Tolerance Standards and Surface Roughness for Various CNC Processes

Precision Levels in Machining Working closely with CNC machining, you’re likely familiar with machining precision. But how well do you know the “tolerance levels” used to express machining precision? Here’s an overview of the standard IT tolerance levels for various machining processes:

CNC lathe (turning machine) cutting brass shafts




IT Standard Tolerance Levels Table

Note: No IT14 to IT18 for basic sizes less than 1mm.

Basic Size (mm) IT01 IT0 IT1 IT2 IT3 IT4 IT5 IT6 IT7 IT8 IT9 IT10 IT11 IT12 IT13 IT14 IT15 IT16
1 0.3 0.5 0.8 1.2 2 3 4 6 10 14 25 40 60 100 140 250 400 600
3 0.3 0.5 0.8 1.2 2 3 4 6 10 18 30 48 75 120 180 300 480 750
6 0.3 0.5 0.8 1.2 2 3 5 8 12 21 33 52 84 130 210 350 550 900
10 0.3 0.5 0.8 1.2 2 4 6 10 15 25 40 64 100 160 250 400 640 1000
18 0.3 0.5 0.8 1.2 3 5 8 12 18 30 48 76 120 190 300 480 760 1200
30 0.3 0.5 1 1.5 4 6 9 15 22 36 58 92 150 240 380 600 950 1500
50 0.4 0.6 1.2 2 5 8 12 18 25 44 70 110 180 290 460 740 1200 2000
80 0.4 0.6 1.2 2 6 10 14 22 32 52 84 130 210 340 540 870 1400 2300
120 0.5 0.8 1.5 2.5 8 12 18 25 36 58 92 150 240 380 600 950 1500 2500
180 0.6 1 2 3 10 14 22 32 46 72 120 190 300 480 760 1200 1900 3000
250 0.8 1.2 2.5 4 12 18 25 36 52 84 130 210 340 540 870 1400 2300 3600
315 1 1.5 3 5 15 21 30 46 66 100 160 250 400 630 1000 1600 2500 4000
400 1.2 2 4 6 18 25 36 52 75 120 190 300 480 760 1200 1900 3000 4800
500 1.5 2.5 5 8 21 30 44 66 100 150 240 380 600 950 1500 2300 3600 5800
630 2 3 6 10 25 36 52 75 120 180 290 460 740 1200 1900 3000 4800 7500
800 2.5 4 8 12 30 46 66 100 150 210 340 540 870 1400 2300 3600 5800 9000
1000 3 5 10 15 35 52 75 120 180 250 400 640 1000 1600 2500 4000 6400 10000
1250 4 6 12 18 40 66 100 150 210 300 480 760 1200 1900 3000 4800 7500 12000
1600 5 8 15 21 50 75 120 180 250 360 580 950 1500 2400 3800 6000 9500 15000
2000 6 10 18 25 60 90 150 210 300 420 670 1100 1800 2900 4600 7500 12000 18000
2500 8 12 21 32 70 110 180 250 360 520 840 1300 2100 3300 5400 8700 14000 23000




Turning

cnc turning brass

  • Description: The workpiece rotates while the cutting tool moves linearly or along a curve to cut. Used for machining internal and external cylindrical surfaces, end faces, conical surfaces, formed surfaces, and threads.
  • General Precision: IT8IT7, Surface Roughness: 1.60.8μm.
  • Rough Turning:
    • Precision: IT11
    • Surface Roughness: Rα20~10μm.
  • Semi-Finish and Finish Turning:
    • Precision: IT10~IT7
    • Surface Roughness: Rα10~0.16μm.
  • Mirror Turning:
    • Precision: IT7~IT5
    • Surface Roughness: Rα0.04~0.01μm.



Milling

5 axis CNC machining steel parts

  • Description: Uses a rotating multi-edge tool to cut the workpiece. Suitable for machining planes, grooves, various formed surfaces, and special shapes like splines, gears, and threads.
  • **### Tolerance Levels for Different Machining Processes

Precision Levels in Machining Working closely with CNC machining, you’re likely familiar with machining precision. But how well do you know the “tolerance levels” used to express machining precision? Here’s an overview of the standard IT tolerance levels for various machining processes:



Planing

Planing involves using a planer tool to perform horizontal reciprocating linear motion on the workpiece, primarily used for shaping parts.

  • General Precision: IT9IT7, Surface Roughness: Ra6.31.6μm.
    1. Rough Planing:
      • Precision: IT12~IT11
      • Surface Roughness: 25~12.5μm.
    2. Semi-Finish Planing:
      • Precision: IT10~IT9
      • Surface Roughness: 6.2~3.2μm.
    3. Finish Planing:
      • Precision: IT8~IT7
      • Surface Roughness: 3.2~1.6μm.



Grinding

Cylindrical Grinding

Grinding involves using abrasives to remove excess material from the workpiece, widely used in precision machining within the manufacturing industry.

  • General Precision: IT8IT5 or higher, Surface Roughness: 1.250.16μm.
    1. Precision Grinding:
      • Surface Roughness: 0.16~0.04μm.
    2. Ultra-Precision Grinding:
      • Surface Roughness: 0.04~0.01μm.
    3. Mirror Grinding:
      • Surface Roughness: Below 0.01μm.



Drilling

Drilling Machines

Drilling is a basic method for hole-making, typically performed on drilling machines and lathes, but can also be done on boring and milling machines.

  • General Precision: Up to IT10, Surface Roughness: 12.5~6.3μm.
  • Post-Drilling Processes: Reaming and boring are often used for semi-finish and finish machining.



Boring

Boring

Boring uses a tool to enlarge a hole or other circular profile’s internal diameter, ranging from semi-rough to finish machining, with single-edged boring tools (called boring bars).

  • General Precision for Steel Materials: IT9IT7, Surface Roughness: 2.50.16μm.
    1. General Boring:
      • Precision: IT9~IT7
      • Surface Roughness: 2.5~0.16μm.
    2. Precision Boring:
      • Precision: IT7~IT6
      • Surface Roughness: 0.63~0.08μm.




IT Tolerance Grades by Machining Process

Machining Method 01 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Grinding X X X X X X X X X
Lapping X X X X X X X X X X
Honing X X X X X X X X X X
Fine Boring X X X X X X X X X X X
Stone Grinding X X X X X X X X X X X X
Diamond Turning X X X X X X X X X X X X
Broaching X X X X X X X X X X X
Reaming X X X X X X X X X X X
Turning X X X X X X X X X X X
Boring X X X X X X X X X X X
Drilling X X X X X X X X X X
Planing X X X X X X X X X
Shaping X X X X X X X X X
Extrusion X X X X X X X X
Stamping X X X X X X X
Casting X X X X X X
Powder Metallurgy X X X X X X
Die Casting X X X X X
Sand Casting X X X X
Forging X X X

The “X” marks indicate the achievable IT tolerance grades for each machining method.

Conclusion

Understanding the tolerance standards and surface roughness achievable with different CNC machining processes is crucial for achieving optimal manufacturing outcomes. Each process, from turning and milling to grinding and drilling, offers unique precision and roughness characteristics suited for various applications.

We’d love to hear your thoughts and experiences with different machining processes. What challenges have you faced, and how have you overcome them? Share your insights in the comments below!

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