Flexible Machining Calls For Flexible Gaging

Where flexibility and versatility are the rule, it should apply not only to machine tools but also to shop-floor metrology equipment. For this contract shop, a modular, flexible gaging system for shaft-like parts helps keep the company agile.


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For many contract shops, today's trends in industry are spurring a re-examination of traditional approaches to production gaging. As many original equipment manufacturers continue to outsource parts production in smaller lot sizes, more frequent changeovers dictate the use of gaging devices that can be easily adapted from one job to another. At the same time, today's universal demand for quality--and for supplier documentation of quality--points to greater use of precision multi-dimensional gaging systems.

To meet the dual demands for flexibility and precision in measuring shaft-like parts, some gage manufacturers have introduced modular multi-dimensional gaging systems that can be reassembled, or readjusted, for use from one part application to another. Such systems are built up by the gage user, who selects from a set of off-the-shelf fixture components and combines them with supplied or existing metrology and display units. For changeovers, the components can be torn down, then reconfigured, generally using only a few hand tools.

Dedicated To Flexibility

Machine Specialties, Inc., Greensboro, North Carolina, is a contract machine shop that specializes in the manufacture of precision metal components for high-speed production machinery, and for customers in the electronics, medical, and aerospace industries. Production typically consists of short runs of small parts.

"The OEMs we deal with have been outsourcing more and more in recent years," says Vice President of Operations Joseph Landry, "but it's mostly in the form of short-run jobseight to ten production runs in lot sizes from 25 to 100 pieces. Currently, we're making very few parts for more than a year or two. Our customers' own product life cycles have shortened. And where they expect quantities to grow into the thousands, they tend to go to manufacturing processes other than machining."

To handle the continual changeovers required in this environment, Machine Specialties makes sure that every new piece of equipment it buys has the flexibility to accommodate a wide range of workpieces, and the versatility to meet different work requirements efficiently.

The company's machine tools reflect this focus. A family of multi-axis machining centers, for instance, is equipped with programmable indexers and tilting rotary tables to allow multiple machining operations, in a single setup, on up to five sides of a workpiece. Among other examples, there are Swiss-style CNC screw machines that can produce a completely finished part in a single operation, and CNC cylindrical grinders that can grind many different diameters on a shaft-like part in a single setup.

"Our strong suit has always been our willingness to do the tough close-tolerance jobs the customer doesn't really want to do," says Mr. Landry. "Today, OEMs are looking for shops that can do this work. But you have to be able to match--or come close to--their in-house costs. If you don't have flexibility, there's no way you can do that."

Gaging Was A Bottleneck

Shaft work--both turning and grinding--constitutes a significant part of Machine Specialties' business. Like other parts the company manufactures, the shafts vary widely in style, and are typically processed in small lots for customers in each of the industries the company serves. Typical shaft length is from 6 to 12 inches (20 inches maximum). Diameters usually range from 3/8 inch to 1.5 inch.

All parts processed at Machine Specialties are first-piece inspected to validate machine programming and setup. The parts are further inspected throughout the production run to ensure continuous process control and, in many cases, to generate statistical process control (SPC) documentation. The inspection is typically performed at a station near the machine by the operator, who is responsible for the quality control of that machine.

In the case of shaft-like workpieces coming off a grinder or a turning machine, inspections were formerly done with a bench micrometer. Although generally accurate, this device had a major limitation. It could measure only one or two diameters of a shaft at a time.

"The bench mike was effective enough with our old manual grinders, which could only grind one or two diameters at a time," Mr. Landry says. "But the CNC grinders we have today grind any number of diameters in a single setup--so with these machines, the bench micrometers represented a real bottleneck." To illustrate, Mr. Landry describes one of the relatively few jobs the company is still doing on a regular basis.

"We grind one shaft that is 12 inches long and has 6 diameters roughly in the 3/4-inch range. All of these diameters must be ground to ±0.0001 inch. With the bench mike, we could check no more than two of the diameters at a time. That meant we had to put the part through the grinder a minimum of three times--do three separate machine setups--to complete the machining. We would grind the first two diameters on a part, check the diameters, make any necessary adjustments, and run the complete lot on those two diameters only. Then we'd set up on the second two diameters, do the checking, and run the same lot through again. We'd also do the same with the third set of diameters.

"We did this--not because the machine couldn't grind six diameters in one cycle, but because the gage could only inspect two diameters at a time."

Flexible Gaging Is A Fix

To eliminate the gaging bottleneck in the shaft grinding operations, Machine Specialties began looking at modular-type flexible multi-dimensional shaft gaging systems that could measure all diameters on a shaft at one time. According to QC Manager Tom Zales, a high degree of flexibility was judged to be the most important feature of such systems. The system that met this test for Machining Specialties was the Quick Set flexible shaft gaging system offered by Marposs Corp., Auburn Hills, Michigan. This system allows the shop to check diameters from 0.2 inch to nearly four inches.

Several other aspects of the system appealed to the shop, convincing them that modular flexible gaging would complement their machining capability. For example, initial investment costs were low, because there was only one fixture assembly to buy. There would be fewer setups to make, because one fixture gaged all diameter sizes. Because all diameters could be gaged at once, the inspection cycle itself would be shorter.

Another advantage to modular, flexible gaging systems was how quickly the system could be acquired. Dedicated gages require a long lead time to design, engineer, and build. Not so with modular systems. Because they consist of off-the-shelf components, suppliers can configure a system for almost every user in a very short time.

