Numerical Control of Machine Tools

Numerical control (NC) is the control of a machine tool by coded numerical data (a program) giving tool movements and machine functions. Computer numerical control (CNC) uses a dedicated computer in the machine controller which stores the program in memory and can edit it.

NC and CNC

Main elements of a CNC machine

1. Part program - instructions in G and M codes.
2. Machine control unit (MCU) - reads the program, interpolates paths, issues commands.
3. Drive system - servo motors with ball screws that move the axes.
4. Feedback devices - encoders or linear scales, resolvers, tachogenerators.
5. Machine tool - slide ways, spindle, tool changer, coolant system.
6. Operator panel - display, keys, override switches.

Program -> MCU -> Amplifier -> Servo motor -> Table
             ^                                 |
             +------- feedback (encoder) ------+

Open and closed loop

• Open loop: MCU sends pulses to a stepper motor; no position feedback. Simple and cheap, but errors are not corrected; used on light machines.
• Closed loop: feedback of the actual position or velocity is compared with the command and the error is corrected. Accurate and used on most CNC machines.

Advantages

Higher productivity and accuracy, repeatability, lower scrap, reduced setup time and tooling, ability to machine complex contours, less skilled labour, and easy integration with CAD/CAM.

Justification

Machine initial cost and programming skills are high, so CNC is justified for batch production of medium volume and high variety, parts with complex shapes or tight tolerances, costly materials (scrap expensive), frequent design changes, and where flexible automation is needed. For very high volume of one part, a dedicated machine is cheaper; for one-off simple parts, a conventional machine is cheaper.

Meaning of the codes

G-codes are preparatory functions (motion and mode); M-codes are miscellaneous machine functions.

Assumptions

• Programming in FANUC-type format, metric units (G21), feed per revolution (G99), diameter programming (X is diameter).
• Work zero (Z0) at the finished face; X0 on the axis. Stock Ø40.
• Roughing passes at Ø36 and Ø32 (2 mm depth per side each) leave 1 mm per side; the finish pass cuts to Ø30.
• The chamfer is at the end of the 30 mm diameter, at Z-50: the diameter reduces to Ø27 at Z-50, so the full Ø30 ends at Z-48.5.

Program

O0100 (TURN DIA 30 X 50 WITH CHAMFER)
N10 G21 G99 G40 G97       (metric, mm/rev, cancel comp)
N20 T0101                 (select tool 1, offset 1)
N30 S800 M03              (spindle 800 rpm CW)
N40 G00 X42.0 Z2.0 M08    (approach, coolant on)
(FACING)
N50 G00 Z0.0
N60 G01 X-1.0 F0.1        (face to centre)
N70 G00 X42.0 Z2.0
(ROUGHING PASS 1 - DIA 36)
N80 G00 X36.0
N90 G01 Z-50.0 F0.2
N100 G00 X42.0 Z2.0
(ROUGHING PASS 2 - DIA 32)
N110 G00 X32.0
N120 G01 Z-50.0 F0.2
N130 G00 X42.0 Z2.0
(FINISH PASS WITH CHAMFER)
N140 G00 X30.0 Z2.0 S1200
N150 G01 Z-48.5 F0.08
N160 G01 X27.0 Z-50.0    (1.5 x 45 deg chamfer)
N170 G00 X42.0 Z2.0
N180 G28 U0 W0 M05       (home, spindle stop)
N190 M09                 (coolant off)
N200 M30                 (end of program)

Notes on the program

• Each roughing pass retracts to X42, Z2 at rapid speed so that the tool does not drag over the cut surface.
• Chamfer is a straight G01 move: it reduces the diameter by $2\times1.5 = 3$ mm over a length of 1.5 mm, giving 45 degree.
• Machining time of one roughing pass: $50/(0.2\times800) = 0.31$ min.

An industrial robot is a reprogrammable, multifunctional manipulator designed to move material, parts, tools or devices through programmed motions for a variety of tasks.

Main parts

Manipulator (links and joints), actuators (electric servo, hydraulic or pneumatic), controller, sensors and the end effector.

Configurations

Degrees of freedom

Each independent joint motion is one degree of freedom (DOF). Three DOF position the wrist at any point of space; three more (roll, pitch, yaw) orient the tool. A general-purpose robot therefore has six DOF.

End effectors

• Grippers: mechanical (jaw), vacuum cups, magnetic, adhesive.
• Tools: welding torch, spray gun, drill, grinder.

Applications

Material handling and palletising, machine loading and unloading, spot and arc welding, spray painting, assembly, inspection, and work in hazardous environments (forging, foundry). Advantages: repeatability, consistent quality, no fatigue, and safety. Limits: high cost and need for programming.