Dongguan Portable Tools as the professional manufacturer of on site machine tools, we design the on site machine tools, including portable line boring machine, portable flange facing machine, portable milling machine and other on site tools according to your requirements. ODM/OEM is welcomed as needed.
On site boring bar as part of portable line boring machine , we can make the boring bar length up to 2000-12000 meters according to different size. And the boring diameter can be customized from 30mm-250mm according to on site service situation.
The processing process of boring bars mainly includes the following steps:
Making materials: First, according to the size and shape of the boring bar to be processed, select the appropriate raw materials for cutting materials.
Hammering: Hammer the cut materials to improve the structure and performance of the materials.
Annealing: Through annealing treatment, the stress and defects inside the material are eliminated, and the plasticity and toughness of the material are improved.
Rough machining: Perform preliminary mechanical processing, including turning, milling and other processes, to form the basic shape of the boring bar.
Quenching and tempering: Through quenching and tempering treatment, the material obtains good comprehensive mechanical properties, including high strength and high toughness.
Finishing: Through grinding and other processes, the boring bar is finely processed to achieve the required size and shape accuracy.
High temperature tempering: Further improve the mechanical properties of the material and reduce internal stress.
Grinding: Perform the final grinding of the boring bar to ensure its surface quality and dimensional accuracy.
Tempering: Tempering is performed again to stabilize the structure and reduce deformation.
Nitriding: The surface of the boring bar is nitrided to improve its hardness and wear resistance.
Storage (installation): After all processing is completed, the boring bar is stored or directly installed for use.
Material selection and heat treatment arrangement for boring bars
Boring bars are usually made of materials with high strength, high wear resistance and high impact resistance, such as 40CrMo alloy structural steel. The heat treatment process includes normalizing, tempering and nitriding. Normalizing can refine the structure, increase strength and toughness; tempering can eliminate processing stress and reduce deformation; nitriding further improves surface hardness and wear resistance.
Common problems and solutions for boring bars
Common problems in the boring bar processing process include vibration and deformation. In order to reduce vibration, multi-edge cutting methods can be used, such as using a boring cutter disc, which can significantly improve processing efficiency and stability.
In order to control deformation, proper heat treatment and adjustment of process parameters are required during processing. In addition, deformation control during hard nitriding is also critical, and quality needs to be ensured through testing and process adjustment.
The boring bar is one of the main core components of the machine tool. It relies on two guide keys to guide and move forward and backward axially to achieve axial feed. At the same time, the hollow spindle performs rotary motion through the key transmission torque to achieve circumferential rotation. The boring bar is the core of the main motion of the machine tool, and its manufacturing quality has an extremely important influence on the working performance of the machine tool. Therefore, analyzing and studying the processing process of the boring bar is of great significance for the reliability, stability and quality of the machine tool.
Selection of boring bar materials
The boring bar is the main component of the main transmission and needs to have high mechanical properties such as bending resistance, wear resistance and impact toughness. This requires that the boring bar has sufficient toughness in the core and sufficient hardness on the surface. The carbon content of 38CrMoAlA, a high-quality alloy structural steel, makes the steel have sufficient strength, and alloy elements such as Cr, Mo, and Al can form a complex dispersed phase with carbon and are evenly distributed in the matrix. When subjected to external stress, it plays a mechanical barrier and strengthens. Among them, the addition of Cr can significantly increase the hardness of the nitriding layer, improve the hardenability of steel and the core strength; the addition of Al can significantly increase the hardness of the nitriding layer and refine the grains; Mo mainly eliminates the temper brittleness of steel. After years of testing and exploration, 38CrMoAlA can meet the main performance requirements of boring bars and is currently the first choice for boring bar materials.
Boring bar heat treatment arrangement and function
Heat treatment arrangement: normalizing + tempering + nitriding. Boring bar nitriding is the last step in the heat treatment process. In order to make the boring bar core have the necessary mechanical properties, eliminate processing stress, reduce deformation during the nitriding process, and prepare the structure for the best nitriding layer, the boring bar needs to be properly pre-heat treated before nitriding, namely normalizing and tempering.
(1) Normalizing. Normalizing is to heat the steel to above the critical temperature, keep it warm for a period of time, and then cool it with air. The cooling speed is relatively fast. After normalizing, the normalizing structure is a blocky “ferrite + pearlite”, the part structure is refined, the strength and toughness are increased, the internal stress is reduced, and the cutting performance is improved. Cold working is not required before normalizing, but the oxidation and decarburization layer produced by normalizing will lead to disadvantages such as increased brittleness and insufficient hardness after nitriding, so sufficient processing allowance should be left in the normalizing process.
