Introduction to machining

Machining is the process of changing the shape, size, and surface quality of a workpiece by removing excess material from its surface. This process usually requires the use of equipment and tools such as machine tools, cutters, fixtures, and corresponding materials.

The principle of machining

The principle of machining is simple: excess material is removed by relative motion between the tool and the workpiece. This process usually includes turning, milling, grinding and other basic processing methods.

Turning: Turning is a process in which the workpiece is rotated and the tool is moved along the axis of the workpiece to remove excess material. This method is mainly used for machining shaft parts.

Milling: Milling is a process in which the cutter rotates and the workpiece moves to remove excess material. This method is mainly used for machining flat and complex-shaped parts.

Grinding: grinding refers to the use of abrasives on the surface of the workpiece for cutting processing methods. This method is mainly used to improve the surface quality and accuracy of the workpiece.

Process route of machining

Analyze part drawing and product assembly drawing: understand the role of parts, structural characteristics, technical requirements, identify the position of parts in the product and assembly relationship with other parts. For example, for shaft parts, need to pay attention to its diameter size, length size, cylindricity, coaxiality and other precision requirements; for box parts, must consider each hole the position precision, the hole and the plane verticality and so on.

Determine the blank type: according to the parts of the material, shape, size, production batch and mechanical properties and other factors, select the appropriate blank type. Common Blanks are castings, forgings, profiles, weldments and so on. Such as machine tool lathe bed generally use casting blanks, shaft parts when the force is larger use of forging blanks.

Draw up the process route: this is the core part of the machining process route, mainly including the following:

Select positioning datum: positioning datum is divided into coarse datum and fine datum. The rough datum is used for the first working procedure, and should be selected to ensure that there is enough machining allowance for each machined surface, and the position accuracy between the non-machined surface and the machined surface meets the requirements. For example, for shaft parts, the outer circle of the blank is often used as the rough datum for machining the center hole, and the selection of the fine datum should follow the principles of datum coincidence, datum unification and self-reference, such as shaft parts often use two center hole as a precise benchmark to ensure that the outer surface of the coaxial.

Determine the processing method: according to the technical requirements of the processing surface, select the appropriate processing method. For example, rough turning and semi-finish turning can be used for the outer surface with low precision, and finishing turning and grinding are needed for the outer surface with high precision. Common processing methods are turning, milling, planing, grinding, drilling, reaming and so on.

Arrange the processing sequence: generally follow the first rough after the fine, first after the main, first after other principles of the base surface. First roughing, removing zero machining allowance, leaving the appropriate machining allowance for subsequent finishing, and then finishing, to ensure the dimensional accuracy and surface quality of the parts. First the main surface processing, such as parts of the working surface, assembly datum surface, etc. , and then the secondary surface processing. The datum plane is machined first to provide accurate positioning datum for subsequent machining. For example, the processing box-type parts, usually first processing bottom as a datum, and then processing other planes and holes.

Heat treatment and surface treatment process arrangement: heat treatment process can improve the mechanical properties of materials, eliminate residual stress, etc. . For example, normalizing and annealing are generally arranged after blank manufacturing and before roughing to improve cutting performance; quenching and tempering are generally arranged after semi-finishing and before finishing, in order to improve the hardness and wear resistance of the parts, aging treatment is used to eliminate the residual stress generated in blank manufacturing and machining, which is generally arranged after rough machining and before finishing. Surface treatment processes such as electroplating, painting, etc. , can improve the corrosion resistance and beauty of parts, generally arranged after machining.

Inspection Process Arrangement: inspection process is an important part to ensure product quality, generally after rough machining, before finishing, after the important process, after the completion of parts and other stages to arrange the inspection process. Inspection methods include measuring tool measurement, instrument testing, flaw detection, etc. .

Determine the machining allowance: according to the parts of the machining accuracy, surface quality requirements, processing methods and production batch and other factors, determine the machining allowance of the surface. The size of the machining allowance directly affects the machining quality and production efficiency of the parts. If the allowance is too large, it will increase the machining hours and material consumption. If the allowance is too small, it may not guarantee the machining accuracy and surface quality of the parts.

Determine the cutting parameters and man-hour quota: according to the processing method, machining allowance, workpiece material, tool material and other factors, reasonable selection of cutting parameters, including cutting speed, feed rate and cutting depth. The choice of cutting parameters directly affects the machining quality, production efficiency and tool life. Man-hour quota refers to the time needed to complete a process, which is an important basis for arranging production plan and accounting production cost.

In the case of a simple shaft part, the machining process may be as follows:

Blanking: according to the size of the parts and machining allowance, select the appropriate bar, using sawing machine or other cutting equipment blanking.

Forging (if necessary) : for the larger force shaft parts, through forging to improve the structure and mechanical properties of metal, so that the distribution of internal fiber organization is reasonable.

Normalizing: eliminate forging stress, improve cutting performance.

Turning:

Rough turning: take the outer circle of the bar as the rough datum, turn the two end surfaces, drill the center hole, and then take the center hole as the fine datum, rough turning each outer circle surface, leaving a certain machining allowance.

Semi-finish turning: semi-finish turning of the cylindrical surface to further improve the dimensional accuracy and surface quality, leaving a smaller machining allowance.

Finish: finish each outer surface to the dimensional accuracy and Surface roughness required by the drawing.

Milling Keyway: milling the keyway on the milling machine with the outer circle and the central hole of the shaft as the positioning datum.

Heat treatment (such as quenching, tempering) : improve the shaft hardness and wear resistance.

Grinding: take the center hole as the fine datum, grinds the outer circle surface, achieves the final dimensional accuracy and the surface quality request.

Inspection: inspect the dimensional accuracy, form accuracy, position accuracy and surface quality of the parts to ensure that they meet the requirements of the drawings.