TPT November 2008

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November 2008 124 ›
Bending ‘off-fall’: minimizing the
necessary evil of tube forming
By Mr Lonnie McGrew, vice president of engineering, AddisonMckee, USA
Introduction
As global steel prices continue to soar, it is essential to develop
solutions that are purpose-designed to expedite manufacturing time
and minimize tube wastage. This article will investigate the methods
of negating bending ‘off-fall’, a complex adversary of tube bending.
‘Off-fall’ is more commonly known as remnants of unusable material,
or planned material scrap or waste.
In today’s cost-conscious times, anyone involved in the tube
bending process may instinctively consider perishables, such as
wiper dies, mandrels and lubricants, to be their greatest enemies.
However, investigation of the scrap bin sitting beside a tube trim
machine in any tube forming facility will quickly establish the real
culprit: namely bending ‘off-fall’.
For example, most vehicle exhaust systems are produced from
stainless steel, which has increased in price per inch by over 30
per cent compared to twelve months ago. It is, therefore, relatively
straightforward to see how even one inch of extra material wasted
per unit can equate to a substantial cost increase per year.
Although bending ‘off-fall’ is unavoidable, there are ways to
minimize the amount required for bending and thus initiate cost
savings. Here, we investigate theoretical tube calculations used by
process engineers and provide some efficiency improvements for
minimizing material waste.
Making theoretical tube calculations
There are three main components that must be considered when
calculating the length of material needed to produce a bent part
(see figure 1). These components are:
• Clamping stock – the extra material needed to sufficiently grip
and bend the first bend of the component.
• Component(s) length – the developed length along the
centerline of the component, including material needed for
subsequent end-forming operations and, if there are multiple
components being bent in one tube, material required for parting.
• Collet stock – the extra material needed to sufficiently grip the
tube in order to rotate and position for bending.
Examining sample figures
It is possible to examine sample figures that are based upon tube
centerline data of XYZ, YBC.
60.3 diameter x 1.75mm wall, 409SS
X Y Z CLR Y B C
150.70 67.60 -7.45
130.40 44.50 0.00 63.50 23.97 13.78
94.10 0.00 0.00 63.50 19.61 -98.62 50.79
15.50 0.00 0.00 48.45 0.00
Determining clamping stock
When determining clamping stock, several factors must first be
considered:
• Will the bender have boost capabilities?
• Does the product require a square end?
• Cosmetic details (grip type – saw tooth, carbide spray, etc)
• Will the extra clamping stock be removed with a saw or stab-cut
type operation?
Where a bender is equipped with boost and it is intended to use the
minimum grip length of 1D, the end of the tube will be out of square.
A good rule of thumb, therefore, is the first straight must be at least
1.5D before the end of the tube is not pulled out of square by the
bending process. If boost is not used, the first straight should be at
least 2D.
Looking at the sample data, it is possible to observe that the first
straight is 23.97mm. This is a 0.40D straight (23.97/60.3). Assuming
the bender used does not have boost capabilities, enough extra
stock will need to be added to increase the first straight to 2D.
Clamping stock formula
• OD = Tube outside diameter
• SLx = Straight length (where x is the first straight)
Clamping stock = (OD * 2) – SLx (without boost)
Clamping stock = (OD * 1.5) – SLx (with boost)
Using the sample data:
Clamping stock = (60.3 * 2) – 23.97 (without boost)
Clamping stock = 96.63mm (extra stock added to existing first
straight)
Components and end-forming stock
Clamping stock Collet stock
 Figure 1: Multi-component bend stick