avoiding common bending problems with common sense

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Avoiding common bending problems with common senseMatch tool design with bending application

By Bob Want

December 12, 2006

Tooling determines the outcome of a bending operation. Whether an application is simple or complex, matching the right tooling with the bending equipment and method will save both timeand money.

As we strive for simplicity in solving manufacturing problems, we are captivated and enthralled withthe flash and glitter of the newest gadgets. Whether by clever marketing or our innate desire to havethe latest and greatest newfangled technological gadgets, the outcome is the same. We not onlybelieve that bigger and badder is better, we positively crave it and will generally stop at nothing to justify it.

When sourcing technology for new manufacturing projects, we want it all. It seems to be in our nature.Not only must the equipment produce good, accurate parts quickly, but it must look good doing it. Wewant, or need, equipment that is bigger, badder, and better than our competitors' technology.

These days manufacturers tend to run lean, and competition is intense. The environment is fast-paced, short-handed, and frantic. We're all trying to do more with less and in less time, and when

we're looking for a technology to help alleviate some of the pressure, an emotional reaction rather than a reasoned response usually wins.

Rather than go with the gut instinct and our need for the flashiest solution, we should try to use amethodical, calculated approach.

All of the factors that come into play in a bending operation are driven by the application itself. Do youhave just a few bend radii and a small number of tube ODs, or do you anticipate a large variety of

bends? Are they simple or complex? What is the total number of finished parts you expect to producein a week, a month, or a year?

These factors, among others, drive the tooling options and design. Simply stated, garden-varietytooling offers little versatility with few options, but it is a good first step if your application can beformed this way. More complex designs have advantages, but too much complexity can adverselyaffect the success of the bending application.

Give similar thought to your bending machine. Its main function is to provide a rigid, stable platformfor the tooling. It also assists in gripping the tube by providing the necessary pressure. Don't bemisled, though. All the pressure in the world can't make up for poorly designed tooling. In fact, toomuch pressure will shatter the tooling. The workpiece dictates tooling design and bender selection,and all must be matched well or they simply will not work together to produce parts successfully.

Tooling Tales

All tube bending operations depend on the tooling. While the majority of applications can be formedwith simple, commodity-type bend tooling and methods, many cannot. It is essential through proper applications analysis to match the tooling type and design with the workpiece requirements.



Matching the bending method, bender features, and type of equipment to the tube's requirements willhelp you avoid costly mistakes and production nightmares. Focus on the fact that successful tubebending is a complex operation because of continuously changing variables that must be manipulatedinto a workable formula. Simply stated, the harder the problem, the more simple the solution must be.

Assume that rotary draw (mandrel) bending has been determined to be the best method for your application. To lay the proper foundation for this, you must at least become familiar with some of thecommon mistakes with regard to the bending equipment.

Your application determines basic criteria for the bender, and the most feasible, profitable manner toproduce it. These criteria include the following:

y Size and wall thickness of the workpiece

y Materials to be bent

y Number of bends in the part

y Proximity of the bends to one another (distance between bends, if any)

y Plane of bend relationship to one another

y Production rates

y Finished part tolerances (such as for wall thinning, ovality, and point-to-point dimensions)

y Centerline radius of the bends

y Initial cost of equipment (plus training, service, support, repair parts, and tooling)

y Return on investment

Bear in mind that the bender needs to have enough power for the job and enough finesse to align thetooling properly to get the job done, and it must have both in equal measure. A machine with its tool-mount surfaces knocked out of adjustment will be less productive, and if it is not corrected, it will ruinthe tooling.

In this case, bigger is not necessarily better. Bigger might be stronger and more rigid, thus providing amore stable platform for holding the tooling in alignment, but if you don't take into account theconfiguration of multiple-bend parts, you can get into trouble with machine/part interference if themachine has a large frame and footprint. Furthermore, adapting small-OD tooling to a large-capacitybender has a substantial cost disadvantage.

Bender Basics

Be very clear on what the present and future requirements are. Obviously, the choice of machine willdiffer radically from a job shop environment to a fully automated production cell.

A CNC machine is not needed if you are making simple, single-bend parts. If the parts have no morethan one centerline radius, it is not necessary to stack multiple tool sets on the machine. An exceptionto this would be if the bends have little or no straight tube length between bends and you need tostack the tooling to facilitate bending with no tooling changeover.

Machine features affect equipment cost, so be certain they are necessary. Remember, keep thesystem as simple as possible.

Before making a purchase, know where to go for support, service, part repair, and operator training,not only at a machine runoff but after the sale as well. Familiarize yourself with the documentationreceived at the time of purchase and how accurate and helpful this documentation is to themaintenance department.

Knowledge is power when considering used equipment. If you don't know what you're looking at, findsomeone who does. Spending money for a consultant isn't a bad idea if you're dealing with unfamiliar makes and models. Purchasing a plane ticket is cheaper than replacing a boat anchor.

The bottom line is that you will invest a substantial amount of money, so do your homework, spendthe money once, and spend it well. Good research is an investment, while a large impulseexpenditure is a disaster.

More About Tooling

Not all rotary draw bend tooling is created equal. It is absolutely vital that you educate yourself tomake the best use of your tooling budget. If you do not understand the bending process, bending



equipment, and part prints, you should not attempt to source bend tooling. Leave this job to anexperienced professional.

The variables affecting tooling design are numerous. Confusion increases exponentially if the persongathering supposedly comparative quotes does not use adequate information and isn't experiencedenough to competently evaluate suggested tooling options. It can be disastrous to go with low-pricedtooling that has no versatility; it can be equally disastrous to think that bigger and badder is better.

