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Computer numerical control (CNC) machining is one of the world’s most widely used techniques for manufacturing parts because of its high precision. One of the key reasons for its success is the relative motion between the CNC workpiece and the tool. We can classify these movements as cutting and feed movements and measure them with cutting and feed speeds. What is cutting speed, and how is it different from feed rate? How do these processing cutting parameters contribute to the success of a manufacturing project? This article will answer all these questions and more.

The difference between cutting speed and feed rate

To help understand these two concepts, let’s consider a simple analogy of a car moving at a linear speed of 60 km/h, with the wheels rotating at 500 rpm. The diameter of the wheel and its rotation make the car move on paved roads. But when you describe the speed of a vehicle, you explain it in kilometers per hour. Cutting speed can be compared to the linear speed of a car, which depends on the wheel’s diameter and the number of turns. It measures the linear distance the tool moves relative to the workpiece in a certain amount of time. Cutting speed is measured in millimeters per minute (mm/min), meters per minute (m/min), or feet per minute (ft/min).

On the other hand, the feed rate can be compared to the rotation of a car’s wheels. It is simply the distance the tool travels during one revolution of the part. We measure it in inches per revolution (inch/rev) or millimeters per revolution (mm/rev). Still using the example of an automobile, a wheel rotating at higher revolutions may use more power and wear faster than a wheel rotating at lower revolutions. This wear is caused by friction and heat between the tire and the road surface. Similarly, spindle speed affects tool life, cutting temperature, and power consumption. Feed rate also affects tool life and energy consumption during machining, but their impact is usually neglected compared to cutting forces. Feed rate, on the other hand, has a greater impact on machining time and surface finish of the machined part. This is important because the choice of cutting parameters affects the product’s final quality. The course of the machining process is different when the cutting speed is low and when it is high. This is why the selection of machining parameters is so important.

Choosing the optimal cutting speed

To determine the optimal cutting speed for a given machining project, the hardness of the workpiece and the tool’s strength must be considered. Hardness determines the material’s resistance to deformation caused by abrasion, dents, or scratches. Harder materials require special attention during machining, as they can easily shorten tool life. In general, the more complex the material, the lower the cutting speed should be. For example, materials such as titanium require lower cutting speeds than steel. The strength of the cutting tool plays an essential role in the allowable cutting speed for a Cutting operation. For example, high speeds can be used when machining tools from high-strength materials such as diamond and boron nitride, while high-speed steel tools require lower speeds.

Chip thinning and optimal feed rates

Chip thinning is a manufacturing defect that occurs when machining a workpiece with a cutting width of less than half the tool diameter. This reduces chip load (the amount of material removed during one revolution of the cutting tool), resulting in longer lead times. One way to reduce the impact of thinner chips is to machine the workpiece at high feed rates. This helps increase productivity and tool life. Now that you understand the difference between feed rate and Cutting speed, you will agree that these two machining parameters are essential in CNC machining. However, even if you choose the ideal cutting speed and feed rate, the success of your project depends on the shop you work with. Chipping affects the appropriate depth of cut.

Increasing the cutting speed based on the hardness of the workpiece material

The hardness of the Cutting tool material also has a big impact on the recommended cutting speed. The harder the drill bit, the higher the cutting speed. The softer the drill bit, the slower the recommended cutting speed.

-Carbon steel

-High-speed steel 

-Sintered carbide

Increase in cutting speed depending on the hardness of the cutting tool

Cutting speeds for material types:

  • Low carbon steel 40-140
  • Medium carbon steel 70-120
  • High-carbon steel 65-100
  • Free machining steel 100-150
  • Stainless steel, C1 302, 304 60
  • Stainless steel, C1 310, 316 70
  • Stainless steel, C1 410 100
  • Stainless steel, C1 416 140
  • Stainless steel, C1 17-4, pH 50
  • Alloy steel, SAE 4130, 4140 70
  • Alloy steel, SAE 4030 90
  • Tool steel 40-70
  • Cast iron- ordinary 80-120
  • Hard cast iron 5-30
  • Gray cast iron 50-80
  • Aluminum alloys 300-400
  • Nickel alloy, Monel 400 40-60
  • Nickel alloy, Monel K500 30-60
  • Nickel alloys, Inconel 5-10
  • Cobalt-based alloys 5-10
  • Titanium alloy 20-60
  • Unalloyed titanium 35-55
  • Copper 100-500
  • Common bronze 90-150
  • Hard bronze 30-70
  • Zirconium 70-90
  • Brass and aluminum 200-350
  • Non-metallic materials not containing silicon 100-300
  • Non-metallic materials containing silicon 30-70

Spindle speed (spindle speed)

After determining the SFM for a given material and tool, the spindle speed can be calculated, as this value depends on the cutting speed and tool diameter:

RPM = (CS x 4) / D


  • RPM = Revolutions per minute.
  • CS = Cutting speed in SFM.
  • D = Tool diameter in inches.

Milling feed rate

Tool feed can be defined as the distance in inches per minute that the work moves into the cutter. On milling machines, the feed rate is independent of the spindle speed. This is great for faster feed rates and for larger, slow-moving tools.

Feed per tooth

Feed per tooth is the amount of material that each tooth of the tool should remove as it rotates and moves toward the workpiece. As the workpiece moves toward the tool, each tooth of the tool moves equally, producing chips of equal thickness. The chip thickness or feed per blade and the number of teeth in the tool are the basis for determining the feed rate. The ideal cutting speed and feed rate are measured in inches per minute (IPM) and are calculated using the following formula:

IPM = F x N x RPM 


  • IPM = feed rate in inches per minute
  • F = feed rate per tooth
  • N = number of teeth
  • RPM = revolutions per minute

For example:

The feeds for end mills used in vertical milling machines range from 001 to 002 feeds per tooth for very small diameter cutters on steel workpieces to 010 feeds per tooth for large cutters in aluminum workpieces. Since the cutting speed for mild steel is 90, the RPM for a high-speed 3/8″ double-round cutter is:

RPM = CS x 4 / D = 90 x 4 / (3/8) = 360 /.375 = 960 RPM. 

To calculate the feed rate, we will choose .002 inches per tooth

IPM = F x N x RPM = .002 x 2 x 960 = 3.84 IPM

Machine feed rate

The machine motion that causes the cutting tool to cut deep into or along the surface of the workpiece is called the feed rate. When cutting metal, feeds are usually measured in thousandths of an inch. Feed is represented slightly differently in different types of machines. Drilling machines with a motorized feed are designed to move the drill bit by a certain amount each time the spindle rotates. If the feed is set to 0.006″, the machine will move 0.006″ per spindle revolution. This is expressed in inches per revolution (IPR).

Threading procedure

Threading guides are an integral part of creating usable straight threads. The threader is already refined and centered when using a lathe or milling machine. Be careful when setting threaders by hand because a 90° threader guide is much more accurate than the human eye. It is very important to use oil when drilling and tapping. It keeps the drill bit from squeaking, the cut is smoother, chips are removed, and the drill and material do not overheat.


Spot drilling prevents the drill bit from overheating and breaking when drilling or tapping. It involves drilling through a portion of the part, then withdrawing the drill bit to remove the chips and allowing the piece to cool. A common practice is to turn the chuck a full turn and then back out a half turn. After each withdrawal of the drill or threader, remove as many chips as possible and oil the surface between the drill or threader and the workpiece.