Making a Work Horse Hum: How to Maneuver Through Tube Bending Quality Problems on a Vertical Compression Bender
Decades of Bending
Vertical compression tube benders have been around for more than 50 years. Historically associated with high volume bending, press benders continue to play a role in production environments where parts consist of 1 or 2 bends. Press benders have stood the test of time mainly due to their speed and ability to produce consistent bends. It is not un- common to get 500 to 900 bends per hour thru a vertical press bender.
The quality of a bend is most often a result of how the dies are set. On a press bender, a ram die (punch) pushes thru a bend while two wing dies provide resistance (cushion) to the process, Figure 1 below. It is the job of the wing dies, in part, to keep the tube up inside the punch die as the punch die makes its way thru the bend.
When the wall factor (tube OD / wall thickness) is low (<30), the skill level needed to setup the tooling is not as critical. However, as the wall factor heads towards 50, the strength of the tubing (or lack of) can impact the overall bend quality produced. With a high wall factor, wing die pressure (cushion pressure), die alignment, and pinch all become critical for successful bending.
Wing Die Pressure
Wing die pressure, sometimes referred to as cushion pressure, is directly related to how much resistance the wing dies apply to the tube as the ram die is pushing against the wing dies. Too little pressure and wrinkles can sprout up. Too much pressure with a high wall factor and a number of unwanted events can occur. For instance, the punch die during a bend cycle can be forced to make contact with the wing dies. When this happens, wrinkles may be produced. Another result of too much cushion pressure can result in crushing the tube beyond acceptable limits. This is rarely seen with a low wall factor. However, with a higher wall factor, too much cushion pressure is possible.
Sometimes you need more wing pressure but the punch is already making contact with the wing dies. In this case shaving .010” off the top of the wing dies may allow you to increase the cushion pressure. However, be careful not to shave too much off the top of the wing dies. Shaving too much off can lead to poor support of the tube thus allowing for the possibility of the tube to ooze out between the punch die and wing dies.
Another point of concern when the wall factor is high is to ensure that the cushion pressure in both wing dies is equal. Uneven pressures can lead to non symmetrical bends results.
As a general rule, the wall factor for vertical press benders can be viewed as follows: you should have at a minimum .025” of wall thickness for every 1” of tube diameter in mild steel. This does not hold true for aluminum. Going beyond this bench mark is possible however the tool profiles and tool setup become more sensitive.
Side to Side Die Alignment
In general, wing dies that slide relative to the wing die holders are guided by bronze wear strips. For applications with low wall factors, the alignment of the wing dies to one another and to the punch die itself are important yet not as critical as when the wall factor approaches 50.
As the wall factor approaches 50, the wing dies should be held to a minimum clearance side to side relative to the groove in the punch die. This is because thin wall tubes provide for little side to side resistance to the bend tooling. When bending a thin wall tube with excessive side to side clearance, .005” can make all the difference in bend quality consistency, Figure 3 below. With a thicker wall tube under the same conditions, tool alignment is not as critical.
Pinch in the Dies
You can have the right cushion pressure dialed in, perfect die alignment all around, yet never be able to achieve the needed bend quality because the tube grooves in the dies are cut incorrectly for the application.
Sometimes referred to as squeeze, the tube grooves machined into the punch and/or wing dies are most important for successful bending of high wall factor applications. Squeeze is associated with the tube grooves that are cut below the diameter of the tube being bent. No squeeze can quickly lead to wrinkles. Too much squeeze can lead to gouging the sides of the tube in the area of the neutral axis. The amount of squeeze is application specific. In one application 1% squeeze may work fine. On the other hand, 8% may work well for other conditions.
Below is a photo of a Pines bender taken around 1955. The photo is of a press bender in production making exhaust pipes.
George Winton, P.E. designs and builds CNC tube fabrication equipment for Winton Machine in Suwanee, GA. He can be reached at firstname.lastname@example.org or 888.321.1499.
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