Whether you're doing manual programming or using CAM (the logic runs parallel), CNC part program preparation follows a five-step sequence. Below, each step is laid out together with the common mistakes worth watching.

The Five Steps

Step 1: Drawing Analysis and Process Planning

Before touching any code, settle these things first:

• Read the drawing: dimension chains, tolerances (±0.05? ±0.01?), geometric tolerances, surface roughness, material (aluminum / steel / stainless / titanium — cutting parameters differ a lot).

  
  

• Define the process route: roughing first, then finishing; face before hole; primary features before secondary. Avoid repeated setups.

  
  

• Decide workholding: vise / clamp / dedicated fixture / 4th-axis rotary table. Consider rigidity, tool clearance, and anti-deformation.

  
  

• Set machining sequence: which face first, which face carries the datum forward.

  
  

📌 Get this step wrong, and everything downstream is rework.

Step 2: Numerical Calculation (Math Processing)

• Calculate all tool center point coordinates (X/Y/Z), especially non-integer dimensions, arc tangent points, chamfers, ball-nose cutter contact points.

  
  

• For complex surfaces CAM handles it, but manual programming still needs: base points, nodes, arc centers, intersections.

  
  

• Factor in cutter radius compensation direction (G41/G42) and length compensation number.

  
  

⚠️ Common pitfalls: drawing is metric but the machine is set to imperial; missing decimal point (on some controls G01 X50vs X50.behave differently); arc I/J/K direction reversed.

Content layers:

1. Program header: coordinate system, absolute/incremental, cancel compensations, tool change, spindle start.

   
   

2. Machining body: roughing → semi-finish → finish, leave allowance per pass.

   
   

3. Program tail: retract, return to zero or tool-change position, coolant off, M30.

   
   

Step 4: Program Verification (Simulation / Dry Run)

Do three things before running on the real part:

• Software simulation: VERICUT / Mastercam backplot / machine's built-in graphics — check collisions, overtravel, table crash.

• Dry run + single block: Machine Lock mode, watch coordinate movement.

• First Article Trial Cut: run one piece in aluminum / modeling board / free-cutting stock, measure, then switch to real material.

⚠️ Skip this step = gambling. One spindle crash costs more than ten reports.

Step 5: Trial Correction and Archiving

• Full First Article Inspection (FAI), adjust compensation / feed / speed against the drawing.

  
  

• Finalize the program with comments, version number, tool list, and fixturing sketch archived together.

  
  

• If tooling or material changes during production, update the program synchronously — don't let "program unchanged, tool swapped" cause a defect.

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• Key Pitfalls by Stage (Summary Table)

One point worth repeating: About 70% of CNC programming effort sits in Step 1 (process)and Step 2 (math). The code itself is the easiest part. A lot of people spend time debugging G-code syntax, but the real accidents almost always trace back to "process not thought through + first article not verified.