Cutting, punching and breaking sheet metal
A brief introduction to the issue of area processing of sheet metal.
Sheet metal breaking, cutting and punching is beside CNC-sheet-metal- bending (folding) and sheet metal welding (spot welding, resistance welding, stud welding) the most commonly used technology for sheet metal processing. Adding the automated CNC fabrication and powder painting assembly satisfies everybody's interested in sheet metal processing.
Cutting is probably the most widely used sheet metal processing at all. We use cutting to produce semi-finished products for further processing as well as directly for end products. Sheet metal punching and cutting are related operations which have much in common. Collectively, these can be described as metal cutting.
During the fabrication, it comes to the breaking of sheet metal while cold (especially thin sheets) as well as at higher temperatures, although the cutting is more difficult and expensive, but actually more efficient and allows for punching of harder materials. Temperatures of up to 600-700 ° C are used for breaking the sheet metal.
In principle it is separating a part of material form another through cutting edges, until it breaks free (cuts) the sheet metal. The edges after the cutting of sheet metal are not completely smooth,because of the elasticity of the material.
Technologies and methods for cutting sheet metal
When cutting sheet metal and related operations processing sheet metal, the so called Punch knives for punching sheet metal are used. We can distinguish:
- parallel blades
- inclined blades
- circular blades
- blades for profiles and rods
The use of parallel knives for cutting sheet metal is probably the most widespread. Cutting sheet metal with oblique and inclined knives has the advantage of less power requirements and is therefore used wherever sequential processing is possible. It is mainly punching and cutting and its realized through two methods.
First comes the stepped arrangement of punch knives, second the cutting of material with a towed cut. This method is not very effective for cutting complex shapes.
Separating sheet metal with circular blades is used for longitudinal cutting of long sheets. Cutting with circular knife may be time consuming, but the surges during the cutting of sheet metal are minimal. The so called vibrating scissors that provide opportunity when punching sheet metal grooves and holes are a very special tool in this category. By cutting with circular blades, we are but limited to a maximum sheet thickness of 10 mm.
Cutting sheet metal with blades for profiles and rods or tubes also has its own specifics, in particular regarding the shape of the material being most frequently square or round. Is technologically demanding to especially maintain the parallel course of cutting force during the shearing process in order to achieve minimal distortion of product ends.
Accurate cutting
A high quality of the product is a desired property while cutting sheet metal. Its mainly about the compliance of sheet metal punching dimensions, minimal unwanted distortion and minimal surface roughness of the cut surface. The customers would mostly appreciate the opportunity to use products of sheet metal cutting immediately and without further modification. A summary of all the quality requirements for products made of sheet metal is generally called a "precision cutting".
During the efforts to improve the quality of metal processing, various methods have developed. We can mention, for example, sheet cutting without will, cutting sheet metal with a holder, sheet metal punching with V-shaped edge and back-pressure or reverse cutting of sheet metal.
Their purpose may be the improvement of quality of surface hole and surface quality, especially through the optimization of stress in the cutting point. The best results so far were achieved with a method of cutting V-shaped projection, which combines stress and compressive strength (triaxial state of stress). It works effectively with thickness to about 7 mm.
Efficiency of Sheet metal cutting
As for Sheet metal cutting, the perhaps the most attention from all methods of processing of sheet metal gets the waste material. Cutting inherently generates waste that is inevitable, but every extra centimeter can in large volumes mean significantly more costly production.
To avoid excessive waste, there can be many ways. It all begins with the actual drawing of sheet metal punching. The operator then must ensure that the clippings are deployed as accurately as possible. This deployment on the sheet metal is also known as a notching plan for punching sheet metal.
Waste is an integral part of the production of products made of sheet metal, Thus it is essential to save the material, since even an industrial demanding CNC production machine attributes about 65% of the cost to the material.
Tools for sheet metal punching
Tools for sheet metal processing through cutting, breaking or punching are generally called stamping tools. In principle, they consist of two parts namely an upper movable blade, which is otherwise called a punch, and the lower fixed blade, which is called the cutting die. In practice, of course, it is a much more complex structure and particular specifics depend on the specific focus of production.
Differentiation according to number of operations:
- simple
- progressive
- merged
- associated
- associated stepped
The progressive machine includes multiple die cutting operations that might be directly linked onto each other or need the intervention of an operator. Ideally, there is an inserted base material on one side (or semifinished product), and on the other side we find a finished product made of sheet metal.
Another form are the merged devices. This allows for example to combine cutting and punching sheet metal, or even with bending and pressing of sheet metal at the same time in one step. Such machines have applications in mass production with standardized parts that do not change.
A special form of cutting and breaking of sheet metal has the use of rubber. It finds its application by very thin sheets of metal. Sheet cutting is carried out by using a product template. It is a relatively inexpensive method that can be combined with towing. The fabrication process is simple and fast, but not suitable for large series of sheet cutting, since it produces an considerable amount of waste and the life expectancy of the template is limited.
The process of sheet metal cutting
Essence of punching Sheet metal material consists in separating the material with opposing edges of blades. The separation of the material is achieved through stress shearing. It is the only process in sheet metal processing, where a violation of sheet metal is desired.
When cut, the sheet metal deforms in the sheared area. We apply appropriate procedures to avoid the creation of low-quality cuts and damaging the blades. Not even a destruction of the equipment for cutting sheet metal because of negligence in manufacturing can be excluded.
The roughness of the cutting surface is directly proportional to the quality of the performance of the procedure of sheet metal cutting and material quality. Another factor is the cutting will, change in thickness around the shearing surfaces or sagging of the material.
These imperfections can never be completely prevented, but they can be successfully and considerably reduced. It is but a must to reckon with the fact that higher demands for quality are reflected in prices of machinery and equipment for cutting and punching of sheet metal, thus increasing production costs. Nevertheless the manufacturer should at least perform periodic calibrations.