TPT November 2008

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 94.10 0.00 0.00 15.50 0.00 0.00

23.97

13.78

63.50 63.50

19.61 -98.62 50.79

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)

 Figure 1 : Multi-component bend stick

Components and end-forming stock

Clamping stock

Collet stock

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N ovember 2008

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