CNC Machining Feeds and Speeds: What Should You Know

CNC Machining Feeds and Speeds
Jack Lie CNC machining expert

Specialize in CNC Milling, CNC Turning, 3D Printing, Urethane Casting, and Sheet Metal Fabrication Services.

Feeds and speeds are important parameters in CNC machining that directly affect the product quality, accuracy, and surface finish, as well as machining efficiency and tool life. But what factors affect these parameters, and how do you determine the optimum speed and feed for your machining process? This article is here to offer a detailed guide on speeds and feeds for CNC machining operations and explains how to calculate the optimal values.

Essential Terms Explained

When starting with the main body of this article, there are several important criteria you need to know:

  1. RPM: Revolutions per minute (RPM) is a measure of how fast a rotating object turns. This represents how fast your tool is rotating, but not how fast the flutes is moving.
  2. Feed rate: Feed rate is the speed at which the cutting tool moves relative to the workpiece, often measured in inches per minute (IPM).
  3. Feed per Tooth: Feed per Tooth is the rate at which the cutting tool moves into the material per tooth on the cutting tool, typically measured in inches per tooth (IPT).
  4. Surface Speed: Surface Speed is the speed at which the cutting tool is moving relative to the surface of the material being cut, typically measured in feet per minute (FPM). This parameter is crucial because a larger tool will cut at a lower RPM at the same speed as a smaller tool with a higher RPM.
  5. Width of Cut: Width of Cut, abbreviated as WOC or RDOC, is the distance the cutting tool travels across the face of the material. It controls how involved the tool diameter is in the cut and the width of the chip.
  6. Depth of Cut: Depth of Cut, shorted for DOC or ADOC, is the distance the cutting tool moves into the material. It controls the length of the flute and thus the height of the chip.

Speeds and Feeds in CNC Machining

Speeds and feeds concern different velocities of a cutting process. Cutting speed is the rate at which the cutting tool advances along the workpiece and is measured in Surface Feet per Minute (SFM). Spindle speed, on the other hand, is the rotational speed of the cutting tool or workpiece and is measured in Revolutions per Minute (RPM).

Getting started with a CNC requires knowledge of various parameters, such as speeds, feeds, chip load, depth of cut, etc., that dictate the machine’s performance. To achieve the best results, one must find the optimal combination of these parameters. Generally, it’s recommended that use high speed with a slow feed rate to produce smooth cuts. However, using an extremely high speed can lead to overheating and tool damage, while a very slow feed rate will increase cycle time and decrease productivity. Thus, setting the optimal value is of great significance to producing clean cuts with high productivity.

Speed parameters of a CNC machine can be divided into two types: spindle speed and cutting speed.

Spindle Speed

The difference between spindle speed and cutting speed is determined by the fact that two circles of different diameters, rotating at the same RPM, will cover different distances due to the larger circle having a larger circumference.

Rotation of cutting tools with different diameters

The spindle speed for a particular material is determined by the CNC machine, cutting tool, and type of material being machined. It is generally recommended to use a high RPM for soft materials and a low RPM for hard materials to maximize performance and tool life. However, running the material at too high or too low spindle speed can damage the workpiece and reduce tool life. Thus, it is best to select the optimal speed for the particular application.

Cutting Speed

Cutting speed is the relative speed between the workpiece and the cutting tool, which is calculated in terms of meters per minute (MPM) or surface feet per minute (SFM). This is the speed at which a particular point of the workpiece passes through the cutting edge and is used to measure the surface removed by the cutting teeth per minute.

Cutting speed is a key factor, along with feed rate and depth of cut, in determining the material removal rate (MRR) during machining. There is an optimum cutting speed for each material that is based on the material, cutting tool material, and expected tool life, which helps to ensure the accuracy provided by CNC machining.

The chart below exhibits optimal cutting speed of carbide end mill for different materials.

Category Materials Cutting Speed (S.F.M) Under 32 HRC Cutting Speed (S.F.M) Above 32 HRC
Cast Iron Ductile Cast Iron 100-400 100-200
Gray Cast Iron 100-400 80-140
High Temp Alloys Cobalt Alloys 60-100 40-80
Nickel Alloys - 50-90
Iron Alloys 80-130 60-120
Super Alloys Titanium Alloys 50-250 90-160
Non-ferrous Materials Aluminum, Aluminum Alloys 800-2000 500-1000
Copper 800-1500 800-1000
Copper Alloys 800-1000 700-1000
Magnesium 1000 700-1000
Plastics, Acrylics, Phenolics 200-800 200-500
Carbon, Graphites 200-400 -
Steels High Steel Strength 50-250 80-180
High Alloy Steels 100-300 80-180
Medium Alloys Steels 150-350 80-180
Low Alloy Steels 100-400 100-200
Stainless Steel Precipitation 80-250 90-150
Austenic 100-350 100-150
Martensitic 100-250 100-175

For more information, you can click this PDF – Selecting the Right Bit/Feeds and Speeds Charts for reference.

