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Automatic or manual?
Automation doesn't solve process problems. People do.
- By Bob Want
- October 9, 2007
- Article
- Bending and Forming
Whether it is as simple as a single CNC tube bender loaded by a robot or as complex as a fully automated line that turns raw coil into a finished and packaged bent tubular product, automated workcells have made their way into nearly every manufacturing theater. Once limited to the automotive industry with its ultrahigh production volumes, automated cells now are found in any industry in which the production rate is a "make or break" aspect of cost-effective manufacturing. As raw material costs and labor costs continue to rise and cutthroat competition forces prices down, automated cells are being used to produce more complex workpieces than ever before.
Trials abound. The demand for higher production comes with an inherent requirement that the process runs nearly continuously. Automation in a tube bending cell must be well-planned and well-executed simply to bring the project to a successful launch. Once the workcell is up and running, the maintenance staff must keep it running for two or three shifts a day, six to seven days a week—and that's another challenge entirely.
Plan, Plan, Plan
Successful tube bending is possible only after analyzing the application and matching the application to the proper techniques, machinery, and tooling. If the application is borderline—that is, just barely successful—when accomplished manually, no amount of automation will make it more successful. In other words, automation doesn't reduce problems; it merely increases the output. If a manual process has a 10 percent scrap rate, under the best conditions a similar automated process has a 10 percent scrap rate. Debugging and simplifying the manufacturing process before automating it is critical.
Taken as a stand-alone operation, tube bending is itself a matter of juggling ever-changing variables. To simplify this facet of workcell planning, initial considerations must include:
- Raw material ductility. Does the tube bend to the desired radius without fracturing? If the material does not have enough elongation, consider increasing the bend radius or using an alternate material, alloy, or temper.
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Also commonly overlooked is raw material consistency. It is absolutely essential that all the tube's properties—the chemical, mechanical, and physical characteristics—be consistent from lot to lot. Make certain from the start that the material selected is toleranced to optimize production, rather than a variable that makes steady, consistent production impossible.
- Bent part configuration. Does the application require more than one bender? Left-handed, right-handed, or both? How many bends are in the part? Does the workpiece have a sufficient length of straight tube between bends, or does it require gripping a bent section to produce the next bend? If so, the application requires stacked tooling. How many stacked sets does it need?
- Bend severity and die tooling selection. Does the application need complete die sets, including internal ball mandrels and wiper dies? Adding one or both of these tools makes bending much more complicated. These tools are in constant friction with the workpiece, so they must be lubricated. They must be set and adjusted properly, and maintained that way. They also must be changed quickly for maximum production efficiency.
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Many tooling design techniques can help meet production demands, but the tooling must be as simple as possible, especially in a high-production-volume environment. Don't throw more technology (and complications) at the application than necessary.
- Bending machine selection. Simply stated, specify at least enough bender capacity to get the job done. If the workcell is expected to run close to 24/7, make accommodations for all manufacturers' preventive maintenance recommendations. Don't simply follow them. Exceed them. Ignoring routine maintenance guarantees trouble when (that is when, not if) the cell goes down. With regard to maintenance, the degree of neglect and the mean time between failure are inversely proportional.
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In applications that require several die stacks, make certain that the machine is rigid enough to keep all the die sets in proper alignment. This often requires overhead tie bar supports for the bend die stack. While offering more strength and support, these tie bars also can interfere with some bent tube configurations. Be certain that you and the machine manufacturer understand the requirements of the proposed bent part.
With regard to several stacked wiper dies, remember that these fragile tools must be held in their proper positions securely. Otherwise they will not last or work effectively. It is often necessary to have a custom wiper bracket (holder) made to keep wiper dies secured and aligned properly and thus achieve acceptable die life and greater productivity.
- Acceptance criteria. Set a benchmark standard for tube acceptance before attempting to calculate the production rate of the automated cell. Criteria must include all aspects from processing raw strip to packaging finished parts. Because each operation hinges on the successful completion of the previous one, every operation must have a reasonable and repeatable go/no-go attribute or tolerance that can (and will) be monitored. Regardless of how this is done—visually, with fixtures, or with sophisticated coordinate measuring equipment—the fact is that all aspects of the process must have some periodic inspection. Other-wise troubleshooting will be impossible.
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This also prevents manufacturing scrap. Empowering the operators to verify part conformance frequently throughout the manufacturing process enables them to troubleshoot the process when they discover nonconforming parts. Doing so efficiently is a matter of checking parts randomly and frequently while they are being manufactured, not after they are finished.
- Above all, keep it simple.
Synchronize the Workcell's Operations
Because tube bending generally is the nucleus of a workcell, the other processes must be synchronized to this process. Determine the optimal cycle time for bending, and adjust the loader speed accordingly. If the cell involves a shear that cuts the tube or, in an extreme case, a tube mill that produces the tube, these operations must be timed so the bender is neither over- nor underfed.
Be aware that the overall speed of the automated cell or line must be based on the fastest tolerable cycle of the slowest operation of the cell. Also be aware that timing is nothing without debugging. Set up and troubleshoot every operation individually before attempting to integrate them in an automated process.
Launch the Cell
Sophisticated animation software can facilitate placing and integrating every piece of equipment in the workcell. Most work flow problems and equipment collisions are caught in the programming stage. However, in the real world it can take some time and program tweaking to achieve the mechanized ballet we strive for in a workcell. When the optimal cycle times are dialed in and benchmarks set for speed and other acceptance parameters, the next critical phase of the start-up begins.
