CNC Cheat Sheet 2026

The 30 highest-yield CNC facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. Which of the following is a key inspection point for the CNC machine's electrical connections during an annual PM? → Check for loose terminals, corroded contacts, and damaged insulation
2. Under OSHA 29 CFR 1910.147, which of the following is NOT an acceptable energy isolation device for CNC LOTO? → A CNC program that commands all axes to a safe position
3. On a drawing, a surface finish symbol with a value of 63 (Ra) means the surface must have a roughness average of: → 63 microinches Ra average roughness
4. Which of the following best describes 'axial depth of cut' (ADOC) in CNC milling? → The depth the tool engages the workpiece along the spindle axis
5. What does a 'datum feature symbol' (a letter in a square box with a triangle) identify on a drawing? → A reference origin surface or feature used for measurement and location
6. What is the purpose of checking backlash during CNC machine preventive maintenance? → To measure lost motion between the servo drive and the axis leadscrew or ball nut
7. Which measurement tool can provide readings in both metric and imperial units? → Digital caliper
8. A part drawing shows the concentricity symbol ◎ with a tolerance of 0.004 A. What is being controlled? → The median points of a feature must fall within a cylindrical zone coaxial to datum A
9. In the 3-2-1 locating principle, which surface receives the fewest locating points? → Tertiary datum — 1 point
10. What is the primary function of a tool length offset (TLO) in CNC milling? → To account for differences in tool length so Z-axis positions remain accurate
11. When programming a G71 OD roughing cycle on a Fanuc control, the 'U' value specifies: → The finishing allowance left on the diameter
12. What is the purpose of a chip breaker groove on an insert? → Curl and break chips to prevent long stringy chips
13. What is the primary ethical obligation of a CNC professional when a conflict of interest arises during g-code & m-code programming activities? → Disclose the conflict to all relevant parties and recuse from the decision if necessary
14. What is the purpose of specifying a 'dwell' (G04) command in a CNC boring cycle? → To pause spindle rotation at the bottom of the bore for a timed period to improve finish
15. A negative rake angle on a turning insert: → Provides a stronger cutting edge
16. In a face milling operation, what does 'radial depth of cut' (stepover) represent? → The width of the cutting path measured perpendicular to feed direction
17. What does the surface finish symbol 'Ra' represent in machining drawings? → Arithmetic average roughness of a surface profile
18. In CNC milling, 'climb milling' differs from 'conventional milling' primarily in: → Whether chip thickness decreases or increases from entry to exit
19. Why is it important to clean the machine after use? → To maintain performance and tool life
20. In a CNC program, what does the block 'N100 G28 G91 Z0' accomplish? → Returns the Z axis to the machine home position incrementally
21. On a CNC blueprint, a dimension shown as 1.500 ± 0.005 means the feature must be between what values? → 1.495 and 1.505
22. Which parameter is verified during a CNC spindle PM to detect bearing degradation before catastrophic failure? → Vibration signature analysis using an accelerometer
23. According to NFPA 70E, arc flash hazard analysis for CNC machine electrical panels is required when: → Any employee must interact with energized electrical equipment above 50 volts
24. A PM procedure for a CNC machine requires 'checking and adjusting the belt tension on the spindle drive.' What tool is used to measure belt tension? → A belt tension gauge (sonic tension meter or deflection gauge)
25. Which code in a CNC turning program specifies the spindle speed in surface feet per minute? → G96
26. Which GD&T control is the most comprehensive and can replace circularity, straightness, and taper controls for a cylinder? → Cylindricity ⌭
27. What is the purpose of zero return in CNC machines? → Set reference positions
28. Which of the following surface finish values represents the smoothest machined surface? → 16 Ra (micro-inches)
29. What does the abbreviation 'TYP' following a dimension on a CNC blueprint mean? → The dimension applies to all identical features unless otherwise specified
30. What is G-code used for in CNC programming? → To control machine operations