How to choose the size datum of axle sleeve parts
Time: 2019-01-12 Writer: mingrui
How to choose the size datum of axle sleeve parts
Selection of part size datum
A datum is the starting point for marking or measuring measurements.The selection of datum directly affects whether the design requirements can meet the feasibility and convenience of the processing technology.
(I) classification of benchmarks
According to the use of benchmark points: (1) design benchmark to determine the parts in the machine position of the point, line, surface known as the design benchmark.Process reference parts in the processing, measurement, inspection of the selected reference.According to the form of the benchmark: reference point on the surface of a point.The axis of the rotary surface on the datum line parts.(3) a surface on the part has the main assembly surface and support surface on the part;The main processing surface (positioning surface, contact surface) of the parts;The plane of symmetry (symmetry in one direction) of a part.
(2) selection of benchmarks
In order to reduce errors and ensure the design requirements of parts, it is best to make the design datum coincide with the process datum when selecting the datum.If they cannot be overlapped, the functional dimensions of the parts shall be marked from the design datum, and the unimportant dimensions shall be marked from the process datum or from the shape analysis method.When parts are more complex, a direction to choose only a benchmark is often not enough, but also some additional benchmark.Among them those that play the main role are called the main datum and those that play an auxiliary role are called the auxiliary datum.There shall be contact dimensions between the main datum and the auxiliary datum.
Selection of dimension datum for shaft bushing parts
In view of the above questions, according to the specific situation of students, we see the following figure milling head axis parts as an example to illustrate this problem.
(a) to understand the parts shown in the parts of the machine function, position and each part of the mutual position.
According to the shaft part drawing (figure 2) and milling cutter head shaft drawing (figure 1) comparison analysis, the left end of the shaft
Connect with V belt pulley by key;The right end is connected with the milling cutter plate through two flat keys, and is fixed with a baffle ring and a screw
Flat shaft.The shaft has two end caps mounted on the shaft section and two bearings mounted on the shaft section through the bearing handle
The shaft string is installed on the body, and then axial fixation of the shaft string is realized through screws and end caps.Thus it can be seen that the shaft is a milling cutter head
It consists of a number of cylindrical shafts with different diameters and a number of small structures.
(2) selection of radial size datum
According to the structural characteristics of this part, the shape is relatively simple and regular, and most of them are composed of coaxial cylinders with different sizes.The steps with different diameters can be used for axial positioning of the parts mounted on the shaft.Due to the need of design, processing and assembly process, this kind of parts often have chamfering Angle, chamfering circle, snail mosquito, snail mosquito back knife slot, grinding wheel overpass slot, keyway and so on.Most of the processing (rough machining, semi-finishing) is carried out on the lathe, a small part of the processing (finishing) is carried out on the grinder or other machine tools.Place the axis horizontally on the main view according to the machining position, and select the axis of the axis according to the radial dimension datum, so as to unify the design requirements and the processing datum.
(2) datum selection of axial dimension
Since there are many dimensions in the length direction, which end face should be selected as the main reference in the length direction?The main function of the shaft bearing is to support the mechanical rotating body, which is used to reduce the friction coefficient in the process of power transmission and keep the shaft center fixed
Parts.Therefore, the performance of all aspects of the shaft directly affects the transmission of power and torque, so the shaft and bearing connection surface of the dimensional accuracy requirements are higher.It can be seen from the figure that the left and right shaft segments of the shaft segment are equipped with bearings, and the axial dimensions of the fixed shaft segment are required to be higher (for the role of axial positioning of the two bearings).So, in the process of machining, the theory of choice on the surface of the shaft segment on which face as the main reference of length direction, but the right end face of shaft section contact the right end face of shaft has a size of 95, so choose the right end face of shaft section for the length direction of the main benchmark, with the aid of size 95 and the right end face of shaft benchmark (auxiliary), then by 400 and the shaft of the left end face benchmark (auxiliary) also contacted.Other symbols within the diagram serve as auxiliary reference points for length (reference points set for ease of machining and measurement).
(iv) look at pictures and use analogy method;More practice, contrast method;Help students to understand the problem.
In order to correctly point out the dimensional datum of each direction in the drawing of the shaft bushing parts, it is necessary to understand the characteristics, functions, mutual positions in the machine and the roles of the structural shapes of the parts.In fact, it is the "analogy method", mechanical processing is a very experiential work, in the actual production practice, analysis and comparison of the original design ideas, add new design ideas.Rely on the "contrast method" design, and look at the drawings of the same type, how these drawings are dealt with on this issue, which is helpful for students to understand the problem of size benchmark selection.Based on this, the author brought the students to practice workshop see some shaft sleeve parts processing and assembly entity, on the network to find relevant video or make your own 3 d courseware for students, let students learn more shaft sleeve parts usage, processing method, experience different processing technology, the working principle of machine or parts, the assembly relation between all parts, connecting method and the structure such as the shape of the parts, only in this way can they truly read machine function, structure features, parts of each component in the mutual position, parts on the role of the various sizes, etc.,Can choose the main and auxiliary datum of each direction of parts properly.
Summarizing
The selection of part datum is determined by the processing technology of the part or the processing conditions on site.Axle sleeve type parts are rotary parts. Considering that most of its processing (rough machining, semi-finishing) is carried out on the lathe, the axis of the main view is generally selected to be placed horizontally for projection.When choosing a benchmark, often with the axis of the parts as the benchmark for the radial dimensions, thus the requirement of the design and machining process of the benchmark (axial parts on a lathe processing, both ends with top to resist shaft center hole) together, and the length direction of the main benchmark often choose important end face (machining accuracy of the highest surface), the contact surface (shoulder) or important processing surface, think in terms of easy processing and measuring set benchmark other benchmark for auxiliary.
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