现代数控机床原理与结构 双语 教学课件 ppt 作者 郁元正 Chapter2 Motion and coordinate systems
Chapter 2 Motions & coordinate system of Machine tools,2.1 Motions of machine tool 2.2 Machine tool drive 2.3 Coordinate systems,2.1 Motions of machine tool,2.1.1 Purpose of motions The purpose of machine tool motions is to remove materials from workpieces to obtain required shapes, dimensions and surface quality. Motion essentials are tool motions, workpiece motions, and cutting motions. 2.1.2 Surface forming motion Surface forming motion is the relative cutting motion between tool and workpiece. Types and quantity of surface forming motion determine shape of workpiece, machining method and tool constructions,Outer circular surface turning,Surface forming motion consists of : Primary motion Feeding motion,1) Primary motion Primary motion causes relative motion between cutting tool and workpiece, and remove material from workpiece. Examples of primary motions: Rotary motion of workpiece (turning) Rotary motion of cutting tools (drilling, milling, boring) Reciprocating motion of cutting tools (broaching, planing),Every machining process must have, and only has a primary motion.,2) Feeding motion Feeding motion causes an secondary relative motion between cutting tool and workpiece, and enables tool can continuously remove material from workpiece to form a desired shape. Examples of feeding motion Axial and radial motions of cutting tool (turning) Step motions of cutting tool or workpiece (planing),Note: Feeding motion can be performed by both cutting tools or workpieces; Two or more feeding motions can co-exist (gear hobbing); Zero feeding motion is possible (broaching).,Thinking,Identify primary motions and feeding motions.,Planing,Broaching,Milling,2.1.3 Assistant motions All motions other than surface forming motions, including plunge motion, indexing motion, alignment motion, fast moving, and other actions. 1) Plunge motion Plunge cutting tool into workpiece to obtain a certain dimension 2) Indexing motion Periodical moving, rotating of worktable or tool post to machining each surfaces of workpiece, e.g. multi-thread screws turning, polygon machining, etc.,3) Alignment motion It is used to obtain correct relative positions between cutting tool and workpiece, e.g. aligning drill to hole center by moving rocker arm of the driller. 4) Fast moving and other actions Fast moving improves machining efficiency. Other actions include direction change, clamping, releasing, etc.,2.2 Machine tool drive To realize those necessary motions for machining, each NC machine tool should have three basic units: (1) Actuators, e.g. spindle, tool post, worktable, etc. They direct perform machining (2) Drive system, e.g. AC/DC motors, step motors, etc. They produce mechanical motion for actuators.,(3) Transmissions. They transfer motion and power from drive system to actuators. Transmissions can be in mechanical, electric, hydraulic and pneumatic,2.2.1 Transmission connection Connection between actuator and power or between actuators by transmission components.,Transmission diagram of CA6140 horizontal lathe,2.2.2 Transmission chain A series of transmission components that consist a transmission connection is known as transmission chain. Two forms of transmission Constant ratio and direction. Variable and direction.,Gears, worm gears-shaft, ball screw-nut,Adjusting gears, clash gears, clutch gear shift,Transmission chains include external and internal relations: 1) External relation Connecting power unit and actuators Driving actuators running at designed speed Determining machining efficiency and surface roughness rather than shape of workpiece.,2) Internal relation Connecting and regulating motions between actuators; Precise transmission ratio is important. Frictional transmission (e.g. clutch) or instantaneous variable ratio (chain drive) is unacceptable;,Screw turning requires stringent transmission ratio between spindle rotation and tool post moving to ensure precise thread pitch.,2.2.3 Transmission diagram Transmission diagram briefly represents relative motion paths of surface forming.,Symbols for transmission diagram,Transmission diagram of screw turning,From NC machine tools, spindle and tool post may be driven by their own motors.,2.2.4 Transmission system diagram: XKA5750 milling machine,Cartesian coordinate (Right hand rules),Descartes (1596-1690) French philosopher, mathematician,2.3.1 Standard coordinate system and direction 1. ISO coordinate system,2.3 Coordinate system for NC machine tools,2. Directions of axes (1) All axes assume that tools move while worktables rest; When worktable moves, it moves along an axis marked with “;,Notes: Worktable motion only be considered during machine tool design stage.,(2) Positive direction- The motion that makes tool and workpiece apart. (3) Positive rotation- Spindle screws into workpiece in right hand thread.,3. Axes 1) Z-axis Definition: Parallel to the axis of spindle, or for those machine tools that has no spindle (e.g. planers), vertical to clamping plane; Positi
