The first important thing to consider is the part you want to produce. The main point is to buy a machine that can complete the processing task with the shortest worktable and the smallest tonnage.
Carefully consider the material grade as well as the maximum processing thickness and length. If most of the work is low carbon steel with a thickness of 16gauge and a maximum length of 10 feet, the free bending force need not be greater than 50 tons. However, if you are engaged in a large number of bottom die forming, you may want to consider a 150-ton machine.
Well, assuming that the thickest material is 1/4 inch, 10 feet of free bending requires 165 tons, and bottomed die bending (corrected bending) requires at least 600 tons. If most parts are 5 feet or shorter, the tonnage is almost halved, which greatly reduces the purchase cost. Part length is important to determine the specifications of a new machine.
Under the same load, the distortion of the 10-foot machine table and slider is 4 times that of the 5-foot machine. This means that shorter machines require fewer shim adjustments to produce qualified parts. Reducing shim adjustments reduces turnaround time. Material grade is also a key factor. Compared with low carbon steel, the load required for stainless steel is usually increased by about 50%, while most grades of soft aluminum are reduced by about 50%. You can always get a machine tonnage table from the press brake manufacturer, which shows the estimated tonnage required per foot length for different thicknesses and materials.
3. Bending radius of parts
When using free bending, the bending radius is 0.156 times the opening distance of the die. In the free bending process, the die opening distance should be 8 times the thickness of the metal material. For example, when forming a 16gauge mild steel with a 1 / 2-inch opening, the bending radius of the part is about 0.078 inches. If the bending radius is almost as small as the thickness of the material, a bottomed die must be formed. However, the pressure required for forming a bottomed die is about 4 times greater than free bending.
If the bending radius is smaller than the thickness of the material, a male die with a radius at the front end that is smaller than the thickness of the material must be used, and resort to the embossing bending method. This requires 10 times the pressure of free bending.
As far as free bending is concerned, the male and female dies are processed at 85 ° or less (smaller is better). When using this set of molds, pay attention to the gaps between the male and female molds at the bottom of the stroke, and excessive bending that is sufficient to compensate springback and keep the material around 90 °.
Generally, the springback angle of the free bending die on the new press brake is ≤2 °, and the bending radius is equal to 0.156 times of the opening distance of the die.
For the bending of the bottomed mold, the mold angle is generally 86 ~ 90 °. At the bottom of the stroke, there should be a gap slightly larger than the material thickness between the male and female dies. The forming angle is improved because the tonnage of the bottomed die is larger (about 4 times of free bending), which reduces the stress that usually causes rebound in the range of the bending radius.
Embossing bending is the same as bending with a bottomed die, except that the front end of the punch is processed to the required bending radius, and the gap between the bottom of the stroke and the punch is smaller than the thickness of the material. Due to the application of sufficient pressure (about 10 times of free bending) to force the front end of the punch to contact the material, rebound is basically avoided.
In order to choose the lowest tonnage specification, it is best to plan for a bending radius greater than the thickness of the material, and use the free bending method as much as possible. When the bending radius is large, it often does not affect the quality of the finished part and its future use.
The bending accuracy requirement is a factor that needs to be carefully considered. It is this factor that determines whether you need to consider a CNC press brake or a manual bending machine. If the bending accuracy requires ± 1 ° and cannot be changed, you must focus on the CNC machine.
The repeatability of the slider of the CNC press brake is ± 0.0004 inches. The precise angle of forming must adopt such accuracy and a good mold. The repeatability of the slider of the hand-controlled press brake is ± 0.002 inches, and the deviation of ± 2 ~ 3 ° is generally generated under the condition of using a suitable mold. In addition to this, CNC press brakes are ready for quick set-up, which is an unquestionable reason to consider when you need to bend many small batch parts.
Even if you have molds full of shelves, don't assume that these molds are suitable for new machines. The wear of each mold must be checked by measuring the length from the front end of the punch to the shoulder and the length between the shoulder of the die.
For conventional molds, the deviation per foot should be about ± 0.001 inches, and the total length deviation should not be greater than ± 0.005 inches. As for precision grinding molds, the accuracy per foot should be ± 0.0004 inches, and the total accuracy must not be greater than ± 0.002 inches. It is best to use precision grinding molds for CNC press brakes and conventional molds for manual press brakes.
6. Bend length
Assuming a 90 ° bend along a 5 × 10-foot 10-gauge low carbon steel plate, the press brake must apply an additional 7.5 tons of pressure to lift the steel plate, and the operator must prepare for a 280 pound straight edge . Manufacturing the part may require several powerful workers or even a crane. Bender operators often need to bend long-edge parts without realizing how strenuous their work is.
Nowadays, a material holding device is suitable for a workshop that performs this kind of work. This device can be improved according to the needs of new and old machines. With this device, only one person is needed to form long-side parts.