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Precision Machining in Aerospace Town

 

VMC cells offer flexible production


By Jim Lorincz
Senior Editor 

 

What founder's story doesn't begin with work done in a garage or in some other makeshift building, with the founder moonlighting in search of customers while he's working a day job?

Aerospace contract manufacturer, Cox Machine Inc. (Wichita, KS), was started in 1954 by Ernest "Bud" Cox, who was an experienced machinist at Cessna Aircraft Co. and Kansas Tool. The company's first home was a 16 x 20' (4.9 x 6-m) building cobbled together on his property from scrap wood. It was equipped with four tools: a mill, a lathe, a shaper, and a drill press. The drill press was a Christmas present from his wife, Ruth Ann.Jason Cox holds a finished stainless socket, part of the latch system that holds the cabin door closed on Cessna business jets.

It took six months for that first job and the first check to come in. As the fledgling company's reputation spread, it outgrew the small building and was moved to a site northwest of Wichita into a building that was purchased from a man who had won it in a poker game. So it might not be too surprising that the building didn't have indoor plumbing and that a "facility" had to be built on the property.

Growth under Bud's son, Steve, who assumed the role of president and general manager in 1972, has been steady through acquisition of CNC technology (first begun in 1972) and more recently in 2002 of a sheetmetal fabricating company, Advanced Manufacturing Technology (formerly John Weitzel Inc.).

Today, Cox Machine occupies a total of 75,000 ft2 (6968 m2) and employs 163 in its two locations. The Wichita facility was completed in 2001 to house the primary machining operations, administrative offices, and support departments including quality, planning, and production, as well as purchasing and receiving for its three divisions. The Wichita facility is laid out so that manufacturing cells can be formed for large production runs, while still maintaining the flexibility needed for small-quantity and prototype jobs. Cox's gundrilling division, Accurate Drilling, is located at the Wichita Division location.

Cox Machine changed its vision statement to include sheetmetal after acquiring the fabricating facility in 2002. The sheetmetal fabrication facility is located in Harper, KS, about 45 miles (72 km) southwest of Wichita. The facility houses forming, fabricating, flat pattern, waterjet, turret punch press, hydroform, and brake-forming equipment for manufacturing webs, ribs, stringers, brackets, and structural parts.

Cox Machine hasn't strayed far from its founder's roots in the city that bills itself as "The Air Capital of the World." According to Jason Cox, chief technical officer, the company has developed a focus exclusively on the aerospace industry, manufacturing parts for leading commercial and general aviation companies.

"We decided a few years back to focus exclusively on aerospace. Primarily we make structural and control-system components and assemblies for Cessna, Spirit AeroSystems, Hawker Beechcraft, Eclipse, Boeing, and a bunch of others," Cox says. "We focus on machining parts that can fit into a 2 x 2' [0.6 x 0.6-m] size. We can provide sheetmetal fabrication up to 6 or 8' or 8' [1.8 or 2.4-m] long."

The company has also expanded its reach beyond Wichita, finding clients in various parts of the world. "We do some work in Turkey and Italy, but I think all of that work ends up back on Boeing products," says Cox.

Jason Cox has watched the company's growth from a slightly different viewpoint, perhaps, than others in the company. He's a graduate engineer, who thinks about tools and the most effective way to use them, about lean and cellular manufacturing, Six Sigma, and Kaizen.

"We really see a continued and strong focus on lean and cellular manufacturing," Cox says. "We see that's the way to go, and we're going to invest in new technology, hard technology and soft technology, like the way our staging areas are set up. That's really where we feel we need to go, and we feel we've done a good job so far, but we see a lot of room for improvement."

A manifestation of this forward thinking can be seen in the company's most recent CNC machining center acquisitions: three VMCs from Bridgeport (Elmira, NY) and two Mini Mills from Haas Automation Inc. (Oxnard, CA). The machining centers figure prominently in cellular configurations to achieve the productivity and flexibility required for its aerospace parts.

Bob McGhay programs one of the twin-pallet Bridgeport APC 1000 VMCs at Cox Machine that are being run as single-machine cells.The three Bridgeport XR 1000 APC VMCs feature integrated front-load automatic pallet changers and 48-tool ATCs with rapid chip-to-chip change times. "We chose the Bridgeports as the way to improve our vertical milling efficiency, because they enable operators to change over and load parts while the spindle is turning and making chips. On the standard vertical, you can't turn the spindle while you're changing over and loading parts, and on these you can," Cox explains.

Cox says that the Bridgeport XR 1000 APC machining centers are each running as individual machining cells. "With the integrated front-load pallet changer, we can actually create a one-piece flow in a cell with one machine. Plus, the 48-tool ATC lets us run a catalog of standardized tools so we don't have to swap out the cutting tools," Cox explains.

"We regard the Bridgeports as cells, so we try to load them with stable and repeatable work, such as the brackets and sockets. Because of this arrangement, we haven't entered new markets, but we have gone after work that's a bit of a specific breed—that is, small-envelope, high-quantity, long-term contract parts.

"It's stable work, repeatable, and high quality. When we operate this way, we're able to invest more in the front end of the cell, into standardization of job and process, into our staging areas. We end up with reduced lead times for the customer and a lot quicker response time to forecast changes or engineering changes, for example. It's the same market, but a specific type of part that we've targeted to put through the cells. And this gives us a competitive advantage over conventional VMCs. We are able to offer dramatically reduced lead times for the parts we run on these machines—which is a huge advantage, here in Aerospace Town or, for that matter, Anywhere Town."

