Secondary Machining--Without The Tears
This automated machining cell is reaping the advantages of balanced operations—and avoiding most of the problems that usually go with it.
Many shops regard secondary operations as something to be avoided whenever possible. Produce the part on one machine in one setup, and you get consistently good parts with none of the costs and opportunities for error usually associated with secondary operations, or so the thinking goes.
When it comes to high volume machining of complex parts, however, the need to optimize throughput is imperative. The advantages of spreading the machining operations over a number of different machines and performing those operations more or less simultaneously to increase the number of parts produced per hour is difficult to ignore. Certainly, that machining strategy more often than not has opened the door to problems in the past.
But what if there were a way to distribute the work without having an operator at every machine loading and unloading parts, without exposing the parts to handling damage or operator errors, without creating machining bottlenecks, without tying up a lot of money in work-in-process inventory—without incurring any of the drawbacks usually associated with secondary operations?
Bronson Precision Products (Bronson, Michigan), in fact, has gone a long way toward retaining the productivity advantages of secondary machining while eliminating most of the disadvantages. Readers who collect antique level-wind fishing reels may recognize the Bronson name. The company started in 1923 as the Bronson Reel Co., a manufacturer of quality fishing reels. In addition to a standard line, it made custom reels for such celebrities as Errol Flynn and Zane Grey.
Bronson turned from fishing reels to more profitable automotive products, and in 1984, the firm was purchased by Royal Oak Industries, Inc., Lake Orion, Michigan. Royal Oak, which has four plants in Michigan, makes engine blocks, cylinder heads, transmission housings, flywheel housings and other large components for diesel engine manufacturers. In 1997, Bronson began making parts for fuel injection systems, some of which go into the engine parts that Royal Oak sells to customers such as Caterpillar and Detroit Diesel. (Bronson continues to also make high-performance engine parts for the automotive aftermarket, as well as a line of air cylinders.)
One of the fuel injection parts that Bronson makes is a housing that contains the electromechanical device that actuates the fuel injector plunger. Made of 303 stainless steel, the housing is primarily a turned part, measuring about 1 ¾ inches in diameter by about 1 ¼ inches long, but it requires milling operations at one end. The part, shown on page 42, is drilled along its entire length. It requires ID and OD turning operations at both ends; drilling, tapping, grooving and pocket milling at one end; broaching of two wrench flats on the largest OD; and final facing and deburring operations. Bronson received orders for the housing, and several other fuel injection system components, with the understanding that order quantities would be modest at the start but grow to hundreds of thousands of parts annually.
When production started, the parts were made from discrete blanks. Initial turning operations were performed on a two-axis CNC turret-type lathe. The susceptibility of the machine to thermal variations made it difficult to hold tolerances. In fact, Bronson discovered that the lathe could not be used to produce the part unless an operator was constantly present at the machine to load and unload parts, turn the part around in the chuck for machining of the back end, inspect the parts, and make adjustments as required to keep the parts within tolerances.
Needed: Better Lathes
Bronson was able to get by with the turret-type lathe for limited quantities of the part. However, the firm was planning a dedicated machining cell capable of producing the fuel injection housings in much larger quantities. Although the firm decided to create the cell using standard CNC machine tools, it became apparent that something more productive than the turret-type lathe would be needed for the turning operations. Specifically, the firm wanted a production lathe that (1) could hold tolerances consistently, (2) did not need an operator in constant attendance, (3) had automatic load-unload capability, and (4) was capable of being integrated readily into a highly automated, lightly staffed machining cell.
The firm found what it was looking for in the G-Series CNC lathe from Wasino Corp. U.S.A. (Rolling Meadows, Illinois). The G stands for gang-tooled, and Bronson chose the G7, the largest model in the series, for its large main spindle motor (7.5 horsepower rating) and gang tooling capacity. Another reason that Bronson chose the G7 lathe is the machine's automatic load-unload system, which consists of a staging system that allows the operator to load multiple blanks in the machine at one time, and an integral gantry loader that takes the blanks from the staging area, loads them in the chuck and unloads them after machining.
