Intermediate Toolmaker Technology (Mechanical Worker Vocational and Technical Training Textbook)

Author: Chief Editor: Zhu Hua
Publisher:
Publish Date: 2003-03-20
Features:
Fragment: The underlined training requires mastering the marking method for box-type workpieces; mastering the key points of marking for large and irregular workpieces. Marking of box-type workpieces in mechanical manufacturing, box-type machining accounts for a significant proportion. Due to the complex machining process and high dimensional and positional accuracy requirements for box-type workpieces, multiple marking processes are typically required according to drawings and process requirements. Marking serves as both a pre-machining process and a basis for alignment during machining (especially in single-piece machining), and the correctness and rationality of marking directly affect the machining quality, efficiency, and economic cost of box-type workpieces. As an intermediate tool fitter, it is essential to master the marking method for box-type workpieces.
I. Key Points of Box-Type Workpiece Marking Process
When marking box-type workpieces, the following requirements should be noted:
(1) Before marking, it is necessary to carefully study the drawings and check the quality of the workpiece blank against the drawings. The relationship between the marked lines and the machining process of each machining area should be analyzed, and the number and sequence of marking should be correctly determined to avoid re-marking lines that will be machined away. The mutual relationship between each machining area and the assembly parts should be analyzed to determine the placement position, marking datum surface, and alignment points during marking. The fixture, fixture method, and safety measures should be determined.
(2) Check whether the machining allowance of each surface is reasonable. If not, re-mark after adjusting the material and continue until all surfaces have reasonable machining allowances.
(3) The marking position of the box-type workpiece should be selected on the side with the most machined holes and surfaces, which simplifies the marking process, ensures marking quality, and improves efficiency.
(4) Box-type marking generally requires marking cross alignment lines. That is, when marking each line, it must be marked on all four surfaces to provide alignment for subsequent marking and cutting processes such as planing, milling, and boring. The cross alignment lines must be marked on long and straight areas, as the longer the line, the more accurate the alignment; the flatter the plane, the easier the alignment. Typically, the cross centerline of the bearing seat holes is marked on all four surfaces of the box-type workpiece as the cross alignment lines.
(5) If the inner wall of the box-type workpiece does not require machining, special attention should be paid to aligning the inner wall during marking to ensure smooth assembly after machining.
II. Safety Measures
(1) Wooden boards should be placed under the box-type workpiece supported by a jack, and the workpiece's center of gravity must be stably positioned within the plane formed by the support points. When adjusting the height of the jack, it is strictly forbidden to manually adjust it to prevent the workpiece from tipping and injuring people.
(2) For workpieces with a high center of gravity, additional auxiliary supports should be added to prevent tipping.
III. Examples of Box-Type Workpiece Marking
Example: Worm Gear Reduction Box Marking
1. Analysis Before Marking
The worm gear reduction box consists of a box cover (Figure 1-1) and a box base (Figure 1-2). Before marking, the quality of the box cover and box base blanks is checked according to the drawing requirements, and the marking number, sequence, and datum are determined based on the machining process.
(1) Determination of Marking Number and Sequence
Marking is performed in four steps. The first marking step is to mark the machining lines of the joint surface between the box cover and box base. After machining the joint surface, the second marking step is to mark the connecting screw holes of the joint surface. After drilling, the box cover and box base are fastened together with connecting bolts and positioning taper pins, followed by the third marking step to mark the machining lines of the two side planes (E, F) spaced 310 mm apart and the other two side planes (G, H) spaced 410 mm apart. After machining the E, F planes and G, H planes, the fourth marking step is to mark the machining lines of all bearing seat holes and the threading holes on the bearing seat end faces.
(2) Determination of Marking Datum
The marking datum for the four marking steps is: The first marking step uses the centerlines II of the box cover and box base as the datum; the second marking step uses the centerlines I—I, III－Ⅲ, and the joint surface machining line II－Ⅱ of the box cover and box base as the datum; the third marking step uses the centerline II after the box is assembled as the datum; the fourth marking step uses the centerline V－V after the box is assembled as the datum.