BEND TOOLING INC.: Rotary-Draw Tube-Bending Tools ~ Die Sets ~ Mandrels ~ Wipers ~ Mandrel-Bending Tools
TECHNICAL FAQ'S
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ANSWERS TO TECHNICAL FAQ'S
I need information about tube bending. Where do I go?
For a good start we recommend our Technical Information section. You will find there a unique resource in our industry, the Bend Tooling Encyclopedia, the most comprehensive glossary of terminology available for tube bending - by far. You will also find in that section sets of formulas for tube bending, tube bending tools, and manufacturing in general. You can also phone or e-mail us with your engineering and troubleshooting inquiries. Click here for contact names and information.
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I do high-quality tube-bending. Don't I need traditional solid-body mandrels and wipers?
Probably not.
Inserted mandrels and wipers perform as well as
traditional solid-body
tools in almost all rotary-draw tube-bending applications except those requiring
high direct pressure and zero-rake of the wiper. In those applications
genuinely requiring high direct pressure (as opposed to those set-ups in which a
high direct pressure setting is compensating for misplacement of the mandrel
nose), complete containment of the tubing material at the point of bend is a
must. Therefore, the mandrel and wiper surfaces cannot have any breaks,
such as that inherent in the design of inserted mandrels and wipers.
Only the continuous surfaces and sheer mass of solid-body mandrels and wipers
provide the critical support at the point of bend to ensure the undisrupted flow
of material as it is plasticized under high pressure. For this reason we
presently recommend purchasing our line of solid-body tools for high pressure
applications. For the other 95% of tube-bending jobs out there, our line
of inserted mandrels and wipers are top-notch performers.
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I use split style links for my mandrels. What are the advantages of Bend Tooling's poppet style links?
We have manufactured the
single-piece poppet link for twenty years. It has
been thoroughly field-tested under the toughest production conditions.
There is no question as to its superiority: The biggest advantage is that
poppet links
last longer than
split links.
The simple reason for that is the neck of a poppet link is machined from a single piece of steel, thus preserving its strength and integrity, whereas the neck of a split link -- as its name suggests -- is divided down the middle into two pieces. So by design the split link is weaker. Competitors who sell the split link try to offset this inherent weakness by making the links out of tool steel. However, a split link of hardened tool steel is brittle, and still not as tough as an alloy steel poppet link.
The integrity of a split link is further compromised, because both sides of its head are hollowed out by a keyway to accommodate a third component: The key that keeps the two halves of the split link aligned. In addition to creating more thin cross-sections susceptible to failure, the need for a key adds another small part to inventory to already small mandrel component. Furthermore, because the head of the link is keyed, the size of the detent system (which brings the mandrel balls back into rigid alignment when retracted from the bend) is severely limited. This is not the case for the single-piece poppet link, which can accommodate a heavier and more durable detent system that improves the performance of the link.
Finally, there is the issue of cost. A poppet link is quicker and less costly to manufacture because of its deceptively simple design. So you get the best of both worlds: A less expensive link that lasts longer.
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What do I need to know about the feathered edge of a wiper die?
Three
things: [1] The material of the wiper, [2] the geometry of the feathered
edge, and [3] whether or not the feathered edge is completely machined or partly
finished by hand. This last consideration is often overlooked, but a
completely machined feathered is critical for optimal performance of the wiper
die. Always insist upon a fully machined feathered edge for all
your wiper tooling, especially
wiper tip inserts.
Click here for a detailed technical article on this topic.
I need my bending dies to last. Why wouldn't I want through-hardened tool steel dies?
For the same reason you wouldn't want a glass hammer.
Bending dies need
to be tough, not necessarily rock hard. In fact, to the extent that
increased hardness increases the brittleness of a die, its life is reduced as it
succumbs to breakage as opposed to wear. Keep in mind that in the
rotary-draw bending process, you are wrapping a soft thin-walled material around
a large solid mass of steel -- i.e., the bend die. A bend die with an
unhardened core is not going to give because a tube with a wall measuring only
in the hundredths of an inch (a millimeter or less) is not exerting a
significant compressive force upon it. All that results by over-hardening
a die, especially one made of tool steel, is to make it more susceptible to breakage.
