The justification questions are many, but here are a few:
- Can a “standard” be used to form the part?
Consider flange lengths, box depth, channel or offset dimensions, special angles, etc.
- Do you have the press brake tonnage required to run a multiple form tool?
- Will that tonnage overload the ram and cause “ram upset”?
- Can the use of a special tool eliminate secondary operations?
The Winged die
The winged type dies use a spring loaded, rotating set of pads to hold the workpiece static while the die moves around the sheet material and punch face, figure 2.
As the punch pushes down through the metal, the winged die rolls to support the workpiece, provide pressure against the punch tip. Because the part remains static it ensures an even forming around the radius and allows for easy over-bending to compensate for springback, which can be significant in a large radius bend.
Rolla-V Die film courtesy of FabSupply Inc.
In this section, we are looking at several different types of bending problems that require a few “tricks of the trade”. These are problems and solutions to issues you will encounter from time to time as a press brake operator. We begin with how to deal with a return flange that is going to physically encounter the body of the punch.
A return flange is defined as a previous flange facing the same direction as the current bend. A reverse flange is also a previously formed bend but formed opposite the current flange.
The window punch can be created by using a standard punch with distance spacers, or by physically cutting away a portion of the tool to create the window in the punch, or both. This is done to facilitate the bending of deep channels and large return flanges, figure 3.
Many times when forming a part the first bend will come out okay, but when the second bend is attempted, the first bend will need to pass through that area which is normally occupied by the punch.
If this final form is attempted without clearance, severe back bending will occur.
Back Bending or Back Breaking can occur for many reasons and is defined as the bending of the sheet in the opposite direction while the sheet is being constrained in the toolset. In figure 4 you see an example of a return flange encountering the body of the punch.
When the part is subsequently removed and the angle checked, you will find the bend angle is open; and there is now a radius in the material that wasn’t there before, figures 5 and 6.
Using a window punch may seem like it is a simple process and it is. But, there are some very tricky considerations that need to be reviewed.
First is the tonnage. It should be apparent that a window punch is by its very nature, very weak at the center. The larger the span becomes, the less tonnage the tool will handle.
Therefore it is recommended that air forming be the method of choice.
Remember that bottom bending can increase the tonnage required by five times that of air forming], and coining can be as much as ten times the pressure of air forming.
Air forming will also let you use the 20% rule to establish the bend radius through die width.
Remember: the wider the width, lower the required tonnage.
In the following example, pay close attention to the relationships between the inside radius (Rp), the Bend Deduction (BD), and the die width. Both of the following examples are solutions to the same problem.
Calculated data for a standard tool set
- Mt = .036
- BD = .0726
- Rp = .060
- Ð = 90°
- Die width optimum = .392
- Tonnage per inch = .3118
The 20% rule applied
- Mt = .036
- BD = .0769
- Rp = .060
- Ð = 90°
- Die width = .472
- Ð = 90° Tonnage per inch = .2173
- Calculating radius = .070
- Radius difference = .010
- BD difference = .0043
- The tonnage with a .392 bottom die = .3118 per inch.
- Tonnage with a .472 bottom = .2173 per inch.
- Tonnage difference = .0945 per inch.
Why should we worry about such a small amount of tonnage? Because a window punch is not capable of spanning any great distance and still resist the bending forces without deflecting. Again, the wider this span becomes, the weaker that it will be in the center.
As the forming process transpires, the punch will “flex” up across the entire center of the window, figure 7.
Because of this deflection, a shim or shims may be required toward the center of the span to compensate for it.
As we discussed in the “die section“, Figure 7 die shimming means that the larger a die width becomes, the greater the number of shims that will be required to effect one degree of change in bend angle.
An excess of shim under the die is not a positive thing; you can badly damage the die by bending it over a pile of shims, creating a reverse bow in the die. This is important because you will be air forming in the widest die that it is possible to use.
Balancing the Tool Load
There is one way to reduce the amount of flexing in a window punch, that is by balancing the load with a form shim.
Notice in figure 8 how the power flows through the tooling and where the flex is going to occur. Add a small pie
ce of the same material, placed at both ends of the punch, and the flexing motion will be greatly reduced.
Balancing the Ram
Optimally, new pieces of form-shim would be used every time to keep the tool load balanced. The reality of that being done in a production environment is pretty slim. The shims will still work, just not quite as well. The second way is to take an already formed shim of the same material and either let them lie in the bottom of the V-die, or double-back-taped to the punch.
The punch will still deflect but will balance at the bottom of the stroke, and add life to the tool and consistency to the parts.Balancing the load of the load can also be used for the press brake bed. When a press brake is being loaded to one side of the machine, it is common to encounter some ram tilt or a machine shutdown.
Trying to achieve a straight bend can be almost impossible even with excessive shimming to the die. When the load is balanced through the use of “forming shims”, add an extra toolset and shim on the opposite side of the press brake; the load will balance, figures 9, 10.
After reviewing this material you should now be able to:
- Define what a special tool is.
- Discuss the costs of these tools and how to justify one.
- Explain the advantages of the winged or Rolla-V die.
- Explain the purpose of a tunnel or window dies.
- Explain how to use a window or tunnel punch.
- Define how to balance forming load.
- Discuss back breaking.
- Explain why air forming is best window or tunnel punch.
- Define a form strip.
- How to balance the load on the ram.
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Next chapter: Forming in Urethane