Machining datum:
Components consist of multiple surfaces, each with specific size and positional requirements. Relative positional requirements between surfaces include dimensional accuracy and relative position precision (such as coaxiality, parallelism, perpendicularity, and circular runout). Studying the relative positional relationships between surfaces of components cannot be separated from datums; without clear datums, the positions of component surfaces cannot be determined. In general, a datum is a point, line, or surface on a component used to establish the position of other points, lines, or surfaces. Datums can be divided into two main categories: design datums and process datums, based on their different functions.
design baseline
The datum used on the part drawing to determine other points, lines, or surfaces is called the design datum. For example, for a piston, the design datum refers to the centerline of the piston and the centerline of the pin hole.
Process Benchmark
The datum used during the machining and assembly processes is called the process datum. Process datum can be classified into positioning datum, measuring datum, and assembly datum according to different purposes.
Positioning datum:
The datum used to ensure the correct position of the workpiece on the machine tool or fixture during machining is called the positioning datum. According to different positioning elements, the most commonly used are the following two types:
Automatic centering positioning: such as three-jaw chuck positioning.
Sleeve positioning:The positioning elements are made into positioning sleeves, such as keyway disc positioning. Other methods include positioning in V-blocks, and positioning in semi-circular holes, etc.
Measurement datum:
The datum used to measure the dimensions and positions of machined surfaces during part inspection is called the measurement datum.
Assembly datum:
The datum used during assembly to determine the position of the part in the component or product is called the assembly datum.
Workpiece Installation:
In order to machine a surface of the workpiece to meet specified technical requirements, the workpiece must be positioned correctly relative to the tool on the machine tool before machining. This process is commonly referred to as “workpiece positioning”.
After positioning the workpiece, due to the effects of cutting forces, gravity, etc., it is necessary to use certain mechanisms to “clamp” the workpiece to maintain its fixed position during machining. The process of positioning the workpiece correctly on the machine tool and clamping it is called “installation”.
The quality of workpiece installation is an important issue in machining, as it directly affects machining accuracy, the speed and stability of workpiece installation, and ultimately the productivity.
To ensure the relative positional accuracy between the machined surface and its design datum, the design datum of the machined surface should be positioned correctly relative to the machine tool during workpiece installation. For example, in the precision machining of ring grooves, to meet the requirements of circular runout between the groove bottom diameter and the skirt axis, the design datum of the workpiece must coincide with the axis of the machine tool spindle during installation.
There are various installation methods when machining parts on different types of machine tools, which can be categorized into three main types: direct alignment method, line marking alignment method, and fixture-based installation method.
Direct Alignment Method:
When using this method, the correct position of the workpiece on the machine tool is obtained through a series of trials. Specifically, the workpiece is directly mounted on the machine tool, and then visual correction is performed using a dial gauge or scribing disc to adjust the workpiece’s position until it meets the requirements. The accuracy and speed of direct alignment depend on factors such as alignment accuracy, method, tools, and the skill level of the operator. Its drawbacks include time-consuming setup, low productivity, and reliance on operator experience, making it suitable only for single-piece or small-batch production. For example, aligning based on a hard reference feature falls under the direct alignment method.
Line Marking Alignment Method:
This method involves using scribed lines on the workpiece or semi-finished product to align the workpiece on the machine tool to the correct position. It requires an additional marking operation. The scribed lines have a certain width and introduce marking errors, leading to observation errors during alignment. Therefore, this method is more suitable for small-batch production, low-precision blanks, and large workpieces where using fixtures is not feasible. For instance, determining the position of pinholes in two-stroke products using a dividing head and scribing method.
Fixture-based Installation Method:
This method involves using fixtures to clamp the workpiece in the correct position. Fixtures, as auxiliary devices on machine tools, are pre-adjusted relative to the tool position before the workpiece is installed. Therefore, when processing a batch of workpieces, there is no need to align each one individually, ensuring the technical requirements of machining efficiently. It saves time and effort and is widely used in batch and mass production. Our current piston machining process employs the fixture-based installation method.
Clamping is the operation of maintaining the positioned workpiece during machining. The device in the fixture that maintains the workpiece’s position during machining is called a clamping device.
Clamping devices should meet the following requirements: they should not disturb the workpiece’s positioning during clamping; after clamping, they should ensure the workpiece’s position remains unchanged during machining, with accurate, safe, and reliable clamping; clamping actions should be quick, easy to operate, and effortless; the structure should be simple and easy to manufacture.
Points to note during clamping: the clamping force should be appropriate; excessive force can cause workpiece deformation, while insufficient force can lead to workpiece displacement during machining, disrupting workpiece positioning.
Metal cutting basics
Turning motion and resulting surface
Turning movement:
During the cutting process, in order to remove excess metal, the workpiece and the tool must make relative cutting movements. The movement of using a turning tool to remove excess metal on the workpiece on a lathe is called turning movement, which can be divided into main movement and forward movement. Give sport.
Main movement:
The movement of directly removing the cutting layer on the workpiece and converting it into chips, thereby forming a new surface of the workpiece, is called main movement. During cutting, the rotational motion of the workpiece is the main motion. Generally, the speed of the main movement is higher and the cutting power consumed is larger.
Feed movement:
The new cutting layer is continuously put into cutting motion. The feeding motion is a motion along the surface of the workpiece to be formed. It can be continuous motion or intermittent motion. For example, the movement of the turning tool on a horizontal lathe is continuous movement, while the feed movement of the workpiece on a planer is intermittent movement.
Surface formed on the workpiece:
During the cutting process, a machined surface, a machined surface and a surface to be machined are formed on the workpiece. A machined surface is a new surface that has been machined away from excess metal. The surface to be processed refers to the surface from which the metal layer is about to be cut off. The machined surface refers to the surface being turned by the cutting edge of the turning tool.
The three factors for cutting are cutting depth, feed rate, and cutting speed.
Cutting Depth:
ap=(dw-dm)/2 (mm), where dw is the initial workpiece diameter and dm is the machined workpiece diameter. Cutting depth, or the depth of cut, is determined based on the machining allowance. For rough machining, the entire machining allowance should ideally be removed in one pass to maximize the product of cutting depth, feed rate, and cutting speed while reducing the number of passes. When the machining allowance is excessive or the rigidity of the machining system or the strength of the tool is insufficient, cutting should be divided into multiple passes. In such cases, the cutting depth of the first pass should be larger, typically taking up 2/3 to 3/4 of the total allowance, while the cutting depth of subsequent passes should be smaller to achieve finer surface roughness and higher machining accuracy.
When machining workpieces with hardened surfaces, such as castings, forgings, or stainless steel, the cutting depth should exceed the hardness or hardened layer to prevent cutting on the hardened surface.
Feed Rate:
The feed rate is the relative displacement between the workpiece and the tool in the feed direction per revolution or reciprocating motion of the workpiece or tool, measured in mm. Once the cutting depth is determined, a larger feed rate should be selected to ensure that the machine tool and tool are not damaged by excessive cutting force, and that the deflection caused by cutting force does not exceed the allowable precision tolerance of the workpiece or result in excessive surface roughness. For rough machining, the feed rate is mainly limited by cutting force, while for semi-finishing and finishing, it is mainly limited by surface roughness.
Cutting Speed:
Cutting speed is the instantaneous speed of a point on the cutting edge of the tool relative to the workpiece surface in the primary motion direction, measured in m/min. After determining the cutting depth and feed rate, the maximum cutting speed should be selected. The direction of cutting development is towards high-speed cutting.
