Numerical control machines or CNC machines basically consist of a machine tool and a computerized system that controls the sequence of operations it performs. The machine tool can be a lathe, milling machine, laser engraver or drill. The computerized control acts by sending commands to a set of motors that ensure the movement of the machine tool in different directions.
However, numerical control can be used to control other types of special-purpose machines, such as winders for motor windings or small electrical transformers and some types of 3D printers.
CNC machines emerged as a commercial product in the 1960s, and have evolved in both their control systems and processing capabilities. Nowadays, there are machines with feedback control, which by means of their sensors identify the spatial location of the tools, allowing them to execute movements with an impressive level of precision.
The characteristics of the machines have evolved in complexity to become complex machining centers capable of performing all the operations necessary to generate a part without it being disassembled from the machine, which means that they can change the tools needed automatically, there are even machining centers that can self-manage the inputs and deliver the finished parts automatically.
When software and hardware technologies are integrated to form a manufacturing system, we have what we call a computer integrated manufacturing system. The part that performs the physical work within such systems are the digitally controlled machine tools. Among the computer-aided technologies involved in the formation of a flexible manufacturing system are: process planning systems (CAPP), production programming and control (PP&C) systems, computer-aided drafting (CAD), CAE engineering systems and computer-aided manufacturing (CAM) systems that are responsible for transforming the full power of the systems involved into manufactured goods. CNC machines are part of CAM systems.
Next, you will review the concepts that are unique to CNC technology.
10.1 Numerical control principles and classification
A CNC machine is a motion control system, through a program that orders the sequence to be performed, the elements to be controlled are one or more servomotors that transmit the controlled movement to the machine of which they form part. A servomotor is a motor specially designed to perform turns at the will of its controller element.
In CNC machines, servomotors have replaced the old cranks that controlled the movement of their axes in manual machines. An axis can be described as a direction of motion and motion can be linear or rotary.
In CNC terms an axis is composed of three elements: the device to be moved, the motion transmission mechanism (usually a ball screw) and the servomotor that provides the motion. Figure 1 shows an example of the elements of a CNC axis, with its motor, its ball screw and the device to be moved (load).
Figure 1. Structure of a CNC axis
GRABCAD COMMUNITY. (2022). Z-as cnc.
Retrieved from https://grabcad.com/library/z-as-cnc-1
For educational purposes only.
Coordinate systems for CNC control
Figure 2 (Groover, 2018) shows the coordinate system used by a CNC system, the Z axis is parallel to the machine spindle and the X and Y axes form a plane perpendicular to the Z axis. With these three axes any point in space can be described. However, some CNC machines use more than three axes, where the additional axes are used to describe auxiliary motions.
Figure 2. Reference coordinate system in a CNC system
Groover, M. (2018). Automation Production System and Computer Integrated Manufacturing (5th ed.). United States: Pearson.
For educational purposes only.
With a fourth axis a rotary table can be described, and a fifth axis can describe a table or a tilting head. Auxiliary shafts are normally identified by the letters a, b, and c, as shown in figure 2. The auxiliary shaft a rotates around the X axis, the auxiliary shaft b rotates around the Y axis, and the auxiliary shaft c it rotates around the Z axis.
Figure 3 (Groover, 2018) shows the CNC coordinate axis system typically used by lathes. The rotation of the workpiece is not controlled by any axis, the movement of the cutting tool is specified in the X-Z plane, while the workpiece rotates, but remains stationary in respect to this plane.
Figure 3. Reference coordinate system on a lathe
Groover, M. (2018). Automation Production System and Computer Integrated Manufacturing (5th ed.). United States: Pearson.
For educational purposes only.
In CNC systems, there can be different combinations of movement between the part being processed and the work tool. Here, it is worth remembering that these systems are used for many types of equipment in addition to machine tools. In some cases, the workpiece can move in respect to the tool, while the tool remains stationary (and rotating in the case of milling machines) or the tool can move depending on the workpiece.
Examples of the first case can be machines that insert components into printed circuit boards (PCB), where the machine head remains fixed, and it is the board that moves. Examples of tools that move relative to the workpiece are plasma cutters that process very large metal plates.
Classification of numerical control systems
Machine tools dedicated to drilling, boring, reaming, internal threading, require the processed part and the tool to be positioned in a relatively fixed manner while the process is being performed. These types of machines are known as point to point (PTP).
