Analysis on Improvement of Stainless Steel Flange Processing Technology

In the actual production we often encounter material for stainless steel flange parts, parts processing will often appear after the size, shape and position tolerance or surface roughness exceeded, the reason for this is because of the stainless steel material itself is determined by the characteristics of the material. For the characteristics of stainless steel materials, the use of specific flange processing technology, not only can save materials, reduce production costs, but also in the production of good practical results.

1. General

With the continuous improvement of the technical level of industrial production, a variety of mechanical equipment is to large-scale, high efficiency, high parameters of the direction of development. In the past, the material of the flange body for QT450, belongs to the easy processing parts, are now changed to stainless steel material, using 1Cr18Ni9Ti material. Stainless steel is difficult to machine the material, in machining, there is a tendency to work hardening, low thermal conductivity, high cutting temperature, fast tool wear, chips and tool bonding phenomenon is serious, chips are not easy to break and other issues. Therefore, the processing of stainless steel flange body on the tool, tooling, equipment, process requirements are higher than the processing of QT450. For the processing of stainless steel flange body characteristics, we have conducted a systematic study of the stainless steel flange processing technology, in order to achieve the purpose of shortening the processing time and improving the machining accuracy.

2. Parts process analysis

2.1 Analysis of the characteristics of the part material

From the material processing technology analysis, metal material 1Cr18Ni9Ti is austenitic stainless steel, austenitic stainless steel in the processing of four characteristics.
Plasticity is very high (δ = 40%), so its machining deformation and hardening are more serious, and the tool surface of the cold welding phenomenon is relatively strong, easy to produce chip tumors at the tip of the tool, affecting the roughness of the machining surface. Cutting force is about 25% higher than 45# steel. The degree of hardening of the processing surface and the depth of the hardened layer is large, to the next process to increase the difficulty.
Toughness, not easy to break the chip, so the cutting energy consumption, cutting force and cutting temperature are also very high.
Thermal conductivity is small, 1Cr18Ni9Ti’s thermal conductivity is about 1/3-1/4 of 45# steel, so the heat generated, and not easy to dissipate, so the cutting area temperature is high, the tool wears fast, easy to produce hardening phenomenon, exacerbate the tool wear and shorten the service life of the tool.
Linear expansion coefficient is large, the size of the workpiece changes, it is not easy to control the accuracy.
Through the above analysis found that 1Cr18Ni9Ti cutting and machining is very poor, its high plasticity, strong adhesion is easy to cause the roughness can not meet the requirements, and its toughness, cutting temperature is very high, the thermal conductivity is also small, the coefficient of linear expansion is large is likely to result in the outer circle and the inner hole size exceeds the limit.

2.2 Example analysis

Φ38₀^+0.034mm bore and Φ44⁰_-0.016mm outer circle requires coaxiality of 0.03mm, the datum B surface and Φ44⁰_-0.016mm outer circle axis is required to be perpendicular to the outer circle, so the outer circle, the bore, the datum B surface and the lower end of the surface to be a clamping process.
The upper end face requires perpendicularity of 0.02mm with the datum B face, so the upper end face should be ground flat with the datum B face as the datum. Since the datum B face can not be contacted with the table of the surface grinder, and the lower end face and the datum B face are clamped at one time, the lower end face can be selected as the datum to grind the upper end face flat.
The sealing groove of the upper face of the flange is 2.4⁰_-0.05mm deep, if the car is good before flat grinding, it is difficult to guarantee its depth when flat grinding, so the sealing groove of 2.4⁰_-0.05mm deep has to wait until the flat grinding and then processing.

2.3 Determine the process route

According to the above analysis, we can determine the machining process route of the parts as follows:
2.3.1 Unloading
Saw the blank Φ100mm×40mm.
2.3.2 Rough turning
Rough turning each part, Φ20mm inner hole and Φ50mm, Φ90mm outer circle turning to size, the rest of the single-sided leave 2-3mm machining allowance, chamfering blunt to remove sharp corners.
2.3.3 Precision turning

• Three-jaw chuck clamp the outer circle, turn the lower end face, finish turning Φ44⁰_-0.016mm outer circle to the size length.
• L=6mm, turn Φ38₀^+0.034mm bore to 18mm depth, chamfer 1×45°. Requirements to ensure that the end face of the datum A perpendicularity 0.02, Φ38₀ ^{+ 0.034}mm hole Φ38₀ ^{+ 0.034}mm and the outer circle of Φ44⁰_-0.016mm coaxiality of Φ0.03, and the hole size, tolerance, roughness in line with the requirements.
• Adjusting head clamp has been turning the outer circle, turning the end face, the size of the thickness of 29 to leave 0.2mm of grinding. The sealing groove of 2.4⁰_-0.05mm deep is not processed.
• Drill 4-Φ11mm holes to ensure that the distance between the holes is Φ70mm, in accordance with the requirements of the drawings.
• Grind the lower end face as the reference and utilize the tooling to hold the upper end face flat to ensure that the thickness of the part is 29mm.
• Turn the three-jaw chuck to clamp Φ90mm outer circle, use to correct the inner hole and the lower endface runout within 0.02mm, turn the sealing groove of 2.4⁰_-0.05mm deep, ensure the size of Φ27.5mm, Φ35.1mm.
• Inspection

3. Selection of tools and process parameters

From the above process, the key part of the part processing in the turning process and grinding process, the following focus on the selection of turning tools and process parameters for a detailed description.

