TPT September 2011

A rticle

• LL = Limiter length (Distance from tangent at which the collet housing will encounter interference) Arc length formula: AL y = (Π * CLR / 180 * DEGREE OF LAST BEND) Pressure die length remaining formula: PL r = PL t - AL y Limiter Length Formula: LL = WL or PL r (whichever is greater) Collet stock length formula: LL + CF + CD - SL x Using the sample data: • SLx = 48.45mm • CD = 30.15mm • ALy = (Π * CLR / 180 * 50.79) = 56.29 • PLr = 176 - 56.29 = 119.71 • LL = 119.71 (PL r is greater than WL) Collet Stock = LL + CF + CD – SL x Collet Stock = 119.71 + 5 + 30.15 – 48.45 Collet Stock with Boost = 106.41mm Note: If the calculated collet stock is a negative number then no additional collet stock material is required. Money-saving strategy 4 For part configurations, where the deepest bend is not the last bend, the pressure die is typically extended past the wiper as the bend cycle completes (see Figure 4). When this situation occurs, a ‘split pressure die’ should be considered. The two halves will act as one die until the last bend. Before the last bend is positioned, the rear pressure is lifted out of the way, allowing the collet housing to approach further towards tangent (see Figure 5). • WL =100mm • PLt = 176mm

Figure 3 Determining collet depth When determining collet depth stock, several factors must be considered: • Rotational torque from tube • 0.5D minimum • Bender manufacturer’s standards Again, the idea is to reduce the amount of tube needed to sufficiently hold the tube during the bending process. For the sample calculations, we will use the 0.5D minimum. Collet Depth Formula: TUBE OD * 0.5 Using the sample data: Collet Depth = 60.3 * 0.5 = 30.15 Collet Depth = 30.15mm Collet stock length Once all the tooling lengths are determined, it is possible to calculate the additional material need for the collet end. At this point it is necessary to decide whether or not boost will be used. With Boost (collet will hold tube during the entire bending cycle): • PL t = Pressure die length (where t is the total length) • PLr = Pressure die length (where r is the remaining length from tangent after the last bend is made) (see Figure 4) • WL = Wiper die length • CD = Collet depth • SLx = Straight length (where x is the last straight) • CF = Clearance factor (5mm) • ALy = Length along the arc (where y is the last bend)

Figure 5 Without boost (collet will release the tube during the last bend): When bending without boost, the pressure die length is not a consideration. As seen in Figure 6, the pressure die can be ‘backed’ out of the way while the tube is positioned for the last bend. As the tube is positioned, the wiper die becomes the only point of interference. Once the tube is in position and clamped, the collet is retracted and the pressure die is advanced.

Figure 4

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S eptember 2011

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