|A flexible hybrid machining system from Heller Machine Tools works on a diesel engine block at Detroit Diesel. |
Walk the floor of any automotive manufacturing facility and see several types of manufacturing systems, ranging from dedicated transfer machines to fully flexible all-machining-center-based configurations. Now, there’s another type of machining system sprouting up on automotive factory floors.
Flexible hybrid machining systems combine flexible system transfer machines and heavy-duty horizontal machining centers. The machining centers do the bulk of the work in a hybrid system, but then other special machines carry out critical finishing operations.
The most important advantage of a hybrid system is that it allows manufacturers to add incremental production capacity as volumes increase, without totally upsetting system production. Also, manufacturers can manage their capital investments carefully and launch at lower volumes. This has been the case at Detroit Diesel Corp., a Daimler company in Redford, Mich.
For its new DD15 heavy-duty engine platform, Detroit Diesel launched a three-phase, multi-million dollar turnkey engine-block manufacturing project that included a flexible hybrid system. The fully automated, gantryloaded system completely machines the shop’s 1,200-lb special grade cast iron engine blocks, ready for assembly, in ten machining operations. A combination of heavy-duty horizontal machining centers, a CNC head-changing machining center and flexible system transfer lines provide Detroit Diesel with batch-of-one capability to produce as many as 32 engine variations without changeover.
|Heller MCH-400D machining centers are at the heart of Detroit Diesel’s flexible hybrid machining system. |
Early in the project, Detroit Diesel worked with its product development colleagues to identify a strong need to have common locating and clamping points across a family of parts. It strived, from the start, to design the DD15 engine for manufacturability.
For the project, the company partnered with Heller Machine Tools, who David Mitchell, manager of manufacturing engineering at Detroit Diesel, said gets a significant share of the credit for the evolution of the process, particularly in the design for manufacturability of the 11, 13, and 15-liter engines with common locating and clamping points. Any of these blocks can go in the same fixture, which is critical to lot sizes of one and important to Detroit Diesel being able to provide forecast model mixes.
The primary machine in the hybrid system is a Heller MCH-400D horizontal machining center built for direct loading of large prismatic parts within a machining system. The machine’s working area can accommodate workpieces 66 in. in diameter and 57-in. long.
In the first engine block operation, a Heller machining center mills crankshaft half-rounds and rough face mills front and rear faces, pan faces, and bearing cap faces, plus drills transport holes. Then, a Heller HCS 450 D Head Change Machine uses a milling head with a gang mill arbor to mill the engine block bulkheads. In the same section of the hybrid system, a Heller Flexible System Transferline (FST) next face mills the rear sides of the block “ears.” For this critical operation, an angular head face mill is used.
|Detroit Diesel opted for a hybrid machining system that lets the company add production capacity incrementally, without totally disrupting current operations. |
According to Mitchell, one advantage of the Heller system is its use of the head-changing machine for one of the process’s critical operations – rough milling the bulkheads. He said that it allows the shop to do a tool change while the machine continues working, which increases uptime. Mitchell adds that other machine builders suggested a special machine that would have had to stop during a tool change.
Direct-loaded and -unloaded automatically by Liebherr gantry loaders, horizontal machining centers perform the next six block-machining operations. To meet tolerances and part feature specifications, Heller chose to present the large prismatic parts to the machining centers in line for series processing.
The blocks then move to another MCH-400D that finishes the cylinder bore water jackets, rough mills head and flange faces, counterbores flange bores, and rough bores cylinder bores.
Relating to the rough cylinder boring process, Mitchell said that Heller offered a version of a standard machining center with a U-axis tool-actuation device that comes through the spindle and allows Detroit Diesel to do an undercut in the water jacket. He pointed out that competing vendor proposals did not include this capability.
In the next operation, a MCH-400D drills and taps holes on the engine block heads and pan faces, then finishmills the pan and bearing cap mounting faces. A subsequent operation includes gun drilling the oil gallery holes and finish milling side bosses.
