Metal Cutting Basics
1. Turning movement and formed surface
Turning motion: During the cutting process, in order to remove excess metal, it is necessary to make the workpiece and the tool perform relative cutting motion.
The movement of removing excess metal on the workpiece with a turning tool is called turning movement, which can be divided into main movement and feed movement.
Main movement: The cutting layer on the workpiece is directly cut off to convert it into chips, thereby forming the movement of the new surface of the workpiece, which is called the main movement. cut
When cutting, the rotational motion of the workpiece is the main motion. Usually, the speed of the main movement is higher, and the cutting power consumed is higher.
Feeding movement: The movement of making the new cutting layer continuously put into cutting. The feeding movement is the movement along the surface of the workpiece to be formed, which can be continuous movement or intermittent movement. For example, the movement of the turning tool on the horizontal lathe is continuous, and the feed of the workpiece on the planer
Movement is intermittent movement.
Surfaces formed on the workpiece: During the cutting process, machined surfaces, machined surfaces, and surfaces to be machined are formed on the workpiece. Finished surface refers to a new surface that has been removed from excess metal. The to-be-machined surface refers to the surface from which the metal layer is about to be cut. The machined surface refers to the surface that the cutting edge of the turning tool is turning.
2. The three elements of cutting amount refer to cutting depth, feed rate and cutting speed.
1) Cutting depth: ap=(dw-dm)/2(mm)dw=unmachined workpiece diameter dm=machined workpiece diameter, cutting depth is also what we usually call the amount of cutting.
Selection of cutting depth: The cutting depth should be determined according to the machining allowance. When roughing, in addition to leaving the finishing allowance, all the roughing allowance should be removed in one pass as far as possible. This can not only make the product of cutting depth, feed f, and cutting speed V large on the premise of ensuring a certain degree of durability, but also reduce the number of passes. When the machining allowance is too large or the rigidity of the process system is insufficient or the strength of the blade is insufficient, it should be divided into more than two passes. At this time, the cutting depth of the first pass should be larger, which can account for 2/3~3/4 of the total allowance: and the cutting depth of the second pass should be smaller, so as to make the machining process more precise. Obtain smaller surface roughness parameter values and higher machining accuracy. When the surface of the cutting parts is hard-skinned castings, forgings or stainless steel and other materials with severe chilling, the depth of cut should exceed the hardness or chilled layer to avoid cutting edges from cutting on the hard-skin or chilled layer.
2) Selection of feed amount: the relative displacement of the workpiece and the tool in the direction of the feed movement every time the workpiece or tool rotates once or reciprocates once, in units of . After the cutting depth is selected, a larger feed should be selected as far as possible. The selection of a reasonable value of the feed should ensure that the machine tool and the tool will not be damaged due to too much cutting force, the deflection of the workpiece caused by the cutting force will not exceed the allowable value of the workpiece accuracy, and the surface roughness parameter value will not be too large. When roughing, the main limit of the feed is the cutting force, and in the semi-finishing and finishing, the main limit of the feed is the surface roughness.
3) Selection of cutting speed: When cutting, the instantaneous speed of a certain point on the cutting edge of the tool relative to the surface to be machined in the main movement direction, the unit is m/min. When the cutting depth ap and the feed amount f are selected, the maximum cutting speed is selected on the basis of these, and the development direction of cutting processing is high-speed cutting.