How to choose a bending machine correctly

Once an improper selection is made when purchasing a bending machine, the production cost will rise, and the bending machine cannot be expected to recover the cost. Therefore, several factors must be weighed in the decision-making process.

Workpiece

The first important thing to consider is the parts you want to produce. The point is to buy a machine that can complete the processing task with the shortest workbench and the smallest tonnage.

Carefully consider the material grade and the maximum processing thickness and length.  Assuming that the thickest material is 1/4 inch, a 10-foot free bending requires 200 tons, and a bottomed die bending (correction bending) requires at least 600 tons. If most parts are 5 feet or shorter, the tonnage is almost halved, which greatly reduces the cost of purchase. The length of the part is very important for determining the specifications of the new machine.

Torsion

Under the same load, the deflection of the worktable and sliding block of the 10-foot machine is 4 times that of the 5-foot machine. This means that shorter machines require fewer shim adjustments to produce qualified parts. Reduced shim adjustment and shortened preparation time.

The 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 get the machine's tonnage table from the bending machine manufacturer at any time. The table shows the estimated tonnage required per foot in different thicknesses and different materials.

Bending radius

When free bending is used, the bending radius is 0.156 times the die opening distance. During the free bending process, the opening distance of the die should be 8 times the thickness of the metal material. 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 that of free bending.

If the bending radius is smaller than the thickness of the material, a punch with a front-end fillet radius smaller than the thickness of the material must be used, and the imprint bending method must be used. In this way, 10 times the pressure of free bending is required.

In terms of free bending, the punch and the die are processed at 85° or less. When using this set of molds, pay attention to the gap between the male mold and the female mold at the bottom of the stroke, and the excessive bending that is sufficient to compensate for the springback and keep the material at about 90°.

Generally, the springback angle of the free bending die on the new bending machine is ≤2°, and the bending radius is equal to 0.156 times the opening distance of the die. For the bending of bottomed concave molds, the mold angle is generally 86 ~ 90°. At the bottom of the stroke, there should be a gap slightly larger than the thickness of the material between the male and female molds. The forming angle is improved because the bottomed die has a larger bending tonnage, which reduces the stress that usually causes springback in the bending radius.

In order to select the lowest tonnage specification, it is best to plan for a bending radius larger than the material thickness 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.

Curvature

Bending accuracy requirements are a factor that needs to be carefully considered. It is this factor that determines the need to consider a CNC bending machine or a manual bending machine.

The repeatability of the slider of the CNC bending machine is ±0.0004 inches, and the precise angle of forming must use such precision and a good mold. The repeatability of the slide block of the manual bending machine is ±0.002 inches, and the deviation of ±2~3° is generally produced under the condition of using a suitable mold. In addition, the CNC bending machine is ready for rapid mold assembly. When many small batches of parts need to be bent, this is an indisputable reason for consideration.

Mold

Even if there are shelves full of molds, don't assume that these molds are suitable for newly purchased 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 female mold.

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 the precision grinding mold, the accuracy per foot should be ±0.0004 inches, and the total accuracy should not be greater than ±0.002 inches. It is best to use fine grinding molds for CNC bending machines, and conventional molds for manual bending machines.

Side length of bending part

Assuming that it is bent 90° along with a 5×10-foot 10-gauge low-carbon steel plate, the bending machine must apply an additional 7.5 tons of pressure to lift the steel plate, and the operator must prepare for the 280-pound straight edge drop. It may require several strong workers or even a crane to manufacture this part. Operators of bending machines often need to bend long-side parts without realizing how strenuous their work is.