Case-hardening, such as nitriding or carburizing, is a good alternative. A high-quality case-hardened die provides you with a very hard surface over an alloy steel surface. This type of die is tougher. It's more forgiving of the sharp blows and impacts to rotary-draw tube-bending while providing a long-lasting working surface -- and usually at significantly less cost compared to a harder but more fragile through-hardened tool steel die. That said, there do remain occasions in which through-hardening is necessary for die. Most often if the die has a thin cross-section that makes case-hardening, especially carburization, impractical. Even then, you should consider a high-quality alloy steel instead of tool steel to improve the toughness of such a die.
For more information about material specification of tube-bending tooling, click here.
Each rotary-draw tooling set-up is unique. So isn't experience a better guide than the 4-Step Set-up procedure?
It's not an either-or situation. Experience is valuable.
Although tube-bending is not a black art, there remain many factors that are not
fully defined. Experience helps to fill the gap. The 4-Step Set-Up
procedure that we detail in our website (click here)
works hand in glove with experience by establishing a systematic means for a
machine operator of identifying values for set-up parameters and gauging which
changes in those parameters produce different results - in particular, those
bend specifications that are out of tolerance. Thus, inputs can be
objectively related to outputs and the foundation for process control in
tube-bending is laid.
In this way experience is leveraged by translating it into a set of values that inexperienced machine operators keep adjusting each element of the 4-Step Set-Up towards to maintain bend quality in all but the most difficult tube-bending jobs.
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How do I set up my mandrel?
The key to the mandrel set-up is its nose. It needs to have the right diameter and nose radius, and it needs to be positioned properly relative to the line of tangency. We recommend this formula for sizing the diameter of the nose for most applications: TOD - (WT x 2.21). (Laminated and extremely thick-walled tubing require deviations from this formula.) As for the nose radius, we will generally recommend a sharp one for thin-walled or high-pressure applications and a more generous one for most other jobs. Finally, the best results in both terms of bend quality and tool life are achieved by aggressive use of the mandrel nose. This means placing it into the bend past the line of tangency. In this way the body of the nose controls of the flow of the tubing material as it becomes plasticized at the point of bend. Click here for detailed instructions on this.
How do I set up my wiper die?
The key to the wiper set-up is its rake. Rake is the angle at which the cavity of the wiper is offset from the inside line of the tube. Normally the rake should be zero for high-pressure bending jobs, and you should ask that your wiper die be cut with an offset-geometry feathered edge (or "aero-cut"). Otherwise, raking the wiper die is advisable to extend its life. Therefore, if you are going to rake your wiper, ask that it be cut with a simple-sweep feathered edge (or "standard cut"). Click here for a technical article on wiper rake.
How do I troubleshoot a tube-bending problem?
First check out our on-line troubleshooting guide. (Click here.) If you need further help, e-mail either Rod Malone (rmalone@bendtooling.com) or Steve Parrow (sparrow@bendtooling.com) with all of the relevant information about your problem. Usually that will include your tube specifications, your bend specifications, the make and model of your machine, and brief description of the problem you are experiencing. Otherwise, you can always phone us a 1-800-284-8823 (toll-free in the U.S. & Canada) or 616-454-9965 (worldwide).
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TO:
Bend Tooling Inc. ~ 1009 Ottawa Avenue, N.W. ~ Grand Rapids, Michigan 49503,
U.S.A.
TOLL-FREE (U.S.A. & Canada): 1-800-BTI-TUBE (1-800-284-8823) ~ WORLDWIDE: 616-454-9965
FAX: 616-454-9958 ~ E-MAIL: info@bendtooling.com
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