29 Key Points to Note in CNC Machining – Yumei

29 Key Points to Note in CNC Machining

1. Main factors affecting cutting temperature and cutting force

• Cutting temperature: cutting speed → feed rate → depth of cut.
• Cutting force: depth of cut → feed rate → cutting speed.
• Tool life: cutting speed → feed rate → depth of cut.

1. Effect of parameter changes on cutting force

• Doubling the depth of cut doubles the cutting force.
• Doubling the feed rate increases cutting force by about 70%.
• Doubling the cutting speed gradually reduces cutting force.
• When using G99, increasing cutting speed has little effect on cutting force.

1. Using chips to assess cutting conditions
   Chip shape and color can indicate whether cutting force and cutting temperature are within a normal range.
2. Notes when turning concave arcs
   If the difference between the measured value X and the drawing diameter Y exceeds 0.8 mm, using a 52° secondary cutting-edge angle (commonly a 35° insert with a 93° main cutting-edge angle) may cause tool rubbing at the arc’s starting point.
3. Chip color–temperature reference

• White: < 200 °C
• Yellow: 220–240 °C
• Dark blue: ~290 °C
• Blue: 320–350 °C
• Purple–black: > 500 °C
• Red: > 800 °C

1. Default G-code settings in FANUC Oi MTC

• G21: Metric input
• G25: Spindle speed fluctuation detection OFF
• G54: Default work coordinate system
• G18: ZX plane selection
• G96 / G97: Constant surface speed control / Cancel
• G99: Feed per revolution
• G40: Cancel tool nose radius compensation (G41/G42)
• G22: Stroke limit detection ON
• G67: Cancel modal macro call
• Others such as G69, G64 require reference to machine documentation.

1. Thread dimensions

• External thread minor diameter ≈ 1.3 × pitch (P)
• Internal thread minor diameter ≈ 1.08 × pitch (P)

1. Thread cutting speed formula
   Spindle speed S = 1200 ÷ pitch × safety factor (typically 0.8).
2. Manual tool nose radius compensation for chamfering

• Bottom-up chamfer:
  Z = R × [1 − tan(a/2)]
  X = Z × tan(a)
• For top-down chamfering, replace subtraction with addition.

1. Feed–speed adjustment
   For every 0.05 mm/rev increase in feed, reduce spindle speed by 50–80 rpm to slow tool wear and stabilize cutting force and temperature.
2. Relationship between cutting speed, cutting force, and tool failure
   Higher cutting speed reduces cutting force but accelerates tool wear, which in turn increases cutting force and temperature, potentially leading to tool breakage.
3. Important tips for CNC turning
   • Economy CNC lathes with variable frequency drives may have insufficient torque at low speeds—avoid heavy cuts unless gear reduction is available.
   • Ensure a tool can finish a part or shift without replacement, especially for finishing large parts.
   • Use higher spindle speeds for threading to improve quality and efficiency.
   • Use G96 constant surface speed whenever possible.
   • High-speed machining pairs high cutting speeds with high feed rates and small depths of cut to keep heat in the chips, not the workpiece.
   • Compensate for tool nose radius.
4. Causes of tool breakage and vibration when grooving
   Excessive cutting force or insufficient tool rigidity are the main causes. Shorter overhang, larger insert contact area, and smaller clearance angles improve rigidity.
5. Common causes of vibration during grooving
   • Excessive tool overhang.
   • Feed rate too slow, increasing unit cutting force.
   • Machine rigidity insufficient to handle cutting forces.
6. Reason for unstable dimensions over time
   Tool wear increases cutting force, which may cause the workpiece to shift in the chuck, leading to dimensional drift.
7. G71 usage note
   In FANUC systems, P and Q values must not exceed the total number of program blocks, or an alarm will occur.
8. FANUC subprogram formats
   • Format 1: P0000000 (first three digits = repetitions, last four digits = program number).
   • Format 2: P0000L000 (first four digits = program number, L + last three digits = repetitions).
9. Arc end-point Z offset effect
   A Z-direction shift of the arc end-point will offset the arc’s bottom diameter by half that amount.
10. Deep-hole drilling
    Grind chip evacuation grooves on the drill to aid chip removal.
11. Tooling hole adjustment
    When using a fixture-mounted drill, rotating the drill slightly can change hole size.
12. Drilling stainless steel
    Use a smaller center drill diameter; for cobalt drills, avoid grinding chip grooves to prevent annealing during drilling.
13. Common blank cutting methods
    • Single-piece cutting
    • Double-piece cutting
    • Full-bar cutting
14. Elliptical threads
    If threads turn out oval, they may be caused by workpiece looseness—lightly recut several passes with the threading tool.
15. Using macros instead of subprogram loops
    On systems supporting macros, macros can replace subprogram loops to save program numbers and reduce errors.
16. Boring with excessive runout
    If a drilled hole has excessive runout, use a flat-bottom drill instead of a twist drill; keep the drill short to improve rigidity.
17. Bore size consistency on a drill press
    Drilling directly with a twist drill may cause diameter deviations, but boring on a drill press usually keeps tolerance within ±0.03 mm.

---