CNCProgramming.ppt

1、CNC Programming:Principles and Application,By Ding Yue-Jiao,Some resources at: ,1. Computer Numerical Control of Machine Tools,Some terms:,Numerical Control Computer Numerical Control Conventional machining Lever Encoder Ball screw Spindle Quill Garden-variety Synonymous,Machine Control Unit Feedbac

2、k loop Machine tool Servomotor Stepper motor Resolution Inherently Downtime Turret Keyway,Some terms:,Wrench Slant-bed CNC Cylindrical Plasma Avionics Discrepancy Nut Climb-cut Shaft Trajectory Tachometer Unplug Backlash Vendor,Part program Mill Lathe Bore Grind Drill press Vertical Horizontal Alter

3、nating Current (AC) Direct Current (DC) Shop Knee and column machines thread,1. Computer Numerical Control of Machine Tools,1.1 What Is CNC?,CNC stands for Computerized Numerical Control and has been around since the early 1970s. Prior to this , it was called NC, for Numerical Control. Simply put, C

4、NC is the automated control of a machine tool by computer and computer program. The computer reads a program ,and instructs several motor to move properly, and then the machine tool makes the axial movement.,. Before CNC Conventional machine tool: It is operated by a skilled machinist who carefully

5、turns the hand wheels and moves the lever to position the workpiece or cutting tool in the proper orientation. CNC machine tools are highly productive.,A CNC machine tool is very similar in construction to a conventional machine tool, e.g. machine base , spindle ,lead screw, table,etc. But a CNC mac

6、hine tool has a motor mounted on each lead screw instead of a hand wheel.,1.2How CNC works?,As you might already have guessed, everything that an operator would be required to do with conventional machine tools is programmable with CNC machines. Once the machine is setup and running , a CNC machine

7、is quite simple to keep running. For example, the axial movements of CNC machine tools are guided by a computer which runs a program and instructs several motors to move in the appropriate manner .,2.Common Types of CNC Machine Tools,Milling Machines: The vertical CNC milling machine , the Vertical

8、Machining Center (VMC) and the Horizontal Machining Center ; Lathes and CNC turning centers Grinders,Milling machines Vertical CNC milling machines , see Figure 1-1(a three linear axes CNC VMC), are easy to load, and their operators have good visibility. But they are difficult to remove chips, and a

9、re not as heavy-duty as their horizontal cousins. In order to produce complex geometry,a fourth or fifth rotational axis can be added, see Figure1-2 (a five-axis VMC) . The difference between a vertical CNC milling machine and a VMC: Usually, a VMC have an Automatic Tool Changer (ATC).But a standard

10、 vertical CNC milling machine has not a ATC, e.g. Knee and column machines .,Knee and column milling machine,Knee and column milling machine,Horizontal Machining Center (HMC) is similar in construction to the VMC, but the tool and the workpiece are mounted horizontally.This configuration allows HMC

11、to remove chips easily, see Figure 1-4.,Sometimes, VMCs and HMCs are available with pallet changers. By this way, the amount of downtime can be reduced.,CNC lathes , as shown in Figure 1-6, have some advantages over the conventional lathes. Firstly, they can cut circular arcs that were all but impos

12、sible on conventional counterparts. Secondly, CNC lathes excel at threading. Lastly, CNC lathes are easy to maintain a constant surface speed at the cutting edge while the diameter changes,Lathe,Lathe,Horizontal Machining Center,A CNC lathe (or called CNC turning center) usually has a automatic tool

13、 changer. One design is using a device called a turret. The turret is mounted on a heavy cross slide and carriage similar to that of a conventional lathes, and then 4-16 cutting tools are mounted on the turret. Another design for automatic tool changing is the tool magazine. A tool magazine carries

14、the cutting tools on a belt and loads each tool onto the tool post when needed according to the part program.,Nowadays, many CNC turning centers are equipped with some enhanced features to make them more productive: 1/ multiple turret; 2/ live tooling : a kind of small milling spindle mounted in the

15、 tool holder; 3/ multiple spindle:a spindle and turret on the left , a spindle and a turret on the right.,1.3 Systems View of CNC,Control System : Technically the control is called the Machine Control Unit or controller, control unit. Controls come in two basic flavors: proprietary and PC-based.,The

16、 control program (or the system program) The part program,A CNC system can be divided into three subsystem: control system, drive system, feedback system.,Drive System: This system is comprised of the motors and screws that will finally turn the part program into motion. The simplest type of drive m

17、otor is stepper motor,but servomotors are more common to use in CNC machines.,Feedback System: The feedback system is used to compare the desired position to the actual position,and make adjustments if any discrepancies are found.,A typical motion control system,Actually, there are two main types of

18、 feedback systems: Open-loop system; Closed-loop system.,A variety of sensors are used with closed-loop feedback system to measure the position of the table or cutting tool and the velocity of the motor or spindle. Encoder is a kind of sensor to measure linear displacements of the table or angular position of shaft.,A rotary encoder with only one set of slots cannot tell which direction it is turning. Another set of slots on the same disc and another optical sensor is needed to determine the d