CNC Machining

CNC MACHINES

Nowadays, machines and machine tools have been designed for these machines in order to reduce the need for agriculture and other human labor and to go into serial production. After other machinery industry and automotive industry, it was needed to be used in agricultural machinery manufacturing. As we mentioned at the beginning, the aim of these designers was to replace the manpower with faster, reliable and efficient tools. There have not been any drastic changes in these machines for many years. But continuous progress has been made. Our era of computer technology has opened an age in metal cutting. This event is often referred to as “Computer Aided Numerical Control”. Its short name is CNC. Such machine tools have spread to the agricultural machinery industry after other industries and have prompted manufacturers to invest in these machines. In this way, the agricultural machinery industry tended to approach European standards, and serial and error-free production started in manufacturing. In this study, introduction of CNC, machine types and programming techniques will be discussed.

HISTORY OF CNC MACHINES:

Numerical control idea II. At the end of the world war, A. B. D. was introduced for the production of complex aircraft parts needed by the air force. Because it was not possible to manufacture such parts with the existing manufacturing benches of that day. To accomplish this, PARSONS CORPORATION and MIT (Massachusetts Instute of Tecnnology) have started working together. In 1952, they performed the first successful work in this field by equipping a CINCINNATTI-HYDROTEL milling machine with Numerical Control. Since then, many machine tool manufacturers have started to produce numerically controlled machines. In the past, vacuum tubes, electrical relays and complicated control interfaces were used in NC machine tools. However, these often had to be repaired or even renewed. Later, miniature electronic tubes and monolithic circuits were introduced, which were more useful in NC machine tools. Rapid developments in computer technology have also affected numerically controlled systems. Nowadays, integrated circuit elements, cheap and reliable equipment, which have been developed more advanced, are used in NC machines. With the introduction of ROM (Read Only Memory) technology, it was possible to store programs in memory. As a result, these systemic developments led to the birth of CNC (Computer Numerical Control). CNC then lathe, drill etc. started to be used widely in machine tools.

WHAT IS CNC?

Computer Numerical Control (Computer Numerical Control), the basic idea of ​​machine tools, numbers, letters, etc. It is a system consisting of symbols and operating with the help of commands coded according to a certain logic and able to program the workbench control unit (MCU).

Besides computerized numerical control, it is possible to preserve the programs as a result of the computerization of the machine control unit, to stop the program at every stage of the part production, to make the necessary changes in the program, to continue the program from where it left off, and to store the program in its last form. Therefore, it is sufficient to load the program once in the control unit. The transfers of the programs to the counter are perforated paper strips (Punched Tapes), Magnetic Tapes, etc. carried out through data carriers.

CNC TOOL BENCHES:

Numerical Control (NC), which will be summarized to NC machine tools before CNC machine tools and will tell CNC machines, is widely applied in all kinds of machine tools used for machining metal and other materials by removing chips. Some of these machines are:

Lathe Machine
Milling Machine
Drilling Machine
Boring Machine (Boring Machine)
Grinding Machine

All NC machine tools have their own capacity, operational capabilities and a number of characteristic features. Therefore, any machining feature that the machine does not have cannot be made to that machine.

Since there is no memory in NC machine tools, block data is read and processed sequentially in such machines. During the manufacture of a workpiece, the machine’s Control Unit reads all the data in a block and performs the required machining operations on the machine. After the operations are completed, it is passed to the next block. This process continues until the end of the program, respectively.

Part programs have different numerical (numerical) and alphanumeric (alphanumeric) values ​​and meanings according to the place and arrangement patterns on the standard paper tape.

The physical design and constructions of CNC machine tools are the same as the NC machines. However, a number of functional features that were practically impossible and economical to do in NC machine tools were added to this type of machine.