The two Haas Mini Mills were chosen to provide the small-part machining capacity in its Small Extrusion Cell. "We set up the Small Extrusion Cell to manufacture 3–4" [76–102-mm] clips and brackets, profiling and drilling the extrusions. We set up a couple of Mini Mills in a true cell configuration with atumble deburr, drill press, and inspection. Everything about the cell was chosen for speed. We set it up with super quick-change tool using snap-on vise jaws and standard tool magazines in the machines."

Cox describes the Small Extrusion Cell's operation: "The changeover time is less than 10 min on the extrusion cell. The first operation is cut on the first mill, while the next operation is cut on the second mill simultaneously. While both mills are running, the machinist has time to countersink, deburr, and inspect the last part that came off the machine. This has allowed us to make significant lead-time reductions. It is not difficult at all to receive raw material in the morning and have parts headed to paint in the afternoon."

Cox feels that adopting the latest CNC technology and cellular configuration will enable the company to overcome a labor market that is really tight when it comes to finding qualified, skilled help. Business is that good.

"We have a terrific workforce here. Our goal is to grow as much as possible with them by improving our technology and without overworking them," he says. "We don't want to make them work twice as hard. We want them to work the same amount, three shifts a day, but produce more product, more efficiently, and at a better quality. Also we want to standardize our processes—our lean and cellular initiatives—and have equipment that lets us do that and use people with fewer skills."

With the increased spindle utilization, increased spindle speed and power, plus the 48-tool and rapid chip-to-chip change capability of the XR 1000 APCs, Cox Machine was able to replace two of its older verticals with a single new Bridgeport.

"It's nice to watch the transition: two older VMCs out, one Bridgeport APC in," says Cox. "Then, when that worked so well, we did it again—two out and one in. And this is what we plan to continue to do. It's this kind of waste removal from the value stream that's a hallmark of lean manufacturing."Stainless bracket is being machined with a carbide rougher on the Bridgeport APC 1000.

"A typical Cox job for a Bridgeport XR 1000 APC would be a smaller envelope size part, roughly 4 in.3 [101 mm3]," says Cox. "It's going to be steel or aluminum and because of the Bridgeport's high horsepower, it's going to be hogged out and high usage. Target parts are machined complete—no secondary operations, like honing or turning.

"We manufacture part of the latch system that holds the cabin door closed on Cessna business jets, called the socket," says Cox. "These parts are about 2 x 3 x 0.5" [50.8 x 76.2 x 12.7 mm] and are manufactured from stainless. We produce four variations for a total of 1100 parts per year."

Cox also manufactures several brackets and stiffeners for Spirit AeroSystems that go into the 737 fuselage. These tie pieces of the structure together and are hidden from view in the finished plane. They are machined from aluminum and range from 2 x 2 x 1" to 3 x 3 x 12" (50.8 x 50.8 x 25.4 mm to 76.2 x 76.2 x 304.8 mm). They produce 350 of each bracket per year. The stiffeners are machined from a 10.5 lb (4.8-kg) block, but have a final weight of less than 0.5 lb (0.23 kg).

"We typically set up all three or four operations of a bracket on one table and a different bracket, again requiring three or four operations, on the other table," Cox says. "The automatic, two-station rotary pallet swaps the tables in and out. Sometimes, we run a larger part like a stair step for Hawker Beech that has two operations. We'll set up one operation on each table — but typically we're going to do the smaller parts. Any required deburring is going to happen as the parts come off the machine. And after that, the parts go out for final processing. Basically, were getting a complete part, minus the sourced finishing operations (NDI, painting, etc.) with the cycle of each pallet."

Cycle times average about 15 min for the brackets. The Bridgeports are equipped with Big Plus spindles offering face and taper contact at the toolholder to increase rigidity, stability, and precision. "We can run extended length face mills considerably faster than we could on a conventional 40-taper machine. Plus, we reduce tool chatter and deburr at the same time," Cox says.

Aluminum is the material most often machined at Cox. "On the brackets we can run a 2" [50.8-mm] diam face mill, 506 ipm [12.8 m/min] with a 0.125" [3.2-mm] depth of cut. If we're running carbide," Cox says, "we can run a 0.750" [19-mm] carbide rougher at 300 ipm [7.6 m/min] with 0.250" [3.2-mm] depth of cut. So, we can hog the material out pretty quickly."

Operators use signal lights at machines to call quality control for buy-off inspection of parts before production run begins.The Bridgeport APCs are capable of holding the precision tolerances that are typically required for its parts and then some. "Some of the Spirit brackets have bearings pressed in before they are installed. We can interpolate the tight-tolerance holes that the bearings press into and hold 0.0005" [0.013 mm] on the bore—which is pretty darn good," Cox says.

"Most of the brackets we machine don't require that kind of precision, but when we do have to interpolate precision bores we know that we can easily do 0.0005". On parts like the sockets, we are only required to hold the profile within 0.010" [0.25 mm], which is very, very easy for the Bridgeports."

Cox Machine's focus on lean is aimed at reducing customer lead time and inventory. "Changeover is right smack in the middle of a lean process," says Cox. "The changeovers we're seeing with the Bridge-ports are under an hour for all the parts, and most of that happens while the spindle is running another part, so it's off line and external. We continue to improve as we increase the standardization of the jobs that run on the machines. Under an hour is not that great a changeover; I'd like to get it below that.

"A few years ago we ran six months' inventory on all our customers' parts, because our setup times dictated that. We'd run six months and then hold the inventory for our customers. Reducing our setups to below the one-hour mark allows us to reduce our batch sizes. Now we run a month's inventory for our customers that may be only 24 to 40 parts. Running a month's parts as opposed to a six-month quantity really makes a difference: it's beneficial to our customers and to us. Our inventory turns go up, our available cash goes up, and the customer gets his parts faster. Nobody loses," Cox concludes.

 

This article was first published in the March 2008 edition of Manufacturing Engineering magazine. 


Published Date : 3/1/2008

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