The electromagnetic field of each engine block hole is automatically compared to database values for tapped and untapped holes. Operators are alerted when readings indicate the holes have not been tapped, reducing the defect rate from 125 ppm to zero.
Developed by a line operator at the Romulus Engine Plant, this semi-automatic piston stuffer greatly reduces repetitive-stress injuries, and – when combined with other error-proofing initiatives – improves first-time quality.
Cam and crank sprockets are placed in the correct orientation on this reusable plastic sprocket along with the timing chain before installation in the engine. Dowels hold the sprockets by their teeth, and slots in the carrier line up with holes in the sprockets so workers can tell at a glance if the unit is assembled correctly.
In 1992, GM's Romulus Engine Plant, where the corporation's 4.3-liter V6 had been in production since 1986, was selected to produce the Gen II V8 for the upcoming C5 Corvette. GM was in the midst of a crisis at the time. Money was extremely tight–the company reportedly had to borrow to meet payroll the year before–market share continued to erode, and quality was a word often spoken but infrequently implemented. "The genesis of the GM Global Manufacturing System was beginning to take shape," says Homi Patel, general manager, Manufacturing Operations at GM Powertrain, "as we discovered an urgent need to establish common practices and procedures worldwide that would reduce the number of variables, increase the flow of information and ideas, and increase quality." GM's money troubles would stall this initiative: it was urgent the company make money in the short term. Nevertheless, Romulus was a beneficiary of this awakening, and it again benefited when GM delayed the introduction of the fifth generation Corvette from 1994 to 1997.
The Romulus Engine Plant builds more than just the Corvette's Gen III engine. It builds a version of the same engine for GM's full-size light trucks, as well as a 4.8-liter V8, two versions of the 6.0-liter V8, and–still–the 4.3-liter V6. Engine capacity is 4,000/day for the V8s and 1,700/day for the V6, though 4,400/day of the former and 1,200/day of the latter are normally scheduled. "We reached 100% of capacity within 12 months of launch–a new benchmark," says Steve Somers, plant manager at Romulus, adding almost as an aside, "The GMT-800 light trucks sold very well from the start. We didn't have much choice but to hit our targets."
What makes the Romulus plant tick? It isn't the Pride Center, a large room within the plant dedicated to recognizing employee contributions and increasing enthusiasm. What's put on display here is a reflection of larger forces at work in the plant. And it's not the and/on boards which let employees see what's happening on the line at a glance. It only signals what's happening along the production line, or alerts onlookers when a maintenance operation is due. Beneath this superficial layer is an eddy that extends from the design staff, to the engine plant, to the assembly plant, to the customer and back again. One that works to incorporate the information that flows along this stream. Here are a few examples:
- Each engine block has "customer feature holes" that are machined in Romulus, but not used until the engine reaches the vehicle assembly line. These include transmission mounting holes, and starter and motor mount holes. Typical discrepancies included cases where the holes were left undrilled, or untapped, which caused consternation–and swearing–on the vehicle assembly line. Previously, the holes had been inspected manually, a process that was about 85% effective. Now each hole is inspected automatically by a machine that compares the electromagnetic field of each hole against the same readings for a threaded or unthreaded version of the same hole. Under the manual system, 125 parts per million were defective, while for the month of June 2002, the automatic system had a zero reject rate, having found all unthreaded assembly plant-only holes before the blocks left this quality station.
- Piston stuffing. It sounds like a fraternity initiation prank. Yet the folks at Romulus know the noise problem created when the piston isn't installed properly, or the rod bolts slip out of position and into the motor. They also know how difficult it is to force the piston into place when you have to apply 100 lb. of pressure at an awkward angle to get it to slide down the bore. Repetitive stress injuries were a given. There had to be a better way. And there was.
The dimple on the piston crown–which must be aligned with the front of the engine–is mimicked by the rod, which is rounded around one cap bolt, but not the other. For this assembly to pass muster, the two hallmarks must be facing in the same direction. The stick used to guide the piston into the bore follows this same pattern, giving the operator an immediate visual cue whether or not the piston and rod assembly are facing the right direction. And the rod bolts are no longer loose, being captured in the rod assembly, eliminating the chance one or more will fall into the engine as it is moving down the line. The operator places a collar over the piston to collapse the rings, attaches the guide stick, places the piston and rod assembly in the bore, and uses a pneumatic "stuffer" to push the assembly into place. Because the pneumatic part rides on tubes that align with the side of the block, allows free movement along their length and utilizes a bumper to match the piston with the bore center, this unit can be adapted to any bore spacing, and allows the operator to process 10% more engines in the same time.
- Intake manifolds were another problem area for the Romulus crew, but one that succumbed to Design for Assembly diagnosis. In order to reduce the potential for incorrect assembly, or problems that might miss an inspection but cause problems in service, the intake manifold was error-proofed. Bolts and gaskets are captured, and the latter are molded for strength. The bolts have acorn heads, bushings, and integral sleeves that help self-center them and eliminate the need for finger-tightening at the start. Room was created for the bolts to be automatically loaded and tightened, and each throttle body is designed around a common centerline. This eliminates the need for multiple manifolds, and bar coding matches the right throttle body with the right engine.
- Similarly, the timing gears and chain are held in a reusable spacer that locks them in the correct timing position. Dowels capture the gears, while keyways align with holes in the spacer. This provides the operator with a visual confirmation that the gears are properly aligned and ready for installation. The spacer mates with the engine only one way, and releases the gears and chain at the correct tension as it moves closer toward the block face.
As a result of these and other process and design improvements, assembly hours per engine have dropped from 1.51 in 2000 to 1.30 today, an improvement of 14%. Assembly scrap costs have dropped 90% from 1999 to 2002, and there is very little provision for engine tear downs at the Romulus plant. Despite a maximum award of $20,000 for an implemented suggestion, Somers says Romulus employees saved GM more than $10 million on a $1.2 million payout last year. And their effect has been felt throughout the system.
According to GM Powertrain's Patel, processes have been commonized among GM's 20 engine assembly plants, and the lessons learned at each shared. "We haven't defined the measures which will make us the benchmark in engine assembly," he says, "we just keep on improving. Assembly time varies with engine design, and the processes used in the plant, though we feel we have a manufacturing and cost advantage because of our ‘cam-in-block' (a.k.a. pushrod) engine design." Enough, in fact, that the 5.3-liter LM7 V8 was rated "Best in Class" for the second year in a row for problems per 100 engines in the J.D. Power Initial Quality Survey, and comfortably ahead of Toyota's 4.7-liter V8. Says Patel, "It's amazing what you can accomplish in 10 years."