These features are as follows;

Part programs loaded on the machine can be stored in the memory of the control unit, they are called up and operated many times.
There is a special power supply that supplies the machine control unit. Even if the machine’s energy is cut, program etc. data is retained.
The changes that are planned to be made on the part program can be made easily and at any time. The modified program is both run and stored in its final form.
Some routine operations are defined as cycles in the program and used where necessary. (Drilling, boring, rectangular pocket milling, step and taper turning, radius turning etc.)
Programming operations that need to be repeated on a workpiece other than loops are written once in the main program (Main Program) and named as Sub Program. During the implementation of the main program, these sub-programs are called where necessary and the process is completed. As an example, we can give the program of ADANA script. A subprogram for the letter A is written here. However, this program is implemented by making coordinate definitions at different X distance. Thus, the normal program is shortened by 40%.
When a program of a part is written, it is normally machined to cutters of a certain type and diameter. During programming, certain dimensional shifts must be made taking into account the cutter diameter. However, with ease of cutter compensation, these shifts are made by the CNC control unit during the operation of the program. When the cutter used is broken and no other cutter of the same diameter can be found, it provides the ease of continuing the program with the cutter of different diameter. The control unit makes the necessary dimensional shifts according to the diameter of the new cutter.
Thanks to the computer, it reaches optimum values ​​in position changes, speed and advances. As a result, ideal working conditions are provided in CNC machine tools. Since the diameter of the workpiece is constantly changing in the face turning process, the number of revolutions must change accordingly (Constant Surface Speed). The resulting surface quality and precision are comparatively good compared to conventional machines (Conventional Machines).
It has the advantage of being able to communicate with many other computers and systems as a result of the use of computers in the CNC control unit.
It is possible to simulate the part program graphically with the help of the Visual Display Unit before starting the part manufacturing.
Replacing cutting tools is done without any manual intervention. For this, rotating turrets (Rotery Turrets) or crawler cutter magazines are used.

ADVANTAGES OF CNC TOOL MACHINES:

Some binding dies used in conventional benches, gauge etc. Compared to the elements, the setting time of the machine is very short.
Adjustment, measurement, control, manual movement etc. time losses due to reasons have disappeared.
Serial and precise manufacturing is possible because the human factor is not very effective in manufacturing.
There is no need for qualified people.
Machine operations have a high precision.
The working tempo of the machine is always high and the same.
All kinds of consumption (electricity, labor, materials, etc.) are minimized.
All kinds of personal errors arising from the operator in manufacturing have been eliminated.
Mold, gauge, template etc. The system is cheaper because expensive elements are not used.
Less space is needed in storage.
Transition to parts manufacturing is faster.
Changes to the part are made only in the relevant part of the program and in series, without changing the whole. For this reason, manufacturing with CNC machine tools has a great flexibility.

DISADVANTAGES OF CNC TOOL MACHINES:

As with any system, there are some disadvantages besides the advantages of CNC machines and systems. These are;

A detailed manufacturing plan is required.
It requires an expensive investment.
The hourly rate of the counter is high.
Compared to conventional benches, they require more rigorous use and maintenance.
High cutting speeds and high quality cutters should be used.
Periodic maintenance should be done regularly by experts and authorized persons.

USAGE AREAS OF CNC IN INDUSTRY:

Today, the part of the industry we call machining is the area where CNC is the most widely used. It was explained above that the problems of this field were caused by the birth of today’s CNC. When a three-axis milling machine was first operated in 1952, it was perfect because it provided the solution to some manufacturing problems of the day. These systems applied to milling machines are then turned into lathes, grinders, etc. applied to machine tools. Today, CNC is used in almost every field where manufacturing is made.

Main areas where CNC is used;

Machining
Fabrication and welding.
Press forward
Inspection and control.
Assembly.
Transport of materials.

CNC TOOL BENCHES:

A- CNC LATHE MACHINES:

In numerically controlled lathes, there are generally two main axes, the X and Z axes. Linear interpolation and curvilinear interpolation feature is sufficient for many profile turning operations in such machine tools. In addition, speed and cutter change, determination of feed rate etc. they have functions.

CNC lathes with larger machining capacities may have 3 or more axes. The third axis may be the axis movement of the machine turret. Especially in industrial type CNC lathes, some design changes have been made in its structural details in order to increase the structural resistance of the machine, to realize more precise manufacturing and to remove the chips from the cutting area.

CNC Milling machines are the machines that have the most processing capability after machining centers in terms of variety of operation capabilities. (Figure B-2) This type of machine has the ability to process at least 3, 4-5 and more axes. All types of these machines are equipped with Continuous Paht Control. The convenience of automatic tool change is another feature. Cutter compensation (Tool Compensation) provides great convenience especially in curved milling operations and molding.

