This article will introduce how to choose the right CNC press brake for your use.
Consider Your Workpiece
The first crucial factor is the part you intend to produce. It’s vital to choose a machine that can handle your requirements with the shortest table and the least tonnage. Evaluate the material grade, maximum thickness, and length. For instance, if you’re mostly working with 16-gauge mild steel up to 10 feet, a free bend force of 50 tons is sufficient. However, if you frequently perform bottom die forming, you might need a 150-ton machine. For 1/4-inch thick material, a 10-foot free bend demands 165 tons, whereas a bottom die bend requires at least 600 tons. For parts 5 feet or shorter, the required tonnage is nearly halved, significantly lowering acquisition costs. The part length is crucial in determining the appropriate machine size.
Managing Deflection
A 10-foot machine’s table and slide will deflect four times more under the same load compared to a 5-foot machine. This means the shorter machine requires fewer shim adjustments to produce acceptable parts, reducing setup time. The material grade is also important; stainless steel typically needs about 50% more load than mild steel, while soft aluminum grades require about 50% less. Press brake manufacturers provide tonnage tables that estimate the required tonnage per foot for various materials and thicknesses.
Bending Radius
In free bending, the bending radius equals 0.156 times the die opening distance. The die opening should be 8 times the metal thickness. For example, bending mild steel with a 1/2-inch opening results in a bending radius of 0.078 inches. If the bending radius is almost as small as the material thickness, bottom die forming is necessary, requiring about 4 times the pressure of free bending. For a radius smaller than the material thickness, use a punch with a front corner radius less than the material thickness, necessitating the impression bending method, which requires 10 times the pressure of free bending.
Free bending uses punches and dies processed at 85° or less, with a gap between the punch and die at the bottom of the stroke to compensate for springback, maintaining the material at about 90°. Typically, free bending dies on a new machine produce a springback angle of ≤2°, with the bending radius equal to 0.156 times the die opening distance.
For bottom die bending, the die angle is usually 86-90°, with a gap at the bottom of the stroke slightly larger than the material thickness. The larger tonnage reduces stress and springback within the bending radius. Stamping bending is similar but requires sufficient pressure (about 10 times that of free bending) to avoid springback.
Plan for a bending radius greater than the material thickness and use free bending whenever possible to select the lowest tonnage specification. Larger bend radii typically do not affect the finished part’s quality or future use.
Precision
Bend accuracy is crucial in deciding between a CNC or manual press brake. If you require a bend accuracy of ±1°, a CNC machine is necessary. CNC press brakes have a slide repeatability of ±0.0004 inches, essential for precise angles, and support rapid tooling changes, ideal for small batch production.
Manual press brakes offer a slide repeatability of ±0.002 inches, with a typical deviation of ±2 to 3° using appropriate tooling. CNC machines are better suited for precise and varied production needs.
Tooling Considerations
Check each tool for wear before assuming they will suit a new machine. Measure the length from the front of the punch to the shoulder and between the die shoulders. For conventional dies, maintain a deviation of around ±0.001 inch per foot, with a total deviation not exceeding ±0.005 inch. Precision-ground dies should have an accuracy of ±0.0004 inch per foot, with a total accuracy within ±0.002 inch. Use precision-ground dies for CNC machines and conventional dies for manual machines.
Length of the Bent Part
Bending a 90° angle along a 5×10-foot mild steel plate may require the machine to apply an additional 7.5 tons of pressure to lift the plate. The operator must manage a 280-pound edge falling. Producing such parts often needs multiple strong workers or a crane. Modern support devices are available to ease this process, allowing one person to form long-side parts efficiently.
By reorganizing and carefully considering these factors, you can select the right press brake that meets your production needs and enhances efficiency.