(2) Tempering. The amount of processing after normalizing is large, and a large amount of mechanical processing stress will be generated after cutting. In order to eliminate the mechanical processing stress after rough processing and reduce deformation during nitriding, it is necessary to add a tempering treatment after rough processing. Tempering is high-temperature tempering after quenching, and the obtained structure is fine troostite. The parts after tempering have sufficient toughness and strength. Many important parts need to be tempered.
(3) The difference between the normalizing matrix structure and the “normalizing + tempering” matrix structure. The matrix structure after normalizing is blocky ferrite and pearlite, while the matrix structure after “normalizing + tempering” is fine troostite structure
(4) Nitriding. Nitriding is a heat treatment method that makes the surface of the part have high hardness and wear resistance, while the core maintains the original strength and toughness. Steel containing chromium, molybdenum or aluminum will achieve a relatively ideal effect after nitriding. The quality of the workpiece after nitriding: ① The surface of the workpiece is silver-gray and matte. ② The surface hardness of the workpiece is ≥1 000HV, and the surface hardness after grinding is ≥900HV. ③ The depth of the nitriding layer is ≥0.56mm, and the depth after grinding is >0.5mm. ④ The nitriding deformation requires runout ≤0.08mm. ⑤ Brittleness level 1 to 2 is qualified, which can be achieved in actual production, and it is better after grinding.
(5) The difference in structure between “normalizing + nitriding” and “normalizing + tempering + nitriding”. The nitriding effect of “normalizing + quenching and tempering + nitriding” is significantly better than that of “normalizing + nitriding”. In the nitriding structure of “normalizing + nitriding”, there are obvious blocky and coarse needle-shaped brittle nitrides, which can also be used as a reference for analyzing the phenomenon of nitriding layer shedding of boring bars.
Finishing process of boring bars:
Process: blanking → normalizing → drilling and rough turning center hole → rough turning → quenching and tempering → semi-finishing turning → rough grinding of outer circle → rough grinding of taper hole → scratching → milling of each groove → flaw detection → rough grinding of keyway (reserving fine grinding allowance) → semi-finishing grinding of outer circle → semi-finishing grinding of inner hole → nitriding → semi-finishing grinding of taper hole (reserving fine grinding allowance) → semi-finishing grinding of outer circle (reserving fine grinding allowance) → grinding of keyway → fine grinding of outer circle → fine grinding of taper hole → grinding of outer circle → polishing → clamping.
Finishing process of boring bars. Since the boring bar needs to be nitrided, two semi-finishing outer circle processes are specially arranged. The first semi-finishing grinding is arranged before nitriding, the purpose is to lay a good foundation for nitriding treatment. It is mainly to control the allowance and geometric accuracy of the boring bar before grinding to ensure that the hardness of the nitriding layer after nitriding is above 900HV. Although the bending deformation is small during nitriding, the deformation before nitriding must not be corrected, otherwise it can only be larger than the original deformation. Our factory process determines that the outer circle allowance during the first semi-finishing grinding is 0.07~0.1mm, and the second semi-finishing grinding process is arranged after the fine grinding of the tapered hole. This process installs a grinding core in the tapered hole, and the two ends are pushed up. One end pushes the center hole of the small end face of the boring bar, and the other end pushes the center hole of the grinding core. Then the outer circle is ground with a formal center frame, and the grinding core is not removed. The spline grinder is turned to grind the keyway. The second semi-finishing grinding of the outer circle is to make the internal stress generated during the fine grinding of the outer circle reflect first, so that the precision of the fine grinding of the keyway will be improved and more stable. Because there is a foundation for semi-finishing the outer circle, the influence on the keyway during fine grinding of the outer circle is very small.
The keyway is processed using a spline grinder, with one end facing the center hole of the small end face of the boring bar and the other end facing the center hole of the grinding core. In this way, when grinding, the keyway is facing upward, and the bending deformation of the outer circle and the straightness of the machine tool guideway only affect the bottom of the groove, and have little effect on the two sides of the groove. If a guide rail grinder is used for processing, the deformation caused by the straightness of the machine tool guideway and the deadweight of the boring bar will affect the straightness of the keyway. In general, it is easy to use a spline grinder to meet the requirements of the straightness and parallelism of the keyway
The outer circle fine grinding of the boring bar is carried out on a universal grinder, and the method used is the longitudinal tool center grinding method.
