Cutting speed (Vc) is a critical parameter in CNC machining. It directly impacts tool life, surface finish, risk of tool burn, vibration marks, and overall machining efficiency. Incorrect speed settings not only degrade part quality but also accelerate tool wear.
What is Cutting Speed (Vc)
Cutting speed refers to the instantaneous linear speed of the tool’s cutting edge relative to the workpiece, measured in meters per minute (m/min). It serves as the primary basis for spindle speed settings while controlling heat generation, tool wear, and surface finish quality.
Core Function
Accurately calculating Vc ensures the tool operates at a safe temperature, minimizing the risk of edge chipping or tool burning, and maintains the desired dimensional accuracy and surface quality.
Standard Cutting Speed Formula
The industry-standard formula is: N = (1000 × Vc) ÷ (π × D)
Here, N is spindle speed in rpm, Vc is the material’s recommended cutting speed in m/min, D is the tool diameter in mm, and π is approximated as 3.14.
In short: choose Vc according to the material, then calculate spindle speed based on the tool diameter.
Typical Cutting Speed Reference (Vc)
Aluminum alloys and copper: 800–1500 m/min, suitable for high-speed, light cuts.
45# steel and standard carbon steel: 180–280 m/min.
Mold steels P20/718: 120–180 m/min.
Hardened steels H13/S136: 60–100 m/min.
Stainless steel 304: 40–80 m/min, to prevent work hardening and tool sticking.
Plastics and acrylics: 1500–3000 m/min, for ultra-high-speed cutting.
Practical Calculation Examples
Example 1: Aluminum part, D10 tool, Vc = 1000 m/min. N = (1000 × 1000) ÷ (3.14 × 10) ≈ 31,800 rpm.
Example 2: Standard steel part, D10 tool, Vc = 200 m/min. N = (1000 × 200) ÷ (3.14 × 10) ≈ 6,360 rpm.
Example 3: Stainless steel, D6 tool, Vc = 60 m/min. N = (1000 × 60) ÷ (3.14 × 6) ≈ 3,180 rpm.
Roughing and Finishing Speed Adjustments
For roughing, reduce spindle speed by 20–30% and increase feed to prevent tool burn and chipping. Finishing can use slightly higher speeds and reduced feed to improve surface quality. Deep cavities, long tools, or extended tool overhangs require more than a 30% speed reduction to minimize vibration.
Influence of Tool Material on Cutting Speed
High-speed steel tools are suited for lower-speed operations due to limited heat resistance. Standard carbide tools work at typical speeds and are ideal for general production. Coated carbide tools can increase cutting speed by 20–50%, offering better heat resistance and wear performance, suitable for high-demand machining.
Common Problems from Incorrect Speed
Excessive spindle speed can overheat the tool, cause rapid wear, chipping, or even burn the workpiece. Too low a speed results in rough tool marks, sticking, work hardening, and reduced efficiency. Stainless steel is particularly prone to hardening if cut too quickly, becoming progressively harder to machine.
Quick Factory Estimation Rules
Aluminum runs fast, steel runs medium, and stainless steel runs slow. Larger-diameter tools require lower spindle speeds; smaller tools can rotate faster. Reduce speed for roughing while increasing feed, and increase speed for finishing while reducing feed.
Conclusion
The essence of cutting speed calculation is first to select Vc based on the material, then calculate spindle speed from the tool diameter, and fine-tune according to the machining conditions. Mastery of these formulas and reference parameters effectively prevents tool burning, vibration marks, poor surface finish, and excessive tool wear, making it essential for CNC programming and practical operation.
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FAQ
What is cutting speed?
Cutting speed is the instantaneous linear speed of a tool relative to the workpiece, directly affecting tool life, precision, and surface finish.
How do you calculate spindle speed based on material?
Use the formula N = (1000 × Vc) ÷ (π × D). Choose Vc for the material, then calculate speed using the tool diameter.
What happens if the cutting speed is too high?
Excessive speed can overheat the tool, cause rapid wear, chipping, or burn the workpiece.
Do cutting speeds vary between materials?
Yes. Aluminum and plastics are suited for high speeds; stainless steel requires lower speeds to prevent hardening; steel falls in the mid-speed range.
How should roughing and finishing speeds be optimized?
For roughing, reduce spindle speed and increase feed; for finishing, increase speed and reduce feed to balance efficiency and surface quality.