Special Rotary Draw Tooling Solutions

There are distinct differences from what would be considered sets of bender tooling and a tooling system. Simply defined a set of tools typically refers to the (generally five piece) family of dies to bend a tube for a given outside diameter and wall thickness, to a single specified centerline radius. A fabricating company may have dozens of tooling sets each specific to the job it is designed for, and the machine designated to perform the job. In the case of a bending job shop they may have hundreds of die sets.

More often than not in this scenario they have been added over time each for a certain application to meet their customer’s requirements. Many times there is little planning or time permitting for tooling to be supplied as compatible with the dies that are already in house for a given bender mount design, let alone for the dies to be interchangeable over various bending machine mounting standards.

Needless to say that over time this results in dies that are frequently grabbed in haste and used for whatever part needs to be bent and out the door yesterday. Further to this, the mind set develops to modify dies on the fly to suit a purpose they were not originally intended for, becomes common practice. It is an understandable situation happening far too often even in the most organized shop with conscientious well intended personnel.

Bottom line, if the product does not get out the door, on time and to meet the customer’s criteria the company fails to deliver, and in this case you do whatever it takes…. Many times when a company goes through the sustained growth that drives capital expenditure in the form of new bending equipment, new opportunities open for tooling considerations as well. The cost associated with new bending equipment typically dictates a major process of budget setting and multilevel approval followed by significant research into options involving everything from performance and support to service and payback. These factors are all given close scrutiny in the due diligence required to make the decision of investment as informed as possible. In many cases the decision for new equipment is driven by acquisition of new work and in these instances new bender tooling is part of this process.

While this generally means an additional big ticket item to the bottom line, it further makes it imperative that you cover all the bases with regard to what options are available. This situation is an excellent opportunity to consider bringing in a tooling system that can be built on later as well as meeting the immediate project needs. A given bending company more than likely over the years has developed a tooling strategy that they find works for their needs. This can be true whether they make the tooling in house, buy the dies as needed from a given tooling supplier or regularly go out for quote from multiple tooling sources as new projects require.

While each of these directions taken has pros and cons there are many important aspects to consider in this decision. For the most part in house made tooling is advantageous in that the schedule of the tooling design and manufacture is controlled. Most companies that go this route use this fact as the primary reason for the decision and unfortunately the design or quality of the tooling can suffer in the process. The company in question is generally speaking in the fabrication business not necessarily in the tool and die business.

Over the years I have worked with many large companies that have spent an incredible amount of time, money and manpower to develop what they consider to be proprietary die designs specifically to address in house die compatibility issues. While some of these designs were fairly straightforward, others have been so overdesigned for versatility that they are unwieldy, fragile and problematic especially in high production situations.

What was started as an integration strategy to stay in control of the processes can easily become a nightmare. The reality is that when problems develop, parts fail to ship or the quality of them regularly fails to meet the needs, even a qualified and capable outside source has its hands tied to come in and offer more than a quick band aid on the fly.

There is also a more insidious aspect that can develop with this inside integration thinking. Many times the company in question can get so caught up in how proprietary and wonderful the design may be (time, money, manpower…remember), that they concentrate so much about keeping their cards to vest that they fail to look to outside sources and notice new technology that could and should be integrated into their processes. Some of the companies I have seen get to this point over the years, are no longer in existence.

Having the die work done outside by a company specializing in the design and manufacture of the tooling to suit the specific machine and the application, ensures that the investment is well placed. While it is always important that tooling cost be considered it should not necessarily be the final determining factor.

The most imperative consideration to tooling begins with understanding the bending process, and the various design options of the tooling in concert with the application needing to be fabricated. Many times these design options will fluctuate the cost radically, whether or not those options are justified or even need to be considered can save or cost you in the long run. As ultimately you may invest as much into the tooling on the project as the equipment, you must develop and maintain a mindset of understanding all of the processes involved. Partnering with a supplier to work with you from the initial application analysis, through installation, set up training and on-site support is a major leg up.

A common problem encountered is that an existing library of tooling is already on hand and a decision will need to be made as to building on the existing die work or to depart from it completely with a new design. The selection of the new machine will have a major impact on this decision, but the first consideration should by all rights be the application itself. If the new project that is to be manufactured is completely different than the current product being made, the decision is an easy and obvious one (new product, new machine, new tooling). Many times however in situations where the new product is similar or even in cases where the new acquisition is to increase the productivity of the same part, this decision still bears more thought than you might think. It should be noted that most bending machine manufacturers have a specific mounting pattern that drives the die design also.

While some of them may be receptive to building a new piece of equipment with a different than their standard mount pattern (to for instance accommodate your current die library), others may not. This fact should not be a deciding factor on selection of the machine manufacturer per se, any more that the decision of die design be based on the bender itself. Once again the first consideration to select the process, the equipment and the tooling design, is the subject application.

