I know that all of the draw bending applications in the tube metal fabrication shops don’t require a wiper, a mandrel, or a pressure-die assist feature.
But understanding them could help a metal fabricator troubleshoot the problems more quickly and accurately.
Four-Step Setup
To implement the 4 step setup properly, you need to understand each step that address just the one aspect of the metal bending quality.
Step One: Moving the Mandrel Forward
Mandrel is a key central tool on a draw bending setup. It’s the main mean of control over the flowing of tubing material in the bend point.
For determining the best placement for your mandrel nose, it‘s crucial that you know the difference between the bend point and the line of tangency.
There are several factors that play a role in identifying proper placement of mandrel.
Using the Pythagorean theorem, calculating the maximum depth is easy.
b = √{r+(d/2)–t}2–{r+(m/2)}2+n
Where, d is outside diameter of the tube, t is wall thickness, r is centerline bending radius, m is mandrel nose diameter, and n is mandrel nose radius.
Step Two: Lowering the Pressure-Die Setup
Pressure die’s work is to keep the tube in place. Whether it’s hydraulic and mandrel, the pressure dies will apply a radial force to the tube at the bending point. As the radial force is applied perpendicularly to the usual axis flow, the drag results.
Too much drag may result in terminal humps & excessive flattening. Thus, your main objective should be to eliminate the drag as much as possible. However, there’s no straightforward formula to determine the optimum settings for each and every shape & material. But a small experimentation would yield substantial benefit.
Step Three: Rake the Wiper Tip
Wiper’s work is to prevent any wrinkle at the end of any bend. If you don’t find any wrinkle at the terminal tail of the otherwise smooth surface inside radius, the wiper isn’t needed. In this case, you can skip this step.
However, you should remember that if the wiper is set properly, the tip will catch the top of the terminal bulge before setting it into a wrinkle.
First, you need to find the natural and ideal resting position of the wiper at 0 rake. Then you’ll need to calculate the maximum rake for the application.
Step Four: Setting the Pressure-Die assist Pressure
Like a wiper, the pressure die assist isn’t required at all times. Its job is to prevent the terminal humps & the excessive flattening on the outside radius.
If that outside radius is okay, there’s no need for the pressure-die assist. Turn the pressure-die assist off or you can set it to the neutral pressure, which actually depends on the machine. Otherwise, you’ll need to start from the neutral pressure and gradually increase the assist till the flaw disappears.
Tube Bending Troubleshooting
The important advantage of this 4-step setup approach is that it can facilitate troubleshooting. Most of the defects in tube bends can easily be traced to this setup.
But the troubleshooting procedure cries out for such procedure in laser cutting, metal welding, metal rolling, and any other stainless steel fabrication, aluminium fabrication or steel fabrication processes.
