Managing rotary-draw tube bending
Best practices minimize variation and downtime
Editor's Note: This article is adapted from a conference presentation made by the author at a previous TPJ Symposium.Most information published about tube bending relates to a specific aspect of the process, such as tooling, control and equipment features, or component technology. This type of information is valuable for keeping tube fabricators up-to-date on what's available and is useful in selecting the right machine and tools for a given application, but little information is published on managing a tube bending operation.
It is necessary to give some thought to how the equipment will be managed and how it will fit into the company's overall management philosophy. Managing equipment for its most efficient operation is a part of managing the complete fabricating operation, which takes into account all the operations, organization, technical support, safety, and quality. Managing equipment is a bottom-up approach in which the equipment fits into an existing scheme of how the process is managed—or at least, how it is perceived to be managed.
In any given plant, management of the tube bending operation probably doesn't differ much from the management of its other manufacturing processes. This leads to several questions: Should it be different? Are some aspects of tube bending unique, and do they require special attention? If so, what are they? Several different manufacturers were surveyed and asked three primary questions:
- What do you do in tube bending that you are really good at?
- What are you not very good at, or where can you improve?
- What change, if management allowed, would help improve your process the most?
Several other questions were asked to help pinpoint specific problem areas that may be common to all manufacturers. This information then was categorized into the five variables of tube bending: equipment (bending machine and control), tooling, setup, material (tube), and lubrication for the tooling. The presumption is that if you manage all the variables of the process, you are managing your tube bending business well. A rating then was given in each of the five areas. The rating is strictly the author's interpretation of what was said during the interview process.
Finally, an overall grade describes how well the company manages its tube bending operation. This grade is not necessarily the average of the five variables; the variables' weights vary from operation to operation. For instance, if a company's core competency is high-volume production of parts that are not difficult to bend, setup does not carry the same weight as equipment.
Equipment. Significant criteria in evaluating how well a company manages its equipment include having a preventive maintenance program (including machine lubrication), having skilled technicians for troubleshooting (mechanical, electronic, and computer software), maintaining an inventory of spare parts, replacing aging equipment, rebuilding machines and controls, and having skilled maintenance personnel who can make quick and correct repairs.
In some plants, maintenance skills have not kept up with the advanced diagnostics in controls and special systems installed on machines by suppliers. Control technology is changing fast, and new systems are being used that some maintenance and engineering personnel know little about.
Tooling. The tooling variable focuses on the condition of the tooling and tool mounting surfaces. Are tube grooves ever checked for wear with a tube groove gauge? Are clamp and clamp inserts replaced or rebuilt? In high-production applications, does the company perform preventive maintenance on tools? Are wiper dies adjusted or changed beforeproblems occur? Are mandrels evaluated for wear and replaced when a certain tolerance is reached? Are special features such as platform dies, captive lip tools, and interlocked tools used appropriately where needed? A company does not have to do all of these to get a good grade, but it must be aware of the benefits of such strategies.
The importance of the tooling variable increases as the bend difficulty increases. Tooling for a 1D bend requires not only more tools than a 3D bend, but the tolerances and maintenance of grooves and grip surfaces are more important too.
Setup. When tools are changed, someone has to set up the tools in the proper relationship to each other and set any pressures that are required. Setup is a skill that requires not only training but also experience. Some people say that a person needs a year of experience to become proficient in setup work, although some people neverbecome proficient in this area. Tooling setup has so many variables that two people can get the same bending result by programming different distance and pressure settings and adjustments.
Setup has two aspects—defects and dimensions. The first requirement is getting a quality bend without defects, and the second is getting it correct dimensionally.
The setup time required for a quality bend varies with the difficulty of the bend. On a simple 2D or 3D bend, a good setup person can set up the tools and get a quality bend on the first attempt. A 1D bend on a 400-series stainless steel being set up for the first time on a three-stack tool set can take four hours to get a good bend on each tool.
The second component of setup is making adjustments in the bend program and tooling settings to get the part dimensionally correct. This usually is checked with a test gauge or fixture. The dimensional aspect of setup relies heavily on experience; it's not easily taught. Also, it can take substantially longer to get a dimensionally accurate part than merely getting a quality part.
Depending on the number of bends and part complexity, it can take from a few hours to three days to get the proper fit. Even after all that work, something could change the next day that requires further adjustments. Setup is not only important for the initial tool change; it is also necessary for making adjustments to maintain quality and accuracy during a production run.