The only thing that is programmed in this machine are the positions where the processing will be performed on the workpiece. The movement of the PTP machine axes is controlled independently and without any relation between them. Depending on the type of machine, relative movement of the head and workpiece can be accomplished either by moving the head and leaving the workpiece stationary or by moving the workpiece and leaving the work head stationary.
An example of PTP systems are machines in which the working head is placed on a gantry system (figure 4), the head is placed "hanging" and the system has two axes to position itself on the workpiece. To achieve PTP motion, servomotors can be used, which only move when the machine head is in the retracted position.
Figure 4. Example of a PTP machine
Iscioglu, E. (2022). CNC ROUTER with aluminum profile.
Retrieved from https://grabcad.com/library/cnc-router-with-aluminum-profile-1
For educational purposes only.
The other type of CNC machines, which move their spindle while the process (milling, drawing, routing, plasma cutting, among others.) is taking place, are known as contour or continuous path machines (figure 5). In this case, the required controls are so-called contour controls.
Figure 5. Example of a contour machine or continuous path
Van, T. (2022). Sparky: CNC Plasma Cutter.
Retrieved from https://grabcad.com/library/sparky-cnc-plasma-cutter-1
For educational purposes only.
A contour machine can also be used as a PTP machine, but it is not economical, as the axes of contour machines are controlled simultaneously and are more complex. These machines need information on speeds and positions of the axes, also the spindle feed rate must be programmed. Examples of this type of machines are two-dimensional and three-dimensional contour milling machines.
The role of numerical control in manufacturing integration
Manufacturing integration occurs when technologies such as CAD, CAE, CAPP, PP&C and CAM come together, with CNC machines forming part of CAM systems. The concept of manufacturing integration is nothing more than the automation of manufacturing systems.
CAD and CAE systems provide the capability to design parts with computer-aided drafting and engineering; CAPP and PP&C systems provide the capability to perform process planning and production control, while CAM systems provide the computer-aided manufacturing part, of which CNC machines are an integral part.
All the aforementioned systems or technologies are linked together using a network and an integrated database with the necessary information for their correct performance. The integration of information allows the design and engineering systems (CAD/CAE) to communicate with the computer aided manufacturing system (CAM) for the generation of the numerical control programs; it also allows the production planning and control system to link with the administration and material handling systems, for the preparation of the raw material required for the realization of the production programs.
10.2 Numerical control machines and tools
The main function of a motion control system is to establish the positional relationship between the part to be worked on and the tool that will be performing the manufacturing operation.
The coordinates of movement and relative position between part and head are encoded in the CNC program and are interpreted by the control section of the machine, which exerts its action on one or more motors (servomotors), which are the actuators that will cause the movements that were encoded in the program.
To achieve the purpose for which a machine tool was designed, it must be able to perform controlled movements in different axes. We already mentioned that a CNC axis is a direction of motion and is an arrangement composed of a servo motor, a ball screw, and the device to be moved.
The figure 6 shows the diagram of a typical CNC axis, to achieve movements in a plane, an axis is built having as support another axis perpendicular to the first one.
Figure 6. Schematic diagram of a CNC positioning system
Groover, M. (2018). Automation Production System and Computer Integrated Manufacturing (5th ed.). United States: Pearson.
For educational purposes only.
CNC systems, in addition to being classified according to the type of motion they perform (PTP or contour path), can be identified by the type of control they perform on the motion itself.
The two most important groups that can be identified are the following:
Open loop systems |
Closed loop systems |
The closed loop system is generally used in machine tools where the resistance force to the spindle movement is large due to the effect of the process it performs; a typical example of this system is the contour milling machines. Open loop system, less expensive than closed loop, is implemented in machine tools where the spindle has a low opposing force to its movement, such as adhesive application heads, plotters, among others.
It is worth mentioning that closed loop control can also be implemented based on the motion accuracy that is required, regardless of the resistance force to the movement of the machine head in question.
The functions that a CNC machine can perform are multiple and in various fields of application, for example, there are 2- and 3- dimensional contour milling machines; routers for woodworking, drilling machines, tapping machines, boring machines, among others. Similarly, there are CNC lathes for cylindrical parts, plasma cutting machines, metal plate bending machines.