Then, oil holes are gun-drilled and pan face holes are drilled, tapped and counterbored. In Op 70, the left and right sides of blocks are drilled, reamed and tapped. In the final in-line section of the system, the front and rear face holes get drilled, tapped and reamed, and a ball mill on an angular head completes a chamfering operation.
Heller FSTs complete the remaining operations. These systems use flexible, configurable machine units mounted on standard basic machines. Heller standardizes the interfaces of these units. That allows parts-related units, such as tool heads and clamping fixtures, to be used throughout. Additional FST highlights include selfsufficient stations with decentralized control principles.
The concept behind Heller’s FST is to build machine tools that can be reconfigured once a program has been completed, and that provides manufacturers a low total cost of ownership (TCO) factor. TCO, according to Heller, not only takes the initial capital investment cost into consideration, but also the repair and maintenance cost over the lifetime of the equipment. And reducing downtime and meantime between failure and repair are key factors.
In Op 120, the next-to-the-last series of block operations, FSTs generate thrust-bearing faces using a line boring bar to finish the crank bores and to gauge crank bore diameters. Another FST finish mills the front faces, and a loader transports the block to the next operation – an MCH-400D finish counterbores, reams and gauges the flange ends.
In the final series of operations, an FST finishes cylinder bores, gauges them, face mills the rear side of the ears, and generates a waterway groove. After this, the blocks are ready for engine assembly.
According to Joerg Sieverding, director of manufacturing engineering, Detroit Diesel recognized a big difference between its transfer lines and the Heller system, not only in flexibility but also concerning plant environment.
“It was key for us to have minimal contamination in the area because engine assembly is located next to the Heller equipment. We would never have considered doing this in the past. But, the machines are so well enclosed that the setup worked,” Sieverding said.
Mitchell added that having the machining and assembly in one area would pay long-term benefits. The layout saves time, and from one location, the assembly personnel know if blocks are in process because they can actually see them offloading and loading – it happens only 150 feet away, and gauge stations are easy to see. In addition, the system layout situates all the human work along one side and minimizes the number of times a fork truck has to come into the system area.
An open exchange of ideas with Heller benefitted Detroit Diesel. Ideas came from both sides that improved the project as it progressed.
For example, the final system layout was by no means the first, nor even the twelfth, that they studied.
“For that kind of effort, you need a good collaborative partner who is willing to help you work out the right layout to fit a big, complex system in a plant that is not a greenfield – one that is willing to contribute ideas. This was the first time we worked with Heller, and the experience grew into a good collaborative relationship,” said Mitchell.
Heller Machine Tools’ MCH class of heavy-duty horizontal machining centers boasts high-torque for serious metal removal, high-speed rapid traverse, and super fast acceleration supported by quick tool and pallet changes and rigid one-piece bed construction for high-precision machining. The flexible machines are designed for palletized production, cell operation, or stand-alone applications.
The MCH-400Ds, such as those at Detroit Diesel, feature massive A-axis trunnions for multi-side machining. The configuration is especially well suited for handling large compacted graphite iron diesel engine components.
Machine pallets change within seconds for simultaneous part loading and machining and to reduce chip-to-chip times further. Depending on customer needs, MCH machines can be equipped with high-accuracy CNC rotary tables with fully capable feed axis, or Hirth-coupled tables. Shops easily access work from three sides for manual or automated loading and unloading.
MCH toolchangers serve a 50 to 100-tool chain magazine or up to 405 tools in a universal rack magazine. Chip-to-chip toolchange (VDI) is 4.1 sec., and machine CNCs choose the shortest tool selection path.
The machines deliver 0.2-micron resolution, 7 microns total true positioning accuracy anywhere in the work zone. This is according to the standard for machine tool positioning accuracy VDI DGQ 3441. And, when axis travels are well within the bandwidth, accuracy is even better, stated Heller Machine Tools.