Computer numerical control (CNC) machining is a subtractive manufacturing process that uses programmed codes to control tool axis movement. The programmed codes include all necessary cutting parameters such as cutting tool movement, spindle speeds, feeds, rotational speeds, etc.
It is important to consider these parameters when developing products for CNC machining operations. The optimization of various parts of the CNC machining process is guaranteed by these parameters. Service life and energy consumption are optimized by cutting speed. The processing time and surface roughness of finished products are determined by the feeding speed.
Therefore, engineers and machinists need to know cutting speed and feed, know the difference between them, and know how to calculate cutting speed and feed. Keep reading.
What is the feed rate?
During one revolution of the spindle, the distance covered by the cutting tool is called feed. It may also be called cutting tool engagement speed and is usually measured in inches/minute or millimeters/minute in milling operations.
For drilling and turning operations, this can be measured in inches/rev or millimeters/rev. Feed speed variation depends on blank material (steel, wood, aluminum, stainless steel, etc.), tool material (HSS cutting tool, ceramic, cermet, etc.) and other cutting factors such as finish surface and characteristics of the CNC machine.
The aesthetics of processed products depend on the feed speed, and therefore optimizing the feed speed is crucial in CNC machining processes.
Selection of optimal transport capacity
Every aspect of the CNC machining process is directly related to feed rate, from safety to productivity, tool life and product quality. When choosing the feed speed, the following factors must be taken into consideration.
productivity
To achieve higher productivity, the feed rate can be increased by sacrificing surface quality. In another case, the cutting speed can be increased by keeping the feed rate stable.
Surface Finish
Better surface quality can be achieved with lower feed speeds. An approximate feed rate can be considered for rough cutting. For example, the feed rate for the finishing process can be assumed as 0.01-0.05 mm/rev and for the rough turning process 0.1-0.3 mm/rev.
Feed Limitation
The available machine tools have feed speeds within the minimum and maximum limits. These machine tools do not allow exceeding the limit, but traditional lathes only allow certain feed options to be applied within this range.
Cutting tool geometry
In addition to feed rate, the surface quality of products can also be influenced by tool geometry. If geometry permits, a higher tool geometry value may be preferred.
Machine tool capacity
Due to the higher feed speed, high cutting forces and high vibrations can occur. The feed speed must be selected depending on the absorption and transmission of high forces and vibrations by the machine tool.
What is cutting speed?
The relative speed between the cutting tool and the workpiece surface is commonly called surface speed or cutting speed. It can also be defined as the linear distance in meters per minute or feet per minute that the cutting tool material travels across the workpiece surface in a cutting operation.
The main parameters of CNC machining such as power consumption, cutting temperature and tool life, etc. are determined by the cutting speed. Cutting speed values vary depending on the material such as carbon steel, carbon steel, aluminum and plastic. Some tools or processes, such as threading and knurling tools, operate at lower cutting speeds than specified.
Selecting the optimal cutting speed
To obtain the best result from the CNC machining process, the ideal cutting speed must be guaranteed. The ideal cutting speed for a specific CNC machining process can be predicted based on the following factors.
Part hardness
A material's resistance to deformation caused by abrasion, indentations, and scratches is commonly called hardness. During the machining process, special care must be taken with harder parts, as they can easily affect the tool's performance. When machining a harder material, slower cutting speeds should be used. For example, titanium requires a lower cutting speed compared to steel.
The strength of the cutting tool
In machining, the thickness of the cutting edge plays a decisive role in the permissible cutting speeds. For example, machining for higher cutting speeds can use cutting tools made from high-strength materials such as diamond and carbon boron nitride. For lower cutting speeds, however, cutting tools made of high-speed steel can be used.
service life
At higher cutting speeds, the softer materials in the cutting tool wear quickly, resulting in shorter tool life. Another crucial factor in determining cutting speed is how long the engineer or machinist wants to work with this tool. Parameters such as tooling costs and quantity of parts produced are mainly taken into account. If these parameters are within the permitted limits, high cutting speed can be used.
Why are speed and feed important in machining?
Speeds and feeds are important in machining because they determine the rate and amount of material removed.
Speeds and feeds can significantly affect the life of a tool.
The difference between feed and cutting speed can be determined by a crucial factor called cutting temperature, as higher cutting temperatures can negatively affect parameters such as tool life and surface roughness.
The sloppy effects of speed and feed are not visible in softer materials (aluminum or resin), where there is a lot of room for error. However, the sloppy effects of speed and feed are visible in harder materials (titanium or Inconel) where there is a limited margin for error.
If there are small deviations between speed and feed, the cutting tool breaks quickly.
Speeds and feeds are mandatory to obtain better surface roughness. When the machine operates at a high value of tool and spindle speed, chatter marks appear on the surface of the material.