3.1 Turning tool material selection

There are many types of tool materials, the most commonly used tool materials are mainly carbide and high-speed steel. As the thermal hardness of cemented carbide is much larger than high-speed steel, so the choice of cemented carbide as a tool material. Cemented carbide cutting tool materials are mainly hard cobalt and hard titanium, its characteristics are as follows:

• Hardness: the hardness of Cemented Carbide with the nature of carbide, number, particle size and change, and with the cobalt content increases and decreases, in the cobalt content of the same amount, the hardness of YG is lower than YT.
• Flexural strength and toughness: the flexural strength of commonly used grades of cemented carbide is in the range of 0.883-1.472GPa. The higher the cobalt content, the higher the flexural strength, when the cobalt content is the same, the flexural strength of YG is higher than YT.
• Thermal conductivity: Since the thermal conductivity of TiC is lower than that of WC, the thermal conductivity of YG is higher than that of YT.
• Coefficient of linear expansion: the coefficient of linear expansion of Cemented Carbide is much smaller than that of high-speed steel, and the coefficient of linear expansion of YT is larger than that of YG, and it increases with the increase of TiC content.
• Cold welding resistance: Cemented Carbide and steel cold welding temperature is higher than high-speed steel, YT and steel cold welding temperature is higher than YG, for cutting stainless steel, due to the Ti element contained in such materials, thermal conductivity is low, easy to cold welding, cutting force, cutting temperature is high, so the requirements of the tool does not contain Ti, and have a better thermal conductivity in order to mitigate the cold welding and reduce cutting temperatures, so processing of stainless steel is easy! Selection of Ti-free YG class carbide, here choose YG6A.

3.2 Selection of tool geometry parameters

Through the analysis of the characteristics of the material, stainless steel toughness, not easy to break the chip, so the cutting force and cutting temperature are also very high, so choose a good tool geometry parameters should be able to reduce the cutting force, reduce the cutting temperature and easy to break the chip.
3.2.1 Front angle
The influence of the front angle on the cutting force
Front angle can be selected according to the machined material and tool material. In order to improve the surface quality of the workpiece can increase the front angle, and a smaller front angle is conducive to improve the durability of the tool.
The effect of the front angle on the cutting temperature
Front angle γo value directly affects the deformation of the cutting process and friction, so it has a significant impact on the cutting temperature, the front angle is large, generating less cutting heat, cutting temperature is low; front angle is small cutting temperature is high.
Front angle on the cutting edge and the strength of the cutter head impact
Increase the front angle of the tool, will make the cutting edge and cutter head strength is reduced, may cause serious chipping, so rough turning tool front angle should not be selected too large, generally select the front angle γ_o = 15-18 °, and fine turning depth of cut is small, the main to ensure that the cutting accuracy, should be selected a larger front angle, generally select the γ_o = 20 ° – 25 °.
3.2.2 Back angle
Increase the back angle can reduce the friction between the back face and the machining surface, can reduce the cutting force, improve the quality of the machined surface, but increase the back angle can also make the cutting edge and the cutter head strength is reduced, the reasonable back angle of the lathe tool α₀ = 6 ° – 8 °.
3.2.3 Negative chamfer
The effect of negative chamfering on cutting force
Negative chamfer can improve the strength of the tool, thereby improving the durability of the tool, but with the negative chamfer increases, cutting deformation, so that the cutting force increases, so should be appropriate to use a small negative chamfer, negative chamfer is through the width of his b_r1 and the ratio of the amount of feed f affect the cutting force.
Negative chamfer on the cutting temperature
Negative chamfer on the one hand, the existence of plastic deformation in the cutting zone increases, the cutting heat also increases, but on the other hand, the tool heat dissipation conditions have been improved. The result of the balance between the two, so that the cutting temperature is basically unchanged.
3.2.4 Main deflection angle
Influence of main deflection angle on cutting force
When machining plastic metal, with the main deflection angle increases, the cutting force increases, about in X_r = 60-75 ° between, when F_z reduced to a minimum, and then with the X_r continues to increase, Fr also increases.
Influence of main deflection angle on surface roughness
Reduce the main deflection angle can reduce the roughness of the machined surface, so the processing of stainless steel parts choose the main deflection angle: X_r = 75 °.
3.2.5 Chipbreaker
In the high-speed cutting stainless steel parts, will produce strip chip, strip chip continuous and often winding in the workpiece or tool, pulling the surface of the workpiece or broken cutting edge, and even back to hurt people, so in general want to avoid the formation of strip chip, in order to prevent strip chip pulling the surface of the workpiece or broken cutting edge, generally should be ground chipbreaking groove, chipbreaking groove radius of arc: r_nb = 2-7mm, slot Width L_nb = 3-6.5mm.

3.3 Cutting amount of choice

Cutting speed
Rough turning selection of cutting speed V_c = 50-70m/min, here choose V_c = 60m/min; fine turning cutting speed V_c = 80-100m/min, choose V_c = 90m/min.
Feed
The smaller the feed, the higher the quality of the machined surface, but the cutting efficiency is about low. Here roughing f = 0.3-0.5mm/r, finishing f = 0.08-0.1mm/r.
Back draft
This piece due to roughing, α_p = 2-7mm, semi-finish turning, due to the margin is greater than 2mm, so divided into two times, the first time, α_p = 1.5-2mm, the second time, α_p = 1-1.5mm, finish turning to leave 0.2mm a machining is completed.

4. Conclusion

Stainless steel is a kind of material that is not easy to process, for the processing of this kind of metal material, it is necessary to analyze its processing technology in detail, the preparation of the process of this kind of material can not be simply confined to the preparation of the process card, but should be from the equipment, tool materials, tool geometry parameters, cutting dosage, etc. to make a detailed specification, the use of flanges processing technology, can greatly improve labor efficiency and achieve good results in production.
Author: Fu Xianliang