If an engineer or a mechanic doesn’t know the concepts and applications of cutting speed and feed for CNC Machining then he is not adept in handling the machines. Even if you have an outstanding CNC programming structure, without properly setting the cutting speed and feed for the workpiece performing CNC machining is quite difficult and worthless as well. With correct cutting speed and feed the material removal rates can be increased thus, enhancing the surface finish and life of the tool. As important is creating a flawless CNC programming structure, so is the need to set up the accurate cutting speed and feed. Otherwise, an ideal machined workpiece cannot be created.
Below, we will first give an insight into cutting speed, feed rate, and material removal rate that are essential for CNC Machining. Then, we will give you a generalized idea about machining speed for most of the metals and plastic materials.
Cutting speed, feed rate, and the material removal rate for CNC machining–
Let us understand the basics of CNC machining such as cutting speed, feed rate, and other variables and their relation with each other.
- Cutting Speed
Cutting speed can be referred to as the speed with which the tool cuts the work. It is usually measured in Surface feet per minute (SFM).
- Feed Rate
Feed rate can be explained as the total distance movement done by the tool in a complete spindle revolution. It is measured in inches per minute (IPM).
- Material Removal Rate
Material removal rate (MMR) can be described as the total volume of the material cut divided by the machining time.
So, to say, a feed rate of 15 IPM will push a tool 5 inches in a single minute and a mil rolled at 50 FPM will travel 50 feet in one minute.
Cutting speed and the feed rate are two of the crucial factors that help us decide upon the surface finish, material removal rate, and necessary power requirements. While determining the cutting speed and feed for CNC machining consideration of the material to be cut is also crucial. Additionally, the tool material, toughness of the workpiece, dimensions, and conditions of the lathe, and cutting depth also play a key role in CNC machining.
Spindle speed and Chip load are also important parameters for CNC machining. The spindle speed can be obtained by dividing the required cutting speed by the workpiece circumference. It is measured in rotations per minute (RPM). Chip load can be defined as the total distance advanced by cutting tool per cutting tooth per revolution.
A generalized idea about Machining speed and feed rates
In this section, we are going to give you a generalized idea about various parameters essential for CNC machining of metals and other materials.
Here, we have mentioned the cutting speed formula, feed rate formula, and other variables primary to CNC machining. The cutting speed formula, feed rate, and other parameters will help you select the appropriate cutting speed and feed.
- Cutting speed formula
Where, D = diameter of the tool
And, S = Spindle speed
- Feed rate formula
Where S = Spindle speed
F = Feed per tooth
N = number of flutes
- Spindle speed formula
Where, V = Cutting speed
D = Diameter of the tool
- Feed per tooth
Where, F = Feed
S = Spindle speed
N = number of flutes
It is important to mention that with a reduction in the flexibility of the material, there is an increase in the cutting speed. Also, with an increase in the strength of the cutting tool material, there is an increase in the cutting speed as well.
Considering the work material hardness, the cutting speed can be expressed as –
Aluminum > Lead > Iron > Steel
Considering the cutting tool hardness, the cutting speed can be expressed as –
High Speed Steel > Carbide > Carbon Steel
Cutting speed is signified by meters per minute, m/min or feet per minute, ft/min. While an excessively higher cutting speed leads to the cracking of cutting tool edge, a significantly slower cutting speed might lead to minimal production rates because of the extra time invested in machining activities. For cutting the metals, the lathe should be set at a certain RPM which is defined by –
Cutting speeds suggested for some of the materials are:
The feed of the lathe is expressed by inches. For a lathe set at 0.010 inches feed rate, the total distance advanced by the tool in the workpiece is 0.010 inch for each of the complete revolutions made by the workpiece. The lathe feed is defined by the lead screw’s speed which is regulated through the change gears. A coarser feed should be used in case of a rough cut intended to discard the excess material. In case of a rougher cut, the surface finish is not given much priority. In case of finishing cut, a solid feed is required to ensure good surface finish while machining the diameter of the workpiece to an accurate size. For an excellent feed rate, it is important to consider the factors such as hole depth, drill size, type of machinable material, finishing and precision, and the type of coolant use. Different Spindle speeds are used for CNC machining of several materials. To sustain the varied spindle speeds the lathe needs to be built accordingly. The speed of the lathe is expressed in RPM, i.e., revolutions per minute.
Feeds for different materials are –
For most of the Aluminium alloys, the roughness cut, i.e., 0.01-0.03 in cut depth runs at a feed rate of 0.005-0.02 IPM or inches per minute. Similarly, the finishing cut, i.e., 0.002-0.012 in cut depth runs at a feed rate of 0.002-0.004 IPM or inches per minute. It is also important to consider that there is a decrease in stock diameter by two times for each cut depth created. For various common CNC machining purposes, a feed rate of 0.005-0.020 IPM is suggested in case of roughing cuts and 0.002-0.004 IPM for finishing cuts.
So, this is how you can effectively set the appropriate cutting speed and feed for performing CNC machining of metals and other materials. Determining the cutting speed and feed prior to conducting CNC machining will wipe out the chances of any time loss, thus increasing the total production rate.
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