CNC machining is a subtractive manufacturing process that involves shearing away material chips until the final product is achieved. So first, machinists need to know the amount of material the machine will shear off in one revolution and the speed at which the CNC machine will move. Here is where the difference between feed rate and cutting speed is important.
When designing the parts for CNC machining, it is important to consider these parameters. This is because they ensure the optimization of different parts of the CNC machining process. While cutting speed is more important in optimizing factors like tool life and power consumption, the feed rate is vital in determining the machining time and roughness of the finished area. This article will compare feed rate vs. cutting speed and explain how to derive each of them.
What is Cutting Speed?
Cutting speed is generally defined as the relative velocity between the surface of the workpiece and the cutting tool. Some experts also define it as how fast the workpiece moves past the cutting edge of the tool. Machinists measure it in surface feet per minute (SFM) or meters per minute (m/min) or feet per minute (ft/min). The cutting speed is quite an important factor in the determination of other parameters of CNC machining, such as cutting temperature, power consumption, tool life, etc. Its influence in these parameters serves as a significant difference between feed rate and cutting speed.
Factors Determining Cutting Speed
It is necessary to ensure that the cutting speed is optimum so that the CNC machining process will give the best part. However, it is possible to predict the optimum cutting speed for a particular CNC machining process by considering other factors. Examples of such factors include:
Workpiece Hardness
One of the most important factors that determine cutting speed is the hardness of the material being cut. The harder the material, the slower the cutting speed, and vice versa. For example, machining materials like steel will require a lower cutting speed compared to aluminum.
Cutting Tool Material
There are different lathe tools used for various CNC machining. Each of these tools is also made with different materials, hence possessing different hardness properties. The cutting tool material will have a significant impact on the cutting speed used in a machining process. If the cutting material is of high strength, the machinist can utilize a high cutting speed to a little detriment. However, softer cutting tool materials will tend to wear out quickly with higher cutting speeds. This will lead to shorter tool life.
The Expected Tool Life
How long the machinist wants the tool to last is another factor that is important in determining the cutting speed. This will include considering variables such as how much the tool cost and the cost of the tool compared to the quantity of parts being produced. If variables like this are favorable, then a high speed might be feasible for use.
Depth of Cut
Cuts that are deeper, removing more material, increasing the load on the tool and increase heat. To compensate, reduce cutting speed. In excess speed at high depth of cuts quickly wears out tools as well as increases the force of cutting and decreases the quality of the surface. However, smaller cuts allow greater speeds.
What is Feed Rate?
Feed rate is the distance which the cutting tool during one spindle revolution. It is also defined as the velocity at which the cutter is advanced against the workpiece. It is measured in either inch per revolution or millimeters per revolution (ipr or mpr) for turning and boring processes. However, machinists use inches per minute or millimeters per minute (ipm or mpm) for milling processes. In calculating the feed rate, the machinist considers the number of flutes (or teeth) the cutting tool has and calculates the feed rate for each tooth.
Factors Determining Feed Rate
The feed rate also affects the same factors that the cutting speed influences. The only difference is that its effects are to a lesser extent. However, the feed rate is important in the final aesthetic look of the machined part (i.e., the surface finish of the machined part). Hence, its optimization is also quite important in CNC machining processes. To determine its optimum value, machinists consider factors such as the ones below:
Cut Width
Any cut width that is less than half the diameter causes chip thinning. Chip thinning is a manufacturing defect where the chipload (amount of material cut by the tool in one revolution) is reduced. Chip thinning could lead to a longer lead time; hence it is important to avoid it. In addition, increasing the feed rate will help reduce the effects of chip thinning, hence, increasing productivity and tool life.
Additional Feed Rate Considerations
Other factors that might influence feed rate include:
- The type of tool.
- The power available at the machine spindle.
- The strength of the workpiece.
- The threads per inch (TPI) for taps, die heads and threading tools, etc.
What is the Difference Between Feed Rate and Cutting Speed?
Due to the close types of definitions both parameters have, it is possible to confuse them with each other. Some machinists would also refer to this parameter as the difference between speeds and feeds. There are quite a few practical factors that serve as the difference between feed rate and cutting speed. Examples of such factors include:
Cutting Temperature and Tool Life
The cutting temperature is a crucial factor that proves a difference between feed rate and cutting speed. This is because the higher cutting temperature can hamper parameters such as the part’s tool life and surface finish. The extent of the effect of both parameters on cutting temperature and tool life differentiates them from each other. It has a comparatively lower effect on the cutting temperature and tool life than cutting speed for feed rate. Hence, the difference between feed rate and cutting speed is the extent of their effect on cutting temperature and tool life.
Surface Roughness and Scallop Marks
Scallop marks are also known as feed marks. These marks always accompany CNC machined prototypes and parts, and they are the main cause of surface roughness. The feed rate has a direct influence on the scallop marks present on any part. Hence, the higher the feed rate, the higher the degree of scallop marks and surface roughness. However, cutting speed does not affect scallop marks; hence it does not affect surface finishes.
Directrix and Generatrix
In geometry, a generatrix is a point or surface that generates a new shape when moved along a given part. The given path through which the generatrix moves is the directrix. In machining, the basic goal is to create geometrical surfaces with aesthetically pleasing finishes and higher accuracy. Hence, these two parameters are required in machining processes. The difference between speeds and feeds is that the cutting speed provides the generatrix while the feed motion provides the directrix.
Other factors that differentiate between feed rate and cutting speed include:
- Units of measurement.
- Impact on cutting force and power consumption.