When a defect occurs during production, how quickly can adjustments be made to bring the part back into tolerance? This speaks to the issue of familiarity with the machine and tools and pinpointing a problem's causes. Companies that make their money doing very difficult bends usually have very good setup people.
Material. Only one word can describe the material variable: consistency. Some processes are vulnerable to material changes both in hardness and dimensions. Companies rely on purchase specifications for quality control. These specifications have tolerances. As long as the raw material is within the tolerance, the material is acceptable.
However, dimensional tolerances can affect the tooling fit, especially that of the mandrel. This can be important when a mandrel fit on a 0.035-inch-thick tube has about 0.009 in. of gap between the mandrel OD and tube ID. ASTM sheet steel tolerances for tubes are ±10 percent of nominal dimension, which means ±0.0035 in. in this case. Even if purchase specifications tightened this to ±0.002 in., at additional cost, of course, the tolerance is still a big problem. A 0.002-in. change in sheet steel thickness is a 0.004-in. difference in tube ID; this change would cause the mandrel fit to change by nearly 45 percent.
Usually sheet steel is fairly consistent. However, if a lot is purchased from a different steel mill or an order goes to a different source such as an unfamiliar service center because of material shortages, all bets are off regarding consistency between material from the known source and material from the unfamiliar source. If you don't know what you have been receiving, how would you know that the problems you now have stem from wall thickness changes that still were within your specified tolerances?
In other applications the quality of the weld seam can make the difference between success and failure. When a weld seam or hardness problem is discovered down the line (that is, in some other operation performed after bending), the costs multiply rapidly—both in material and in the time spent on every operation performed in the manufacturing process until the defect causes part failure. The important question is this: Has your company evaluated the cost of inspection in relation to the cost of remedying the problem later? Or do you just ignore the material variable?
Lubrication. In rotary draw bending, three tools are required to bend. These are the bend die, clamp die, and pressure die. They move with the tube during bending. Depending on the support needed for the tube, up to two more tools are used—an inside mandrel and a wiper die. These tools are stationary and the tube slides on them.
Most companies recognize that these tools should be lubricated to prevent metal-to-metal contact, which causes galling, scratching, and wear. What often is not recognized is that a good lubricant allows the tube to slide on these tools with minimal friction. More friction makes it more difficult to maintain a good grip with the clamp die to prevent the tube from slipping during bending, which causes many more quality problems.
Most companies view the lubrication of tooling during bending as a necessary evil. Some consider it so evil they refuse to lubricate some tools. The lubrication issue is not just to lubricate, but to use the proper lubricant and apply it in the correct place.
The importance of lubrication goes up exponentially with the degree of difficulty in making the bend. Lubricant quality is more important for a 1D bend than it is for a 3D bend. Different lubricants are required for aluminum from those needed for carbon steel and stainless steel, and higher-quality lubricants are required for stainless steels than for carbon steels. Also, a mandrel requires a better lubricant than a wiper die does.
Some companies do not lubricate tools at all. Either they lack of knowledge regarding the importance of it or they simply do not want to bother with the mess lubricants usually make.
Some companies use a coolant approach to lubricating. They pour a water/oil mixture over the tools, which has little lubricating effect but sure cools the tools that have become overheated because of the friction load. The companies that really have to pay attention to lubricant quality are the ones that bend to 1D and are bending low-ductility materials. In this case, the lubricant's quality can make or break the bend. Literally. A high-quality lubricant can facilitate a good bend; using a low-quality lubricant can cause the tube to fail, or break, during bending.
Evaluating the Companies
Four companies were selected for this survey. All are well-managed and highly successful. Each one makes different products, and they vary both in company size the amount of tube bending done. Some companies have a long history of bending tubes, while others are relatively new to tube bending. After the survey was completed, the information was evaluated, and the author rated the companies on each of the bending variables with the following grade scale:
D= Below Average
E= Needs Attention
One important observation was that the management of the bending operation was most influenced by the background and skills of the person directly responsible for the bending. Whether it was a conscious management decision to put this type of person in this area of responsibility because this is how the company wanted the business managed is not known for sure, but it would seem logical.
