Cutting and Feed Speed in Drilling: What to Know?

When you start drilling, your first thought is how fast the drill bit needs to go to do the job in the best way possible. So you look for the best speed tables for HSS drill bits relative to materials. But that's not the final goal, or at least not just that. It will be completely insufficient to use this element as the only parameter: anyone who works professionally with drilling operations knows there are other parameters to evaluate. Like cutting speed and feed rate.

No, they're not the same thing but they fall into the varied world of formulas and definitions for structured drilling. It's impossible to proceed quickly in this world of high-precision operations without knowing how to calculate speeds and feeds.

That's why today we'll talk about two essential (and often confused) parameters: cutting speed and feed per revolution. What exactly are we talking about? What are the formulas for calculating these values? Keep reading to deepen these essential points.

What Is Cutting Speed in Drilling?

By this term we mean the speed at which the cutting edge of the drill bit traverses the material you're drilling, removing the chip. In practice, it's the peripheral speed of the cutting edge. This parameter is measured in meters per minute and represents a decisive element for achieving adequate work quality. In fact, each material you want to drill has its ideal cutting speed. For example, mild steel is drilled at about 25-30 m/min, while aluminum can reach 200-300 m/min.

Want to find a rule? The greater the hardness, the lower the cutting speed. Stainless steel requires lower speeds. Productivity and quality of the hole are the two parameters influenced by cutting speed, but don't forget that this number also influences tool life: drilling hard metal at an inadequate cutting speed can lead to premature wear.

The parameters that determine cutting speed are always determined by field testing, but you must remember that there are conditions that lower the speed by several percentage points: large diameter reduces speed by 10-20%, deep drilling by 20-30%, and lack of lubricant can reach 50% less. What is the calculation formula? Vc = (π × D × n) / 1000.

What are the parameters? Vc stands for cutting speed (m/min), π is pi (≈ 3.14), D is the diameter of the drill bit in millimeters, n is spindle speed in revolutions per minute (RPM). Everything is divided by 1000, conversion factor from millimeters to meters.

Definition of Feed Rate

Unlike the first metric, feed rate in drilling is the speed at which the drill bit penetrates the material. This element is usually defined in millimeters per revolution (mm/rev) or millimeters per minute (mm/min). To summarize, we can say that feed rate indicates how deep the bit goes with each complete revolution.

Why is feed rate fundamental? Simple, if you overdo it you risk breaking the bit, straining the spindle (with resulting breakage) and creating imprecise holes with poor finishes. When feed rate is too low, the bit slides, overheats the workpiece, wears unnecessarily. Optimal feed rate varies based on several parameters:

1. Material to drill.
2. Hole diameter.
3. Type of bit.
4. Presence of coolant.
5. Type of hole.

For example, when you drill mild steel with HSS bits you use feeds between 0.1-0.3 mm/rev, while for aluminum you can go faster and reach 0.3-0.5 mm/rev. The formula for calculating feed rate: Vf = f × n where f stands for feed per revolution (mm/rev) and n for spindle rotation speed (revolutions/min or RPM).

What's the Difference Between These Measures?

Cutting speed and feed rate in drilling are not the same thing, but both measures are fundamental for quality drilling. The cutting parameter (Vc) is the tangential velocity at which the cutting edge of the drill bit moves relative to the material. Feed (f or Vf) is the speed at which the drill bit penetrates deep into the material, how deep it goes with each revolution or in the time unit. Where do you find these numbers? Usually in the reference tables of the various bits. For example:

• C40 steel with HSS drill bit Ø10mm: Vc = 25 m/min, f = 0.15 mm/rev.
• Gray iron with HSS drill bit Ø10mm: Vc = 20-25 m/min, f = 0.20-0.30 mm/rev.
• Aluminum with HSS drill bit Ø10mm: Vc = 80-120 m/min, f = 0.25-0.40 mm/rev.

What Are the Related Parameters?

Spindle rotation speed or RPM is fundamental for both measures, but in different ways. Cutting speed (Vc), for example, is directly proportional to revolutions per minute. The more revolutions you make with the spindle, the mechanical element that holds the drill bit, the higher the cutting speed. Therefore, revolutions per minute determine the cutting speed along with the diameter of the drill bit. With feed rate there's also a relationship: by increasing RPM the bit penetrates the material faster