Since 2000
History of CNC milling machines
History and evolution, from the industrial cutter to the hobby model
The mechanical processing of milling is well known and considered a very normal industrial operation, that is a cold mechanical working, which works by chip removal, just like turning and drilling, and which is carried out through the action of a rotating tool on its axis, the milling cutter , on a piece in advance motion, which is “sculpted” to the desired shape.
More difficult to establish is the troubled history of the milling machine, probably born in an obscure way in some artisan workshop in the early decades of the 1800s and quickly developed into the common practice we know today.
We are around 1750, and at the origin of the milling machine there is the lathe, often, to file the pieces more quickly than could be done by hand, rotating files were mounted on it.
However, we are talking about a very simple and coarse processing.
Towards 1760 we find the first real milling machines, that is, you do not come back with a special tool, but machines with this pure exact purpose.
In 1814 in the United States, in the two federal arsenals, a much more advanced model was developed, for the milling of hexagonal bolts, in any case, still in these decades, milling was seen as a way of saving time on roughing and then completing hand finishing work.
They were later inventions, such as the three-axis movement integrated by Brown and Sharpe in their exceptional model of 1861, to make possible a finished job and pave the way for further innovations, which were continuous in the years preceding the WWI.
One of the first cutter patents is dated 1885,
It is towards the end of the First World War that the continuous search for accuracy in processing reached a capital stage, it was in fact in these years that the concept of relative dimensioning, that is of measurements, was deepened conducted on the piece all starting from a single reference point, and that the normal precision of the machines reached i hundredths of a millimeter, they were the dawn of numerical control of machines today taken for granted.
With pantographs they allowed to shape the movements of the machine by tracing the lines of a model, it was possible to create, already in the 1930s, huge milling machines such as the Cincinnati Hydro-Tel, at this point almost identical to those of today if we leave out the computerized control.
Technology after the war it was marked by the culmination of the development of servomechanisms, and by the birth of technologies
digital. Originated by investments military research,
the technology spread more rapidly precisely in the industrial and mechanical sector, in this as in many other cases typical of the 40s and 50s of the last century.
In the following decades, numerical control evolved towards computerized control of machinery, until the technological explosion of the 1980s which, with the personal computer, brought digitally controlled machines even to the smallest shops.
In the twentieth century, the technological acceleration due to the spread of information technology and microelectronics has allowed companies to have new technologies which, used in an appropriate way, allow them to meet market needs and to optimize quality while minimizing costs.
The numerical control machine tool (MU / CN) is at the base of the computerization process of the production cycle, for the production in small and medium series at minimum cost.
Thanks to it it has been possible to significantly reduce the preparation time of the machine for the processing of different pieces with the simple replacement of a precise program elaborated by a CAM program and transferred via magnetic support (USB key), or directly via network cable, to the operating machine, thus replacing the prehistoric perforated tapes used by the first CNC machines.
The programming language and the CAM
The ISO code
Any numerical control machine, whether it is a milling machine, a lathe or a grinding machine, needs to receive "instructions" on how to move its axes or mechanisms, these instructions are written in a special format also called ISO code or machine code.
The ISO code is a special and relatively simple machine language that the CNC is able to understand and execute.
This language was originally used to program the machining directly on the machine, with an alphanumeric keyboard, without the use of CAM software.
It is a sequence of simple commands, which the machine executes in order, and some other standard instructions, such as the number of spindle revolutions and the movement speeds in various situations (work, rise, descent, rapid, etc. ).
The most common language is the G Code or ISO Code, a simple language developed for the first CNC machines in the 1970s.
The CAM
The definition of CAM is: Computer Aided Manufacturing, refers to the use of various software packages to create ISO code and to operate a CNC, based on 3D model (CAD) of data.
In the past, when the processing was complex, it was realized that having to program directly on the machine using the ISO codes manually, became very complex, long and difficult to modify.
It is thanks to the CAM programs that it has been made possible to convert the drawing of the machining to be performed (2D or 3D), directly into ISO code, with the possibility of simulating the machining and checking the accuracy of the tool path. It is important to note that in reality it is not the CAM that directly controls the machine, but only creates the code of the path to follow.
CAM programming, like 3D modeling, requires knowledge and experience in program management, development of machining strategies, knowing in depth the functions and tools to be used in each situation we can obtain the best results.
