A Progressive Movement In Tube Forming
A Progressive Idea
A progressive die set used in a stamping process is common. It involves a spool of bulk material being fed into a series of dies that quite often transform the flat raw material into a useful component. The idea of a series of dies to form a part has been around for a long time.
The concept of forming a tube via a progressive set of dies may be more common then you think. Take for example the process of forming a tube itself from a flat strip; see Figure 1. In this case, the flat strip is transported thru a series of dies all aimed at moving the strip towards a circular cross section. Without the incremental or progressive approach to changing the flat strip, a round cross section would be next to impossible to achieve. Thus a progressive approach is critical to forming the metal consistently.
Nevertheless, the progressive concept itself is used in other areas of fabrication including tube end forming and tube bending. For example, forming the end of a tube or even bending a tube sometimes is accomplished thru a series of operations.
One Hit or Two?
When it comes to end forming, manufactures would prefer to make an end form in one hit…if at all possible. However, sometimes trying to form the end of a tube in just one hit can cause problems. Thus, trying to do too much with one tool can lead to a Cp and Cpk that is undesirable. Take the case of forming a simple cone; see Figure 2.
The end form itself looks simple enough. However, if the required angle is too severe, inconsistent results can occur from trying to produce this in one hit. In the case of the cone, a single hit process may cause too much axial force, causing a bead like feature to protrude behind the intended cone; see Figure 2.
This is where a progressive process (two hit process) can be useful. Taking this in two hits where the first hit approaches the desired form will allow the metal to cold flow in a more controlled manor thus preventing any unwanted axial forces.
Sometimes you may be on the fence between 1 or 2 hits and with extensive computer analysis you should be able to figure out the most efficient path for production needs. However, the proof most always comes with building the tooling and making some live tests. This is where I have found that a sharp machinist with an eye for metal forming can give some excellent counsel.
Bending: The Pendulum Process
Bending a tight radii on a roll bender is not always that common. There are other more efficient bending processes that will allow for bending of a tight radii. However, to a certain extent, bending a tight radii on a roll bender can be accomplished thru a progressive process.
Take for example a Ø3.25 x .160 wall steel tube; see Figure 3. Bending this to 13” centerline radius in one shot on a roll bender will produce instant wrinkles. However, starting with a straight tube, a 13” centerline radius can be achieved by working the metal in a series of back and forth steps all leading toward a 13” CLR. The part shown in Figure 3 lends itself more so to a roll bender vs. a draw bender only because the bend angle is larger than 180°. To get this part off of a draw bender would take some doing whereas a progressive forming process via CNC roll bender may be advantageous.
George Winton, P.E. designs and builds CNC tube fabrication equipment for Winton Machine in Suwanee, GA. He can be reached at gwinton@wintonmachine.com or 888.321.1499.
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