There are also CNC machines for inserting components into electronic boards, grinding machines for flat and cylindrical surfaces, adhesive applicators, among others. In addition, there are machining centers, which are machines capable of completely processing apart from raw material to finished part, and there are even some that can load and unload the part automatically.
Several additive manufacturing methods, such as filament 3D (FDM) printers, also use numerical control to perform the operation.
To learn about numerical control machines, check the following video:
Haas Automation, Inc. (2021, May 14). Haas' ToolRoom Mill Re-design and the New TM-0. Haas Automation, Inc [Video file]. Retrieved from https://www.youtube.com/watch?v=ddR_Exb7qK8
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10.3 Fundamentals of metal cutting and machining parameters
The most common metal cutting processes are grouped into what we call conventional machining, in which a tool with a cutting edge is used to mechanically remove (cut) the material to obtain a specific shape. Three main machining processes are recognized, lathing, drilling, and milling. Cutting during machining is basically the shear deformation of the material resulting in the formation of the burr (chip), when the burr is removed a new surface is exposed. The cutting process mainly targets metals; however, wood and many plastics are also machined by cutting.
The importance of machining materials by cutting is explained with the following table (Groover, 2018).
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Figure 7 shows a set of lathing cutting tools called burins.
Figure 7. Set of tools for lathing
Cutting tools have one or more cutting edges. Single-edged tools are used in turning operations, multi-edged tools are used in milling and drilling operations. Single-edged tools consist of two parts, one cutting part that produces the chips and the other called the shank. Figure 8 illustrates the parts of a single-edged or burin tool.
Figure 8. Elements of a burin or single-edged tool
Among the most important parts of a cutting-edge tool are the face (surface on which the burr slides, also called the parting surface); the leading edge is the surface in front of which the exposed surface of the workpiece passes (also known as the striking surface); the leading edge (the part that makes the cut); and the tip of the tool is the place where the leading and trailing edges intersect.
When cutting occurs, the tool tip penetrates below the surface of the workpiece. Generally, the tip of the tool is rounded at a radius called the nose radius.
Tools with several cutting edges usually perform the motion in respect to the workpiece in a rotational manner.
The figure 1 shows a set of rotary tools with multiple cutting edges used in the cutting, drilling and milling operation on a CNC machine.
Image 1. Set of rotary tools for CNC
Machining parameters
To perform a machining operation, it is necessary that between the workpiece and the cutting tool there is a relative movement. The speed of the primary motion is called cutting speed on the other hand, the tool needs to perform a lateral movement along the work, this movement is called feed speed or simply feed rate.
For example, the speed at which the workpiece rotates on a lathe is the cutting speed, and the speed at which the graver advances along the workpiece is the feed rate. In a milling machine, the rotational speed indicates how fast the tool rotates in its toolholder and the feed rate is given by the speed at which the work table where the material is clamped moves. The next parameter is the depth of cut, which is the distance the tool penetrates the original surface of the cut. Cutting speed, feed rate and depth of cut are the so-called cutting parameters.
It is important to note that, when a CNC program is made, we must calculate two parameters of utmost importance for the good surface finish of a machining, as well as to increase the tool lifetime, these parameters are as follows:
These two parameters are calculated based on other tool data provided by the tool manufacturers, as the manufacturer designed and researched how the tool material behaves under a specific workload. The data to be provided by the tool designer may be the following:
Other parameters that we can calculate and that will be useful to analyze the work we are carrying out are mentioned below.
Revolutions per minute (n)
The revolutions per minute is the number of revolutions or turns that the cutter will make in one minute; to calculate this parameter, we must know the diameter of the cutter, as well as the cutting speed, the latter will be given by the tool manufacturer based on the tool material and the material to be cut. The formula for calculating the revolutions per minute is as follows.
Where:
n = Revolutions per minute
Vc = Cutting speed expressed in meters per minute
D = Cutter diameter expressed in meters per minute
The constant of 1000 represents the number of millimeters in one meter.
If the English or imperial system is used, we have the following:
Where:
n = Revolutions per minute
Vc = The cutting speed expressed in feet per minute
D = Cutter diameter expressed in inches
The constant 12 represents the number of inches in one foot.
Cutting speed (Vc)
The cutting speed is defined as the distance the periphery of the tool travels per unit time. In other words, the cutting speed is how far the perimeter of the tool moves in one minute. To know this distance, it is important to know the tool diameter and RPM.
