Workpiece

The  first important thing to consider is the part you are going to produce.  The point is to buy a machine that can perform the machining task with  the shortest workbench and the smallest tonnage. Careful  consideration of material grades and maximum processing thickness and  length. If most of the work is a low-carbon steel with a thickness of 16  gauge and a maximum length of 10 feet (3.048 m), the free bending force  does not have to be greater than 50 tons. However, if you are engaged in a large number of bottomed die forming, you should probably consider a 160-ton machine. Assuming  that the thickest material is 1/4 inch, the 10-foot free bend requires  200 tons, and the bottomed die bend (corrected bend) requires at least  600 tons. If most of the workpiece is 5 feet or less, the tonnage is  almost reduced. Half, which greatly reduces the cost of acquisition. The length of the part is important to determine the specifications of the new machine.

Flexing

Under the same load, the 10-foot machine table and sliders are four times more flexed than a 5-foot machine. This means that shorter machines require fewer gasket adjustments to produce qualified parts. Reduced shim adjustment and reduced preparation time. Material grades are also a key factor. Compared  to mild steel, the load required for stainless steel is typically  increased by about 50%, while most grades of soft aluminum are reduced  by about 50%. You  can always get the machine's tonnage table from the bender  manufacturer, which shows the estimated tonnage required per foot length  for different thicknesses and materials.

Bending radius

When free bending is used, 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 16 gauge low carbon steel using a 1/2 inch  (0.0127 m) opening distance, the part has a bend radius of about 0.078  inches. If the bend radius is almost as small as the material thickness, a bottomed die must be formed. However, the pressure required to form the bottom die is about 4 times greater than the free bend.

If  the bending radius is less than the material thickness, a punch with a  front end radius smaller than the material thickness shall be used and  the embossing bending method shall be applied. Thus, 10 times the pressure of free bending is required. In terms of free bending, the punch and the die are machined at 85° or less (small dots are preferred). When  using this set of dies, attention is paid to the gap between the punch  and the die at the bottom end of the stroke, and excessive bending  sufficient to compensate for the rebound to maintain the material at  around 90°. Typically,  the free bending die produces a rebound angle of ≤ 2° on the new  bending machine and a bending radius equal to 0.156 times the die  opening distance.

For bottomed die bending, the die angle is typically 86 ~ 90°. At  the bottom end of the stroke, there should be a gap between the male  and female molds that is slightly larger than the thickness of the  material. The  forming angle is improved because the tonnage of the bottomed die is  larger (about four times the free bending), reducing the stress that  normally causes rebound during the bending radius. The  embossing bend is the same as the bottomed die bending except that the  front end of the punch is machined to the desired bend radius, and the  embossing gap at the bottom end of the stroke is less than the material  thickness. Springback  is substantially avoided by applying sufficient pressure (approximately  10 times the free bend) to force the front end of the punch into  contact with the material.

In  order to select the lowest tonnage specification, it is preferable to  plan for a bend radius greater than the thickness of the material and to  use the free bend method as much as possible. When the bending radius is large, the quality of the part and its future use are often not affected.

Curvature

The  bending accuracy requirement is a factor that needs careful  consideration. It is this factor that determines whether a CNC bending  machine or a manual bending machine needs to be considered. If the bending accuracy is required to be ±1° and cannot be changed, it is necessary to look at the CNC machine. The  CNC bending machine slider repeatability is ±0.0004 inches, and the  precise angle of forming must be such precision and good mold. The  hand-controlled bending machine slider repeatability is ±0.002 inches,  and a deviation of ±2~3° is generally produced under the condition of a  suitable mold. In  addition, the CNC bending machine is ready for rapid loading, which is  an unquestionable reason when it is necessary to bend many small batch  parts.

Mold

Even if there are full-frame molds, 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  shoulders of the die. For  conventional molds, the deviation per foot should be around ±0.001  inches and the total length deviation should be no more than ±0.005  inches. For  precision grinding dies, 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 a fine grinding die for a CNC bending machine, and a conventional die for a manual bending machine.

Bending part length

Assuming  that a 5 x 10 foot 10-gauge low carbon steel plate is bent 90°, the  bender must have an additional 7.5 tons of pressure to push the steel  plate up, and the operator must prepare for a 280-pound straight edge  drop. . Manufacturing  the part may require several strong workers or even a crane. Bender  operators often need to bend long-edge parts, but they don't realize how  hard their work is.