining axes continue to maintain their accuracy targets, in fact surpassing them, as the scale’s positioning accuracies are achieved in conjunction with laser equipment and CNC compensation.
“These absolute linear scales are saving us time, as we can see right away all positions at turn-on, where the axes are and what the skew is between the two gantry sides, which keeps bad things from happening,” explained Zimmermann. “In fact, all our encoders here at Major Tool are Heidenhain—even in the other 14 machines we have retrofitted here since 2010. Though again, this was our largest and we’re happy with its success.”
For more information from Heidenhain Corp., go to www.heidenhain.us, or phone 800-233-0388.