Where:
Vc = Cutting speed expressed in meters per minute
n = Revolutions per minute
D = cutter diameter expressed in meters per minute
The constant of 1000 represents the number of millimeters in one meter.
In case of using the English or Imperial system we have the following:
Where:
Vc = The cutting speed expressed in feet per minute
n = Revolutions per minute
D = Cutter diameter expressed in inches
The constant 12 represents the number of inches in one foot.
Feed rate (Vf)
Feed rate, also known as feed rate, is the distance the cutter travels linearly per unit time, it can be expressed in mm/min or in/min. The formulas for calculating the feed rate are as follows:
Where:
Vf = Feed speed in mm/min or in/min.
n = Revolutions per minute
f = Feed rate per revolution in millimeters per revolution or inches per revolution
The feed rate per revolution (F) is a data that can be obtained from the tool data sheets, which will be calculated based on the tool material and the material to be cut.
Another formula for calculating the advance is as follows:
Where:
Vf = Feed rate in mm/min or in/min.
fz = Feed rate in millimeters per tooth or inches per tooth
n = Revolutions per minute
z = Number of cutter teeth
The feed rate per tooth (fz) is a data that can be obtained from the tool data sheets, which will be calculated based on the tool material and the material to be cut.
The formula for calculating the feed rate per revolution(f) is as follows:
Where:
f = Feed rate per revolution in millimeters per revolution or inches per revolution
Vf = Feed rate in mm/min or in/min
n = Revolutions per minute
The formula for calculating the feed per tooth (fz) is as follows:
Where:
fz = Feed rate in millimeters per tooth or inches per tooth
Vf = Feed rate in mm/min or in/min.
n = Revolutions per minute
z = Number of cutter teeth
Volume of material removed per unit of time (V)
It is possible to calculate the volume of material removed per minute in millimeters or inches with the following formula.
Where
V = Cubic millimeters per minute or cubic inches per minute
ae = Width of cut
ap = Depth of cut
Vf = Linear feed rate in millimeters per minute or inches per minute
If we wanted to know the volume in cubic centimeters per minute, the formula would be expressed as follows:
CNC systems solve the problem of motion and position control with the help of a computer. Its main application is in the field of machine tools and its main components are the programs that store and encode motion and position data, the computer or control that interprets them and the process to be controlled, which are usually the servomotors that will provide motion to the work piece and tool.
CNC systems according to the operation for which they are designed are classified into two main groups: point-to-point (PTP) systems and contour systems, the former are simpler, since they only control the position in which the machine head is set to perform its process. Contour systems are more complex, since they must control several axes at the same time, which are dependent on each other. In contouring systems, the head performs its process on the workpiece while there is a relative movement between the two.
Depending on the way in which the control is performed, we can classify CNC systems into two other groups: open-loop control systems and closed-loop control systems. The more accurate and complicated of the two is the closed loop system, which feeds back to the control element the relative position data between workpiece and work head and corrects it if it does not match the programmed position. Open loop systems perform motion and position control without receiving any feedback.
CNC machines are part of the technology known as CAM, computer-aided manufacturing, and are at the heart of manufacturing systems integration. The integration of manufacturing systems is the automation of manufacturing systems through the use of computer-aided technologies.
Numerically controlled machine tools can be designed to perform many processes. One of the main applications of this type of machine is metal cutting processes, but there are also CNC machines for processes such as wood cutting, plate bending, adhesive application, electronic component placement, among others.
After you have studied the previous topics, get ready to move on to the details of programming CNC systems in the next topic.
Make sure that you:
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To learn more about CNC machines, watch the following video:
To learn more about CNC machine shop, watch the following video:
To learn more about CNC cutting, we recommend reading:
To learn more about CNC machine tools, we recommend reading:
To learn more about CNC machining, we recommend reading:
Through a search for information and classification, the student will describe the fundamental concepts of computer numerical control and will solve exercises to calculate machining parameters.
To understand the fundamentals of computer numerical control and to calculate the machining parameters of operations performed by various types of rotary tools (drilling, milling, among others).
Requirements
Read Chapter 7. Computer Numerical Control from the textbook.
Instructions
Individually
Deliverable(s)
Document with the development of the activity and evidence of the calculations performed to obtain the results of the indicated exercises.
Evaluation criteria