BEND TOOLING INC.:  Rotary-Draw Tube-Bending Tools ~ Die Sets ~ Mandrels ~ Wipers ~ Mandrel-Bending Tools

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Step #1:  Mandrel Nose Placement


The mandrel is the central tool in the draw-bending set-up.  It is your primary means of controlling the flow of tubing material at the point of bend.  To understand the best placement of the mandrel nose it is useful to understand the difference between the line of tangency and the point of bend.


The line of tangency is a theoretical plane fixed in space.  It is perpendicular to the plane of bend and divides the bend arc from the tangent.  On the other hand, the point of bend is an indefinite region surrounding the line of tangency in which the tubing material is in a plastic state [Figure #1].


In this region the tube wall will thicken along the inside radius (intrados) and will thin along the outside radius (extrados) as the clamps draw the tube around the bend die.  The purpose of the mandrel nose is to cover this region of flowing material and ensure a consistently round cross-section by mitigating the simultaneous compression and stretch of the tube wall.


Because the point of bend extends past the line of tangency, so too must the mandrel nose to support it [Figure #2].  If it does not, then the compression and stretching of the tube wall is uncontrolled resulting in buckling and excessive flattening.





While more direct pressure die pressure can often remedy the buckling along the inside radius that will also exacerbate the flattening of the outside radius because of the additional drag it causes on the outside radius.  Therefore, the mandrel nose placement past the line of tangency is critical, because it then can both eliminate buckling and minimize flattening.


Commonsense informs us that there is a limit to how deep the mandrel nose can be placed past the line of tangency into the bend.  At some point the outside line of the mandrel nose will intersect the outside radius of the tube bend and force the tubing material to form over the contour of the nose and perhaps the balls.  What intuition does not tell us is that this point is relatively deep into the bend and that, in most instances, tube-bending machine operators are not using the mandrel nose aggressively enough.


Several factors come into play in calculating how deep past the line of tangency the mandrel nose can be placed:

  • Tube diameter (d)

  • Wall thickness (t)

  • Centerline radius (r)

  • Mandrel nose diameter (m)

  • Mandrel nose radius (n)

Using the Pythagorean Theorem you can calculate this maximum depth (b):




b = SQRT{ [r + (d/2) - t ]2 - [ r + (m/2)]2 } + n     [FORMULA DETAILS]


For instance, for a 2-inch tube diameter x .049-inch wall thickness x 4-inch centerline radius application with a standard diameter mandrel, the maximum depth would more than 5/8 of an inch.  Usually, a placement somewhere between one-half and two-thirds of the maximum is best.  The less rigid the material the deeper the placement must be.


This executes the "Forward Mandrel" principle of this set-up procedure.  At the end of this first step, you should have a bend that is maintaining a consistent cross-section throughout most of the arc.  The most serious problem you might have is buckling throughout the inside radius.  If your mandrel nose is stable at the proper depth past the line of tangency and its diameter is not undersized for the application, then go onto the next step to determine the correct direct pressure die setting.  It is important to ensure that your mandrel nose is not undersized for the bend you want to make.  There is a practical limit to how small a mandrel nose diameter can be and still be effective.


A properly sized mandrel nose is critical to mitigating flattening on the outside radius of the bend and buckling on the inside radius.  The formula for the standard nose diameter (m) is:


m = d - ( t x 2.21 )     [FORMULA DETAILS]


If ease of loading is an important consideration, then the standard diameter (m) can be reduced by .005" for every inch of tube diameter (d).  But if your mandrel nose diameter is substantially smaller than this, deeper placement of it past the line of tangency will not sufficiently compensate for its undersized condition.  As noted above expect problems with buckling and flattening as a result.


If you have other problems such as terminal wrinkles on the inside radius or a terminal hump on the outside radius, continue on.  These problems will be fixed later in the set-up.  next page

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