Effect of Jet Pressure on Surface Roughness of Workpiece in Strengthening and Polishing Process

Abstract: In order to discuss the relationship between strengthening and polishing process and surface roughness of workpiece, the effect of the jet pressure on surface roughness of workpiece is analyzed from mechanism. The validation of the theory analysis is verified by using experiments． The results show that in order to guarantee the surface roughness of workpiece，the jet pressure should be set as about 0. 4 MPa for meeting the requirements and considering the various factors．

Key words: rolling bearing; ring; surface roughness; strengthening and polishing; jet pressure

Enhanced grinding technology is a precision machining technique based on composite machining methods that endows metal materials with fatigue resistance, corrosion resistance, and wear resistance. It is a new method that combines enhanced plastic machining and micro cutting grinding. By using a nozzle, a high-strength shot peened strengthening grinding fluid is high-pressure sprayed onto the surface of the workpiece, causing random and equal probability collisions on the surface. The shot peening causes elastic-plastic deformation of the surface layer and generates residual compressive stress. At the same time, the shot peening performs initial grinding on the surface of the workpiece; Grinding powder generates lateral forces with the surface of the workpiece under high pressure, and performs micro cutting on the surface of the workpiece; And it is accompanied by a series of complex effects such as suspension, cooling, cleaning, and lubrication of the strengthening liquid, thereby achieving the effect of strengthening and grinding the surface of the workpiece. Compared with traditional processing methods, enhanced grinding technology also has advantages such as low energy consumption, low cost, and good efficiency, which meets the needs of modern processing and is also a trend for future development.

1. Mechanism analysis

During the strengthening grinding process, due to the effect of shot peening, the surface of the workpiece undergoes plastic deformation. In the initial stage of processing, the amount of deformation generated is greater than the cutting amount of the abrasive. Therefore, the effect of shot peening on the surface of the workpiece must be considered first. Shot peening is sprayed onto the surface of the workpiece through a nozzle under high pressure, and the surface of the workpiece is continuously compressed due to the impact of high-speed shot peening. In the case of the same material, assuming that the speed of each shot peening during processing is the same, the energy generated when it collides with the surface of the workpiece can also be considered the same. Ignoring the influence of other factors, in the elastic region of the workpiece, according to the calculation formula E=kx2/2 of the elastic performance, it can be seen that the compression amount on the surface of the workpiece is approximately equal each time it is impacted (Figure 1), that is, a spherical crown shaped depression with a diameter smaller than the diameter of the shot peening is left on the surface of the workpiece, and the shape of the sprayed surface is enveloped by a large number of spherical pits, as shown in Figure 2. Usually, the higher the spraying pressure, the greater the energy obtained by shot peening, and the deeper the pits generated by the shot peening on the surface of the workpiece. However, compared with the previous process of strengthening grinding, the surface of the workpiece becomes much neater.

Figure 1 Workpiece Plastic Deformation Diagram

Figure 2 Shot peening

The reinforced grinding fluid mixed with abrasive particles is accelerated by a high-pressure pump and sprayed out through a nozzle, shooting at a very high speed towards the surface of the workpiece being processed. Due to the rapid relative motion between abrasive particles and the workpiece, the surface of the workpiece is subjected to significant impact and shear forces, thereby removing the material. This is achieved through the interaction between abrasive particles and the workpiece surface (high-speed collision shear effect). When abrasive particles are sprayed on the rising edge of the arc shaped contour formed after shot peening (Figure 3), the protruding part of the workpiece surface is cut and the surface tends to be flat; When the abrasive particles are sprayed on the descending edge of the arc formed after shot peening, the abrasive plays a role in grinding and scratching the surface of the workpiece because the spraying direction is the same as the descending direction (in the same direction). Therefore, at this stage, the higher the injection pressure, the greater the cutting effect of the abrasive located on the rising edge of the workpiece surface arc contour in the strengthening grinding fluid, and the interaction between the abrasive located on the falling edge and the workpiece surface is also stronger, which is prone to scratching the workpiece surface.

Figure 3 Grinding Particle Spray

As the strengthening grinding process continues, the depth of shot peening decreases as the workpiece reaches its yield strength, and the protrusions formed on the surface of the workpiece gradually become flat. The unevenness becomes smaller and smaller, and the arc shaped protrusions on the surface of the workpiece are continuously cut. The geometric shape error and surface roughness of the workpiece are gradually improved, and the impact of spraying pressure on the surface roughness of the workpiece is becoming smaller and smaller, Until the required machining accuracy is achieved.

2. Testing

2.1 Testing and detection equipment

The main equipment used for strengthening grinding processing experiments is the centerless strengthening grinding machine for bearing rings. This machine is composed of an electromagnetic centerless fixture system, an integrated high-pressure spraying and recycling system for reinforced grinding materials, a three-dimensional adjustment mechanism for high-pressure nozzles, a safety protection device, a frame and its accessories, an operating interface and control system, etc. The surface roughness of the processed workpiece is measured using the Nissan OLS4000 Olympus laser confocal microscope, and the average Ra value measured multiple times is used as the surface roughness value of the workpiece.

