The knowledge you need to know before buying a CNC sheet metal bending machine:
Table of Contents
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. If most of the work is low carbon steel with a thickness of 16 gauge and a maximum length of 10 feet, then the free bending force does not need to be greater than 50 tons. However, if you are engaged in a large number of bottomed die forming, perhaps a 150-ton machine tool should be considered.
Well, assuming that the thickest material is 1/4 inch, 10 feet free bending requires 165 tons, and 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.
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. Material grade is also a key factor. Compared with low-carbon steel, the load required by 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 sheet metal bending machine manufacturer at any time. The table shows the estimated tonnage required per foot in different thicknesses and different materials.
3. The bending radius of the part
When free bending is used, the bending radius is 0.156 times the opening distance of the die. During the free bending process, the opening distance of the die should be 8 times the thickness of the metal material. For example, when forming 16 gauge mild steel with 1/2 inch opening distance, the bending radius of the part is about 0.078 inches. If the bending radius is almost as small as the material thickness, 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 less 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.
As far as free bending is concerned, punch and die are processed at 85° or less (smaller is better). 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 sheet metal 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 mold, 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 (about 4 times that of free bending), which reduces the stress that usually causes springback in the bending radius.
The imprint bending is the same as the bending of the bottomed die, except that the front end of the punch is processed to the required bending radius, and the gap between the punch and the die at the bottom of the stroke is smaller than the material thickness. Since enough pressure (approximately 10 times of free bending) is applied to force the front end of the punch to contact the material, springback is basically avoided.
In order to choose the lowest tonnage specification, it is best to plan for a bending radius larger 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.
Bending accuracy requirements are a factor that needs to be carefully considered. It is this factor that determines whether you need to consider a CNC sheet metal bending machine or a manual sheet metal 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 sheet metal bending machine is ±0.0004 inches, and the precise angle of forming must use such precision and a good mold. The repeatability of the slider of the hand-controlled sheet metal 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 sheet metal bending machine is ready for quick mold assembly. When you need to bend many small batches of parts, this is an indisputable reason to consider.
Even if you have shelves full of molds, don’t assume that these molds are suitable for the newly purchased machine. 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 sheet metal bending machines, and conventional molds for manual sheet metal bending machines.
6. The side length of the bent part
Assuming that it is bent 90° along a 5×10-foot 10-gauge low-carbon steel plate, the sheet metal bending machine must apply an additional 7.5 tons of pressure to lift the steel plate, and the operator must drop the 280-pound straight edge. Get ready. Several strong workers or even a crane may be required to manufacture this part. Operators of sheet metal bending machines often need to bend long side parts without realizing how strenuous their work is.
Nowadays, there is a kind of supporting device suitable for workshops engaged in this kind of work, and this kind of device can be improved according to the needs of new and old machines. With this device, only one person is required to form long-side parts.