TPT November 2008

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125 November 2008 ›
Efficiency improvements: strategy 1
In this example, 120.6mm of total grip stock is a safe starting
condition. Once the tube is developed, it will be necessary to
shorten the actual grip length by 5mm and re-run the trial.
If there is no slippage, shortening by another 5mm can be
implemented prior to re-run. This process can then be repeated
until slippage occurs, at which point 5mm to 10mm should be
added back to the tube.
Shortening the tube for bending trials
When shortening the tube for bending trials, the tube need not
be physically cut in order to shorten it relative to the first straight.
Instead, it is possible to simply ‘tell’ the bender that the tube is
longer than it really is and the bender will adjust the tube back,
effectively shorting the first straight.
Once the final length is determined and the tube is physically cut to
length, it is then necessary to ‘tell’ the bender the actual length.
Determining component(s) length
To determine the length of stock required for the components,
the distance along the centerline path will need to be calculated.
In addition to the centerline length, any additional material for
subsequent end-forming operations will need to be added. In the
case of multiple components, this includes material required for
splitting or parting operations.
Minimum requirements for trimming
There are also minimum requirements for trimming. If there is a plan
to saw, a minimum of 3mm + the saw blade width will be required.
On the other hand, a stab-cut process will need about 15mm + the
blade width.
• SLx = Straight length (x is the straight number)
• ALy = Length along the arc (y is the bend number)
• NC = Number of components in tube
• CW = Cut width
• EF = Material length required for subsequent end-forming
Arc length formula:
ALy = (Π * CLR / 180 * degree of bend)
Component length formula:
[(SL1 + AL1 + SL2 + AL2 + SL3… + EF) * NC] + [CW * (NC-1)]
Using the sample data:
(Assuming 3mm for end-forming and 4.75mm cut width)
[(23.97 + 15.27 + 19.61 + 56.29 + 48.45 + 3) * 3] + [4.75 * (3-1)] =
509.27mm
Component length = 509.27mm
Determining collet stock length
In order to determine collet stock length, several tooling components
must be evaluated: pressure die, wiper die, and collet.
Determining pressure die length
Pressure die calculations are straightforward and are based on the
deepest bend. The pressure die is typically set to travel at 1:1 ratio
with the perimeter of the bend die. The calculation can be simplified
as follows:
Pressure die length formula:
Π * CLR / 180 * deepest bend
If this were the pressure die’s final length, it would leave a deep
impression at the end of the bend. Therefore, it is necessary to add
an additional amount to ‘support’ the tube at the end of the bend.
This is typically 2D.
Pressure die support adder formula:
2 * tube diameter
Using the sample data:
Pressure die length = (Π * CLR / 180 * deepest bend) + (2 * tube
diameter)
Pressure die length = (3.14 * 63.5 / 180 * 50.79) + (2 * 60.3) =
176.89
Pressure die length = 176mm
Efficiency improvements: strategy 2
If the tube in the collet is gripped through the last bend, the end
of the pressure die will normally be the first point of interference
the collet housing encounters (see figure 2). This is especially
true if the ‘deepest bend’ is not the last bend. If the pressure die
is shortened, then the tube length may likewise be shortened. As
stated above, 2D support is normal but in some cases can be
reduced to 1.5D.
Determining wiper die length
Because many tooling manufacturers are producing wipers in high
volumes, they have standardized lengths. Typically, insertable tip
lengths are:
• 100mm for tube diameters < 76mm
• 127mm for 76 tube diameter
• 1.5D for tubes larger than 76mm
• Minimum length for specials should be 1.5D
Using the sample data:
Wiper die length = 100mm
 Figure 2
Interference point