Scratching Surface Flaws With Automation
Machining parts complete on CNC multi-spindles, with automatic load and unload, is proving the key to nick- and scratch-free parts for this tier-two auto parts maker.
Stay with a proven process or go for greater performance and flexibility with newer technology? That was the decision facing Autocam Corp. (Kentwood, Michigan), a manufacturer of precision metal components, when one of its auto industry customers increased the size of an order for a part for a fuel injection system. The equipment that the shop was using to process the part was already running at capacity, which meant that to increase production for the part the shop either had to buy more of the same kinds of equipment—the part was being machined in multiple operations across several machines—or come up with a better way to make it.
Autocam was reluctant to simply expand the existing production process. "The fuel injection part is a good example of today's more demanding parts," explains Rick Koenig, Autocam product manager. "It cannot be made complete in one setup on our multi-spindle cam automatics. We run it over several pieces of equipment to machine all of the required features. Secondary operations are expensive, however, therefore we prefer to avoid them where possible to keep costs down."
Then there was the matter of flexibility. In the world of auto part manufacturing, frequent changes in order quantities are a fact of life. When order quantities decline, firms serving that market need to be able to quickly change over their production equipment to other jobs. The additional secondary-operation machines that Autocam would have to buy represented a sizeable investment, yet they were not particularly easy to redeploy.
A New Direction
Autocam decided that it would continue to produce fuel injector parts by the existing process. However, the additional order would be processed on CNC multi-spindle screw machines capable of producing the fuel injection part complete in one setup. The firm reasoned that the investment in CNC multi-spindle screw machines would be offset in part by the savings achieved by not having to perform secondary operations on the parts—in particular a final, costly, OD-grinding operation that the existing process needed to remove the nicks and scratches that the parts acquired during processing.
In addition to enabling Autocam to produce the part complete in one setup, the CNC multi-spindle screw machines also better suited the firm's flexibility needs. "The multi-spindle cam automatic is a very robust, repeatable machine, but it takes a long time to set up for the next job," Mr. Koenig explains. "You have to order the cams, wait for them to be made . . . it can take weeks to get to the point where you can make trial cuts. By contrast, the CNC multi-spindle screw machine can be changed over to run a new part in minutes to hours.
"The same thing is true of adjustments," Mr. Koenig continues. "To make an adjustment on a cam machine, the operator must remove the tool, make the adjustments, put the tool back, cut a part . . . it can take hours. By contrast, adjustments can be done in minutes on the CNC screw machine. Because of the speed and ease with which changes can be made, the CNC screw machine encourages tweaking to optimize performance. Make an adjustment. If it works, fine. If it doesn't, simply revert to the previous program settings."
Accordingly, Autocam purchased several MultiDECO 20/8, CNC, eight-spindle screw machines from Tornos Technologies U.S. Corp. (Brookfield, Connecticut). Each machine forms the heart of a highly automated machining cell (shown above) capable of making the fuel injector part complete in one setup so that no secondary operations are required.
Surface Flaws Unacceptable
A major consideration in developing the cells was that the fuel injector parts must be free of scratches and nicks. Such flaws routinely occur when processing bar stock through screw machines and when the machined parts knock against each other in tote bins. In the original process, the OD of the fuel injector part must be ground to remove such surface flaws. One goal of the new process was to eliminate all such secondary operations. Therefore the cell must process the material, both bar stock and finished parts, in such a way as to avoid marring the surface.
That's no small order considering how bar stock is fed to a multi-spindle screw machine. First of all, the screw machine has feed fingers that grab the bar stock and pull it from the bar feeder into the machine. The process frequently results in longitudinal scratches in the bar OD that can only be removed by the grinding of the finished parts.
