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A machine shop recently faced a challenge relating to producing large mold cores for the tooling it uses to manufacture the long-lasting, top-performing timing belts that have made the company a leading supplier of such products to automotive OEMs. These cores are drum-shaped components that may be as large as 28-inches in diameter or as small as 2 inches in diameter. Regardless of size, these cores feature a series of contoured grooves, evenly spaced along the length of the workpiece, that look like teeth on gear. A larger core may have as many as 115 of these teeth.
The problem was finding an effective way to grind these teeth to a 16-rms finish on the largest of these cores. The solution was a surface grinder outfitted with special fixturing and an indexer. This grinder, the ProGrind 1267 Easy from Jones & Shipman (Farmington, Connecticut) accelerated the entire grinding process while enabling lights-out operation.
Although the company has an immense product portfolio, the actual machining space at its facility can be characterized as a quintessential job shop. The machining area covers about 40,000 square feet. This location produces tooling for the company’s global operations. It deals extensively with large parts such as mandrels and drums of varying sizes that may be used to manufacture parts such as timing belts. Primarily serving the automotive segment, the 17-person shop also performs a good deal of fabrication work.
At first, the way to attain the desired finish on the large cores was not readily apparent because the previous method could not accommodate diameters that large. The manufacturing engineer at the company explains the limitations associated with tooling setup and the grinder that had been used for this specific application.
“That grinder, which we still use for mid-sized work, can comfortably accommodate 4-inch to 11.5-inch diameters,” he says. It could not handle the largest diameter core. “To grind about 0.03 inch off the circumference of this particular drum, however, we were dealing with diameters ranging up to 28 inches.”
For this family of parts, the OD is essentially the same—just the number of teeth varies.
In June 2006, the new grinder joined the company’s four other NC machines. Strong points from the company’s perspective were the grinder’s distinctly different programming techniques and intuitive interface, as well as the ballscrew-driven table, which is designed to speed the grinding process while yielding favorable flatness. Another noteworthy feature is the EasyControl, which has enabled the company to coordinate axes to a greater extent when applied to four-axis work. According to the builder, the control’s capability to interpolate axes simultaneously enables precision grinding of complex shapes, angles and surfaces.
Drawing upon the capabilities of the icon-based EasyControl software and GE Fanuc touchscreen, the “self teach” grinder produces high levels of straightness, flatness and parallelism. As the “Easy” name implies, the control system is equipped to guide the operator through a series of menus to digitize positions and create programs.
It is helpful to possess a decent knowledge of speeds and feeds and general understanding of grinding, the company says. However, the control is said to be so easy to use that most people can grasp it fairly quickly.
Customarily dealing with “onesies,” the shop doesn’t often run multiples of a certain part. However, according to the company, grinding different-sized rolls does not create setup bottlenecks. “You can access various menus by the touchpad and select different profiles,” says one machinist. “The grinder requires minimal input from the operator. The user need only select the form and how many to grind. Using this interface, you can change sizes and commence based off that.”
Aside from the relative ease with which the company is able to program its new grinder, it cites the automatic dressing and grinding cycles as key ingredients in easing setups, holding part accuracy and facilitating 24/7 production. The company can initiate straight or formed wheel dressing via the touchscreen at any time in either manual, single-cycle or multi-cycle mode with the ‘Dress on Demand’ feature.
At start-up, three prompts appear: Manual, Dress or Grind cycle mode. In manual mode, the grinder operates as a hand-controlled machine, with manual dress and no setup procedures. The user can adjust speeds and feeds via the touchscreen. Dress mode offers five standard wheel shape options—flat, slot, facing, Vee form and full ISO with one or two diamonds as needed. Dressing increments, speeds and frequency are selectable via the menus. Grind mode allows one or as many as 20 grind cycles, as the complexity of the component dictates.
The 28-inch-diameter parts are mounted on a Nikken indexer and tailstock, with special fixturing between the centers. Dressing occurs at the start of the cycle and at the feed change point. There are two cycles—a rough grind and a finish grind. During the roughing process, two teeth are machined, and then dressing occurs. The finishing process entails three teeth, and then dressing.
The manufacturing engineer explains the operation in further detail:
“If there are 100 teeth in the part (and two grinding cycles), then most of the material is roughed out first,” he says. “In that pass, we’ll grind one tooth; the machine will index to the next one; we’ll grind a tooth. After the machine is done with the second tooth, it comes over and dresses. After progressing to the third and fourth teeth, the machine will return to the dress position and so on. Finish grinding is similar, but because we remove less material, we can grind three teeth before the machine has to move to the dress position.”
Instead of incorporating a single-point diamond that would customarily be used to dress a wheel, the surface grinder is equipped with two diamond rolls. These wheels are formed using two diamond discs accurately lapped with 0.004- to 0.005-inch radii. A diamond roll basically resembles a disc with diamonds impregnated all the way around it. Recommended by Jones & Shipman for the process, these diamond discs are initially more costly. However, they are said to last longer and expedite the grinding process, in turn compensating for the extra expense.
Right now, the ProGrind is primarily reserved for grinding fairly large parts involving heavy cuts. On average, the grind time hovers around 35 hours for the large cores, says the company. The smaller grinder averages approximately 45 hours on smaller parts, mainly because it is not appropriate for larger parts.
For instance, the company explains that roughing one particular mold on the smaller grinder would take about 40 hours. Using the ProGrind, however, the shop can complete the same roughing cycle in about 22 hours—nearly half of the time formerly required.blog comments powered by Disqus