Feed Rate

Feed rate is the velocity at which a cutting tool is moved over a material in a single spindle revolution. It is represented in distance per revolution (DPR) units. It is determined by the RPM of the cutter, the chip load, and the number of flutes of the cutting tool. In general, a slow feed rate with a high RPM produces cleaner and smoother cuts, while a high feed rate may result in a rougher cut with a lower surface finish.

Factors Affecting Speeds and Feeds in CNC Machining

ParametersRelation with speed or feed
Spindle speedDirectly proportional to the feed rate
Number of flutesDirectly proportional to the feed rate
Chip load/Feed per toothDirectly proportional to the feed rate
Cutting speedDirectly proportional to spindle speed
Tool diameterInversely proportional to spindle speed

1. Cutting Tool Material
The cutting tool material affects its ability to withstand strong cutting forces. Carbide cutting tools can withstand greater forces than HSS tools and can be machined with faster speeds and feed configurations.

2. Tool Deflection
The deflection of the tool affects the quality of machining. A tool with a longer shank is susceptible to deflection under strong cutting force and cannot be used for high speeds and feeds configurations. A tool with a minimum stick-out has a small chance of deflection and can be used to deliver comparatively stronger forces.

3. Tool Diameter
Tool diameter is an important factor to consider when setting the optimal RPM for a machining process. A larger tool diameter can help complete the operation faster than a smaller one at the same RPM setting.

4. Cutting Teeth
The number of flutes (cutting teeth) of a tool is also an important factor in setting the optimal feed rate. A multi-flute cutter distributes the cutting force among all the flutes, reducing the risk of damaging the cutting tool and increasing the optimal feed rate.

5. Chip Load
Chip load, also referred to as feed per tooth, is the thickness of the chips removed during a machining process and is important for setting the optimal feed rate.

6. Workpiece Material
The type of material of the workpiece also affects the optimal speeds and feeds. Generally, hard materials require a slower feed rate than soft materials. Machining hard metals requires greater shearing force and a slow RPM and high-torque configuration. Soft materials, such as wood, require a high RPM to glide the cutting tool through the workpiece.

7. Stiffness of the Machine
The stiffness of the machine determines the maximum cutting force that can be transmitted to the workpiece. CNC machines with rigid chassis can provide stronger cutting forces.

Considerations for Cutting Speed in CNC Machining

Optimum cutting speed is essential to increase the efficiency of CNC machining. This is due to the fact that CNC machining is a software-controlled process, and the cutting speed depends on various factors. Some of the most important ones are the life of the tool, the type of lathe tool, and the material being used.

  • Tool Life
    Tool life is a fundamental factor in cutting speed considerations. Tool life basically refers to the period during which a cutting tool works effectively. Thus, the service life of the tool is essential in determining the cutting speed and accuracy of the workpiece.
  • Cutting Tool Material
    Different types of lathe tools can be used with the CNC machine, and the strength of the cutting tool plays a role in the optimum cutting speed. For instance, the cutting tool made of high-strength material can be used for high-speed cutting, while cutting tools made of soft material can be used for low-speed cutting.
  • Workpiece Material
    The material being used can be metals, wood, glass, etc., and the thickness and hardness of the material have an impact on the cutting tool and the cutting speed. If the material is too soft, then the cutting tool material will cut the material quickly with high accuracy and cutting speed, but for durable and harder materials, the cutting speed will be slow.

Considerations for Feed Speed in CNC Machining

Feed rate is an essential factor for achieving the desired finish of a workpiece. Machinists must consider a few factors when determining the optimal feed rate, such as the surface roughness, the cut width, the thread pitch on screws, and the type of tool being used for cutting the material.

  • Surface Roughness of Workpiece
    The feed rate increase with the growth of surface roughness. To get a smooth surface finish, the feed rate must be slowed down.
  • Cut Width
    If the cut width is less than half the diameter, the chip may be thinned, causing manufacturing defects. Increasing the feed rate can help reduce this issue.
  • Thread Pitch
    The feed rate is proportional to the thread pitch on screws being used.
  • Tool Types
    The type of material being fed largely determines the type of tool used to cut material. So, the tool types used must fit the material being fed.

