April 2008 Edition

machining centers

Cox thinking out of aero box

Bridgeport centers run as individual machining cells for Kansas firm

Jason Cox has watched his company's growth from a slightly different view than perhaps others in the company. As the chief technology officer of Cox Machine Inc. of Wichita, KS, he's a guy who thinks about tools and their most effective use, about lean and cellular manufacturing, and about Six Sigma and Kaizan.

"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, like our Bridgeport machining centers, 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."

Bridgeport is key

A manifestation of where Cox wants to go can be seen in two recent acquisitions, two Bridgeport XR 1000 APC vertical machining centers featuring integrated front-load automatic pallet changers and 48-tool ATCs with rapid chip-to-chip change times.

"Before we had standard three-axis vertical mills that were aging," Cox says. "We've chosen the Bridgeports as the vehicle to drive our efficiency improvements in our vertical milling operations. On the standard vertical, you can't turn the spindle while you're changing over and loading parts, and on these you can. So one Bridgeport easily replaces two conventional verticals, and that allows us to grow our business without unnecessary workforce or footprint expansions."

Cox explains the company occupies 75,000sqft and employs 163 in two locations: In Wichita they do all of the machining and assembly, and in Harper, just a short run down the road, the company does all of its fabrication in a facility Cox purchased.

"Primarily, we make structural and control system components and assemblies for Cessna, Spirit AeroSystems, Hawker Beach, Eclipse, Boeing and a bunch of others," Cox says. "But we've come to realize that focusing just on Wichita wasn't broad enough, so we looked beyond and found clients in various parts of the country — and beyond. We do some work in Turkey and Italy, but I think all of that work ends up back on Boeing products."

Cox explains that business is so good, yet it is impossible to find qualified help.

"Our challenge is to grow with the people we have without over-working 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, if the need be."

Technology's the answer

The Bridgeport XR 1000 APC machining centers feature twin 40" x 20" palettes and 12,000rpm, 40.2hp, 40-taper Big Plus direct drive spindles. The XR 1000 APCs are quick with a rapid traverse rate of 1,890ipm and X, Y, Z acceleration/ deceleration of 138", 177", and 177"/sec2, respectively. And with X, Y and Z travels of 20" x 24" x 23.6", a workload capacity of 660lbs per pallet, and the ability to accommodate workpieces up to 13.8" tall, the XR 1000 APCs are suitable for a diverse range of jobs and machining applications.

Cox says, "Between the increase of spindle utilization from 52 to 93 percent, the increased RPM and increased horsepower, plus the 48-tool ATC with a rapid chip-to-chip change capability, we were able to replace two of our older verticals with a single Bridgeport XR 1000 APC.

"It's nice to watch the transition: two older VMCs out, one Bridgeport APC in. 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. Translating that into numbers, we will have roughly $22 million in sales for 2007, and our growth target is $35 million by 2010."

Typical Cox work

A typical Cox job for a Bridgeport XR 1000 APC would be a smaller envelope-size part, roughly 4" cubed, says Cox. It would 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" and are manufactured from stainless steel. We produce four variations for a total of 1,100 parts per year."

'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.' Jason Cox

Cox also manufactures several brackets and stiffeners for Spirit AeroSystems that go into the Boeing 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". They produce 350 of each bracket per year. The stiffeners are machined from a 10.5lb block, but have a final weight of less than 0.5lb.

"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, we're getting a complete part, minus the sourced finishing operations (NDI, painting, etc.) with the cycle of each pallet."

In detail

Cox explains that cycle times of about 15 minutes are average for the brackets. Regarding precision and rigidity, the Bridgeports are equipped with Big Plus spindles offering face and taper contact at the tool holder to increase rigidity. Because of the increase in rigidity, stability and precision, "We can run extended length face mills (2" and 3") considerably faster than we could on a conventional 40 taper machine. Plus, we reduce tool chatter (and debur) at the same time," Cox says.

In aluminum, which is the most-often material machined, Cox says on the brackets they can run a 2" diameter face mill, 506ipm with a 0.125" depth of cut.

"If we're running carbide," Cox says, "we can run a 0.750 carbide rougher at 300ipm with 0.250 depth of cut. So, we can hog the material out pretty quickly."

Holding tolerances is another bright spot for the Bridgeport APCs.

"Some of the Sprit 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" 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 .010", which is very, very easy for the Bridgeports. We're running shrink-fit holders on everything with solid carbide tools or inserted tools, so in aluminum our tools are going to last through 10 to 15 jobs. It's not like parts per tool; it's more like jobs per tool, and its 10 to 15 jobs before we have to change.

"As you know, we are focused very keenly and putting a lot of time into lean, which translates into reduced customer lead time. Changeover is right smack in the middle of a lean process. The changeovers we're seeing with the Bridgeports 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-month's 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. 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."

Single-machine cells

Cox notes that what's evolving is really interesting: the Bridgeport XR 1000 APC machining centers are each running as individual machining cells.

"So, with the integrated front-load pallet changer, we can actually create a one-piece flow in a cell with one machine," says Cox. "Plus, the 48-tool ATC lets us run a catalog of standardized tools so we don't have to swap out the cutting tools. This helps, obviously, in that we run the same tools all the time. Since we've looked at the Bridgeports as cells, 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. 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're 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."

Bridgeport Machines Inc.

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