CNC (Computer Numerical Control)

Question 1

What does CNC stand for?

Answer 1

Computer Numerical Control—machines follow coded instructions (G/M codes) to move tools and make parts.

Question 2

Give three common CNC machine types.

Answer 2

CNC mill, CNC lathe (turning centre), and CNC router (gantry).

Question 3

Basic CNC workflow (simple order).

Answer 3

CAD model → CAM toolpaths → post‑process to G‑code → set workholding/zeros → run (dry‑run first) → inspect.

Question 4

What is G‑code?

Answer 4

The programming language used by CNC machines (e.g., G00 rapid, G01 feed move, M03 spindle on).

Question 5

What is an M‑code?

Answer 5

Machine function commands such as spindle/coolant on/off or program stop (e.g., M03, M08, M30).

Question 6

Define axes on a 3‑axis mill (right‑hand rule).

Answer 6

X = left/right, Y = front/back, Z = up/down (tool moves in +Z away from work).

Question 7

Define axes on a 2‑axis lathe.

Answer 7

Z = along spindle (length), X = radius/diameter direction; the work rotates, tool moves X/Z.

Question 8

Absolute vs incremental moves (codes).

Answer 8

G90 = absolute from the datum; G91 = incremental from the current position.

Question 9

Metric vs inch units (codes).

Answer 9

G21 = metric (mm); G20 = inch (in).

Question 10

Rapid vs feed move (codes).

Answer 10

G00 = rapid non‑cutting move; G01 = linear cutting move at feed F.

Question 11

Arc moves (codes and centres).

Answer 11

G02 = clockwise, G03 = counter‑clockwise; I, J (and K) define arc centre offsets in X, Y (Z).

Question 12

Common canned drilling cycles (mill).

Answer 12

G81 spot/straight drill, G83 peck drill (chip break/clear).

Question 13

What is a tool change command?

Answer 13

Txx to select tool number; M06 to execute tool change (varies by controller).

Question 14

Spindle and coolant basics (codes).

Answer 14

M03 spindle on CW, M04 CCW, M05 off; M07/M08 coolant on (mist/flood), M09 off.

Question 15

What is a work offset (example)?

Answer 15

Work coordinate like G54 defines the part datum relative to machine zero; others include G55–G59.

Question 16

What are tool length offsets (mills)?

Answer 16

Values that compensate for tool length so Z positions are accurate (e.g., G43 Hxx).

Question 17

Why is a safe Z/clearance plane important?

Answer 17

Prevents collisions with clamps while rapiding between features.

Question 18

Climb vs conventional milling (simple).

Answer 18

Climb: cutter pulls into work (better finish on tight machines). Conventional: pushes against; safer for backlash‑prone setups.

Question 19

Three typical 2D mill toolpaths.

Answer 19

Pocketing, contour/profile, and drilling/peck drilling.

Question 20

Two 3D finishing strategies (mills/routers).

Answer 20

Parallel (raster) and scallop/contour finishing of curved surfaces.

Question 21

Router workholding—two methods.

Answer 21

Vacuum table (with spoilboard) and mechanical hold‑downs/clamps (add tabs).

Question 22

Why add tabs or micro‑joints on routing?

Answer 22

They hold parts in place during through‑cuts to prevent movement/tip‑ups.

Question 23

Lathe basic operations (three).

Answer 23

Facing, turning (profiling), and drilling/boring; also grooving/parting and threading.

Question 24

What is a postsprocessor in CAM?

Answer 24

It converts CAM toolpaths into machine‑specific G‑code dialect for your controller.

Question 25

Give a simple safety block/header example.

Answer 25

(Example) % ; G21 G90 ; G17 ; G40 G49 G80 ; (units mm, absolute, XY plane, cancel comps/cycles)

Question 26

What is cutter radius compensation?

Answer 26

G41/G42 offsets toolpath by tool radius so small size tweaks can be made without reposting (wear comp).

Question 27

Chip load concept (formula).