There are hundreds of types of CAM programs, the best known are Mastercam, GibsCam, RhinoCam, some are specific to the type of processing, see ArtCam in the object sector or CopperCam for milling printed circuits.
The PostProcessor
While the ISO Code is in fact a "standard" language, machine manufacturers are free to customize it, to incorporate additional functions specific to each machine: for this reason the G-Code created for one machine may not be suitable for another. .
Some manufacturers have even developed their own programming languages, such as Heidenhain, Mazak / Mazatrol.
Consequently, to translate the tool paths calculated by the CAM software into a specific language for the single machine, an additional "bridge" software is required, called Postprocessor, which, once correctly configured, is responsible for translating the generic code into a language specific to the machine for which it was compiled.
This allows any CAM to calculate routes for any machine.
Flow cycle
The following is the summary of how a CAD-designed piece is transformed into a finished piece.
Physically the CAM program can be present in a computer near the CNC milling machine, it is the operator who uses the milling cutter who uses the CAM, since he is aware of all the characteristics of the material to be machined and of the various milling tools present in the machine.
In other situations the CAD and CAM stations are located in a technical office, and from here the tool path is sent to the machine via a network connection or storage media such as USB sticks.
In the latest generation CNC milling machines, thanks to increasingly sophisticated machine consoles, it is possible to design and carry out CAM processing directly on the machine, this requires a high level of operator specialization.
"Physical" evolution of the cutter
The transformation of the mechanics of the machine was extraordinary.
The first manual cutters were a pride of mechanical technology, inside there were dozens of mechanical pieces that made them up; gears, shafts, gearbox, guides, kinematics, cams, etc. etc.
Each movement was mechanical and only thanks to the kinematics it was possible to perform certain movements of both the axes and the automatic feeds.
The same speed variation of the spindle was possible thanks to a belt system or a gearbox with gears.
All these mechanical parts had different negative sides: they needed lubrication, were subject to wear and running play, breakages and periodic maintenance, as well as a high cost of the machine itself.
With the technological evolution of electronics and servomechanisms, many of these mechanical parts have been completely replaced and with more functionality.
Now a simple three-phase electric motor controlled by an inverter allows for an unlimited spindle speed range.
Servomotors controlled by special drivers allow interpolated movements with millesimal precision.
The machine thus becomes much simpler mechanically, but highly more complex electronically.
We have therefore gone from mechanical problems due to vibrations of moving parts or play of the kinematic mechanisms to problems deriving from electronic interference, program “bugs” or breakage of electronic boards.
If previously a manual mechanical milling cutter had practically an unlimited duration bound only to the replacement of easily rebuildable mechanical parts, now modern CNC milling machines have their life cycle bound to the procurement of the electronic boards that compose them, and these do not remain in production for long. weather.
In fact, however, today's CNC cutters allow much more complex and faster machining than the old manual mechanical cutters.
Components of a numerically controlled machine tool
The main parts that make up a numerically controlled machine tool are: steering units, guides, servomechanisms and transducers.
The unity of government
The unity of government it is the system that receives and / or creates the tool paths, then processing them into signals that will control the servomechanisms that will physically move the machine axes.
The evolution of this unit has been remarkable over the last few years, allowing the control of up to 7 interpolated axes with the management of complex processes.
The number of axes in a CNC milling machine normally varies from 3 to 7 axes, the management of a 3-axis milling machine is relatively simple in terms of programming the tool paths, from 4 axes up, it is necessary to use a more CAM. complex and specialized, which allows the control of collisions between axes and workpiece.
Incorrect route planning can cause serious and costly damage to the machine.
On the market today the most common controls are: Fanuc, Siemens, Selca And Heidenhain, also in hobby world, simple controls were born, which however have respectable functionality.
An industrial control is basically composed of a console consisting of an LCD monitor where it is possible to see the ISO code, the machining, the position of the axes, the machining speed and all the active controls of the machine.
The console then interfaces with the electronic panel of the controller, where the power drivers for each single axis reside.
The driver is an electronic board that electrically controls a servomotor.
In addition to the drivers, there is an inverter that adjusts the rotation speed of the electrospindle, a whole series of control relays for the auxiliary devices of the machine, such as solenoid valves, tool change unit, coolant and safety systems.
The entire control panel is managed by one or more PLCs that communicate with the control console.
