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Bottlenecks in the inspection process can be disastrous when executing a cellular machining strategy. But high-volume manufacturers in Indiana and Nebraska have recently implemented a new gaging system that allows machining cells to operate smoothly and without costly interruptions.
In each department of its plant, Mitchel & Scott Machine Company (Indianapolis, Indiana) has posted a sign that reads, "Quality is built into a product, not inspected into it." But this motto hardly seems appropriate for a company that maintains a battery of more than 17,500 active gages. Indeed, with approximately 250,000 square feet of manufacturing space and more than 300 employees, inspecting parts represents a major aspect of the firm's work.
The firm ships more than 20 million components and assemblies each year to manufacturers of heavy trucks, off-road vehicles, agricultural equipment and automobiles. To meet clients' strict quality requirements, the firm's inspection equipment includes a coordinate measuring machine, 16 optical comparators and numerous gages for measuring contours, surface finish and roundness. The company also maintains a fully equipped inspection station in each of its production departments—plus an environmentally controlled gage lab.
Mitchel & Scott recently acquired a new type of laser gage that has enabled the firm to substantially reduce its reliance on conventional gages, as well as the time required to take measurements. This device is the LaserMicro100 micrometer manufactured by Blum LMT (Fort Mitchell, Kentucky).
"We bought the laser gage to help us keep up with the flow of parts, such as fuel pump drive shafts, supercharger shafts and water pump shafts," says Bill Steward, the firm's gage coordinator. "We installed it in our first manufacturing cell and dedicated it to a single part family. "We were able to run eight different part numbers through it, and when we added a second cell, the laser handled that as well."
The sizes of these shaft parts span the range from 0.625 inch to 1.25 inch diameter and 3.50 inch to 4.50 inch length. Parts are composed of 1215 and 1144 free machining steel and typical tolerances are ±0.0025 inch to ±0.004 inch. The standard machine finish for these parts is 125 Ra. During production—at not less than 15-minute intervals—a sample is pulled. Each part incorporates 17 different diameters to be measured.
Before obtaining the laser micrometer, the company's operators had to use many different types of conventional gages and micrometers. "The operator needed to write down the gage data first and then manually input this information into the computer for downloading to our central SPC system," says Mr. Steward. Given the scope of the firm's operations, this was a very time-consuming process.
Using only one laser gage, Mitchel & Scott has now eliminated 41 conventional gages. "Instead of running a family of eight parts and having to cope with as many as 70 gages, we simply select the part program from the laser's touch screen, load the part and press the start button," says Mr. Steward. "The SPC data are downloaded directly to our central computer without operator intervention. This method eliminates the possibility of errors in manual data input."
When the company first received the new gage, Blum representatives spent 2 days in the company's plant to oversee installation and provide training. Final acceptance tests conducted on samples from the various part families resulted in variations of not more than 0.000021 inch. A number of different parts with surface finishes ranging from 16 Ra to 125 Ra were measured. Double-X and triple-X master setting disks were used during the initial assessments, and the results correlated very closely with those made by digital super-micrometers.
"Having a non-contact gage precise enough to detect the appearance of tool lines has become an invaluable tool in our process control," says Mr. Steward. "It allows us to address an upstream problem before it leads to scrap parts downstream." This process is assisted by the laser gage's color-coded readouts that include red, yellow and green indicators. When the readout drifts into the yellow zone, the operator knows that variations in the part's size or surface finish are moving toward the tolerance limit. This allows tooling issues to be addressed in real time and before any bad parts are produced.
Mitchel & Scott's shaft-production cells employ four distinct machining operations. After the initial forming and shaving operations, the parts proceed to a CNC turning operation, a drilling operation and—finally—to a polygon milling operation. Gaging is performed immediately after the forming/shaving operation to ensure that all subsequent steps are performed on shafts that conform to the specified OD tolerance. In this process, four multi-spindle, automatic machines feed parts to the laser micrometer.
"With the laser gage, we can achieve quick changeovers to obtain the efficiencies we'd hoped for when we adopted our cellular approach," says Mr. Steward. "This removes the inspection process as a factor in our cycle time. Our overall cycle time is now the same as the machine cycle time—nothing more."
York, Nebraska, is the home of a precision machining facility operated by Hamilton Sundstrand, a United Technologies company. This facility employs 225 people who run three shifts a day. Operations include three- and four-axis CNC milling, drilling, CNC turning, precision grinding, spline cutting, metal finishing and assembly. Part materials include stainless, carbon and tool steels, cast iron and nickel-based alloys including Inconel, Nitronic and aluminum. Generally, prismatic parts and components produced by the firm are smaller than a 12-inch cube, and round parts are typically smaller than 12 inches in diameter and 14 inches in length.
Charlie Sanders, the firm's area manager for technical support, is closely involved in the production of shaft-type parts for rotational applications, including many that are used for aerospace engines.
"To meet plant-wide initiatives, we amended our machining strategy to include supply-chain production cells," says Mr. Sanders. "In the process, we invested significant capital and resources in technology improvements. We looked at everything—new tooling, grinding wheels, machine tools and advanced gaging. We needed to squeeze every bit of waste out of our production stream. One island in that stream is metrology."
Many shaft-type parts produced in this cell are critical performance components for aircraft electrical power systems. At Hamilton Sundstrand's Electric Power Enterprise in Rockford, Illinois, rotor shafts produced in Nebraska are assembled into generators before being tested and shipped. These rotor shafts are finish ground on Studer S40cnc cylindrical grinders.
On certain critical OD dimensions of these parts, three different points must be checked and constantly validated. The shafts range from 0.630 inch to 10.165 inch in length and from 0.788 inch to 3.741 inches in diameter. Diameter tolerances are from 0.0002 inch to 0.0005 inch and surface finishes range from 6 Ra to 32 Ra. With an annual production volume of approximately 15,000 shafts—and with each shaft requiring 12 inspections—this equates to making approximately 180,000 individual measurements per year.
"We knew that we needed a gaging technology that would allow us to meet these kinds of demands while keeping costs down with predictable quality," says Mr. Sanders. The company's previous method involved using bench comparators that were not compatible with a cellular manufacturing strategy.
"With the comparators, it might take 15 to 30 minutes simply to gather and prepare the gages," says Mr. Sanders. "If you had three different ODs to check, you'd need three different bench comparators with a disk set. Also, with the bench comparators, you have a range of only 0.002 inch, so you must reset them for each operation. With the laser gage, you simply select the program, load the part, push the button and walk away. The entire process takes less than a minute."
Mr. Sanders also notes that recalibration time is a factor. This is done at least once per shift and perhaps four times per day. In this vein, the laser gage recalibrates itself within 10 seconds compared to about 3 minutes for the bench comparator. "With the laser micrometer, we reduced variation down to about 0.00002 inch—well below acceptable Hamilton Sundstrand standards," says Mr. Sanders.
Using this new inspection technology, the company has substantially improved the speed and accuracy of its manufacturing process. According to Mr. Sanders, this is currently paying some big dividends. "The laser technology has contributed to a 75-percent reduction in defects that our machining operation has realized over the past 3 years," he says. "This not only enhances our operation but also adds real value to the parts we deliver to our customers."