Potential tooling incompatibility from the existing dies to additional ones needed for the new project can be more than exasperating, it can cripple the project before it gets off the ground. Though there are several different methods of tube bending that are used commonly, we will be discussing specifically tooling design using the rotary draw (mandrel) bending method.

Rotary draw bending is by nature more involved and complicated but subsequently the most versatile. It is the only method that is suitable to produce high quality wrinkle free bends in tight radius thin wall tubes. It is by far the most used for any application where support is needed to control the stretching and compression of material and simultaneously prevent the tube from collapsing.

Bending Tool

Typical Rotary Draw Bending Set

Typical Rotary Draw Bending Set

The set consists of:

  • The Bend Die…Rotates with the tube forming it to the correct radius.
  • The Clamp Die…Grips the tube against the bend die to prevent slipping.
  • The Pressure Die…Moves forward with the tube forcing it to conform to the radius of the bend die.
  • The Mandrel…Internal to the tube supports the tube at tangent prevents collapse of the tube.
  • The Wiper Die…Rides between the tube and the bend die controls the compression side on the bend.

In the process of bending the tube we must control its natural reaction to the process of inner wall compression and outer wall thin out. In a very simplistic explanation the mandrel being inside the tube supports it from collapsing in the bend process while the wiper die prevents the inner wall compression from bunching up and forming wrinkles. In any real world scenario however it is far from that easy.

The first step in the process is t0 ascertain the tooling requirement for the subject application. Before this however determining the basic feasibility of the bends required together with the tube material. Simply put, will the material be able to form at the centerline radius needed based on its elasticity or elongation percentage. Over the years what were once solid limitations as to how tight a certain material could effectively be formed, with the advent of today’s cutting edge equipment and tooling, these rules have grown more generous.

Regardless of this fact the limitations are real and failure to research this aspect of the project at the onset of it is a drastic mistake. As we consider the natural reaction of the tube to the bending process is outer wall thin out and inner wall compression, the tooling must provide the support to the tube to control this. This fact means that the tooling in the case of the mandrel and the wiper die are in a fixed position while the tube is drawn across and over them. This in turn crates drag to the tube compounding the prevalence of outer wall thin out.

In the case of materials with low elongation, the yield point is reached and the tube fractures. The balance point then becomes how much support to offer the tube so as not to go so far as to induce enough drag to fracture the tube. Even with material types with low elongation there are strategies that can be effectively employed to achieve success. The bender selected for the project will need to have all the effective options possible for this. Some of these bender options would include, though not limited to: -Incremental pressure die assist, both in speed and pressure -Anticipated mandrel extraction -Carriage/ collet boost -Automatic mandrel/ wiper die lubrication -Overhead tie bar system from multi points to the bend die spindle and the wiper die bracket All of these options work together with the proper tooling to control the tube.

The obvious goal is to effectively form the tube without fracturing and with the least deformation possible. The discussion of these bender options and their implementation fall outside of the scope of this paper, but suffice it to say are an integral part in the success of any bending project. Let us assume then that the homework has been done and the machine selected for the project is equipped as outlined and the best platform possible to get the job done. We then move to the process of dialing in the tooling itself based on production of the application at hand.

To begin it is best to gage the severity of the application. This will determine whether a wiper die will be needed and will further allow the decision of how many balls will be required on the mandrel to support the tube.

Again, we must provide the amount of support needed without crossing the fine line of too much drag induced in the process. We will then calculate the Wall Factor of the tube. O.D. divided by tube wall thickness equals….Wall Factor. (the higher the wall factor the more severe the subject bend is). This must then be further considered with the D of Bend. The bend centerline radius divided by the tube O.D. equals…The D of Bend. (the lower the d of bend the more severe the subject bend is). Once again these aspects must be then weighed with the tube material elongation percentage as well.

The lower the elongation percentage the less support/ drag the tube will withstand, the result of which is tube fracture. The benefit of providing an interchangeable tooling system is immediate to the project at hand but also provides a game changing platform to build on as needs change. The implementation of tooling that can readily interchange pieces of a die set, be repositioned easily in a stack of die sets to accommodate change overs is an incredible time saver. While in some cases it will mean more individual components the setup and die change over time reduction alone is more than worth it.

There are other further benefits as well. Let’s draw a scenario for the subject project at hand. Our goal will be to produce perhaps forty different part configurations in three different tube sizes. Let’s further assume that each of the different tube sizes will have parts required that have for example three different centerline radius bends. While not all of the bent parts in the project have more than one bend in them, in our project we will be bending perhaps 70% of the parts with three or more bends in each. It is entirely possible that some of the subject parts may have bends of different radius dimensions in the same piece of material. This will by necessity be stacking multiple sets of dies on the bender.

The CNC machine will then position the tube automatically to the correct die set in the stack for each bend moving it forward and rotating it between the bends based on the XYZ coordinate data programed in for each part. There will be certain parts in the project simple enough to require a straight forward single die set, and in those cases the machine will go through the same motions as noted but with no need to transfer up to a different level in the stack. Here is a visual example of a tube that will represent parts typical to our hypothetical project.

