First, the basic concept and harm of burrs
A burr is a residue that exceeds the thickness of the material itself at the separation point when the material is cut.
Burrs are a headache for both designers and manufacturers. In the technical documents or design drawings, you can often see the explicit provisions and eye-catching prompt words - "No burrs allowed"; For manufacturers, it is not so easy to achieve this simple "no burrs allowed" rule, often regardless of cost, and often because the effective method of removing burrs is not well known.
As we all know, cold blanking parts are generally with burrs, burr height according to the allowable value is divided into f (precision), m (medium), g (rough), according to the general technical conditions of cold stamping parts JB/T4378.2 execution, are also familiar with the quality of the punching surface and burr height and punching die blade clearance value related, in order to reduce the Angle and tilt of the shear surface, When removing the tear and improving the smoothness of the cut surface, further measures must be taken to solve it.
From further analysis, it is known that the generation of stamping burrs depends on the shape of the part, the material (hardness, composition, organization and other physical properties), the thickness, the processing conditions (speed, pressure), the lubricant (composition, viscosity), the mold (material, hardness, wear resistance) and other factors.
The burr height generated by the general PRESS shear processing is about 1-5% of the thickness of the billet, and the burr height will even exceed 10% of the thickness of the billet when processed by the hydraulic FINEBLANKING PRESS (hereinafter referred to as FB). It has also been reported that the value of burrs produced by processing FB in the height direction is about 2.5 to 5 times larger than that in the width direction. The research shows that the main factor of the burr height generated by FB processing is the difference between the mold clearance and its own structure. Even if the same mold is used, such as the use of wear-resistant die materials and the selection of high-quality oil with special lubrication properties during processing, etc., it is more favored.
Whether from the safety performance, or from the mechanical properties, burrs will inevitably bring harm. As a metal products hate burrs, it will not only affect the appearance, cause human injuries, oil valves and other action components are extremely sensitive, and even invisible burrs will cause equipment accidents, such as burrs of silicon steel sheet parts, power generation equipment and aerospace equipment are absolutely not allowed.
At present, according to the drive source, the press can be divided into two types of mechanical and hydraulic, the total pressure of the small mechanical press with high production capacity has reached 250t~300t, of course, as far as the speed is easy to control, there is no doubt that the hydraulic type is ahead of the mechanical type. At present, there are about 1400t large presses used in the automotive industry, and the average capacity of a hydraulic press sold in Japan from 2001 to 2005 is about 5230t, the shear speed is 5-60mm/sec, and the thickness of processing materials ranges from 5-15mm, and even up to 19mm (Switzerland). In recent years, the press has shown a large-scale and high-speed trend, and the high-speed models produced in Japan generally have a stroke number of 200t type in 150, 250T type in 80, 800T and above 20-40.
The billets show the following trends: (1) to large, thick materials and high strength; (2) To electronic components, small, thin and micro development. It is the so-called polarization, so for us, it is necessary to fully understand the mechanism of burr generation, and consider the corresponding burr control and treatment methods.
Second, burr formation mechanism
In the stamping process, the force acting on the billet and the phenomena generated in the deformation process of the billet should first consider the following points:
1) Material and its supporting method;
2) Cut outline shape;
3) The shape of the die edge;
4) blanking processing conditions.
These factors are closely related to burr generation.
Methods for controlling and reducing burrs
It can be seen from the cutting mechanism of the front joint that burrs of 1-2% height of the billet thickness are inevitable. As shown in FIG. 5, the burr height depends on the wear of the tips of the two tools, that is, due to the blunt radius of the knife edges of the two tools, the burr height will be almost the same as the value of the blunt radius. Since the wear of the tips of the two tools has been recognized on the burr height, it is necessary to monitor the re-grinding time of the tools. The more concentrated the tension on the blank, the smaller the burr, but with the increase of the press processing frequency, the tool tip becomes dull, the compression stress increases, the burr also increases.
Fourth, the removal of burrs countermeasures
Before talking about the countermeasures for removing burrs, a concept is once again stated, that is, it is generally believed that the burrs after FB processing will not be too large, but this is a misunderstanding, and it should be corrected that the burrs generated after FB processing are larger than the burrs of general punching processing. Because the situation caused by the processing of burr with FB can be confirmed by observing the fragment, when the fragment shows a smooth section, there is often a fact that there are more traces of broken section in the product.
Here's how to remove burrs.
The usual method of removing burrs is to use grinding or sanding, nothing more than sandpaper, emery cloth, polishing wheel (sheet), impeller and other tools, but for mass production, mainly rely on machinery, the most popular to calculate the belt sander, multi-head conveyor belt sander grinder, should become the first choice for removing large burrs. When the burr is large, with the belt grinding to remove, of course, is not scientific, because the burr under high pressure dumping when the aggravation of the significant wear of the belt, so the use of multi-head grinding is an effective method, that is, first with coarse sand, and then with fine sand until Z with fine sand belt.
Even the removal of burrs, but also a high-speed trend, the feed speed of 20m per minute, the use of sand belt and brush and 7-head combined sand belt grinder with workpiece turning device, in Germany, the United States and Italy to grind the friction sheet successfully, there are also composite forming processing, For example, in Europe, it is gradually popularized to use resin brushes injected with abrasive liquid or directly use metal brushes to remove burrs. Japan is also using a composite processing method, called the brush with a special note deburring machine.
In short, deburring equipment is divided into wet type and dry type, there are many kinds, when it comes to deburring, there are electrolysis, heating, high water pressure, magnetic sand, roller finish, shot blasting, ultrasonic oscillation, chemical method, sand strong flow method, etc., but also have their own uses.
Of course, burrs are not good, first of all, because the tip will hurt the hands; Secondly, equipment failure is often caused by burrs in components. For general burrs, solid tools and methods such as files or grinding wheels are also commonly used to deal with them. However, for FB machining parts, due to large burrs and stickiness, they require a certain cutting force and pay attention to component deformation and dimensional error, so elastic tools are mostly used. Among them, abrasive belt grinding is a more common deburring method, because abrasive belt grinding has the dual advantages of heavy grinding and flexible grinding. Due to the increasing growth of FB processing, relief processing and surface processing, due to the large burrs and the requirement of a certain roughness of the processing surface, especially for flat parts, the use of multi-head abrasive belt composite grinding machine is a wiser choice.
V. Concluding remarks
In short, for the post-processing of stamping parts, it is now clear that belt grinding is a process that cannot be avoided, because the equipment has almost no connection with the shape and size of the parts. Since the equipment is incorporated into the production line in a very simple step, it is expected that as a professional factory of belt grinding, it will install belt grinding machines and other connecting devices at the discharge end of the FB connection in the future for deburring and grinding. Until the processing of the finished product, it will certainly become the direction of development.
