10. CNC Hardware Basics

Transcription

1 CAD/CAM Principles and Applications 10 CNC Hardware Basics 10-1/10-20 by P.N.Rao 10. CNC Hardware Basics 10.1 Structure of CNC machine tools Table 10.1 Some design criteria for CNC machine tool design [Rogers] Machine Response Type of command signal Input configuration Maximum feed rate Static accuracy Dynamic accuracy Magnitude of load Range of travel Weight of moving members Power source Component characteristics Undamped natural frequency Power requirement Friction characteristics Inertia Stiffness Amount of backlash Speed range Bandwidth Operating and cost considerations Reliability Maintainability Cost of operation Capital investment Installation requirements

2 10-2 Computer Aided Design and Computer Aided Manufacturing Fig Heavy machine tool structure of a CNC machining centre (Courtesy Makino Milling Machines Co. Ltd., Tokyo, Japan)

3 CNC hardware basics, 10-3 Fig Schematic of a concrete bed of a CNC Turning centre Fig Concrete bed of CNC Turning centre GF NDM-16 with slant bed (Courtesy George Fischer, Switzerland) 10.2 Spindle design

4 10-4 Computer Aided Design and Computer Aided Manufacturing Fig Spindle design for a CNC turning centre Fig Spindle design with an integral spindle motor and cooling system for a CNC Machining centre (courtesy of Hüller Hille GMBH, Germany)

5 CNC hardware basics, 10-5 Fig Spindle assembly with the slideways of CNC vertical axis machining centre Fig Bifurcated column structure for CNC Machining centre to improve torsional rigidity (see also Fig. 10.1) 10.3 Drives The various drives used in CNC machines are: Spindle drives to provide the main spindle power for cutting. Feed drives to drive the axis as per the programme.

6 10-6 Computer Aided Design and Computer Aided Manufacturing Fig Typical torque speed characteristic of a 50 kw AC spindle motor used for CNC Machine Feed drives: The feed drives that are used in CNC machine tools are DC Servo motors AC Servo motors Stepper motors Linear motors Fig Typical step motor system. Precise step systems have feedback loop (dotted line) using encoders or resolvers.

7 CNC hardware basics, 10-7 Fig Step motor with permanent magnet rotor. Stepper motors have a number of benefits, which call for their use in motion control applications. Low cost Ruggedness Simplicity in construction High reliability No maintenance Table 10.2: Comparison of Ball Screws vs Linear Motors Characteristi cs Max speed Ball Screw Linear Motor 30 m/min (lead 120 m/min typical (180- dependent) 240 m/min possible) 0.5-1g 2-10g Max acceleration Static 9-18 kgf/m 7-27 kgf/m stiffness Dynamic 9-18 kgf/m kgf/m stiffness Settling time 100 ms ms Max force 26.7 kn 9.0 kn/coil

8 10-8 Computer Aided Design and Computer Aided Manufacturing Reliability hours hours 10.4 Actuation systems Lead screws: Table 10.3 Lead screw efficiencies Efficiency (%) Type High Median Low Recirculating Ball screw - nut Acme with metal nut* * Since metallic nuts usually require a viscous lubricant, the coefficient of friction is both speed and temperature dependent. Fig Lead screw with Acme nut

9 CNC hardware basics, 10-9 Fig A recirculating ball screw and nut arrangement (Courtesy THK Co. Ltd., Japan). Fig A recirculating ball screw and nut arrangement with external return tube (Courtesy THK Co. Ltd., Japan).

10 10-10 Computer Aided Design and Computer Aided Manufacturing Fig Pre-loading of the recirculating ball screw and nut arrangement The recirculating ball screws have a number of advantages compared to the conventional type of screws. Fig Conventional slide way systems used in machine tools Linear Motion systems

11 CNC hardware basics, Fig Antifriction guideways used in CNC machine tools (Courtesy THK Co. Ltd., Japan). Fig Ball bush used for linear movement in CNC machine tools (Courtesy THK Co. Ltd., Japan).

12 10-12 Computer Aided Design and Computer Aided Manufacturing Fig Use of recirculating ball screw and the LM device for axis movement in the bed of a CNC machine tool (Courtesy Makino Milling Machines Co. Ltd., Tokyo, Japan) 10.5 Feedback devices Encoders Linear scales

13 CNC hardware basics, Fig The closed loop control system used for the control in a CNC machine tool Optical rotary encoder Fig The encoder disc for rotary position measurement. Absolute encoder

14 10-14 Computer Aided Design and Computer Aided Manufacturing Incremental encoder Fig The absolute encoder disc for rotary position measurement. Fig Operation of a digital rotary encoder for position measurement

15 CNC hardware basics, Fig The encoder disc for rotary position measurement. Linear scale Fig The linear scale fixed to the machine tool structure for direct position measurement.

16 10-16 Computer Aided Design and Computer Aided Manufacturing Fig Principle of optical grating for position measurement in linear scales Axes - Standards Co-ordinate system Fig Right hand co-ordinate systems

17 CNC hardware basics, Fig Finding directions in a Right Hand Co-ordinate System and also the positive directions for rotary motions Designating the Axes First axis to be identified is the Z-axis. This is followed by the X and Y axes respectively. Z-Axis and Motion

18 10-18 Computer Aided Design and Computer Aided Manufacturing Fig Vertical axis Milling Machine or Machining centre X-Axis Y-Axis Rotary Motions

19 CNC hardware basics, Fig Axes designation for CNC turning centre with twin turrets and driven tooling. Fig CNC Horizontal axis boring mills in 3 and 4 axes versions

20 10-20 Computer Aided Design and Computer Aided Manufacturing Fig axes CNC Vertical axis machining centre configuration.