The spindle speed is the rotational frequency of the spindle of the machine, such as drilling machines, lathes, mills, and routers, measured in revolutions per minute (RPM).  The spindle speed of CNC drilling refers to the rotational speed of the spindle of the CNC machine tool during drilling operations.  The preferred spindle speed for drilling is determined by working backward from the desired cutting speed (SFM or m/min), which may be defined as the rate at the workpiece surface, and incorporating the diameter of the cutter or workpiece. CNC drilling is computerized process in which a cutting tool with sharp teeth rotates and feeds into the workpiece axially and create a hole with diameter equal to that of the tool (drill bit). In a drilling operation, the tool diameter is used. Drilling Spindle Speeds Directly affecting drilling efficiency, tool wear, and aperture accuracy, it needs to be comprehensively set based on material hardness, tool diameter, and machine performance.

Metric Drilling Spindle Speed Calculation Formula: n (RPM) = (1000 × V)/(π × D)

Imperial Drilling Spindle Speed Calculation Formula: n (RPM) = (12 × V)/(π × D)

Where:

• n = spindle speed (rev/min, RPM)
• V = cutting speed (m/min or ft/min)， Determined by the durability of the cutting tool;
• D = drill tool diameter (mm or in)

General Drill Spindle Speed Range

CNC drilling spindle speeds typically range from 1200 to 4500 rpm, with the specific speed determined based on the drill diameter and material hardness.

• Small drills (such as center drills) can reach speeds up to 1200 rpm, with feed rates of approximately 60-80 mm/min.
• Conventional drilling: For a 25 mm diameter, the speed is typically 300-450 rpm, and the feed rate can be adjusted to 70% of the drill diameter.
• High-speed drilling: When using a high-speed spindle (such as the RBZ series), speeds can reach 16,000 rpm and torque up to 0.28 Nm, making it suitable for efficient drilling of small holes.

Typically, smaller drill diameters increase speeds; larger diameters require lower speeds to avoid vibration. When drilling hard materials such as alloy steel, the speed should be reduced to 300-450 rpm to minimize tool wear. High-speed drilling requires a constant temperature cooling system to prevent spindle overheating and affect accuracy.

The cutting speed of CNC drilling is the linear velocity of the cutting edge of the drill bit, measured in meters per minute (m/min). The cutting speed reflects the relative motion speed between the cutting edge of the drill bit and the contact point of the workpiece, which directly affects the material removal efficiency and machining quality.

Metric Drilling Cutting Speed Calculation Formula: V (m/min)=(π× D × n)/1000

Imperial Drilling Cutting Speed Calculation Formula: V (ft/min)=(π× D × n)/12

Where:

• V = cutting speed (m/min or ft/min)
• D = drill tool diameter (mm or in)
• n = spindle speed (rev/min, RPM)

In CNC drilling, it is generally prioritized to maintain a large back-cut (ap = D/2), then balance feed rate and cutting speed. For example, for a 10mm drill diameter, the cutting speed is typically controlled within the range of 50-150m/min. Generally, larger drill diameters increase cutting speeds. Higher spindle speeds also increase cutting speeds. For harder materials, lower cutting speeds are required to minimize tool wear.

Feed rate for CNC drilling is the velocity at which the cutter is fed, or the speed of the cutting tool’s movement relative to the workpiece as the tool makes a cut, typically measured in inches per minute (IPM). Drilling feed rate is determined by the type of cutting tools, surface finish desired for milling parts, power available at the spindle, rigidity of the machine and tool setup, characteristics of the material to be cut, workpiece strength, cut width, etc.

Metric Drilling Feed Rate Calculation Formula: F (mm/min) = n×fz

Imperial Metric Drilling Feed Rate Calculation Formula: F (in/min, IPM) = n×fz

Where:

• fz = feed per revolution (mm/rev or in/rev), from tool/vendor
• F = feed rate (mm/min or in/min)
• n = spindle speed (rev/min, RPM)

Drilling Feed The feed rate is closely related to drill diameter and rotational speed. As drill diameter increases, the rotational speed must be reduced to maintain a stable cutting speed. Otherwise, excessive cutting forces or overheating may occur. Workpiece material properties such as hardness and strength directly influence feed rate selection. For example, for harder materials (such as stainless steel), the feed rate should be reduced to reduce drill wear, while for softer materials (such as aluminum alloy), the feed rate can be increased to improve efficiency. Coolant can reduce cutting temperatures, indirectly affecting feed rate. Insufficient coolant flow and pressure require a reduction in feed rate to prevent overheating and chip accumulation. The geometric parameters of different drills (e.g., straight drills and twist drills) (such as the helix angle and number of flutes) affect chip evacuation efficiency. If chip evacuation is poor, the feed rate should be reduced to prevent clogging.

CNC drilling cut time refers to the total time required from start to finish when drilling a hole using a CNC machine. It is primarily determined by the hole length, number of holes drilled, and spindle feed rate.

Drilling Cut Time Calculation Formula: Tc = L / F (If you know feed rate F) or Tc = L / (n  × fz) (If you know RPM and feed per rev)

Where:

• Tc = Cutting time (min)
• L = total drilling travel length (mm or in) along Z
• F = feed rate (mm/min or in/min)
• n = spindle speed (rev/min, RPM)
• fz = feed per revolution (mm/rev or in/rev)

Generally, higher material hardness increases cutting resistance, requiring a lower spindle speed or higher feed rate. Larger drill diameters increase cutting speeds (vc), but considerations should be given to heat dissipation and vibration. In addition, adequate cooling during the drilling process can extend tool life and reduce machining time.