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Machining center-based cells are used in the VarityKelsey-Hayes plant in Fowlerville, Michigan. Although high production of anti-lock brake (ABS) system components are required, the company wants to have the flexibility to make changeovers as new products are developed. One cited benefit of cells rather than transfer lines: if a machining center goes down, the other machining centers in the line keep producing parts whereas if a station in a transfer line goes down, the whole thing is down.
"In gross numbers," says Dennis Boyer, director of Advanced Manufacturing for VarityKelsey-Hayes (Livonia, MI), "a transfer line will out produce any stand-alone machine." Yet Boyer and his colleagues have 60 Makino A55 machining centers—admittedly arranged in cells, but they are stand-alone machines—producing ABS components at a plant in Fowlerville, MI. And as they look at other upgrades to the company's manufacturing facilities in North America, utilizing machining centers is among the considerations being made. Boyer has experience with both transfer lines and cells; prior to joining Kelsey-Hayes he was with the then GM Delco Moraine Div., where he was involved in the installation of a transfer line for 2.4 million units of ABS on an annual basis.
He explains that the big difference is not in the gross numbers, but in the net numbers. In the case of a transfer line, machine utilization is the key thing. He said that 70% is what can be expected out of a transfer line—"if you're good at it." "At Fowlerville," he says, "we started at 85% and we're getting better."
The issue is this. Say there is a hard-tooled transfer line that's setup to make 1 million parts per year. If one station in the line goes down, then the whole thing is down. Period. But if there are 10 cells setup to do the million parts, and one of the cells goes down, then 90% of the capability is still running. Which, in Boyer's view, is a definite advantage.
Keeping things up and running from a maintenance standpoint can be better with the machining center approach, Boyer says. He explains: "You've got one type of machine and a set of tools. So the technicians need to know one machine, one control, and the tools. In a transfer line, there are a variety of things." For example, there are heads for drilling and milling, box spindle clusters, motorized spindles, and so on. It is unlikely that one person can know a whole lot about many of those different things. "Even though a stand-alone machine may be somewhat complicated, it is simplified when you know all about that one thing rather than having to deal with a whole bunch of different things. This know-how is what allows you to become lean and in the end means improved reliability."
There is a fundamental difference in the cellular approach to high-volume production versus a transfer line. In the case of the transfer line, say it takes 10 different tools to make a part. So it is typically a matter of getting 10 spindles in place and putting in the tools. With the cellular approach, it might be a matter of a single machine with a single spindle and 10 tools in a toolchanger. In the case of the transfer line, Boyer notes, it is the slowest operation that paces the entire line. Improvements to the nine other stations isn't particularly advantageous. In the cellular approach, on the other hand, any improvement in any of the operations can be beneficial. That is, if a second can be saved during the processing of the third tool, then it means that the fourth and following tools are employed that much quicker. So improvements anywhere are beneficial. "The good news about the cellular concept," Boyer says, "is that any place you can make an improvement, you can see it."
But then, of course, there's the other hand: "But if you don't optimize every operation, then you lose."
So the lesson here is that going to a flexible approach requires attention to detail.
Attention to detail has paid off at Fowlerville. Boyer says that they have actually realized 38% greater net production from the cells than had been anticipated. He estimates that about a third of that gain results from running the spindles at a higher speed—the maximum is 8,000 rpm—than the original estimates, estimates that were based on typical speeds used on conventional transfer lines and dial machines (e.g., 3,000 to 5,000 rpm).
When asked about going to high-speed machining, Boyer replies, "We're looking at it for every possibility." He notes, "Machining investment is a large burden to handle and still get the new products in the customers' hands at the right price so we can make a profit." Boyer explains, however, that much of the machining that's performed at Kelsey-Hayes is drilling, which is not where the real gains for high-speed machining can be realized. "The big benefit is in contour-milling type operations," he remarks. But he adds, "We have to look at what higher speeds will do for us in drilling, reaming and boring."
Cut Non-cut Time
Fitted to a CNC machine, OptiMil can boost productivity by 20 to 40%, according to the folks at OMAT Control Technologies (Jerusalem, Israel).
The reason: It monitors cutting conditions in real time and, based on the tool load measured, adjusts the feedrate to its highest level. Consequently, it can increase the programmed feed in the case of air-cutting, or it can reduce the feed in cases when overload conditions are detected.
Benefits include reduced downtime thanks to this monitoring, which can extend tool life or stop the machine if tool breakage is detected.
The SV400 from Mori Seiki (Irving, TX), a vertical machining center that offers an X, Y, Z machining envelope of 23.6 x 16.9 x 118.1 in., is engineered for speed.
It is equipped with a direct-drive spindle motor that provides a maximum output of 30 hp; it can rotate at up to 12,000 rpm thanks to a 2.7-in. spindle bearing. Durability is enhanced by encasing the spindle cylinder and motor in an oil jacket so there's assurance of cooling.
The rapid traverse rate: 1,260 ipm. Tool change time is just 0.9 sec.; chip-to-chip: 2.8 sec.