SHAFT MACHINING

Shaft parts are one of the typical parts in machining. Shaft parts are revolving parts whose length is larger than diameter, and its surface mainly consists of several cylindrical surfaces, conical surfaces, holes and threads. It's a little complicated, but do not worry about it. Let's talk about "shaft" together today.

The spindle of a machine tool is a typical shaft part, and it is one of the key parts of a machine tool. It transfers the furniture which is at the end of the spindle to the workpiece or tool by means of swirling motion and torque. Therefore, the spindle requires torque and bending moments during operation, and it also requires a high precision of rotation. Therefore, the manufacturing quality of the spindle will directly affect the working accuracy and service life of the entire machine.

A series of technical requirements are specified in the spindle part drawing, such as dimensional accuracy, shape and position tolerances, surface roughness, contact accuracy and heat treatment. All these are to ensure that the spindle has a high rotation accuracy and rigidity, good abrasion performance and dimensional stability.

The following describes the functions and technical requirements of the main parts of the spindle:

(1): Bearing journal: The roundness tolerance of the two bearing journals A and B is 0.005 mm and the radial runout tolerance of A and B is 0.005 mm; The contact ratio of the bearing journals 1∶12 conical surface is ≥ 70%, the surface roughness Ra is 0.4 mm, and the dimension accuracy of bearing journals is IT5. Since the bearing journals of principal axis is used to install block bearing, it is the assembly datum of the main shaft parts, so its manufacturing precision directly affects the rotary precision of the spindle.

(2): The precision of the working surface of the spindle: The working surface of the main shaft refers to the centering surface of the clamping props or furniture, such as the Morse hole, the outer end of the shaft end or the outer circle of the flange. And the demands for them are: The dimensional accuracy, geometric accuracy and contact accuracy of the inner and outer tapered surfaces, the concentricity of the centering surface on the bearing journal, the perpendicularity of the positioning end surface to the neck axis, etc. Their influence on the accuracy of machine tools will cause the clamping error in the furniture or workpiece. The radial runout of the near-spindle end and the radial round jump at 300 mm from the end face are also stipulated in the spindle technical requirements. In addition, in order to ensure good contact between the cone hole and the top cone handle, the contact area shall be verified by coloring method using a standard taper plug gauge.

(3): The accuracy of the secondary shaft neck and other surfaces: The minor axis of a spindle refers to the surface of parts such as gears and bushings. The rank of their dimensional tolerances is generally IT7, and their roundness tolerance is 0.01mm. The thread on the spindle is usually used to fix parts or adjust the bearing clearance. When adjusting the ending beat of the nut, it will cause the bearing ring to be tilted, so that the radial runout of the spindle will increase. This will not only affect the machining accuracy of the workpiece, but also reduce the service life of the bearing. The grade of tolerance of the spindle thread is generally 6g grade. The coaxiality tolerance of the main journal is not more than 0.025~0.05mm, the relative end faces of the nut is less than 0.025mm on the 500mm radius.

(4): The roughness of each surface of the spindle: The roughness requirements for each surface of the spindle of different precision machine tools are as shown in the standard.

(5): Hardness of each surface of the spindle: The journal surfaces, working surfaces and other sliding surfaces of the spindle are subject to varying degrees of friction. When sliding bearing, the journal and the bearing bush are rubbed, and the wear resistance of the journal surface is required to be high, and the hardness may vary depending on its material. For example, the hardness of the tin bronze surface should be greater than 60HRC. When using steel sleeve bearings, the surface hardness of the journal should be higher. If the spindle of the boring machine is nitriding on the surface, its hardness is greater than 900HV. When rolling bearings, friction is jointly borne by bearing rings and rolling elements. Therefore, the journal should not have high wear resistance, but it is still required to improve its hardness, so as to improve its assembly process and assembly accuracy. The hardness of the surface of the journal is generally 40~50HRC.

For the centering surface, because the accessories and the chuck are often removed, it is easy to bruise and affect the contact accuracy. So it must have a certain wear resistance. In order to improve the phenomenon of surface pulling and prolong the term of the machine tool which has high precision, the hardness of the centering surface is generally required to be higher than 45HRC.

The main shaft material usually selects 45Ta, 65Mn, 40Cr and other grades of steel. The hardenability of 65Mn and 40Cr is better. High mechanical property and wear resistance can be obtained after quenching and tempering. When the spindle is required to work at high precision, high speed and heavy load, 18CrMNTi, 20Cr, 20Mn2B and other low carbon alloy steels can be selected. After carbonizing and quenching, the quenching surface has compressive stress, which can improve the fatigue resistance, but the process of heat treatment is poor and the possibility of deformation is greater. The precision spindle can selected 38CrMoAL nitriding steel, its surface hardness and fatigue resistance are higher. And the nitriding layer also has many advantages, such as corrosion resistance and small deformation during heat treatment.

The hairy embryo of the spindle always use forgings. Free forging is often used when the production batch is small, and the required equipment is simple, but the accuracy of blank is poor, and the allowance should be more than 10mm; Die forging can be used to forge more complicated blanks with less machining allowance. And it is beneficial to reduce the amount of machining labor. So it is widely used in batch production. Precision drop forging is an advanced technology in forging production. It can forge blanks with complex shapes and high precision. In addition, spindle blanks of multi spindle automatic lathe with gathering stock of seamless steel tubes are also adopted.

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