The runout of the tapered hole is a major finished product accuracy of the boring machine. The final requirements for the processing of the tapered hole are: ① The runout of the tapered hole to the outer diameter should be guaranteed to be 0.005mm at the end of the spindle and 0.01mm at 300mm from the end. ② The contact area of the tapered hole is 70%. ③ The surface roughness value of the tapered hole is Ra=0.4μm. The finishing method of the tapered hole: one is to leave an allowance, and then the contact of the tapered hole reaches the final product accuracy by self-grinding during assembly; the other is to directly meet the technical requirements during the processing. Our factory now adopts the second method, which is to use a cap to clamp the rear end of the boring bar M76X2-5g, use a center frame to set the outer circle φ 110h8MF at the front end, use a micrometer to align the outer circle φ 80js6, and grind the tapered hole.
Grinding and polishing is the final finishing process of the boring bar. Grinding can obtain very high dimensional accuracy and very low surface roughness. Generally speaking, the material of the grinding tool is softer than the workpiece material and has a uniform structure. The most commonly used is cast iron grinding tool (see Figure 10), which is suitable for processing various workpiece materials and fine grinding, can ensure good grinding quality and high productivity, and the grinding tool is easy to manufacture and has a low cost. In the grinding process, the grinding fluid not only plays a role in blending abrasives and lubricating and cooling, but also plays a chemical role to accelerate the grinding process. It will adhere to the surface of the workpiece, causing a layer of oxide film to form on the surface of the workpiece quickly, and play a role in smoothing the peaks on the surface of the workpiece and protecting the valleys on the surface of the workpiece. The abrasive used in boring bar grinding is a mixture of white corundum powder of white aluminum oxide and kerosene.
Although the boring bar has achieved good dimensional accuracy and low surface roughness after grinding, its surface is embedded with sand and is black. After the boring bar is assembled with the hollow spindle, black water flows out. In order to eliminate the grinding sand embedded on the surface of the boring bar, our factory uses a self-made polishing tool to polish the surface of the boring bar with green chromium oxide. The actual effect is very good. The surface of the boring bar is bright, beautiful and corrosion-resistant.
Boring bar inspection
(1) Check the straightness. Place a pair of V-shaped irons of equal height on the 0-level platform. Place the boring bar on the V-shaped iron, and the position of the V-shaped iron is at 2/9L of φ 110h8MF (see Figure 11). The tolerance of the straightness of the entire length of the boring bar is 0.01mm.
First, use a micrometer to check the isometry of points A and B at 2/9L. The readings of points A and B are 0. Then, without moving the boring bar, measure the heights of the middle and two end points a, b, and c, and record the values; keep the boring bar axially stationary, turn the boring bar 90° by hand, and use a micrometer to measure the heights of points a, b, and c, and record the values; then turn the boring bar 90°, measure the heights of points a, b, and c, and record the values. If none of the detected values exceeds 0.01mm, it means it is qualified, and vice versa.
(2) Check size, roundness, and cylindricity. The outer diameter of the boring bar is checked with an outside micrometer. Divide the full length of the polished surface of the boring bar φ 110h8MF into 17 equal parts, and use an outside diameter micrometer to measure the diameter in the order of radial a, b, c, and d, and list the measured data in the boring bar inspection record table
Cylindricity error refers to the difference in diameter in one direction. According to the horizontal values in the table, the cylindricality error in a direction is 0, the error in b direction is 2μm, the error in c direction is 2μm, and the error in d direction is 2μm. Considering the four directions of a, b, c, and d, the difference between the maximum and minimum values is the true cylindricality error of 2μm.
The roundness error is compared with the values in the vertical rows of the table, and the maximum value of the difference between the values is taken. If the boring bar inspection fails or one of the items exceeds the tolerance, it is necessary to continue grinding and polishing until it passes.
In addition, during the inspection, attention should be paid to the influence of room temperature and human body temperature (holding micrometer) on the measurement results, and attention should be paid to eliminating negligent errors, reducing the influence of measurement errors, and making the measurement values as accurate as possible.
If you need the on site boring bar customzied, welcome to contact us for further information.
