In mechanical products , many important components such as bearings, gears , crankshafts, camshafts, piston pins and universal joints are subjected to grinding after heat treatment. In contrast, the power consumption per unit of cutting area during grinding far exceeds other processing methods, and most of the heat converted will enter the surface of the workpiece, thus easily causing changes in the metallographic structure of the machined surface. In the case of improper selection of process parameters, cooling methods and abrasive conditions, the workpiece is prone to a relatively deep metallographic structure change layer (ie tempering layer) during the grinding process, accompanied by large surface residual stresses, even This leads to cracks, which is the so-called grinding burn problem.
Surface layer burns on parts will cause a significant drop in product performance and life, or even no use at all, causing serious quality problems. To this end, the production company on the one hand to reduce and avoid the phenomenon of grinding burns by implementing correct and scientific process specifications; on the other hand, strengthen the inspection of parts and components, find unqualified workpieces in time, and judge the ongoing grinding process situation.
However, for a long time, the inspection of the surface burn of the workpiece, in addition to the simplest visual method, is to use the traditional method that has been used for many years - pickling method, that is, apply acid on the surface of the part to be inspected or immerse it in It is filled in an acid tank prepared according to regulations. After that (or after removing the workpiece), the degree of grinding burn is judged according to the different colors presented on the surface. Generally speaking, if the color does not change, it indicates that the situation is normal; and when the color turns gray, it indicates that there is a burn condition, and as the color becomes deeper and deeper, it indicates that the surface of the workpiece is higher due to the higher temperature. Cutting burns is more serious.
Although the traditional inspection method is simple and easy, it has great limitations, mainly because the surface of the workpiece is etched by acid, even if it is a problem-free part, it can no longer be used. The traditional method of implementation is actually a destructive check.
As can be seen from the above description, the pickling method is essentially a qualitative test, and it is difficult to quantitatively explain the degree of wear and burn.
In view of the above two points, when the traditional method is adopted, only the sampling method can be adopted, and the sample is small, and it is difficult to make a more accurate evaluation and improvement of the executed process.
The theory shows that the acid washing test can only reflect the decrease of hardness caused by the change of metallographic structure, and the residual stress on the surface of the workpiece cannot be reflected. It is insufficient to fully reveal the extent of grinding burn.
On the other hand, due to the use of acid, companies have increased the burden of eliminating environmental pollution; the standardization reliability of traditional inspection methods is low, making it more difficult to develop operability assessment criteria.
A Novel and Efficient Method for Grinding Burns——Magnetic Bomb Method
1. Working principle
The magnetic bomb method, BN (Barkhansen Noise Method), is a test method based on the physics Barkhansen effect discovered in 1919. It can effectively test grinding burns. In recent years, the test instruments developed by the magnetic bomb method have been gradually applied in the surface grinding burn detection of parts, and the advantages are fully demonstrated.
It is well known that those parts that have been burned and burned are mainly made of ferromagnetic materials. Under normal conditions, its magnetic order (embodied in the magnetic domain structure of polycrystals) is regularly arranged. However, as mentioned above, changes in metallographic structure and large residual stresses that may occur after grinding burns will cause changes in the magnetic sequence within the magnetic domain structure. The Barkhansen effect states that the coercive (magnetic) force, that is, the strength of the magnetic field required to change the polarity of the inverted magnet, is related to the degree of lattice structure misalignment and residual stress of the ferromagnetic material. This is the mechanism by which the BN method is used to detect the surface burn of a part to be inspected. Figure 1 is a schematic diagram showing the working principle of a detecting instrument developed on the basis of the BN method.
Figure 1 Schematic diagram of the working principle of BN detection instrument
In Fig. 1a, the effect of the magnetic field formed by the "gate"-shaped inductor coil on the steel to be tested depends on the actual condition of the surface burn of the workpiece, and the magnetic field formed around the workpiece causes the probe to A corresponding electrical signal is generated in the induction coil of the test area, and this signal is directly related to the extent to which the workpiece is burned. Figure 1b is a block diagram of the composition of the test instrument developed according to this. The arrow reflects the whole working process: the corresponding magnetic field is generated by the induction coil, and the corresponding detection signal (called B signal) is generated in the sensing head through the workpiece to be inspected. ), the B signal is subjected to processing steps such as amplification and filtering, and finally displayed and output.
Auto Lock Steel Measuring Tape
Auto Lock Steel Measuring Tape,Abs Auto Lock Measuring Tape,Auto Lock Tape Measure 5M,Auto Lock Measuring Tape
Shangqiu Keming Industrial & Measuring Tools Co., Ltd , https://www.keminghandtools.com