2.2 Test piece

The test piece used in this experiment is the outer ring of an angle contact ball bearing that has undergone heat treatment and precision machining. The material is GCrl5 bearing steel, with an outer diameter of 72.00 mm and a hardness of 60-62 HRC.

2.3 Strengthening grinding fluid and formula

The strengthening grinding fluid used in industrial processing generally consists of strengthening steel shot, grinding powder, and strengthening fluid, among which strengthening shot is mainly cast steel shot and bearing steel shot. Meanwhile, due to the fact that the size of steel shots directly affects the surface roughness of the workpiece, various sizes of steel shots are usually mixed for use. Here, two types of steel pellets with a total of 7 sizes will be mixed for testing. The experimental grinding powder is mixed with three different types of brown corundum in a certain ratio, while the strengthening solution is mixed evenly with ethylene glycol amine solution, detergent, and water in a certain ratio.

2.4 Experimental Plan

This experiment used 11 bearing rings, one of which was an untested original ring (used as a sample for comparison), numbered 0 #. Here, it can be assumed that its injection pressure is 0; The remaining 10 rings are numbered from 1 # to 10 #, with corresponding injection pressures of 0.1, 0.2, 0.3,..., and 1.0 MPa, respectively; The other process parameters remain unchanged and are set as follows: spraying time of 5 minutes, workpiece speed of 150 r/min, nozzle aperture of 10 mm, and distance between nozzle and workpiece surface of 45 mm. Strengthen and grind the 1 # to 10 # rings under the corresponding injection pressure set, and then observe the processed rings under the OLS4000 Olympus laser confocal microscope.

2.5 Experimental Results and Analysis

The surface morphology of the 1 #, 4 #, and 9 # rings after the test is shown in Figure 4. From the graph, it can be seen that as the injection pressure gradually increases, the plastic deformation on the surface of the ring gradually increases; The surface roughness of the ring also varies with the change of injection pressure.

Figure 4 Surface morphology of 1 #, 4 #, and 9 # rings

The variation trend of surface roughness of the 0 # to 10 # ring with spray pressure is shown in Figure 5.

Figure 5 Changes in surface roughness of the ring with injection pressure

As shown in Figure 5:

(1) Under the set processing test conditions, the surface roughness value of the bearing ring is the lowest at a spray pressure of 0.4 MPa, reaching Ra 0.138 μ m. This is because under this spraying pressure, the pits generated by shot peening on the surface of the ring can almost be completely removed by the cutting action of the abrasive material, and a stable equilibrium state is reached between them.

(2) As the injection pressure gradually increases from 0.4 MPa, the surface roughness of the ring also gradually increases; Starting to reach a horizontal state at 0.8 MPa; When the pressure increases again, the surface roughness of the ring increases significantly. This is because during the same period of time, the higher the injection pressure, the deeper the pits generated by shot peening on the surface of the ring, and the longer it takes for the abrasive to cut these protrusions. Therefore, the higher the injection pressure, the greater the surface roughness value of the ring; When the pitting caused by shot peening on the surface of the ring reaches equilibrium with the cutting effect of the abrasive, the surface roughness also tends to balance with the change of pressure. That is, when the pressure is 0.8 MPa, the surface roughness of the ring reaches an equilibrium state; When the pressure increases again from 0.8 MPa, the cutting effect of the grinding material is relatively smaller than the pits generated by the strengthening compression of shot peening. The surface roughness of the ring is mainly determined by the pits generated by shot peening compression. Therefore, when the pressure exceeds 0.8 MPa, the surface roughness value of the ring significantly increases.

When the injection pressure gradually decreases from 0.4 MPa to 0, the surface roughness of the ring no longer decreases with the decrease of injection pressure, but instead shows a continuous upward trend. The main reason for this is that when the injection pressure is too small, the strengthening compression generated by shot peening on the surface of the bearing ring is very small. At the same time, the micro cutting effect of the abrasive on the surface of the ring is also continuously reduced, and can even be ignored. Therefore, the surface roughness changes very little compared to the unprocessed ring.

4. Conclusion

The experimental results show that the enhanced grinding technology can effectively reduce the surface roughness of the ring, making it an efficient, reliable, and low-cost machining process. In the process of strengthening grinding, in order to ensure the optimal surface roughness of the ring, and under the conditions of meeting the requirements and considering various factors comprehensively, the spraying pressure should be controlled at around 0.4 MPa.

2024 February 4th Week KYOCM Product Recommendation:

Self-aligning Ball Bearing:

Self-aligning ball bearings have two rows of balls, a common sphered raceway in the outer ring and two deep uninterrupted raceway grooves in the inner ring. They are available open or sealed. The bearings are insensitive to angular misalignment of the shaft relative to the housing, which can be caused, for example, by shaft deflection.