Answer 27

Feed (mm/min) = chip load per tooth × number of teeth × RPM.

Question 28

Step‑over vs step‑down—effect?

Answer 28

Step‑over (sideways) and step‑down (vertical) affect tool load, finish, and cycle time.

Question 29

Entry strategies—why ramp/helix instead of plunge?

Answer 29

Reduces tool load and improves chip evacuation and surface quality.

Question 30

Dry‑run/air‑cut—purpose.

Answer 30

Simulate motion above the part or with spindle off to verify paths before cutting material.

Question 31

Three pre‑run checks on a mill/router.

Answer 31

Correct work offset zeroed, tool length/diameter set, and clamps clear with safe Z verified.

Question 32

E‑stop and interlocks—why critical?

Answer 32

They allow instant stop and prevent operation when guards/doors are open—reducing injury risk.

Question 33

Chip/coolant management—two tips.

Answer 33

Use appropriate coolant or air blast; remove chips frequently to avoid recutting and heat buildup.

Question 34

Wood/acrylic routing—extraction note.

Answer 34

Use dust/extraction and PPE—fine dust and melted chips can be hazardous and foul motion.

Question 35

Lathe datum setting (simple method).

Answer 35

Touch off X on a known diameter and Z on the face; set work offset so programmed sizes match measurements.

Question 36

Mill datum setting (simple).

Answer 36

Probe/edge‑find X/Y on a corner and touch Z off on the top surface or gauge block; store in G54.

Question 37

What is runout and why care?

Answer 37

Spindle/tool concentricity error—causes poor finish, size error, and tool wear.

Question 38

Soft jaws—why use them?

Answer 38

Custom‑bored jaws hold irregular parts accurately and repeatably on the lathe or mill vise.

Question 39

Parallels and vise stop—purpose.

Answer 39

Parallels raise the part for through‑cuts; a stop repeats the part position for multiple pieces.

Question 40

Simple G‑code line—explain: ‘G01 X25.0 Y10.0 F600’.

Answer 40

Linear feed move to X25 Y10 at 600 mm/min using the current Z and tool.

Question 41

Program end commands (two).

Answer 41

M30 (end and rewind) or M02 (end).

Question 42

What is toolpath simulation used for?

Answer 42

To check for collisions, gouges, and stock limits; estimate cycle time before running the machine.

Question 43

Tolerance vs surface finish—link to toolpaths.

Answer 43

Tighter tolerance and finer finish usually require smaller step‑over/step‑down and slower feed.

Question 44

Three common CNC controller brands (examples).

Answer 44

FANUC, Haas, Siemens (syntax may vary).

Question 45

What is machine home vs work zero?

Answer 45

Home is machine reference (limits/encoders); work zero (G54 etc.) is the part datum used in the program.

Question 46

Backlash/looseness—two symptoms.

Answer 46

Size variability on reversals and visible marks when climb milling on a loose machine.

Question 47

Basic aluminium milling starter settings (rule‑of‑thumb).

Answer 47

Use sharp carbide, moderate RPM, light step‑over/step‑down, and coolant/air blast; adjust from test cuts and manufacturer data.

Question 48

Router sheet cutting—kerf and finishing tip.

Answer 48

Account for tool diameter in offset; add lead‑in/out and tabs; finish with a final spring pass for clean edges.

Question 49

What is a spring pass?

Answer 49

A light repeat contour to remove elastic deflection marks and improve size/finish.

Question 50

Inspection after CNC (two actions).

Answer 50

Measure critical sizes (e.g., with caliper/micrometer) and check deburr/edge break to workmanship standard.

Question 51

School safety reminder (router/mill).

Answer 51

Keep hands clear, secure loose clothing/long hair, wear eye protection, and never open guards during operation.

Question 52

Exam tip: When asked to justify a CNC approach, name three points.

Answer 52

Material and workholding, required tolerance/finish, and the safest/most efficient toolpath strategy.