The processing of three dimensional (3 Dimension) workpieces with the ideal profile and optimum features is successfully accomplished. The cutters used are hard metal and titanium coated tips with radii and high cutting speed. (Figure B-3)

C- CNC MACHINING CENTERS:

Such CNC machines are equipped with point to point and continuous track control (CPC). Manufacture of workpieces with such complex and numerous operations is carried out in a bind.

a- Horizontal machining center
b- Vertical machining center

CHARACTERISTIC FEATURES OF CNC MACHINING CENTERS:

3 or even 4 surfaces of prismatic workpieces can be machined simultaneously in one bond.
Face milling, drilling hole enlargement reamer and tapping, profiling, angle drilling, etc. transactions can be made.
The cutters to be used are placed in the magazine part of the workbench and used in the operations required in the program. Magazines have a cutter capacity of 10-30-60-80 or more.
Robot arms and equipments are used in the connection and unwinding of workpieces. Thus, time losses in this area are eliminated.
D- CNC DRILLING MACHINES:

CNC drill benches are not much different from conventional types in terms of process functions. They are not designed as CNC drill benches themselves, but as a small capacity vertical machining center. The movements of the workbench are in the X and Y axes and the movement of the cutter is in the Z axis.

There are many possibilities in such machines. (The head to which the table fixed cutter is attached can move in the coordinate axes. There can be more than one machine spindle and table.) They are especially useful in simple milling, drilling and boring operations.

PROCESS STAGES IN DRILLING OPERATIONS:
The cutting tool is sent to the X and Y coordinates of the target point to be drilled. When the target point is reached, the cutter quickly approaches the workpiece surface up to a safe distance (Clearance Height). The drilling of the workpiece is started. If the hole is deep, the cutter is removed a bit and the chips are emptied and the drilling continues again. At the end of the drilling process, the cutter is quickly taken out of the piece. The cutter is sent to the coordinates specified for the next hole. Mirror image and repeat cycles are widely used in CNC drill benches. The X and Y coordinate values ​​required for the mirror image are entered. In the repeat cycle, the X and Y coordinate definitions are made for the number of repetitions and the operation. Thus, the program is considerably shortened. Values ​​related to different cutter sizes are entered into the relevant unit of the machine control unit as cutter length compensation. With the ease of automatic cutter change (ATC), a great speed is gained in the production of a large number of cutter.

E- CNC GRINDING MACHINES

High precision and surface quality should be obtained in cylindrical and plane grinding processes. Therefore, it is very suitable for the basic philosophy of Numerical Control especially in terms of technology. Unfortunately, the use of NC in this area has been in recent years. Manufacturer companies that have successfully solved some of their special problems related to grinding have developed their own NC systems by integrating with Numerical Control manufacturing.

Standard control designs used in lathes and milling machines are not useful in grinding machines. Therefore, different solutions are needed in the control systems of grinding machines than other types of machines. These are: Sometimes high sensitivity up to 0. 1 micron. A wide range of pacing. Feed speeds of 0. 02 mm / min. With 60 m / min. ranges from. Special grinding cycles related to grinding processes. Gradual progress increase, waiting, swinging, routine, stone sharpening, etc. are examples of these cycles. Automatic cutter compensation after grinding the grinding wheel. Linear and curvilinear (Circular) interpolation should be done without slowing down. Grinding of the grinding wheel in any contour deviations. Data entries and corrections can be made easily to the program. Compensation is simpler because the amount of cutter used in grinding machines is not much.

F- CNC PRESS AND STAPLE DRILLS:
Position changes with CNC presses and punch drills are made in the form of two-axis continuous trace control and at high values. Programmable course progress can be changed according to the thickness of the sheet materials. In general, the parts manufactured in these machines are similar. Therefore, program memories have a wide range of combined and repeatable programming capabilities.