It stands to reason that if we look simply at one in this scenario that we will require a different bend die for each bend radius needed. Each of these bend dies might need multiple gripping lengths dependent upon the part configuration needed. The system we will build will then have the ability for each bend insert to interchange into each bend die in a common . Every bend die insert will have the same bolt hole pattern allowing for a minimal number of hand tools and time to change to a different length or surface finish. All the bend dies will be exactly the same height with the machine mount provided on both the top and bottom so that they will all sit on the machine the same (or in any position in the die stack) regardless of or of centerline radius.

It will be possible to preset certain die stacks if needed to load on the machine as one unit to further facilitate quick die changeover for part changes in production. All the mandrels will be made where possible with the same length of shank and thread size. And all wiper dies with a uniform mounting pattern. Die sets or stacks could be pre-assembled based on tube part number prior to the machine operators shift based on the daily production requirement for the job. The benefit for the long term building on this foundation is substantial as well.

In an all too familiar scenario of part dimensional changes in production and additional parts thrown into the mix, the interchangeability of the die system is obvious. Now assume that your customer (or a new customer entirely needing an emergency prototype etc.) throws in a part of the same but a different radius. The additional dies needed by your tool supplier will simply (and quickly) be a new bend die body and a wiper die rather than an entire new set. The ability to have this versatility work for you is an immediate return on investment. The long term benefit can be the start of an entire new direction of thinking for production problems far too familiar to many.

The Building Blocks of the System

The Building Blocks of the System

The Bend Die

The bend die is the backbone of the tooling system we will develop and build upon. For obvious reasons the type 6 die shown on the previous page is in most cases the best to consider for the versatility of the removable grip design. This is also a beneficial design for tooling strength as the grip insert bolted in is fully supported by the body of the bend die itself, rather than in the design of the type 1 die (inserted spool) where the grip is left unsupported for a portion of its length.

Either of these bend dies offer greater versatility in that the grip area can be exchanged with one of a different length or even a different surface finish. The grip can be made with directional serrations to aggressively grip the tube even on the shortest gripping length. These serrations can be made with different peak spacing and height to make them finer and thus minimize the amount of surface marking to the tube. An alternative tube groove finish that can provide even the shortest grip with the ability to firmly hold the tube is a tapered knurl finish.

While this has the impression and appearance somewhat to a typical knurling process. The ‘knurl” is machined into the surface of the die. While this will still mark the tube surface in the bending process, the marks are less noticeable and it is possible through a secondary operation to reduce them to even less of an issue. As we discussed, the bends get more severe as the tube OD increases, the bend radius decreases and the wall thickness of the tube gets thinner, all of these aspects draw on the decision of the bend die design. The grip length and surface finish being simply one of them.

The break point on when to change from a relatively simple design of the type 1 to the type 6 is based on the amount of grip insert that will be unsupported. As the radius of the type 1 die gets tighter the amount of material that will fully support the grip area is reduced. If that reduction means more than a third of the length of the grip insert has no backing the grip can in extreme situations weaken, deflect or even break.

Type One Bend Die

More often than not we would move to a type 6 design and be done with it. As we are building a system to ensure optimal compatibility we are going a slightly different route. It has been found that using the attributes of both these dies in a hybrid design can bring about the most versatile and strongest platform This will be the first design point we will build on. First however we need to get some other basics considered. There are several schools of thought regarding Interlocking dies that should be factored in as well at this point.

Type One Bend Die

Reverse Interlocking

Reverse Interlocking

Non Interlocking

The most obvious advantage of interlocking tooling is that it self-aligns to a degree but most importantly the alignment of the tooling to itself (once the hangers are adjusted and locked down), is consistent set up to set up. These hangers for the clamp and pressure dies respectively once set should not be removed from the tooling, ensuring that every time they are put into the tooling set the alignment is there.

This makes any adjustment or tuning of the tool set changing from one to the next minimal if needed at all. The interlock “shoulders” if you will, are also very beneficial to the die rigidity on the machine. This is due to the fact that the interlock will make the overall diameter of the bend die larger. This can be very important on setups of dies where we will be stacking as many as four or five high on the machine.

Non Interlocking

Whether or not to utilize interlocking in your tool system “matrix” does to a point depend on the company’s machine operator preference. It was typical especially in old school bending houses that as the interlocking die design can obscure the machine operators line of sight to the point of tangent where the bend is taking place and the relation of one tool to another in the setup, that some preferred non interlocked dies.

It should be noted that in most of these cases that these were very low volume high degree of difficulty applications where absolutely every single bend had to count and high production non-existent. Incorporating interlock design in today’s fast paced production environment where quick accurate tool changes must go without a problem and minimize downtime is a must to consider.

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