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Flexible, Capable, Production Machining

If you’re looking at running medium to high-volumes of powertrain components and don’t want to be stuck with an expensive monument when part requirements change, you might consider this new approach to flexible systems engineered by Heller.

Although one might think that the ModuleLineSystem (MLS) developed by Gebr. Heller Maschinenfabrik GmbH (Nürtingen, Germany; in the U.S.: Troy, MI; www.heller-us.com) is simply another flexible production system for prismatic parts, the man who is responsible for developing it, Bernd Zapf, thinks that it is not merely an incremental change to existing systems but, in his words, “a completely new development.”

In explaining the MLS approach, Vincent Trampus, vp of Sales at the U.S. operations, explains that systems for (primarily) powertrain components are either transfer lines, which are designed for a specific workpiece (with, perhaps, a bit of changeability engineered in) or flexible systems, which are generally rather costly (with the rule of thumb, in effect, the greater the flexibility, the bigger the price tag). Given the prevailing conditions in the global auto industry—fluctuations in demands for various products, increased model variety, and faster product-development cycles—the goal was to find an approach to machining that would have the fundamental production efficiencies of a transfer line with the flexibility of a flexible system. And the economic factor is a big consideration as well, but one not just predicated on the basic investment cost, but on the TCO, or “total cost of ownership,” a metric that, Trampus explains, is of key concern particularly among European automotive manufacturers. Breaking this down, he says that people might look at the initial investment cost in relation to the cost-per-piece, but there are other factors that should be considered, including operating costs, maintenance costs, and the costs associated with using the equipment for another product. For example, it may be that a manufacturer considers that it will take 10 years to pay for a system, but in order to do so, there will be a need to have an overall equipment effectiveness (OEE; generally calculated by multiplying the availability of the machine by the performance (actual compared with potential) and the quality of the parts produced) on the order of 80 to 85%. A problem could be that before the 10 years are up it may be that the part being produced is being replaced, so unless there is a means by which it can be easily reconfigured (easier, of course, for a machining center-based flex system than a transfer line) there are additional associated costs.

 

Machining modules

The basic element of the MLS is a machining module. There are the MC10 and MC20 single-spindle machines, which offer strokes of 630 mm and 800 mm, respectively. These machines are comparatively compact, with a width of just 1.8 m. (A small footprint is an advantage of the MLS, a point we’ll look at shortly.) Then there is the MCT10, a variant of the MC10, as it has twin spindles. There is the MC200, which is a beefier version of the MC20 (e.g., it can accommodate a 4:1 gear box to produce 800 Nm of torque, and can handle oversized tools for applications including cylinder or crank boring or deck milling). Then there is the MPC, which is engineered to handle multispindle heads. The ancillary equipment, including the tool magazine, tool changer and the workpiece changer, are all common. In addition, there is a trunnion device that is fitted with a swivel table so that there are two additional axes (A and B) so that combined with the basic three of the machine, there is five-axis and five-sided machining capability in a single setup. What’s more, there are various spindle types available (e.g., for cast iron or aluminum) and the MLS can be setup for wet machining (with a sheet metal flume below the spindle), for dry machining (with a chip conveyor in the place the flume would go) or for minimum-quantity lubricant (MQL). 

An important element of the MLS is the automation system, which is used primarily for part transfer, but which can be deployed for handling tools. This system uses a linear motor. The power is transferred to the motor by the contact rails. The control signals are transferred wirelessly. Not only is this design one that minimizes maintenance requirements (e.g., there are no trailing cables that can wear), but it helps facilitate system expansion (or contraction) as required.

 

Costs & capacities

An advantage of the architecture of the MLS versus a transfer line is one that can be realized at the very start of a project. That is, in cases where it is necessary to prove out the process capability by producing parts on the actual equipment that will be used to produce the parts, the transfer line arrangement calls for the transfer line. This is a considerable initial investment. What’s more, as Trampus points out, when programs are launched, it isn’t a matter of going from 0 to full-production with the flip of a switch, that it generally takes several months—to as much as a year or longer—to ramp up to capacity. This means that the equipment is running way below capacity, yet the price tag isn’t in any way diminished. With the MLS, a single station including the part transfer automation can be installed. The process can be validated, then, when additional equipment is required, then other units can be installed. So at the start of the program, compared with a transfer line, the MLS initial investment can be just 1/6th.

While this is not dissimilar to the case of other machining center-based systems, Trampus says that in cases where there is overhead gantry loading, the prove-out portion of the process tends to be far more costly and complex than is the case with the MLS. (Why? Because with gantries the complete anticipated automation package is typically purchased up front.) What’s more, given the efficiencies of the MLS system that Zapf took into account from the start, there are various advantages. For example, according to Trampus, if you take a flexible system and compare it with an MLS with the same capacity, the MLS will save, on average, 37% of the floor space. Overall, when looking at systems with a 50,000-part-per-year capacity, the MLS, Trampus claims, is 18% less expensive.

There are various factors that come into play in achieving this improved efficiency. While the machine performance specs are certainly reputable (e.g., a 60 m/min rapid traverse rate; acceleration of 8 m/s2), the goal was to look at ways to make automotive-style production more efficient, not for features of the machine to be faster. Zapf and his colleagues (there were 40 people on the development team) calculated that if they upped the axes performances, they could achieve a productivity gain on the order of 1 to 2%. But if they reduced the machine idle time by providing a faster tool change and chip-to-chip time, the improvement would be about 6%. So they went to work on how the tools are changed. Essentially, the tool magazine is located above the spindle. The next tool needed is pre-staged in the tool exchange unit so that they’re calling the tool-to-tool time “zero,” the chip-to-chip time is a mere 2.4 seconds, which is important when there are frequent tool changes, as can be typical of many powertrain components (blocks, cases, gear boxes, differential carriers, etc.). Similarly, there is a component changer that stages the next part to be processed outside the machining area so that there is a swap of workpieces in 10 seconds or less.

So say an MLS system is setup with multiple machines. Parts can be processed in a serial manner or in a parallel manner. Or in some cases, there can be a mix. Entirely different types of parts can be run at the same time. Then when there is a change in production, machines can be added or subtracted from the system in a comparatively straightforward manner.

While it might seem like a typical machining center-based flex production system, the keen attention to how to make the lowest total cost of ownership as regards components and performance is something that makes the ModuleLineSystem atypical.

 

MTBF, MTTR & OEE & TCO

When a machine goes down for whatever reason, there is an effect on overall equipment efficiency (OEE). That, of course, has an effect on the total cost of ownership (TCO). So it is key to have a minimal mean time to repair (MTTR) and longer mean time between failure (MTBF). One of the ways that Heller engineers facilitate this is by designing the machining centers such that when the spindles need to be replaced, the spindles are removed from the back of the machine, not the front. They’ve developed a track that bolts into the back of the machine. A carriage is put on the track and it is used to remove and replace the spindles. This is in contrast to having tow motors and other gear in the front of the machine. What’s more, by doing this from the back of the machine, the material handling automation that runs in the front is unimpeded so that other machines in the system can continue work.