For punchers, punch shapes have features that can perform various operations from simple to complex profiles. Commonly used staples for this can be standardized and readily available. Again, such machines have features of automatic replacement of the staples. These are the 36 most commonly used stations of staple turrets. Parts to be manufactured with the help of computer and placed on sheet metal plates. Thus, optimum part placement is ensured so as to cause the least waste. The transportation of the parts and the application to the workbench, the removal of the finished products and the residual parts are done with the help of programmed carriers.

G- CNC POINT WELDING MACHINES:

Numerical Controlled Spot-Welding Machines have been put into practice especially in the automotive industry in recent years. Welding of parts on transfer lines is accomplished with great success compared to manual welding. Parts with complex geometry are welded in series and desired completeness.

The program is organized and repeated with the production of the first track. This process is called ‘Playback’ technique. In other words, each part of the part to be welded is adjusted manually and entered into the memory via the keyboard. Thus, auxiliary coordinate calculations are eliminated. Most CNC spot welding machines are networked with a CNC welding line. During the process, the necessary welding program is called from the memory and the welding process is performed.

The CNC in these machines is basically machine control rather than operation control. The head that performs operations in oxyacetylene, plasma or laser cutters is the bench shaft. If a welding torque is installed instead of these heads, a CNC welding machine is obtained. The use of robots in welding processes is the most successful and common application.

OTHER TYPE CNC TOOL MACHINES:

Due to its high efficiency and precision, CNC is widely used in all kinds of manufacturing systems today. It also provides great ease and time savings in automation with its automatic cutter changing features.

The types of workbenchs where CNC is used today are not only those mentioned above. However, these are the most widely used. Apart from these, CNC types are used:

Three-dimensional measuring and control machines
Tool sharpening machines
Saw benches
Assembly systems
EDM machines
Coating benches
Material handling systems
Laser cutting machines
Tube bending machines
Spinning Benches
Flame cutting machines
MAINTENANCE IN CNC TOOL BENCHES:

The review of bench and equipment at certain periodic intervals is called MAINTENANCE. When electronic circuit elements used in CNC machine tools are compared with those used in conventional machines, it is seen that they are in high numbers. Rigorous use and healthy work environments are required for electronic components.

In parallel with this much development of machine tools and systems, some facilities related to their maintenance and repairs have also been developed. Any malfunctions that may occur in the machine (overload of the engine, insufficient lubrication, dirty filters, overheating etc.) are notified to the operator in the form of signals or messages on the machine control panel.

As with all matters, the initiative is left to the operator. Maintenance and negativity should be eliminated as soon as possible. Otherwise, it is not possible to manufacture parts by running any program in such a machine.

THE IMPORTANCE OF CLEANING AND MAINTENANCE IN CNC TOOL BENCHES:

As mentioned earlier, because CNC systems are expensive systems, any malfunctions that may occur must be detected and eliminated immediately. Otherwise, when the malfunctions reach large dimensions, they will be costly to eliminate and the machine will be out of production, resulting in significant disruptions in production.

Fault Diagnosis method is used to detect faults in a short time. In this diagnosis, answers are given to the questions posed by the machine control unit. According to the responses of the operator, the malfunction is determined by the control unit.

In general, failures occurring in CNC machines are caused by simple reasons such as dust, excess oil, moisture and heat. Also, due to similar reasons such as vibration, there may be loosening in the connection points of the circuit elements. It is often enough to check these connections and fit them in the proper position.

Sensitive surfaces of position measuring systems may be oiled or dusty. Generally, cleaning of these parts is sufficient to eliminate malfunctions. Therefore, mechanical impact, crushing, burning, etc. Malfunctions other than damages are eliminated with very simple dusting and cleaning processes.

For the reasons described above, the cleaning of the environments where CNC machines are located is extremely important. The axis movement areas of the machine must be clean and sufficiently oily. It should be opened and cleaned when necessary, otherwise it is inevitable that the machine will be out of service for a long time due to the simplest reasons. Today, maintenance is done at two levels;

A-Planned maintenance,
B-Preventive maintenance

Maintenance at both levels should be done by technical staff who are experts in this field. Those people;

Adequate technical and practical knowledge, especially in the field of electronics,
To a certain professional experience,
Digital electronics knowledge,
Ability to measure and use test devices,
Adequate technical information about mini and micro computers,
must have.

