The shape and size of the blank depends essentially on the shape and size of the part.
The main difference between the part and the blank is that a certain machining allowance, that is, the blank machining allowance, is added to the surface to be machined. Errors are also produced when the blank is manufactured, and the dimensional tolerance of the blank manufacturing is called the blank tolerance. The processing margin and tolerance of the blank directly affect the labor of machining and the consumption of raw materials, thus affecting the manufacturing cost of the product.
Therefore, one of the development trends of modern machining is to make the shape and size of the blanks as close as possible to the parts through the refinement of the blanks, and strive to make less and no cutting. The blank machining allowance and tolerance are related to the manufacturing method of the blank. The production can be determined by reference to the relevant process manual or related enterprises and industry standards.
After determining the blank machining allowance, the shape and size of the blank, in addition to the blank machining allowance attached to the corresponding machined surface of the part, also consider the influence of various manufacturing factors such as blank manufacturing, machining and heat treatment. In the following, only the mechanical processing technology should be analyzed to determine the problems that should be considered when determining the shape and size of the blank.
(1) Setting of crafts
Some parts, due to the structure, are not easy to be clamped during processing. For the convenience of clamping, a boss can be formed on the blank, the so-called craft. The process lap is only used when the workpiece is clamped. After the part is finished, it is usually cut off, but it can be retained if it does not affect the performance and appearance quality of the part.
(2) Adoption of the overall blank
In machining, parts such as three-watt bearings in the spindle components of the grinding machine, connecting rods of the engine, and opening and closing nuts of the lathe are sometimes encountered. In order to ensure the processing quality of these parts and the convenience of processing, the whole blank is often made, and it is cut after processing to a certain stage.
(3) Adoption of the fitting blank
In order to facilitate the clamping during the processing, for some small-shaped parts with relatively regular shapes, such as T-shaped keys, flat nuts, small spacers, etc., multiple pieces should be combined into one blank, after processing to a certain stage or most surface processing After the completion, it is processed into a single piece.
After determining the type, shape and size of the blank, a blank map should also be drawn as the product pattern for the blank production unit. Drawing the blank map is based on the part drawing and adding the blank margin to the corresponding machined surface. However, the specific manufacturing conditions of the blank should be considered when drawing, such as the hole on the casting, the hole in the forging and the minimum casting and forging conditions of the flange, the flange, etc.; the draft of the surface of the casting and the forging (draft) Slope) and fillet; the position of the parting surface and the parting surface. And the two-dot chain line is used to represent the surface of the part in the blank drawing to distinguish the machined surface from the non-machined surface.