Calculations for Optimal Speeds and Feeds in CNC Machining Process

When determining the best speeds and feeds for a machining process, it is important to consult the reference catalog provided by the tool manufacturer for the optimal cutting speed (SFM).

Likewise, for a particular workpiece material, the optimum chip load for a cutting tool can also be gained from the manufacturer’s tool catalog.

To determine the optimal spindle speed and feed rate for different machining operations, the figures can then be used to perform the necessary calculations.

Calculations for Milling Operations

CNC milling

To calculate the optimal spindle speed for a CNC milling operation, you can use this equation:

S = (Vs x 12)/(π x D)

where S is the spindle speed in revolutions per minute (RPM), Vs is the cutting speed in surface feet per minute (SFM), and D is the diameter of the cutting tool in inches.

For metric units, you can calculate the spindle speed using the equation below:

S = (Vs x 1000)/(π x D)

Where Vs is the cutting speed in meters per second (m/sec) and D is the diameter of the tool in millimeters (mm).

To calculate the optimal feed rate (in inches per minute), you can use the following equation:

Feed Rate = Spindle Speed x Number of flutes x Chip load

The product of multiplying chip load with the number of flutes gives the cutting feed in inches per revolution (IPR). Therefore, feed rate (IPM) can also be calculated by multiplying spindle speed (RPM) and cutting feed (IPR).

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Calculations for Lathe Operations

Turning operation on a lathe
Image Credit: Mellowpine

Machining operations on a CNC Lathe are distinct from other operations because instead of a spinning tool, lathe machines feature a rotating workpiece. As such, the lathe cutting tool is significantly different from router bits or end mills. Despite this, the speed and feed calculations for turning operations are quite similar to those of milling and drilling, with the only difference being the diameter taken into account for calculations.

Lathe machines feature a rotating workpiece, so the optimal spindle speed is calculated using the machined diameter of the workpiece instead of the tool diameter. The optimal spindle speed for a turning operation can be calculated using the following equation:

S = (Vs x 12)/(π x D)

Where S denotes the spindle speed in RPM, Vs denotes the cutting speed in SFM, and D denotes the cut diameter in inches. It is worth noting that the circumference of the workpiece or cutting tool is equal to the product of π and its diameter. Therefore, the spindle speed can also be expressed as the ratio of cutting speed at the tool-workpiece interface and its circumference.

Moreover, as lathe tools used for turning operations are mostly single-point cutting tools, the chip load is equivalent to the cutting feed per revolution (in inches per revolution).

Feed rate (IPM) = Spindle speed (RPM) x feed per revolution (IPR)

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Calculations For Drilling Operations

An illustration of CNC drilling machine
Image Credit: Mellowpine

For drilling operations, the speeds can be calculated using the following formula:

S = (Vs x 1000)/(π x D)

where Vs is the cutting speed in meters per second (m/sec) and D is the diameter of the tool in millimeters (mm).

The feed rate is calculated by multiplying the spindle speed and the feed per revolution:

Feed rate (IPM) = Spindle speed (RPM) x feed per revolution (IPR)

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Frequently Asked Questions (FAQ)

Cutting speed is the speed (usually in feet per minute) at which a tool is cutting the work, while feed rate is the tool’s distance travelled during one spindle revolution.

The optimal feeds and speeds depend on a number of variables, including:

  1. The material being machined
  2. The tool material, geometry, and coating
  3. The machine rigidity
  4. The rigidity and grip of tool holders
  5. The rigidity and grip of workholding
  6. The machine spindle horsepower and torque at different RPM

No. Calculated speed and feed do not always ensure perfect results. Variables such as rigidity of the machine, tool stick-out, vibrations of the machine, etc., may vary from one setup to another, so it is necessary to perform test runs and tune the parameters according to your setup in order to achieve desired results.

Ideal feeds and speeds are important for longer tool life, better surface finish, and faster machining speeds (through higher material removal rates).

The spindle speed (RPM) can be calculated by dividing the cutting speed at the tool-workpiece interface by its circumference.

The optimal speeds and feeds for the process should be calculated by using the formula for drilling operations since drilling operation on a lathe is conducted by using a drill bit. When determining the RPM for the process, it should be the spindle speed of the chuck because the lathe consists of a rotating workpiece.

Depend on Runsom for Your CNC Machining Projects

By now, you may have a good understanding of the concept of cutting speed and feed rate in CNC machining, but it requires extensive skills and knowledge in actual practice. Why not just turn to an expert for your manufacturing demands? Runsom Precision, a specialist in the CNC machining industry, owns a team of highly skilled engineers and technicians who have rich experience in CNC machining operations and CNC programming. Reach out to us today to solve your machining matters or get an instant quote right now!

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