In non-standard equipment and fixtures, the filament screw is widely used. In order to improve the service life of the screw, the quenching screw is widely used. The screw is mainly ground, and the precision is guaranteed by the thread grinding machine.Machining threads on thread grinding machine is the most commonly used cutting method to obtain high precision and low surface roughness in thread machining at present. With the widespread application of high precision quenched thread parts, the advantage of grinding threads has been fully reflected.In real production, in order to obtain high precision and surface quality of the screw, it is necessary to have a deep study on its grinding technology.Take the lead screw encountered in production practice as an example for detailed analysis.
2. Selection of lead screw materials and heat treatment
The screw material directly affects the machining process and the mechanical properties of the workpiece after heat treatment.Therefore, a very important problem in the manufacture of high-precision filament screws is the reasonable selection of materials, usually from alloy tool steel, alloy structural steel, carbon tool steel to choose.According to the technical requirements of the parts as shown in Figure L and the comprehensive heat treatment process and machining process, 65Mn is selected. The material can obtain higher hardness of 55 ~ 60HRC, and the lead screw has high strength and wear resistance.
The heat treatment process of 65Mn material screw is quenching, tempering, cold treatment and tempering.High temperature aging shall be carried out for the lead screw after rough grinding (rough turning), and low temperature aging shall be taken after semi-fine grinding (fine turning), in order to eliminate the stress generated in the machining process and improve the stability of the lead screw.
In the process of heat treatment, the lead screw should pay attention to avoid bending deformation, try not to use straightening process, if necessary, can only use heat straightening.Because at room temperature straightening screw, although it appears to be straightened in a short time, but the second day or once the grinding process will produce bending deformation.
Preparation before grinding
3.1 Control of benchmark
Screw process benchmarking is at the ends of the center hole, center hole general should use type B center hole, it can prevent the end face bruised and affect the precision of the center hole, at the same time the center hole should reach 6 o ~ 65 HRC hardness, the precision of the center hole is the key to ensure accuracy of ball screw, in front of the coarse grinding, fine grinding process is must arrange center Kong Xiu research after quenching and aging process.The contact area between the center hole and the center is required to be 75% in rough grinding and above 80% in fine grinding.The method chosen here is to use the six-sided hard alloy tip scraping on the machine tool. Its edge has the function of micro cutting and extruding, which can correct the geometric shape error of the center hole. Moreover, it has high efficiency, long tool life and roughness up to Ra0.8 m.The axial pressure on the lead screw should not be too large during grinding to avoid deformation of the lead screw.
3.2 Adjustment of machine tools
In addition to the correct machine tool adjustment according to the instructions, accurate selection and collocation hanging wheel, correct for the machine tool lubrication, should be the top back before and after grinding machine tool, improve the correctness of the linearity, Angle 60 ° taper surface and the surface roughness, and strictly control the radial runout, thus ensuring top before and after the machine and screw center hole rate of good contact between, in order to improve precision of the leadscrew grinding.
3.3 Selection of grinding wheel
The grinding wheel with white corundum abrasive and ceramic binder is generally suitable for grinding hardened steel bars.Because it is hard and brittle, in the grinding process is not easy to blunt, blunt abrasive grains are easy to be broken into a new sharp edge, a good cutting;It has low heat and grinding force, which can reduce screw deformation, burn and crack.The shape accuracy of grinding wheel can be maintained for a long time.In order to obtain a lower roughness value, the grinding wheel with fine grain and lower hardness can be selected.
Selection of grinding method and grinding dosage
4.1 Grinding method
There are two kinds of grinding for precision lead screw: smooth grinding and reverse grinding.Smooth grinding is beneficial to the full pouring of coolant and can reduce the thermal deformation of lead screw in grinding area.The reverse grinding is not conducive to the full pouring of coolant, the screw is easy to deformation, but the cutting performance is good.Through the process test, it is found that the reverse grinding method starting from the head rack should be avoided, because the thermal deformation of this method will lead to the elongation of the lead screw toward the tail rack, resulting in the increase of grinding amount on the right side of the screw profile and the gradual decrease on the left side.Therefore, the grinding method starting from the head frame is not only good in cooling performance, but also difficult to generate hot deformation of the lead screw, and the feed direction of the grinding wheel makes the lead screw tensile stress, which can effectively reduce the radial runout and vibration of the lead screw, and is conducive to obtaining higher accuracy and roughness.
4.2 Selection of grinding dosage
The correct grinding amount can improve the precision and surface quality of the screw, and reduce the surface defects such as ripples and burns.The selection of grinding dosage should fully consider the material and heat treatment hardness of the workpiece, surface roughness, particle size and hardness of the grinding wheel, coarse and fine grinding and other factors.
The linear speed of the commonly used grinding wheel in thread grinding is usually 30 ~ 35m/s. The higher linear speed can be chosen when the pitch is small and the precision is high.Due to the high hardness of this screw, the speed of grinding wheel should be reduced appropriately.
Rough grinding, in order to improve productivity, can use a larger workpiece speed and cutting depth, fine grinding, in order to obtain high precision and surface quality, should use a smaller workpiece speed and cutting depth.The workpiece speed is optional 1 ~ 10r/min, and the cutting depth is optional 0.01 ~ o. 03mm.
In summary, the manufacturing of high-precision hardened filament leads depends on the reasonable selection of materials, the reasonable arrangement of hot and cold processes in the manufacturing process to maintain the stability of their dimensions, and the selection and control of relevant factors in grinding process.It has been proved by production practice that the products produced by the above grinding process are in conformity with the design requirements and the quality is stable.