Company A—Scholarly in Setup
Company A has been bending for more than 40 years. It has 18 machines in the facility, of which six are fully automatic. All of the other machines are CNC with manual loading and unloading. Its expertise is in doing the tough jobs like 1D bends and dual-wall tubes with compound clamps. The company relies on its engineering technology personnel for know-how and places a high value on setup and setup troubleshooting. The company has a strong core of setup technicians who do nothing but setup, leaving the actual bending to operators. Because these bends are difficult, the company puts much effort in tooling fits, and even has developed its own drawings for mandrels.
However, it has no program for evaluating wear and changing tools before a problem shows up. It is interesting that this company still feels as though it could use more setup training and wants to learn more about tooling.
The technician who supervises the bending operation came up through the ranks and is a very good setup person. His skills and ability are in setting up tools on the machines and understanding how to adjust for defects.
The bending operation is organized such that one highly skilled setup person supervises several bending machines. The bending performed at this company requires a high skill level, and management prefers to have specialized people do the setup work rather than have operators rotate through the job, which would cause high turnover.
It is not surprising that this supervisor wanted more training for setup people even though they were already very skilled. The negative side of this is that not as much attention gets paid to the other variables, and the total operation suffers from it.
The company pays little attention to the equipment. Spare parts and the supplier's inventory sometimes are evaluated, but no plan is in place. The philosophy seems to be "If it isn't broke, don't fix it." When a machine does break down, the company is caught by surprise and generally experiences long downtimes getting it repaired because of a general lack of maintenance knowledge and the low availability of spare parts. Maintenance personnel are not very well-trained in troubleshooting machine problems. The company had a preventive maintenance program for the machines, and this program proved its value in reduced downtime. However, the program was scrapped several years ago.
There is no quality check of material. Tubes are purchased by specifications, and if problems arise, the staff generally is unaware of them.
A lot of effort went into evaluating tool lubrication, both in the type and method of application.
The criteria for the lube is that it had to be clean, environmentally compatible, meet disposal requirements, and assist bending. Although the evaluation was originally driven to clean up the mess, the company also focused on preventing higher tool wear and other problems associated with using poor lubricants.
Ratings for this company are:
This company could easily achieve an A- or B+ if it managed the equipment variable like it used to. The company's knowledge in setup covers up its shortcomings in many other areas.
Company B—Needs Remedial Training in Setup, Lubrication
Company B has nine years of bending experience, but the facility reviewed for this article has just two years of bending experience. The facility has four machines operating 24 hours a day, seven days a week. The machines are all automatic and dedicated to a product line, but some tool changeovers are required for flexibility. Usually when tools are changed or a part is modified, the company has setup problems that require outside help.
The company spent a lot of time streamlining the equipment and tools to make the operation more efficient. It has a very strict preventive maintenance (PM) schedule for its equipment and stocks an adequate supply of spare parts. Its bending knowledge and equipment familiarity help the company keep downtime to a minimum.
An engineer oversees the bending operation and is highly motivated to get the equipment operating at peak efficiency and keep spare parts in order. The engineer understands the machine and tooling relationships very well and has minimized tool changeovers by stacking different tool sets for different jobs for maximum flexibility and minimum tool changeover time. The bends are not difficult, and with a little ingenuity, he has eliminated some mandrel and wiper die tooling.
The equipment supplier did the initial setup and developed the bend programs. One person was trained on the machine and controls and also received some maintenance training. From that point on, whenever a part had to be changed or the program revised, problems arose. Nobody was trained well enough in setup to make adjustments or corrections. Further training programs were sporadic and not well-attended by operators or technicians. There was no real dedicated effort to develop this skill in-house.
The facility has eliminated mandrels and wiper dies on some of the product lines based on development work on the process. It does not check tools for wear and reacts only when a quality problem comes up. Surprisingly, it does not use a lubricant on the mandrels and wiper dies that still are used. The decision to run dry comes at the cost of high tool wear. The bends are not that difficult, so the company feels justified in the tradeoff.
Seam finding is a critical part of the bent tube quality. The tube supplier visibly marks the seam at the mill for optical identification later at the bending equipment. The tube supplier is at the plant every morning to check the material before it goes into production. So in essence, the tube supplier has become the material inspector. For the most part, tolerances on these tubes are not a problem, so accurate records of what is received are not as important as the fact that the material meets purchase specifications and the seam is marked properly.