- Also, the motion that generates cutting speed and feed rate is different (cutting motion and feed motion, respectively).
| PARAMETER | CUTTING SPEED | FEED RATE |
| Definition | Speed of the cutting tool edge across the workpiece surface. | Rate at which the cutting tool advances into the workpiece. |
| Units | Surface Feet per Minute (SFM) or Feet per Minute (ft/min) or Meters per Minute (m/min) | Inches per Revolution (IPR) or Millimeters per Revolution (mm/rev) for turning; Inches per Minute (IPM) or Millimeters per Minute (mm/min) for milling. |
| Primary Influence | Tool life, power consumption, cutting temperature. | Machining time, surface finish, chip load. |
| Factors Affecting | Workpiece material (hardness), tool material, depth of cut, desired tool life. | Type of tool, surface finish requirement, cut width, Threads Per Inch (TPI) for threading, number of flutes/teeth. |
| Impact on Cutting Temperature & Tool Life | Major influence. Higher speed = more heat, faster tool wear. Direct correlation. | Moderate influence. Less impact than cutting speed, but affects tool wear indirectly through chip load and friction. |
| Impact on Surface Finish | Indirect influence. Primarily affects surface through chip formation and potential for chatter. | Direct and significant influence. Higher rate = increased roughness and potential for scallop marks. Chip thinning at low feed rates also affects finish. |
| Impact on Cutting Force & Power Consumption | Major influence. Higher speed = greater cutting force and power required. Proportional relationship. | Moderate influence. Affects force and power through chip load and material removal rate. |
| Geometric Analogy | Generates the generatrix (the path of the cutting edge). | Generates the directrix (the path of the tool’s advancement). |
| Motion Generated | Cutting motion (rotary or linear). | Feed motion (linear). |
| Calculation | Involves spindle speed and tool diameter (e.g., SFM or ft/min = (π * diameter * spindle speed (RPM)) / 12). | Involves spindle speed and feed per tooth/revolution. (e.g., IPM = feed per tooth * number of teeth * RPM). |
How to Determine Cutting Speed and Feed Rate
This picture shows all the parameters involved in the determination of cutting speed and feed rate. You’ll notice that the spindle speed is the foundation of determining both the cutting speed and feed rate. Also, the feed rate involves two formulas before arriving at the final answer. First, you have to determine the feed per tooth. That value is then used to determine the feed rate of the cutting tool.
Conclusion
Determining the optimal feed rate and cutting speed, along with factors like depth of cut, tool type, and desired surface finish, might be the factors that enhance your CNC machining process to get an adequately machined part. However, there is no need for you to worry about any of these production issues when you outsource to RapidDirect. With our experienced machinists and CNC machine programmers, you will always get the best-machined part every single time you work with us. So, reach out to RapidDirect today for all your CNC machined needs.
FAQs About Feed Rate and Cutting Speed in CNC Machining
These four variables control the way your CNC machine eliminates material. Here’s a breakdown of the four factors:
The Spindle Speed (RPM): How quickly the spindle rotates.
Cut Speed (SFM and M/min): How fast the cutter’s tip moves across the material. It is the most important factor that affects the life of the tool. If it’s too fast, the tool wears out fast.
Feed Rate (IPM or mm/min): How fast the cutter is moving forward into the material. Most importantly, it affects the finish of the material and how long the work is completed.
Feed per Tooth (IPT or mm/t, to mill): How much material each tooth of the cutter takes out per revolution. This allows you to calculate the overall feed rate.
Why do they matter? The right settings are essential to get a great result. Incorrect settings can result in damaged tools, poor quality of finish, and more time for machining. Always refer to the recommended settings for the specific material you are using and tooling.
These terms are all related to the speed at which the cutter is moving. However, they’re expressed in different ways:
RPM (Revolutions Per Minute): How fast the spindle is rotating. Imagine it as the speed of an engine in an automobile.
SFM (Surface Feet per Minute): How fast the cutter’s blade travels through the materials’ surface. This is the actual cutting speed, usually used by imperial units.
m/min (Meters Per Minute): Same as SFM. However, it uses metrics instead.
ft/min (Feet Per Minute): Same as SFM with imperial units, which are often employed for turning operations.
Why so many units? SFM, m/min and ft/min all refer to the same thing: cutting speed, but in different units. RPM, however, refers to the spindle’s speed of rotation and has to be converted into cutting speed by utilizing the diameter of cutter.
Make use of these formulas to convert:
– SFM or ft/min = (RPM x π x Diameter) / 12 (Diameter in inches)
– m/min = (RPM x π x Diameter) / 1000 (Diameter in mm
A bigger cutter operating at the same RPM will have a greater cutting speed (SFM, m/min, or ft/min). This is due to the outside edges of the cutting device travels a longer distance in each rotation.
Cutting speed (N), measured in RPM, defines spindle rotation frequency. Cut velocity (Vc) however, is the tangential speed of cutting edges on the workpiece. It is measured in SFM or m/min. Vc is determined by RPM and the diameter (D): Vc = πDN / 1000 (D in mm). A greater diameter will result in higher Vc for a given RPM. For instance, a 25mm diameter tool running at 1000 RPM will have a Vc of about 78.5 m/min, whereas one with a 50mm diameter at the same RPM will have twice the Vc (approximately the 157 m/min). This directly affects the life of the tool as well as heat and removal of material. Vc and feed rates also influence each other to control chip formation and surface finish. A higher Vc usually necessitates greater feed rates to ensure the consistent thickness of chip. Ignoring the relationship between Vc and diameter can result in excessive wear on the tool and poor results. Understanding it can optimize machining efficiency and high quality.
