Step #2:  Direct Pressure Die Setting

In this next step you will execute the "Low Pressure" principle. If you have set the mandrel nose sufficiently forward into the bend so that it does most of the work in controlling the shape of the tube, then the only work the pressure die needs to do is to apply enough pressure so that tube does not pull away from the bend die during the bend.  (If it does pull away, then the inside radius will buckle into the gap created.)  Intuition suggests that not much pressure is needed to do this job.

Whether mechanically or hydraulically, the pressure die applies a radial force upon the tube at the point of bend.  Because this radial force is perpendicular to the natural axial flow of material in the draw-bending process, drag results.

Too much drag and you will have terminal humps and excessive flattening of the outside radius.  Too much drag is usually the culprit when the tube slips from the clamp dies.  Thus your objective is to eliminate as much drag as possible by determining the lowest direct (radial) pressure needed to prevent buckling.  Generally the more rigid the cross-section of the tube, the more direct pressure is needed to hold it against the pressure die.

Most mild steel, stainless steel, aluminum, and copper round tube applications 3 inches and under in diameter will require only 400-1,200 pounds per square inch (PSI) of direct pressure to accomplish this.

Square and rectangular tubes (especially if bent the "hard-way") will require higher pressures.  So will copper-nickel, super-alloy, and some stainless tubes.  Unfortunately there is no simple formula for calculating the optimum setting because such factors as tube shape and size, wall thickness, centerline radius, degree of bend, and material rigidity must all be accounted for.  An additional problem is that the pressure reading from the machine may not reflect what is actually applied to the tube.

However, some time spent here on trial-and-error will yield benefits.  A rule of thumb can help to establish a good starting point.  Assuming a typical 3-inch capacity machine, set the direct pressure at 400-500 PSI* for the softest round tubular material such as soft aluminum, soft copper, and low-carbon hydroformable steel.  Increase the initial setting to 600-700 PSI* for high-carbon steels, stainless steels, hard aluminum, and hard copper.  For the most rigid mateials, such as titanium, nickel stainless, superalloys like Iconel, and non-round tubing of any material, an 900-1,000 PSI* setting is a good place to start.

If continuous wrinkling or a buckle forms on the inside radius, increase the direct pressure setting in increments of 50 to 100 PSI* until this problem disappears.  If there is no deformation on the inside radius, consider dropping the direct pressure.  Once you determine the minimum direct pressure your application requires, you can rely upon this value for future set-ups of the same application and as a guideline to other applications.

At the end of this step you will have combined the "Forward Mandrel" and "Low Pressure" concepts behind this set-up to produce an acceptable bend.  Some imperfections may still be present, such as a terminal inside radius wrinkle or terminal outside radius hump.  These can be fixed by the last two steps of the set-up procedure.