The integral workpiece banking system is a carousel made up of columns of stacked parts (photo on page 40). Each stack is supported on a base plate and contained by three retaining rods that quickly and easily adjust to the diameter of the part being turned. With the part banking system, an operator can load multiple parts at a time in one or more lathes and attend to other tasks away from the machines.
The integral gantry loader, which is the second part of the machine's automatic load-unload system, automatically takes workpieces one at a time from the staging area and delivers them to the chuck. A blank that requires turning on both ends is turned on the front end, removed from the chuck by the gantry loader, delivered to a workpiece-reversing station, turned around, and returned to the chuck for machining of its back end.
The combination of the part banking system and gantry loader enables the machine to run for long periods unattended. And it is ideally suited to Bronson's housing machining cell, because instead of simply delivering the completely turned blank to an exit chute, the gantry loader delivers it to a conveyor belt that takes it to an inspection station, from which it continues toward downstream machining operations.
Matt Kroll, director of technology for Royal Oak Industries, explains some of the reasons for choosing the G7s for the housing cell. "In the first place, the rigidity of the machine provides the precision and fine-finish capability we need for our parts," he notes. "The X-axis ballscrew has an oilbath, which helps keep the operating temperature of the lathe uniform. That makes for repeatable results hour after hour and shift after shift. "The gang tooling permits faster tool indexing times than, say, a tool turret, making for faster part cycle times," he adds. "Frequently, going from one tool to the next involves only a ½-inch move.
"The G7 also represented a substantial savings over alternate configurations," he continues. "A conventional CNC lathe the size of the G7 would have cost $80,000 to $100,000. A separately purchased gantry loader, whether supplied by the lathe builder or a third party, would have cost another $100,000. The G7 comes equipped with the gantry loader as a standard feature and costs substantially less than purchasing a lathe and a gantry loader or robot separately, making it a real bargain. Also, operation of the lathe and gantry loader is more tightly integrated since both are controlled by the lathe's CNC control."
Some Basic Considerations
Bronson created the housing machining cell with throughput, part quality and flexibility in mind. Where turning centers capable of producing the part complete in one setup could have been purchased for the job, the firm opted instead to produce the housing in a sequence of operations on conventional CNC machine tools. Blanks for the housings are loaded (in multiples) into the CNC lathes by hand. From that point on, however, the housings travel from machine to machine on conveyor belts and are automatically loaded in and unloaded from the machines, minimizing the number of operators required for the cell.
Conventional CNC machine tools were chosen for the cell because they were less expensive than more sophisticated multi-axis turning centers. And although some operations can be performed simultaneously on turning centers, larger numbers of conventional CNC machines offered greater machining capacity, plus the ability to have the various machining operations happening simultaneously throughout the cell—analogous to the operation of a multi-spindle screw machine—for greater throughput.
Another reason for using conventional machine tools was flexibility. Bronson was gearing up to produce about 900,000 housings annually, but there was a possibility that orders might fall short of projections—which turned out to be the case. If there was a danger that machines in the cell might be idled because of reduced or deferred orders, Bronson wanted to be able to remove them from the cell and put them to work elsewhere in the plant. In fact, five G7 lathes were originally installed in the cell, but as time passed, only four were needed to keep up with production requirements. The fifth lathe was removed from the cell and put into production elsewhere. When demand picks up, the fifth lathe can be easily reinserted in the cell.
The Cell In Detail
The housing machining cell at Bronson starts with a pair of Amada band saws that produces discrete blanks from bar stock. Another cell at Bronson, dedicated to machining a family of injector sleeves, uses as its raw material discrete blanks that are turned and drilled on several of the plant's Acme multi-spindle automatics. Why not produce the blanks for the housings on the multi-spindles as well? Royal Oak Industries' Mr. Kroll explains that the plant was able to reduce the cycle time for the sleeves—as much as 50 percent for one part number—by roughing them on the multi-spindles. However, the estimated time saved by roughing the housings on the multi-spindles did not justify the additional handling, and more importantly, the potential for error that the extra operation would create. He adds that the main bore of the housing is drilled on the G7 using an insert-type drill in a fast 7 seconds, which provides little opportunity for a significant time savings. (Also, producing the housing blanks on the multi-spindles would have required four machines and two operators.)