The electronic panel is normally placed in the rear part of the machine, protected from dust and oil, and is often refrigerated by a small air conditioner that regulates humidity and temperature inside.
The guides
The guides are the parts on which the slides slide, they are the most important part that conditions the precision of the machine tool.
The types of guides most used are:
Of the cast iron on cast iron (or dovetail) type, it is one of the first systems used in both mechanical milling machines and lathes.
Easy to make, low cost and with the possibility of adjustment.
It is a system still used on low cost manual machines
They can be coated with composite material to reduce friction.
Prismatic guides with recirculating balls, thanks to the reduction of costs for their production, are the most used system in today's machine tools.
They have a precision of less than a hundredth of a millimeter, low coefficient of friction, possibility of adjusting the play, long operating life thanks to the continuous recirculation of the balls during movement.
The servomechanisms
The servomechanism is the device that allows to transform an electrical signal into a mechanical movement, it is the organ that allows most of the movements of the machine, such as the movements of the axes and the tool change, the servomechanism receives the signals from the check.
Composition of a servomechanism
An amplifier is an electronic board that receives signals from a control unit, amplifies them and transforms them into certain waveforms. It also checks that all motion signals are executed by the control unit (closed loop system).
A power unit, this unit receives the previously amplified and modulated signals and outputs the current and voltage necessary to operate the control unit. Amplifier and power unit are normally integrated in a single board also called "Servomotor control driver"
The control unit is the final part that transforms electrical energy into mechanical movement, it is generally an electric motor combined with a transducer (encoder), it is generally called "Servomotor".
Normally, servomotors they work according to the feedback principle, where the control quantity at the input is compared to the quantity at the output, measured with some kind of transducer. Any difference between the actual value and the desired one is amplified and used to operate the system in the direction necessary to reduce or eliminate the error.
Motion transmission
For the transmission of motion, in machine tools, ball screws are used, because they transform a rotation movement into a translation movement with maximum efficiency.
The balls circulate in the raceway and are brought back to their starting point by means of return pipes (ducts), which can be internal or external.
Compared to the trapezoidal screw, the friction is considerably reduced and the energy to drive the screw (or nut) is correspondingly reduced.
Given the very low friction, it is possible to obtain a mechanical efficiency of 90% with consequent less wear, long life and excellent reliability in positioning accuracy.
Control systems
In NC machine tools, there are two fundamental systems:
OPEN LOOP (open loop)
CLOSED LOOP (closed loop)
Open loop control
It is a control technique as opposed to feedback control. It is distinguished from it by the absence of a direct measurement of the quantity to be controlled, since the input of the system to be controlled is calculated on the basis of the known characteristics of this system and on the possible measurement of the disturbances acting on it. The signals from the control unit are transformed directly into movements and therefore positions, without control with respect to the position to be reached.
Benefits
The absence of a direct measurement of the quantity to be controlled has a positive impact on the cost and time of implementation of the controller, as well as on the weight.
The ability to eliminate sensors for the measurement of the quantity to be controlled it also entails an advantage in terms of reliability, as the correct functioning of the control system will not depend on the functioning of the sensor or on the data acquisition system read by the sensor.
The absence of delays in the reading of the measured output (due to the dynamics of the system or of the sensors) guarantees a better promptness of response.
Disadvantages
The need to develop an accurate mathematical model results in experimental tests on the system with a consequent increase in development costs.
Poor robustness to parametric variations in the model, due to component aging.
Poor robustness in the presence of disturbances acting on the system.
This system is normally used on CNC cutters for hobby use due to the low cost and the easy availability of the control drivers.
The servomotors used are stepper motors, their accuracy is still excellent, considering that they are currently used on all printers.
Closed loop control
In a closed loop control system the input function is determined on the basis of the behavior of the system, as expressed by the trend of its outputs. This type of control is called feedback control as the outputs are returned to the input. In this type of system there are one or two feedback loops that constantly detect the difference between the instantaneous position and the one to be reached, both for displacements and for speeds.
Benefits
Accuracy of movements with the possibility of setting the margin of error.
Immediate shutdown of the system in the event of an error due to mechanical or electrical problems
High possibilities of setting the acceleration / deceleration speeds of the motion units
Disadvantages
High system cost.
Greater attention in the wiring of the servomotors to avoid electrical interference.