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Tube Bending, Mandrel Bending & More

This is some great information from http://www.bendtooling.com/faq's.htm. 

 
Do I need a mandrel?
 
It depends.  The rotary-draw method of tube-bending is characterized by the use of a mandrel, because it is a process that fixes the line of tangency in space and therefore makes it practical to fixture a mandrel there to control the flow of material at the point of bend.  (See the Tube Bending Encyclopedia for more detail.)  However, not all tube-bending jobs need a mandrel.  One is required only when the tubing material lacks the strength to support itself at the point of bend.  That is why thick-walled tubes bent on large radiuses generally do not need a mandrel.
 
The factors that determine whether a tube is strong enough to be bent without a mandrel are tube diameter, wall thickness, centerline radius, degree of bend, and tubing material.  With this information we can accurately determine whether or not you need a mandrel for your job and if so, how balls the mandrel must have.  There are also a couple of rules of thumb you can use to get a quick take on whether a mandrel is likely.  The first is to calculate the wall factor.  Divide the tube diameter by the wall thickness.  If the result is 15 or more, a mandrel will probably be needed.  The second is to calculate the "D" of bend.  Divide the centerline radius by the tube diameter.  If the result is 1.5 or less, a mandrel is almost always needed.
 
It should also be noted that some tube benders will dispense with a mandrel if bend quality is not an important consideration.  In those cases, heart-shaped dies are often used to flatten the tube in the plane of bend to give it additional resistance to the wrinkling on the inside radius and the flattening on the outside radius that would normally occur without a mandrel.  However, compromises in quality, process control, and production rates have greatly limited this substitute for mandrel bending in recent years.
 
Do I need a wiper?
 
 Like the mandrel, it depends.  The primary purpose of the wiper to prevent the hump that forms at the end of the bend during the rotary-draw process from setting into a permanent wrinkle.  (A secondary purpose is to function as backstop opposite the pressure die in a high-pressure application.)  If the tubing material is strong enough, the hump will not set upon completion of the bend. 
 
The same factors that determine the need for a mandrel also apply to the wiper:  Tube diameter, wall thickness, centerline radius, degree of bend, and tubing material.  However, there is a marginal range of bends that require a mandrel but not a wiper.  Generally speaking if the wall factor is 20 or more and if the "D" of bend is 3 or less, a wiper will be needed.
 
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.
 
 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.
 
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.

 

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