Difference between feed and cutting speed
Although both feed and cutting speed affect the overall performance of the machine, they actually refer to two different things. To get the best results from your CNC machine, it is important to understand the difference between the two.
1. The main difference between feed rate and cutting speed is that feed rate is the speed at which the tool moves through the workpiece whereas cutting speed is the speed at which the cutting edge of the tool moves.
In other words, feed rate is a measure of how quickly the tool moves through the material, while cutting speed is a measure of how quickly the tool actually cuts.
2. Cutting speed provides a generatrix and is usually measured in m/min or ft/min and denoted by Vc. The feed rate provides a guideline and is usually measured in mm/rev or mm/min and denoted by s or f.
3. During machining, cutting speed affects cutting force and energy consumption. However, the feeding speed has no influence.
4. Cutting speed does not play a role in the deviation of chip direction from orthogonal. However, the feed rate almost always influences the actual chip direction.
5. Cutting temperature, tool wear and tool life are less affected by feed speed. However, the cutting speed is greatly influenced.
6. The formation of waves or feed marks on the machined surface does not directly affect the cutting speed. However, the wavy marks on the surface of the finished product directly influence the feeding speed and therefore directly represent the surface roughness.
The following cutting speed and feed diagram provides a clearer understanding of the differences.
|
parameter |
cutting speed |
feed rate |
|
Generating and conducting |
The guide line is created by the cutting speed |
The generator is created by the feed |
|
Movement units and S Hidden treasure F form |
Measured in meters per minute (m/min) or feet per minute (ft/min) and denoted by Vc |
Measured in meters per revolution (mpr) or inches per revolution and indicated by s or f |
|
chip D Direction |
No effect if there is a deviation from the orthogonal direction of the chip |
Affects the actual direction of chip flow |
|
Cut F Energy and energy consumption |
Affects cutting force and energy consumption |
No influence on cutting force and energy consumption |
|
Surface roughness and wavy lines |
Not directly connected to the notches or marks created on the machined surface |
Connected directly to irregular markings on the finished surface |
|
Cutting temperature, tool life and wear |
Strongly influenced |
Less affected |
How are feed and cutting speed determined?
The table above shows all the parameters needed to determine cutting speed and feed. Spindle speed is the basic requirement for determining cutting speed and feed. The final feed can be obtained in two ways: first, by determining the feed per tooth and, second, by determining this feed per tooth to determine the tool feed.
Other important considerations
Spindle speed limitation
Surface feet per minute (SFM) based on material and cutter diameter are required to define cutter speed in RPM. Sometimes calculating the speed of small tools and/or certain materials results in an unrealizable speed.
In this case, the machine operator must operate the tool at the maximum machine speed required for the diameter and maintain the required chip load. In this way, optimal parameters can be achieved at maximum machine speed.
Nonlinear path
In general, feed rates are considered linear movements, that is, linear distances traveled. However, there are some cases where feed rates on an arc or circular interpolation path (outer diameter or inner diameter) are taken into account. As the depth of cut increases, the tool pressure angle increases, resulting in a nonlinear path. Tool engagement is greater at the inside corners than at the outside corners.
Interaction between cutting speed and feed
During machining, the cutting tool compacts the surface of the part and removes a thin layer of material in the form of chips. The relative speed between the workpiece and the cutter is desired to transmit the required pressing force. The primary relative velocity was generated by the cutting speed, which helps visualize material removal.
To take into account the material removed from the entire surface of the part, the cutter or part must be fed with another synchronous movement, the so-called feed movement (which varies depending on the milling process), in the intended direction. These simultaneous actions of feed and cutting speed together with the feed movement meet the basic requirements of machining.
University Degree
Speeds and feeds are needed to optimize different parts of CNC machining parameters such as tool life, power consumption, time and roughness. The interaction between cutting speed and feed is of great benefit to the production of CNC machined parts. To determine speeds and feeds, engineers and machinists must fully understand them.
Common questions
Are cutting speed and feed the same?
No, cutting speed and feed are not the same. Cutting speed is the speed at which the cutting tool moves through the material being cut. Feed rate is the speed at which the material to be cut passes the cutting tool.
What does SFM mean in machining?
SFM, also called surface speed, stands for surface feet per minute and is a common unit of measurement for cutting speed in machining. Indicates the number of feet a tool can travel over a workpiece in one minute. The higher the SFM value, the faster the cutting speed.
Why do engineers and machinists need to consider cutting speeds and feed rates in CNC machining?
Engineers and machine operators need to consider cutting speeds and feed rates during CNC machining as they directly affect the quality of the final product. If the cutting speed is too slow, the material will not be cut correctly and there is a risk of burrs or other imperfections.
