Common Conversions Every Machinist Knows
You will use some conversions so often that you should know them by heart. Here are the ones that come up every day in a machine shop.
Length
- 1 inch = 25.4 mm (exact)
- 1 thou (mil) = 0.001 inch = 0.0254 mm
- 1 micron = 0.001 mm = 0.00004 inch
- 1 foot = 304.8 mm
Pressure and Stress
- 1 MPa = 145 PSI (roughly)
- 1 ksi = 1,000 PSI = 6.895 MPa
- 1 bar = 14.5 PSI (roughly)
Temperature
- Room temp: 68°F = 20°C
- Water boils: 212°F = 100°C
- Quick formula: °C = (°F − 32) × 5/9
Surface Finish
- 1 µm Ra = 39.37 µin Ra
- Standard CNC finish: 63–125 µin Ra (1.6–3.2 µm)
- Fine finish: 16–32 µin Ra (0.4–0.8 µm)
Tip: Keep a small reference card at your workstation. Even the best machinists double-check their conversions. Getting it right the first time saves scrap and rework.
Hardness
- Mild steel: ~80–90 HRB (too soft for HRC)
- Heat-treated 4140: ~28–32 HRC
- Tool steel (hardened): ~58–62 HRC
- HRC 60 is very hard. Most cutting tools are above HRC 60.
Weight and Torque
- 1 lb = 0.4536 kg
- 1 kg = 2.205 lbs
- 1 ft-lb = 1.356 N·m
- 1 N·m = 8.851 in-lbs