Scratching also occurs where the bar leaves the bar feeder and enters the screw machine. Each spindle collet of the multi-spindle screw machine closes on the front end of a bar extending from the bar feeder, causing the bar to rotate in the bar feeder at the spindle rpm. When it is time for the bar to feed forward, the collet opens. During this brief interval, the collet no longer rotates the bar so that the rotation of the bar begins to slow. Also, the bar, which was previously centered and supported by the closed collet, tends to sag to the bottom of the slightly larger opening created by the open collet.
After the bar feeds forward for the next part, the collet re-closes on the bar. In a small fraction of a second the collet re-centers the bar and resumes driving it at the spindle speed. The action of the collet coming into contact with the bar produces spiral or circular scratches in the bar.
To solve the problem of the scratching of the bars, Autocam approached Pietro Cucchi America Inc. (Elk Grove Village, Illinois), manufacturer of bar feeders for multi-spindle screw machines. Autocam was already sold on a space-saving feature of the firm's bar loader: Unlike older bar feeders for multi-spindle screw machines that feed the bars into the screw machine's stock reel, the Pietro Cucchi bar loader feeds the bar directly to the machine's spindles, eliminating the need for a stock reel. The result is a much more compact arrangement, saving all of the space normally occupied by the stock reel.
A standard feature of the Pietro Cucchi bar feeder addressed the problem of the longitudinal scratching of the bars. Many multi-spindle screw machines have feed fingers that pull the bar into the screw machine. The bar feeder takes over the function of incrementally feeding the bars to the multi-spindle screw machine, pushing them into the machine from the rear. This allowed Autocam to remove the feed fingers from the screw machine, eliminating the source of the longitudinal scratching of the bar.
The rear-feed feature provides still another major advantage: The back end of the bar is secured in a rotary spring collet on the bar pusher, and when the bar has been consumed, the bar pusher retracts to the back of the bar feeder where a gripper unit extracts the remnant from the bar pusher, drops it, and inserts a fresh bar. Removal of the remnant at the rear of the bar feeder eliminates the danger of damage to the screw machine tooling, the possibility of jamming the chip conveyor and damage to finished parts.
To solve the problem of the scratching of the bar stock where it enters the collets of the multi-spindle machines, Autocam and Pietro Cucchi worked together to develop a plastic bushing that mounts in the spindle. The bushing keeps the bar centered in the collet opening (prevents it from sagging to the bottom) and keeps it rotating at the same speed as the spindle. The circular scratches formerly caused by the slight misalignment of the bar relative to the collet and the difference in their rotating speeds have been largely eliminated.
According to Autocam's Mr. Koenig, the bar loader is about 3 percent faster than bar loaders that attach to stock reels. Bar feed-out consistency is better too. "Prior to the Pietro Cucchi loader, the variation in the feed-out of the bar was as much as 500 to 600 microns," Mr. Koenig recalls. "That variation has been halved by the new loader, to 300 microns, which in turn has halved the amount of bar stock that must be faced off to ensure uniform part length."
In the cell, the bar loader is attached to a Tornos Bechler MultiDECO 20/8 eight-spindle CNC screw machine (photo at left). The CNC multi-spindle screw machine is the first such machine for the Kentwood plant. It combines the speed and tooling capacity of an eight-spindle cam automatic with the flexibility and faster setup time of a CNC machine. That results in a machine capable of producing very complex parts, in relatively small quantities (compared to traditional screw machine run sizes), and in relatively short time frames because no cams or special tools are required to produce the part.
Absolutely No Contact
As mentioned earlier, nicks and scratches, which could affect the performance of the fuel injector parts, are not acceptable. Accordingly, not only must scratch marks be avoided, for example, at the point where the rotating bar is fed from the bar feeder into the screw machine's spindles, or when a part is transferred from a spindle to a counter-spindle, but the completely machined parts must also be prevented from coming into contact with each other as they exit the machine.
