THE DESIGN OF A CNC MILL FOR PRODUCT PROTOTYPING
Abstract
Rapid prototyping is widely used to reduce time to market in product design and development. Today\'s systems are used by engineers to better understand and communicate their product designs as well as to make rapid tooling to manufacture those products. Computer Numerically Controlled (CNC) milling machines are part of this technology. This project will present the design of a small CNC machine, and production, and analysis of a small CNC machine. This machine has the characteristics demanded by the industrial and academic designers. Studying the existing machines aided in setting specifications for the new design. Comparing the performance of the new machine with existing machines will improve future designs. Table of Contents 1 Chapter 1 - Introduction & Problem Solution 1 1.1 Solution Methodology 2 2 Chapter 2 - Performance Metrics of Numerically Controlled Machines 4 1 2.1 Geometrical Errors 4 2.1.1 Backlash 9 2.1.2 Scaling Mismatch 10 2.1.3 Squareness Error 12 2.1.4 Cyclic Error 13 2.1.5 Lateral Play 15 2.1.6 Reversal Spikes 16 1 2.1.7 Stick Slip 18 2.1.8 Vibration 19 2.1.9 Master-Slave Changeover 20 2.1.10 Straightness 22 2.1.11 ASME Standard Test Method 23 3 Chapter 3 - Performance Evaluation of Existing Machine 25 3.1 Discussion o f Measurements of Microkinetics Performance 26 3.2 Discussion o f Measurements of Prolight Performance 31 4 Chapter 4 - Design Specifications for the New Machine 36 5 Chapter 5 - Design of the New Machine 39 5.1 The Hardware 40 5.1.1 The Structure 40 5.1.2 X & Y Axis 41 5.1.2.1 Axis Motor 43 5.1.2.2 Axis Actuator Hardware 45 5.1.2.3 Rolling Contact Bearing 48 5.1.2.4 Motor Mounting 54 5.1.2.5 Linear Slides 56 5.1.3 Z Axis 61 5.2 The Software 5.3 Driver and Electronics 6 Chapter 6 - Measurement of Performance of the New Mill 7 Chapter 7 - Discussion of Results 8 Chapter 8 - Recommendation for Future Work Appendices A. G & M Codes B. Calculation Sheet for the Ball Screw C. Important PartsofEMC.INI File D. Diagram ofThe Driver’s Circuit E. Calculation and Selection o f the Stepper Motor F. Engineering Drawings of GVSU Mill References Table of Figures Figure 2.1.1 the hardware required for the Renishaw ballbar test. 5
Figure 2.1.2 feed in, out, angular overshoot arcs and the data capture arcs. 6 Figure 2.1.3 the data capture range of the ballhar transducer is approximately 2mm. 7 Figure 2.1.4 a plot o f time vs. transducer travel shows the period of machine acceleration and how it would affect the integrity o f the data collected. 7 Figure 2.1.1.1 an example of positive backlash. 9 Figure 2.1.1.2 the interpolation of the inward step in the ball bar plot. 10 Figure 2.1.2.1 an example of a scaling mismatch error. 11 Figure 2.1.3.1 positive and negative squareness. 13 Figure 2.1.4.1 an example of cyclic error. 14 Figure 2.1.5.1 an example of a lateral play in the y axis. 15
Figure 2.1.6.1 an example plot of a reversal spikes error. 16 Figure 2.1.6.2 an example o f the effect of a reversal spikes error on the actual circle milled on the part. 17 Figure 2.1.7.1 stick-slip error as shown on a diagnostic problem. 18 Figure 2.1.7.2 the effect of stick-slip on the machined part. 19
Figure 2.1.8.1 a typical plot showing vibration error. 20 Figure 2.1.9.1 a master-slave changeover error as captured by the ball bar diagnostic plot. 21 Figure 2.1.9.2 master slave changeover every 45\". 21 Figure 2.1.10.1 three distinct distortions in the plot caused by an error in the y axis straightness. 22 Figure 3.1.1 a plot of the ballbar test on the Microkinetics CNC express. 27 Figure 3.1.2 representation of the angular error and how it can cause a scaling mismatch error. 29 Figure 3.2.0 diagnostic plot of the proLIGHT on the same scale as the Microkinetics. 32 Figure 3.2.1 a plot of the ballbar test on the proLIGHT CNC machining center. 32 Figure 3.2.2 duplex arrangement angular contact bearings. 34 Figure 5 a solid model of GVSU mill. 39 Figure 5.1.1.1 the structure of GVSU mill. 40 Figure 5.1.2.1 the X, y axis including the linear slides. 41 Figure 5.1.2.1 the axis drive system. 42 Figure 5.1.2.2.1 lead screw and nut. 45 Figure 5.1.2.2.2 ball screw and nut. 46 Figure 5.1.2.3.1 deep groove ball bearing. 48 Figure 5.1.2.3.2 the driver and the follower pulley diameters and distance. 51 Figure 5.1.2.4.1 timing belt, and timing pulleys. 54 Figure 5.1.2.5.1 illustration of the dovetail slides. 56 Figure 5.1.2.5.2 illustration of the linear ball bearing slides. 57Figure 5.1.2.5.3 illustration of the crossed roller bearing slides. 58 Figure 5.1.2.5.4 the guided linear sliding system. 59 Figure 5.1.3.1 the spindle assembly. 61 Figure 5.3.1 the drive rack and the G201A inside. 66 Figure 6.1 the first diagnostic plot of the new machine using a 50 mm ballbar. 69 Figure 6.2-1 diagnostic plot of the second test on a 100 pm plot scale as the first test.72 Figure 6.2-2 diagnostic plot of the second test on a 50 pm plot scale. 72 Figure 6.3 diagnostic plot of the final test. 74 Figure 7.1 percent deviation from the compromised performance values. 79 Figure 8.1 self aligning linear bearing may cause unwanted movement of the axis 82 List of Symbols and Abbreviations CNC Computer Numerical Control mm millimeter m meter pm micro meter 9 theta, the value quoted for squareness by the diagnostic software Dy the wavelength of the cyclic sinusoidal error ASME American Society of Mechanical EngineersCW Clockwise CCW Counter-Clockwise ISO International Organization for Standardization JIS Japanese Industrial Standard oz.in. ounce per inch RPM Revolution Per Minute VAC Volts of Alternating Current Ibf pounds of force lb pounds of weight Deg. degree CMM Coordinate Measuring Machine DC Direct Current Fa axial force L lead of a ball screw (inches) T torque e efficiency n pi(p belt inclination angle C distance between centers of pulleys Ri radius of the motor pulley Ri radius of the screw pulley rad radians F B m a x the maximum radial force a angle of warp of smaller pulleycoefficient of friction between pulley HP Horse Power AFBMA Anti Friction Bearing Manufacturers Association P equivalent load Fr applied constant radial load V rotation factor X radial factor Y thrust factor L fatigue life expressed in millions of revolutions C the basic dynamic load rating NC Numerical Control CAD Computer Aided Design CAM Computer Aided Manufacturing DOS Disk Operating System PCI Peripheral Component Interconnect EMC Enhanced Machine Controller API Application Programming Interface NIST National Institute of Standards and Technology GUI Graphical User Interface MDI Machine Device Interface PC Personal Computer
TIL Transistor - Transistor Logic.