This was the experience at Machine Specialties. "To order it," says Mr. Zales, "we simply called Marposs and told them the kinds of parts we were making. They then presented us with a package that would cover the required application range. The only variables were the number of diameters we needed to inspect, and whether we wanted to check between centers, or locate on the diameter and check."

Of three fixture bases available with the Quick Set system, Machine Specialties selected the mid-length base to provide flexibility for checking longer parts and to accommodate capabilities of the plant's grinders and turning machines. Components were provided to measure as many as seven diameters in a single setup.

The shop quickly learned another lesson about modular gaging. Initially, the shop used dial and digital indicators, which it had on hand, with the gaging system, although the supplier had strongly recommended programmable column gages. "At first, we considered them too costly," said Production Manager Robert Simmons. "It wasn't long before we changed our minds. With six or eight diameters on a shaft, it was simply too cumbersome to read all of the dials--which were positioned in various directions." Clearly, this new bottleneck violated the shop's focus on flexibility and efficiency.

Eight new columns were sent on a trial basis, but the company didn't hesitate to place a purchase order once the quick-reading displays were in use on the shop floor. Today, Marposs-supplied pencil probes (transducers) are used with the gage fixture to take the part measurements. These are, in turn, tied to Marposs programmable column gages, which display the measurements.

Machine Setup Reduction

For Machine Specialties, the major benefit of the flexible gaging system is a reduction in grinding-machine setups and part handling time. "Now, you set up the gaging fixture one time to check all the diameters on a shaft," says Mr. Simmons. "Then you run a first part off the grinder, check it in the gage, and make any necessary adjustments to the machine for all the diameters at once. When these are done, you run the complete lot of parts one time, and you're finished."

Compared to the old bench-mike gaging routine with a typical six-diameter shaft and a 50-piece run, Mr. Simmons estimates that the shop is probably now saving 6 hours of setup time and 2 days of part handling time in just a single parts run. Overall time savings, of course, magnify the value of this move to modular gaging. Working in sync with Machine Specialties' advanced manufacturing processes, the flexible gaging system is at once helping to increase the uptime of costly equipment, and to reduce part turnaround times.

Fast Gage Setup

For Machine Specialties, another important feature of the flexible gaging system is how quickly it can be set up. "In 15 minutes30 minutes topsI can reconfigure it to gage any shaft we make here," says Mr. Zales. "To measure four diameters, for example, all you do is set up four measurement arms, four pencil probes, and four columns."

According to Mr. Simmons, a machinist who is generally familiar with gages can learn to set up a flexible gage proficiently in about three attempts. "Building up the fixture is very straightforward, and there are features for positive part centering and positive probe zeroing that make mistakes unlikely. A major benefit of this is that we can use our existing level of operator expertise, and not have to worry about specialized training." That's efficiency.

Although individual machine operators set up the gage themselves, a backup strategy is used for validation. After the first workpiece off the machine is gaged, it is double-checked with a bench mike, optical comparator or coordinate measuring machine (CMM) in the shop's Quality Assurance cleanroom. If the readings match, it confirms that the flexible gage is set up properly." Mr. Simmons notes that, next to the company's CMM, the flexible gaging system has proved to be the most accurate gage in the house.

Every Diameter On Every Shaft

Because the flexible gage fixture is easy to set up, Machine Specialties is able to use it for all of its shaft productionboth from turning operations and from cylindrical grinders. The fixture is reconfigured at least once and, more often, twice or more each day. In some family-of-parts applications, the only alteration needed between parts may be a change in the column programming.

Using the fixtures is likewise a simple matter, allowing the shop to inspect every shaft it produces. "It takes only about 10 seconds to check six diameters on a 12-inch shaft," Mr. Simmons reports. "And by checking every part, we get more complete data that allows us to maintain even better control of our process."

To make use of the measurement data for statistical control, Machine Specialties uses the output capabilities of the programmable column gages. Measurement data is downloaded to a personal computer, which generates SPC documentation used internally to help improve or fine-tune machine processes. The reports are also often used to satisfy increasing customer demand for SPC documentation.

In addition to basic inspections of part length and diameters, the flexible shaft gaging system is versatile enough to be used for inspecting geometric attributes like taper, roundness, and concentricity.

"Let's say I have a shaft with only two diameters to check, but I have a potential to check for seven," says Mr. Zales. "I can use my extra measuring arms to check these two diameters in several different locations."

For Machine Specialties, the pencil probes and column gages used with the flexible gaging system are also providing added versatility. "A big advantage of the probes and columns is that you can move them to other machine setups around the shop to check parts other than shafts," says Mr. Simmons. "We mount the probes in existing bore gage fixtures, and set up the columns to read the measurements. This is our own creative way of getting extra mileage out of our investment in the columns."

The Aim Is Maximum Uptime

For Machine Specialties, flexible shaft gaging is one new element in an array of up-to-date production equipment calculated to maximize manufacturing flexibilitywhile containing costs, increasing productivity, and meeting today's pressures for continuous quality improvement.

Mr. Landry cites the company's recent purchase of new CAD (computer aided design) and CAM (computer aided manufacturing) software as another investment targeted at the same purpose. By allowing programmers to write inspection programs for the company's CMM directly from a CAD model, the CMM is ready to measure the first part off any production run.

"Everything we buy," Mr. Landry says, "is geared to allowing our machine spindles to cut metal more hours, and to be down fewer hours waiting for information. Like the CAD/CAM software, the new flexible gaging system has been of real value in enabling us to do that."

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