One Workpiece
The first important consideration is the part you want to produce, and the key is to purchase a machine that can accomplish the machining tasks with the shortest worktable and the smallest tonnage.
Carefully consider the material grade, maximum processing thickness, and length. If most of the work involves low carbon steel with a thickness of 16 gauge and a maximum length of 10 feet, the required bending force does not need to exceed 50 tons. However, if a significant amount of work involves bottoming dies for forming, a 150-ton machine should be considered.
Now, assuming the thickest material is 1/4 inch, a 10-foot free bending requires 165 tons, while bottoming dies bending requires at least 600 tons. If most of the workpieces are 5 feet or shorter, the tonnage is roughly halved, significantly reducing the acquisition cost. The length of the parts is quite important in determining the specifications of the new machine.
Two Deflection
You also need to consider the deflection that may occur in the machine. Under the same load, the deflection of a 10-foot machine's worktable and ram is four times that of a 5-foot machine. This means that a shorter machine requires fewer shim adjustments to produce qualified parts. Reducing shim adjustments also shortens setup time.
The material grade is also a critical factor. Compared to low carbon steel, stainless steel typically requires an increase in load of about 50%, while most grades of soft aluminum reduce the load by about 50%. You can always obtain a tonnage chart from the bending machine manufacturer, which shows the estimated tonnage required per foot length under different thicknesses and materials.
Three Part Bend Radius
Next, focus on the bend radius of the part.
When using free bending, the bend radius is 0.156 times the die opening distance. In the process of free bending, the die opening distance should be eight times the thickness of the metal material. For example, when using a 1/2-inch die opening to form 16 gauge low carbon steel, the bend radius of the part is approximately 0.078 inches. If the bend radius is almost as small as the material thickness, bottoming dies forming is required. However, the pressure required for bottoming dies forming is about four times that of free bending.
If the bend radius is smaller than the material thickness, a punch with a front-end radius smaller than the material thickness and the assistance of embossing bending method are needed. In this case, the pressure required is 10 times that of free bending.
For free bending, the punch and die are processed at 85° or less (slightly smaller is better). When using this set of tooling, pay attention to the clearance between the punch and die at the bottom of the stroke, as well as enough compensation for springback to maintain an approximately 90° bend.
Generally, the springback angle generated by the free bending die on a new bending machine is ≤2°, and the bend radius is 0.156 times the die opening distance.
For bottoming dies bending, the die angle is generally between 86 to 90 degrees. At the bottom of the stroke, there should be a gap slightly larger than the material thickness between the punch and die. The forming angle is improved because bottoming dies bending requires higher tonnage (about four times that of free bending), reducing the stress that typically causes springback within the range of the bend radius.
Embossing bending is similar to bottoming dies bending, except that the front end of the punch is machined to the desired bend radius, and the clearance between the punch and die at the bottom of the stroke is smaller than the material thickness. By applying sufficient pressure (approximately 10 times that of free bending) to force the front end of the punch to contact the material, springback is essentially eliminated.
To select the lowest tonnage specification, it is best to plan for a bend radius larger than the material thickness and use free bending whenever possible. Larger bend radii often do not affect the quality of the finished part and its future use.
Four Bending Accuracy
Bending accuracy requirements are a factor that needs careful consideration and determines whether you need a CNC press brake or a manual press brake. If the bending accuracy requirement is ±1° and cannot be varied, you must focus on a CNC machine.
The repeat accuracy of the CNC press brake ram is ±0.0004 inches, and precise angles require such accuracy and good tooling. The repeat accuracy of a manual press brake ram is ±0.002 inches, and under proper tooling conditions, it generally produces deviations of ±2 to 3 degrees. Additionally, CNC press brakes are ready for quick tooling changes, which is an undeniable consideration when you need to bend many small batch parts.
Five Tooling
Even if you have a full rack of tooling, don't assume that these tools are suitable for the newly purchased machine. You must check the wear of each tool by measuring the length from the front end of the punch to the die shoulder and the length between the die shoulders.
For standard tooling, the deviation per foot should be around ±0.001 inches, and the total length deviation should not exceed ±0.005 inches. For precision-ground tooling, the accuracy per foot should be ±0.0004 inches, and the total accuracy should not exceed ±0.002 inches. It is best to use precision-ground tooling for CNC press brakes and standard tooling for manual press brakes. Indeed, we often make the mistake of only buying a new press brake without purchasing suitable tooling to go with it.
Six Length of Bent Edges
One often overlooked factor is the length of the material's bent edges before the press brake.
Let's say you bend a 90-degree angle along a 5x10 feet 10-gauge low carbon steel sheet. The press brake would need to apply an additional 7.5 tons of pressure to lift the steel sheet and the operator must be prepared for the drop of a 280-pound straight edge. Manufacturing such a part may require several strong workers or even a crane. Press brake operators often need to bend parts with long edges without realizing how physically demanding their job can be.