Mr. Kroll adds that there are no labor costs associated with the saws since the CNC lathe operators oversee their operation as well. The blanks are brought a short distance to the lathes and loaded in the load station of each of the four G7s by hand (photo on page 40). Up to 30 housing blanks can be banked at one time. (The number of cylindrical blanks that the machine can accommodate varies with their size.)
The G7's integral overhead gantry loader obtains a blank from the blank staging area and delivers it to the chuck. The gantry loader's arm has two (upper and lower) grippers: one removes the just-turned blank from the chuck, and the other inserts the next blank to be turned (photo on page 38). Typically, if a part requires turning on one end only, the gantry loader would deliver the turned part to an exit chute alongside the blank-staging area. However, because the injector housing requires turning on both ends, the gantry loader delivers the part turned on one end to a reversing station that turns the part end for end. The gantry loader then retrieves the upended part and returns it to the chuck for machining of the back end.
In the injector housing cell, four identically tooled and programmed G7s are used to turn the injector housings. The lathes are arranged in a straight line. (See diagram on page 42.) Instead of delivering the turned housings to an exit chute, the gantry loaders deposit them directly on a conveyor line. The conveyors for the two upstream lathes and the two downstream lathes converge on an inspection station positioned midway along the lathe line.
Automatic Gaging Station
A gantry arm at the gaging station removes the turned parts from the conveyor lines and inserts them into a gage that automatically checks all diameters, the thread on the part's small OD, and one or two gage lengths. When the gaging station detects that turned dimensions are trending to an out-of-tolerance condition, it automatically signals an offset change for the appropriate tool on the appropriate lathe.
From the inspection station, the parts move onto an adjoining conveyor line that carries them to one of four Kira vertical machining centers arranged in a row parallel to the row of lathes (photo on page 42). Each machining center has an indexing table with a row of three housing fixtures mounted on each end. While the machine is busy machining three housings, a gantry arm removes the three previously machined housings from their fixtures at the "idle" end of the indexing table and replaces them with fresh parts to be machined. Operations performed on the housings include drilling, tapping and counterboring of four holes; and milling of a roughly rectangular, ½-inch-deep pocket and a corresponding O-ring groove.
From the four machining centers, the housings are conveyed to a pair of broaching machines that broach two 1-inch-long wrench flats in the large OD of the part. "We could have milled the flats on the machining centers, Mr. Kroll explains, but that would have added substantially to the cycle time for the parts at that station. In the interest of increasing throughput, the broaches were added to the cell for the one operation.
Finally, the housings go to another Wasino lathe for a critical facing operation. The front face of the part must be held to a datum surface on the inside back of the part to within plus or minus 0.0004 inch. (The front face has some stringent flatness and parallelism requirements as well.) A second and final operation on the lathe is removal of any burrs created in the broaching operation with a grinding wheel.
The housing machining cell was installed before the economy nosed downward. But with the current emphasis on holding down costs wherever possible, Mr. Kroll's decision to use flexible, more conventional—and less costly—CNC machine tools instead of more sophisticated equipment for the cell is looking better every day. In particular, he is pleased with the savings achieved on the CNC lathes with their built-in gantry loading system. "By buying the G7s with their built-in gantry loaders, we probably saved about $50,000 per unit over buying a comparable lathe and a load-unload system separately," he estimates. "Multiply that by five, which is the number of lathes we needed at the time, and you've got a savings you can feel pretty good about."
"Looking at the cell as a whole, by automating the transfer of parts from machine to machine and the loading and unloading of parts at each machine, we've been able to staff the cell with a relatively small crew given the number of machines involved, holding down labor costs," Mr. Kroll continues. "We have also eliminated the potential mistakes that can occur when machines are manually loaded. Manual handling of parts is greatly reduced, and with it damage due to rough handling. In addition, costs associated with storing and keeping track of incomplete parts have been eliminated. Production is less affected by interruptions because of things such as breaks, shift changes, lunch and so forth. Best of all, production is more consistent, and we have better control over part quality.
"Automating the secondary operations on the housing line has been a big success," Mr. Kroll sums up. "We expect our automated cell to meet our needs well into the future, but if our production needs should change, the cell is flexible enough so that we can easily and inexpensively reconfigure it to meet our future needs."
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