Train the Operating and Maintenance Crews. Procedures for operations and maintenance training vary considerably from company to company. Some companies have three separate groups—one for tooling setup, a second for machine maintenance, and a third that operates the cell and monitors production. Considering the cell is likely intended to operate for two to three shifts per day, seven days a week, it is obvious that consistent output among all shifts requires consistent training among all personnel.
Without maintenance training, equipment operators can do nothing more to solve bending problems than find the few knowledgeable troubleshooting personnel and inform them of the problem. Likewise, the tooling setup staff's responses to problems can be limited. "Let's replace the tooling!" is a fast, easy, yet ineffective response if the trouble starts elsewhere.
Ideally, all personnel receive the same training and a set of written procedures so that everyone learns a single approach to troubleshooting.
Keep Accurate, Thorough Records. Equally important—and harder to implement—is an accurate method of monitoring uptime and downtime, cycle speeds, scrap rates, maintenance procedures, tooling settings, and changeovers. This data is necessary for making informed decisions regarding equipment and tooling condition. Adjusting procedures based on this information can help minimize downtime and anticipate catastrophic failure.
Any steps for developing constantly updated (and consistently formatted) records are valuable. The records themselves are invaluable if they are frequently analyzed and used for planning preventive maintenance activities.
If nothing else, good recordkeeping prevents running out of consumable tooling items (wiper dies and mandrels, for example). How many times do you need to run out of $50 wiper tips, causing the multimillion-dollar workcell to shut down and leaving your best customer stranded, before you realize that you should have a supply of all tooling and a steady flow of consumable items on recurring blanket orders? Proper documentation analysis and proactive process implementation are the only ways to prevent consumable shortages.
Troubleshoot to the Source
While it is just human nature to rush to get a high-speed workcell or production line up and running as quickly as possible after a problem arises, it is necessary to trace the cause of it all the way to its source. Follow the process back to the first step that did not meet the acceptance criteria. The principal reason for doing this, of course, is to solve the problem and not merely address the symptom. Allowing the problem to continue unchecked means it will compound later on. Resolving the problem now also prevents repeated and excessive downtime later.
If the workcell is a completely automated line, it might involve a tube mill; straightening, punching, and forming machines; a weld seam detector; a bending machine; hydroforming press; laser cutting system; welding station; and, of course, a material handling system. The complexity and speed of such a system means that a small problem early in the process has the potential to get completely out of hand in the blink of an eye. Constant quality monitoring and proper and pragmatic problem-solving are not just advised. They are required.
Bob's Troubleshooting Tips
Troubleshooting a bending operation is like troubleshooting any other manufacturing process—keep records, watch for changes, and keep symptoms and causes separate.
Consumables. Pay attention to consumable usage. For example, an increase in wiper tip reorder frequency is not a problem. More than likely, it is a symptom of a problem.
A wiper die's support comes from the tube groove of the bend die contour. Considering that the wiper is just 0.003 inch thick at the tip if machined correctly, it is easy to understand that this tip will conform to the bend die regardless of its condition, new or worn. If the bend die's groove contour is worn, the wiper tip life will be a fraction of what it should be. How many tips did you discard last quarter after premature failure? How many replacement bend die bodies would have prevented this?
Short wiper die life also can indicate that the tooling mounting surfaces are misaligned. This misalignment, in turn, can be caused by excessive and uneven strain on the bender, which is a result of excessive bender force. This doesn't just prematurely wear or ruin the tooling; it has the same effects on the bender itself.
Ovality. Bend ovality, the degree to which the bent tube is out of round, also is a symptom and also can have one of several causes.
The most common (and generally wrong) step to reduce ovality loss is to replace the balls on the mandrel and leave it at that. The mandrel, like the wiper tip, is in constant contact with the workpiece. The bending process develops a tremendous amount of pressure and wears out the mandrel balls.
While replacing the mandrel balls will improve bend ovality, this isn't necessarily the best course of action.
If correctly made, a mandrel shank (the cylindrical section the articulated ball-and-link assembly attaches to) is generally 0.005 in. greater in diameter than the ball segments. The size difference and the proper placement of the leading corner of the shank (slightly ahead of the bend tangent) dictate that the shank does the initial forming and bears the greatest load in the forming process. Generally speaking, if the shank is worn and the new ball segments are the same diameter as (or larger than) the shank, all the load falls on the ball-and-link assembly, resulting in premature link failure and mandrel breakage.
How many broken links could have been prevented, and how much downtime would have been uptime, if this condition were correctly diagnosed in the first place?
These issues with wipers and mandrels are simply two common problems in high-speed automated workcells. Wipers and mandrels are consumable items, so their premature wear often is treated lightly. Although they are small, they shouldn't be regarded as insignificant. A little knowledge and some preventive actions would extend the service lives of these items and increase the workcell's uptime considerably.
About the Author
Bob Want
194 W. Dakota Ave.
Denver, CO 80223
303-777-7170
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The Tube and Pipe Journal became the first magazine dedicated to serving the metal tube and pipe industry in 1990. Today, it remains the only North American publication devoted to this industry, and it has become the most trusted source of information for tube and pipe professionals.
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