Round-The-Clock High-Precision Work

Round-The-Clock High-Precision Work

One company is holding 10 to 15 in unattended machining and subassembly workcells

"Five years ago we started a project to create a high-precision machining and subassembly workcell that had almost no labor component," said PeterRonge, director of manufacturing at Peer Inc. (www.peerinc.com). "Today we have three cells producing a total of 750,000 to one million parts per year. The cells run 24 hours a day, sevendays a week, are operated by only nine people, hold tolerances at 10 µ to 15 µ, and have 100 percent inspection. Theonly reason this project was successful is that we chose to team up with key players likeMori Seiki Co. Ltd. (www.moriseiki.com), Fanuc Robotics Inc., (www.fanucrobotics.com) and Mitutoyo American Corp. (www.mitutoyo.com), and made an agreement with them that we would not be shopping around from cell to cell. We developed an atmosphere of a team focused on getting that first cell to run, with the payoff being if they got it to run then they got the future business."

Ronge was hired by Peer to make the project happen. His approach was to put everyone involved on the same level. Instead of trying to point fingers and cancel or devalue purchase orders when there were problems, the emphasis was put on doing whatever was necessary to solve the problems and get the first workcell producing.

"We set up strategic partnerships with our suppliers and showed them that we're not going to make our money by squeezing them, we're going to make our money by producing product," said Ronge. "We didn't say ‘we're going to hold you to this' or ‘you didn't deliver so I'm canceling the P.O.' We tapped the shoulders of people with the expertise we needed and said, ‘I need this thing to happen this way' or ‘I need to load that part in that way.' The machine tool guy did his magic and the robot guy did his magic and the hydraulics guy did his and the CMM guy did his and after two years of very hard work we had our first functioning workcell."

The workcell they created is dedicated to a specific application, but Ronge pointed out that the cell is very flexible and can be used for any type of manufacturing including the mix of machining and subassembly. He noted that there were many problems getting that first cell operational, but the second cell was much easier and the third cell took little more than issuing the P.O.s. They expect to add a fourth, and possibly a fifth cell later this year with virtually no problems.

All of the outside support organizations are located less than an hour from Peer, so getting emergency service is quick and easy. Peer's Information Technology department also enabled the outside suppliers to get access to the workcells via communications lines, including webcams in the cells. The automation people can remotely access their cell controller and adjust the programs while the cell is off-line.

"It's amazing what it's taken to get to this point and it would never have happened if everybody was not on board 100 percent," said Ronge.

What's in the cell
Each 30 ft. by 10 ft. (9.1 m by 3.1 m) work cell consists of two machining areas and one subassembly area. Each machining area has a Mori Seiki vertical machining center, a Fanuc machine-tending robot, a compressed air blowout system, a Mitutoyo CrystaApex C CMM, a deburring wheel and two small conveyor belts that lead to and from the subassembly area. The subassembly area has a bushing press, bushing dispenser, and a visionequipped Fanuc transfer robot.

How it works
An operator loads palettes of castings onto the load table. The machinetending robot takes a casting from the table and loads it into one of eight fixture stations in the machining center. The machining center cuts a hole in the casting. The robot removes the machined casting from the fixture station while jets of compressed air clean the part, and puts it on a conveyor for delivery to the subassembly area. When the machined casting reaches the subassembly area, the transfer robot loads it into the bushing press, gets a split bushing from the bushing dispenser, and puts the bushing on a small rotating table which turns the bushing while the robot scans for the split. The transfer robot then moves the bushing to a press that installs the bushing into the hole on the casting, after which the transfer robot removes the casting with inserted bushing from the press and puts the assembly on the conveyor belt leading back into the machining area.

The machine-tending robot takes the part from the conveyor and puts it back into one of the eight stations in the fixture where it is machined to its final form. The finished part is removed from the fixture, deburred and put into the CMM fixture.

The CMM measures the hole diameter, X-location, Y-location and Zheight of the part. The measurements are recorded in the MeasureLink database and displayed on a screen with color-coding. Part information is color-coded green, yellow or red depending on whether or not the part is within tolerance. Parts that are outside tolerance are moved by the machine-tending robot to a red ink daubing station to flag it as rejected, then the part is placed on the work-completed palette. Good parts also are placed on the work-completed palette.

CMM and in-process inspection
Incorporating the CMM as a process tool rather than as an after-thefact inspection instrument, provides two benefits: First, the 100 percent inspection afforded by in-process CMMs helps to keep costs down by allowing machining to be done roundthe-clock with some shifts unattended. If a robot were to put a part into the wrong position or onto a chip or other contamination the CMM inspection would trigger rejection of the part. Second, and perhaps more importantly, the 100 percent inspection allows Peer to make full use of its customers' tolerances. Without inspection, a very high process capability (Cpk) is necessary to ensure zero defects. Moving to the high Cpk could require the use of a much more expensive process such as grinding or lapping. In Peer's case, the shop can center its process and allow the CMMs to weed out the tails beyond the normal curve.

Closing the loop is costly
The system incorporates an Ovation Engineering Inc. (www.ovationengineering.com) EZ-Comp controller that can take out-of-tolerance data from the CMM, and use it to modify the G-code in the machining center controller to adjust for tooling wear and other problems. "The Ovation automatic adjustment worked until we put the robotics in place, because the robots have a different way of thinking and prioritizing," said Ronge. "The robot will start doing things that it shouldn't because it should be waiting for the automatic adjustments to be calculated and made, but it is doing things in the correct way to satisfy its prioritization by moving parts the way we set it up to move them, and this causes a conflict with the Ovation system. There is often a lag that creates over-compensated adjustments. It works if we slow the machining down, which we did in the beginning, but that is not an acceptable solution."

Instead of closing the inspection-adjustment loop through programming, Peer does it manually. "Once the initial setup is made, there is very little adjustment we have to make and there is usually at least one qualified person per shift that can address those situations," added Ronge. "The guys can see what's happening (due to rejected parts marked with red ink) and pull up the history and make adjustments very quickly. If the operator is not around and the number of rejected parts exceeds a predetermined amount, the machine shuts down, but if the operator is around he can see the trend and make adjustments on the fly. We're looking at other ways to close the loop programmatically, but it is not a significant problem at this point."

How to hold tight tolerances in an unattended mode
"If I read that someone is holding 10 µ (machining aluminum unattended) I would think they are blowing smoke, but that is exactly what we are doing," said Ronge. "The key is in the material. We worked with a local foundry to develop an aluminum alloy that is extremely stable and allows us to hold those tight tolerances unattended. We also spared no cost in our tooling. We teamed up with some good tooling suppliers and we have diamond tooling that has performed very well for us. Our suppliers have spent hours here during that two-year development cycle testing to see what kind of performance we would get from their tools, and now they are bringing new things to the table to enable us to run at a constant rate. We're looking to reduce our cycle times with new cutter technologies—coatings and better tooling—and also with better workholding."

When you get it right, don't mess with it
Peer knows that there may be new technologies or software that may help to reduce cycle times, but the shop's managers are are very careful about making changes that might interrupt their process or have a negative impact overall. Reinventing the wheel costs money and the managers at Peer believes that if they can keep their current process running at close to 100 percent, they are far ahead of the game.

After the two years they spent in intensive development, Peer has run successfully for the last three years with high quality, high productivity and low labor costs. If they need to increase production, they clone their polished workcell setup instead of trying to completely reinvent a better one.

Ronge said the current workcell configuration is flexible enough to adjust easily to a change in the shop's current or older products, or to new products that the shop might need to produce. By partnering with outside experts, Peer achieved its objectives and now is reaping the rewards.

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