Ratings for this company are:
With some dedicated setup training and some tool lubrication, the rating could go to an A. The company is not experiencing major problems now, but as the machines get older and tooling wears, more training of shift workers will be required.
Company C—Send the Setup Staff to Study Hall
Company C has been in the tube bending business for 50 years. It has 40 tube bending machines; most operate three shifts per day. All machines have manual loading and unloading. About half the machines are CNC with stacked tools, and the rest are single-axis benders for making a single bend in a tube.
Its expertise is in working with a customer to develop a given product and bending very difficult—some would say impossible—jobs. It has high-volume lines in which machines are dedicated to one product and other lines in which tools are changed more frequently. Nearly all bending is to 1D in either 400- or 300-series stainless steels.
This company is managed well. The bending operation supervisor is well-rounded in all aspects of equipment and setup and is positioned high enough in management to get the results he needs. If the company is not doing something, it's a conscious decision and the consequences have been evaluated.
The company designs its own tooling and has it made to its specifications. All wear tools are on a PM program to be changed before defects appear in bent tube. The machines are also on a PM program for general maintenance. The company has identified major wear areas of the machines and stocks spares for replacement; however, it does not perform major overhaul work. On older machines it contracts control rebuilds to upgrade features and make them more user-friendly.
The machine operators also do the tool setup and adjustments when quality problems appear. The problem is that the company does not have enough well-trained operators for such a large operation on three shifts. This causes delays in getting things back up and running when the right people are not available. The company has high labor turnover in this category and needs to implement a good training program to bring operators up to speed faster.
This company does no material control. If a material problem appears, it usually can be traced back to the lot number, and the whole lot is returned to the supplier. In the meantime the company loses production.
Currently the company is evaluating various tool lubricants in an attempt to make the operation cleaner. Management does not believe in lubricating the wiper die because it fears the lubricant will get on other tooling and cause clamping problems. This practice obviously causes other problems in tool wear and clamping. Management considers tool lubrication to be a problem rather than a means to control the process more efficiently.
Ratings for this company are:
This company has a few good setup people but not nearly enough to take care of the large number of machines it has. The current objective is to improve operator training in setup, but turnover will make this a never-ending program. Management recognizes the need but will have a difficult time achieving it because of the large number of people who have to be trained. An alternative would be to set up a separate classification for this skill so management can choose the people who have the necessary skills and invest the training in them.
If the company could achieve the training it is after, it easily would get an A rating.
Company D's Lesson: Least-cost Equipment Purchases Can Be a Costly Mistake
Company D has been bending for about three years. It has four CNC benders with manual loading and unloading operating two shifts per day. It does high-speed, high-volume production on long runs. The bending is not difficult, so setup is not a critical factor in evaluating this company's performance.
The company got into tube bending as a value-added process and had no experienced tube bender operators. Management's main equipment-purchasing criteria was cost; it spent the least amount of money possible. The company now has problems with controls it can't fix and receives little support from the supplier. The company would like to replace equipment, but it doesn't have enough business to justify purchasing new machines.
The company does not perform any scheduled PM on the machines even though management expects high production volumes with no downtime. It has some equipment idle time but can't take on more difficult short-term jobs because the staff lacks expertise in setup and the company would lose too much time.
The company has high labor turnover in the bending area. It has a difficult time keeping qualified people on the job, which costs the company down the road because it doesn't have the ability to do more complicated work. Despite all of this, the company carved out a market segment where it is successful.
It keeps spare parts for the tooling but does not do any PM on tools. The company checks incoming material for hardness because that is a critical factor in bend quality. Tools are lubricated because they are supposed to be lubricated, but little attention is paid to the lubricant's quality.
Ratings for this company are:
This company has difficulty managing its equipment because it lacks expertise in this area. Management's initial mistakes in equipment selection will take a long time to resolve. If it can find a way to manage the setup variable, the company could proceed to solve other problems. It could achieve a B+ with some dedicated training in setup or if it solved its equipment problems.
What Grade Would Your Company Earn?
The concept of making an operation as efficient as possible by controlling manufacturing variables is not new. However, it is still an interesting way of evaluating how well the operation is managed. Any company can do a self-analysis of its bending operation by putting together a team to assess how well the company manages these variables. After the team assesses the strengths and weaknesses of the bending operation, it is imperative to select a person to implement the necessary changes in managing the bending process.
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