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. 57
Figure 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 Engineers
CW 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