(1) first face after the hole. First processing plane, after processing holes is the general rule of box processing. Plane area is large, stable and reliable with its positioning; from the point of view of processing difficulty, the plane is easier than the hole processing. Most of the support holes are distributed in the box on the outer wall plane, the first processing of the outer wall plane can be cut to the casting surface unevenness and sand and other defects, so as to reduce the lead off the drill bit, to prevent the tool from crashing edge, etc., on the hole machining is favorable.
(2) roughing and finishing separately, first rough and then fine: box are castings, machining allowance is larger, more metal removed in roughing, thus the clamping force, cutting force are larger, cutting heat is also more. In addition, after rough machining, the workpiece stress redistribution will also cause deformation of the workpiece, therefore, a greater impact on the machining accuracy. For this reason, the roughing and finishing processing separately, is conducive to the workpiece deformation caused by various reasons after machining is fully exposed, and then eliminated in the finishing process. Rough and finish machining separate principles: for poor rigidity, batch size, the requirements of high precision box, generally rough and finish machining separately, that is, in the main plane and the support holes in the rough machining of the main plane and the support holes after the finishing process. In this way, can eliminate the rough machining caused by internal stress, cutting force, cutting heat, clamping force on the impact of machining accuracy, and is conducive to a reasonable choice of equipment.
Roughing and finishing separately, will make the machine tool, the number of fixtures and fixtures and the number of times the workpiece is installed to increase the cost, so the single piece, small batch production, precision requirements of the box is not high, often roughing and finishing combined in a process, but must take corresponding measures to reduce the deformation of the machining process. For example, after rough machining, loosen the workpiece, so that the workpiece is fully cooled, and then with a smaller clamping force, with a smaller amount of cutting, several times the tool for finishing.
(3) the process is centralized, the first after the second. Box body parts on the high mutual position requirements of the hole system and plane, generally try to focus on the same process to ensure that their mutual position requirements and reduce the number of fixtures. Fastening threaded holes, oil holes and other secondary process arrangements, generally in the plane and support holes and other major machining surface finishing before processing.
(4) reasonable arrangement between the process of heat treatment Box parts of the complex structure, wall thickness is not uniform, therefore, in the casting will produce large residual stress. In order to eliminate residual stress, reduce the deformation after processing and ensure the stability of precision, so, after casting must be arranged for artificial aging treatment. Artificial aging process specifications are: heating to 500 ℃ ~ 550 ℃, insulation 4h ~ 6h, cooling rate of less than or equal to 30 ℃ / h, out of the temperature is less than or equal to 200 ℃. Ordinary precision box parts, generally in the casting after the arrangement of 1 time artificial aging treatment. For some high-precision or particularly complex shape of the box parts, after rough machining, but also to arrange for 1 time artificial aging treatment, in order to eliminate the residual stress caused by rough machining. Some precision requirements are not high box parts blank, sometimes do not arrange aging treatment, but the use of roughing and finishing processes between the parking and transportation time, so that the natural aging. Box parts artificial aging method, in addition to heating and holding method, can also be used vibration aging to achieve the purpose of eliminating residual stress.
2, the selection of benchmark
(1) the selection of coarse benchmark: the role of coarse benchmark is mainly to determine the non-processing surface and processing surface positional relationship, as well as to ensure that the machining surface of the residual amount of uniformity. In the selection of coarse datum, usually should meet the following requirements: First, in order to ensure that all machining surfaces have a margin under the premise of the important holes should be uniform, the thickness of the hole wall as far as possible, and the rest of the parts have the appropriate wall thickness; Secondly, loaded into the box body of the rotary parts (such as gears, bushings, etc.) should be with the box wall with sufficient clearance;
Thirdly, pay attention to keep the box body of the necessary external dimensions. . In addition, it should also ensure stable positioning and reliable clamping. Rough datum of the box parts are generally used above the important holes and another hole farther apart as a rough datum to ensure uniformity of the hole machining allowance. According to the different types of production, the realization of the spindle hole as a rough reference workpiece installation is not the same. Mass production, due to the high accuracy of the blank, you can directly use the important holes in the box on the special fixture positioning, rapid installation of the workpiece, high productivity. In a single piece, small and medium batch production, the general accuracy of the blank is low, according to the above approach to select the coarse datum, often resulting in the box shape skewed, and even local
Machining allowance is not enough, therefore, usually used to scribe the method of finding the right for the first process, that is, the spindle holes and their centerline for the coarse datum for the scribing of blanks and checking, and corrected if necessary, corrected the hole allowance should be enough! , but not necessarily uniform.
(2) the choice of fine datum: fine datum selection is generally used to unify the benchmark program, often with the assembly of box parts benchmarks or specially processed side of the two holes for the positioning datum, so that the entire machining process benchmarks are unified, the fixture structure is simple, benchmarks do not coincide with the error is reduced to a minimum or even zero (when the benchmarks are coincident).