Figure 1 Bend tooling by design can be as simple or elaborate as the application it is intended for.

Bend tooling by design can be as simplistic or elaborate as the application it is intended for (seeFigure 1). As a basic design rule, versatile tooling costs more to design and more to produce.

The bend die can be as simple as the type 2 one-piece (see Figure 2), the most cost-effective dieavailable. Its limitations, however, can be a problem for some part configurations.

Figure 2

Bend dies differ in cost based on the complexity of their design and the amount of support they can provide.



The principal limitation of this configuration is that the grip area of the die is not removable, so the griplength is set and nonadjustable. The upside is that this die is cost-effective and strong. In the eventthat the die is constructed with a grip length of, for example, four times the tube diameter (4D), thiswould be the minimum straight length of tube that two adjacent bends could have. Generally, the nextoption most fabricators consider is to go shorter on the grip length for versatility without cost increase.

As the grip length decreases, the amount of pressure needed to hold the tube securely increases,which can cause tube surface marking and increase tooling wear. In the instance of very short grips,an aggressive surface can be machined into the tube groove surface. These can be fine or coarseradial serrations (directional buttressed grooves). Many grades of flame-applied ceramic carbidecoatings are also common in the tube groove of the grips.

A machined-tapered knurl pattern provides optimal grip without the radial marking typically associatedwith serrations. One drawback to these grip finishes is cost. Another is increased die wear, becausethe die grips the tube with less surface area. It is a better option to consider a bend die design thathas bolted-in and removable grip areas, such as the type 1 and type 6 dies.

The first time that you have to replace an entire bend die because the serrations in the tube groove of the nonremovable grip are wearing will generally be the last time you permit it to happen. Note thatthe optional reverse interlock alignment feature shown in Figure 1 can be added to all tool designsshown. The advantage is obvious, as is an understandable increase in cost. Note the simpleconstruction of the type 1 die, also called an inserted spool die (Figure 2). This is cheaper to makethan the type 6 die. The difference in these two designs is a matter of support. On the type 1 die, atleast two-thirds of the length of the removable grip should be supported from the bend die body. Atype 6 die can support the total insert length if necessary for additional ridgidity.

This said, if the overall diameter of the bend die itself gets smaller because of a decrease in centerlineradius, less of the insert is supported. Under extreme conditions the insert can actually break from thepressure exerted from the clamp die. In this situation, a type 6 die is the next option, as the insert isfully supported from the bend die body.

Because these type 1 and type 6 dies have a removable grip design, the same bend die body can beused for various insert lengths and finishes, including compound clamps to grip a previous bend whenno straight is available. In well-supplied tool cribs, a large inventory of, for example, 2.0-in.-OD bend

die bodies with different centerline radii dimensions are designed to accept all present and futurebend die insert lengths and finishes.

The initial cost per tool is more, but the cost saving from not having to buy multiple inserts of the samelength for every bend die body and interchangeability of the tooling may offset the higher initial cost.

Selecting a Bending System

The bender type you choose affects your bend die cost and design substantially. Most rotary drawmachines, with the exception of some stacking designs, are engineered to have a standard centerlineheight for the tool set.

The term centerline height is best defined as the dimension, measured vertically, from the machinesurface (or die boss) that the bend die mounts on to the center point of the tube groove.

This means several things in regard to tool design. First, centerline height establishes the overallheight of the tool itself. Also, the die often must be symmetrical about its centerline, which in turnmeans a larger piece of material for the die.

Most rotary draw benders operate either clockwise (CW) or counterclockwise (CCW) as they bend thetube. It is common for a bent part configuration to dictate the rotation required to form the tube withoutmachine/part interference (a collision of the tube with the bender). In extreme situations, both a CWand CCW bender may be required. In both cases, the tooling must be reversible and have machinemounting (counterbore and drive keyways) on the top and the bottom. Some specialty machines bendCW and CCW. Such machines are extremely sophisticated and priced accordingly.

Another way the centerline height affects die design and cost concerns rigidity of the bend die as it'smounted on the bender. A rule of thumb is that if the centerline radius of the bend die is the same asor less than the centerline height of the bend die, the bend die stability under load in the bend cyclewill be compromised.



denominator for the problem is completely elusive. Don't tear out what is left of your hair. You are notalone.

If you were asked to provide the wall thickness of a particular application, how would you respond?Would you:

1. Say 16 gauge?

2. Check the bend print and then reply 16 gauge?

3. Check what is marked on the tubing?4. Look at the purchase order for the tubing?

5. Find a set of calipers and measure it?

While tube OD and wall thickness variations are not new problems, they are far more common thanyou may realize. If the price of a certain material type fluctuates radically by the pound, it is likely thetubing may be within spec for wall thickness but be on the thin end of that spec so it's profitable for themill.

For some tube fabricating applications, this variation is of little concern, but in mandrel bending, IDdimension variation is a big issue. For this reason, you need to pay attention to what you order andhow you order it. Always confirm that what you received is what you ordered. If you need to stockmultiple mandrels that differ slightly in finish diameter to consistently meet your cosmeticrequirements, and if additional labor-hours are required to sort and segregate the material by IDdimensions, you might need to build more margin into your next quote, or risk running the job atmarginal or zero profit ... ouch!

Let's remove two words from our vocabulary: assume and standard. Take every possible step to beinformed. Pay attention to the incoming materials' specifications or pay the price from your bottomline.

avoiding common bending problems with common sense

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