Autocam solved the latter problem with a completely automatic part-unloading system. As finished parts exit the machine, they are automatically gaged to catch defects caused by unpredictable problems, such as incomplete cuts caused by chip bird nests. Defective parts are automatically ejected. Good parts are placed one at a time on a tiny conveyor that carries them through a cleaning operation (photo on page 38).
"Actually, it's a double solvent wash to ensure that we remove all of the cutting oil from the parts," explains Steve Schoonbeck, Autocam manufacturing engineer. "Most of the oil is removed in the upstream stage. The second stage gets any remaining traces. Once a week or so, we'll remove the upstream solvent for cleaning, replace it with the downstream solvent, and start with fresh downstream solvent. Cycling the solvent through both stages helps us hold down total consumption."
Parts are conveyed through the washer to an exit station where a Faraman tabletop robot, made by Samsung Electronics, removes them one at a time and loads them in a pallet mounted on a two-pallet indexing table (photo sequence on page 38). The robot is programmed to insert a part in each pallet hole. When the pallet is full, the table indexes 180 degrees, which moves the full pallet to an opening in the robot enclosure where it can be safely removed without interrupting the operation, and presents a fresh pallet to the robot.
The robot is portable. When it is no longer required for the fuel injector part, it can be quickly and easily moved to another machine, if necessary, to provide automated unloading capability for another job. Mr. Schoonbeck is impressed with that flexibility. "We use a lot of X-Y tables at Autocam (tables that move in X and Y to position a hole for the next finished part), but they're usually dedicated to specific jobs," he notes. "The robots are a real improvement since we can redeploy and quickly adapt them to other jobs when the need arises.
"Also, the robots can do a lot more than simply put parts in a pallet," he continues. "They can present the part to a wire brush or grinding wheel, run it through a gage, put it in a box . . . perform dozens of different tasks that might be difficult and/or dangerous for an operator. When we finish running one job, we can move the robot to another job that involves a totally different set of handling tasks.
"In most screw machining operations, as the finished parts exit the machine into a basket, the operator spot checks them, puts them in a tote or otherwise takes care of them," he adds. "With the robots, our operators can focus on the machining side of things and let the automation take care of the part handling."
Autocam's decision to produce the additional fuel injector parts on CNC multi-spindle screw machines, instead of by the existing process, turned out to be the right move. By producing the parts complete on the CNC multi-spindles, the firm has eliminated secondary machining operations on the part and avoids the wear and tear that goes with moving them through the shop—a paramount consideration.
Mr. Koenig reports that the cycle time for the part produced on the CNC multi-spindles is shorter than for those made by the original process. Work-in-process inventory has also been eliminated. For competitive reasons, Mr. Koenig would not discuss the original process in detail. He explained, however, that producing the parts on the CNC multi-spindle screw machines has made it possible to avoid the final OD grinding operation required for parts produced by the original process, for a significant savings.
Mr. Koenig attributes the savings to the smooth interaction of the CNC multi-spindle and the bar loader. "Without the bar loader, we would have a much more difficult time accomplishing what we do in the screw machine to eliminate the secondary operation."
A Work In Progress
Mr. Koenig adds that although secondary operations for the parts produced on the CNC multi-spindles have been eliminated, the process is not perfect. "We are presently doing 100 percent inspection because we still occasionally get scratched parts," he explains. "However, we are confident that with a little tweaking and some minor changes to the process itself, we will eliminate the scratches and that the sorting operation will no longer be necessary. We're not there yet, but we're very close."
Selecting the right bar feeder can be one of the most important decisions a shop can make. Bar feed systems help improve productivity, throughput and quality, but in order to achieve the most benefit from them it is essential that a bar feed system be matched to the particular needs of the turning operation.
Some primary factors are often overlooked when considering how to justify the implementation of a bar feeder for turning operations.
Barstock is versatile raw material. It’s easily held, easily fed and has significant capacity for many parts per bar. Usually we think of barstock and bar feeders in relation to turning machines. What about applying the advantages of automatically fed barstock to a vertical machining center?