Maintenance on CNC machines is schematically as follows;

WEEKLY CARE:
In such periodic maintenance, short test programs are run. The number of revolutions and feed rates to be given to the machine spindle are tested by entering between minimum and maximum values. The findings obtained in these tests are noted, their causes are tried to be eliminated. It is investigated whether all fans on the bench and peripheral equipment provide adequate air circulation. The perforated paper strip driver and wheels are checked. The reader organizes the head, channel and nails.
MONTHLY MAINTENANCE:
In the monthly periodic maintenance, a workpiece program is tested by running it without removing chips and axis movements are monitored. Places that need to be manually lubricated are lubricated. It is checked whether all circuit connections are properly installed. The casings of the measuring systems are removed, if any, dirt and oils are cleaned.
SIX MONTHLY MAINTENANCE:
The speed, voltage and errors that are interdependent with each other are measured. The data obtained are compared with the values ​​measured in the first assembly. Air and oil filters etc. elements are checked, cleaned and, if necessary, replaced with a new one.
ANNUAL MAINTENANCE:
It is checked whether each circuit and circuit element in CNC systems is perfect. The cleanliness of the contact points is reviewed. Doors and seals are checked to see if fastening screws are tight. The bench console is cleaned with a vacuum cleaner or soft brushes. The working status of the perforated paper tape reader, the precision values ​​of the machine are checked. The power supply (Power Supply) voltage output is checked for suitability. By testing a part program, the accuracy of all functions is investigated.

PROTECTIVE MAINTENANCE IN CNC TOOL BENCHES:

Maximum attention should be paid to preventive maintenance on CNC machines, as well as other types of maintenance. Because simple negligence and mistakes will cause the staff of the machine to be out of service.

Regarding preventive maintenance, the following points must be strictly followed;

Healthy parts should not be tampered with and their settings should not be changed.
Any signals or messages given are strictly followed. These may be related to rust, loose connections, contaminated contacts.
Necessary parts are carefully lubricated. Excess oil is never used. Excess fat is more harmful than lean.
Diagnostics and tests must comply with the instructions.
Instead of repairing the damaged parts, it should be preferred to replace them with new ones.
Preventive maintenance has three main functions;

1-Cleaning
2-lubricating
3-Control

MAIN FAILURE ZONES IN CNC TOOL BENCHES:

The most common failures and regions in CNC machine tools are as follows.

Machine tool axis drives
Hydraulic and pneumatic elements
Control circuits
Measuring and transfer systems
Digital data processors
Logic connections
Input / Output units
IDEAL WORKING ENVIRONMENT AND CONDITIONS FOR CNC TOOL BENCHES:

Highly clean environmental conditions are required for CNC machines to work properly. Ideal working environment conditions recommended by machine tool manufacturers;

These issues should be taken into consideration since the machine control units are affected by temperature, light, humidity, vibration and voltage changes.
Working environment temperature and humidity should be among the values ​​recommended by the machine manufacturer.
Vibration should be below 0.5 grams.
Acceptable voltage variations should be + 10%, -10%
Voltage drops should be a maximum of 2.5 waves (20 MS).
If the working environment conditions of the machine do not comply with these standard values, the manufacturer may not fulfill the maintenance obligations.

MAINTENANCE OF LUBRICATION SYSTEMS:

Another one of the systems that need to be maintained on CNC machines depends on the healthy operation of the lubrication system.

The most frequent maintenance is the replacement of oils, completion of missing oils, and cleaning or replacement of filters when their periodic period expires. The lubrication of the elements related to the closed internal systems of the workbench is done automatically. In some small-scale educational CNC machines, lubrication of the recording-skid systems is done manually.

Another point to be considered in lubrication is to avoid excessive lubrication. It covers the outer surfaces of sensitive electronic circuit elements, preventing their healthy operation, with excessive lubrication oil waste in CNC benches. For this reason, the level lines should never be exceeded in the completion of the missing oils.

PROTECTION OF BALL SPINDLE AND REGISTRATION-SLIDE SECTIONS:
Ball Screws, which are the motion transmission elements used in CNC machine tools, and the slide-slide systems are enclosed in closed housings. These enclosures are vinylex or spiral protectors. These have features that can be opened and closed according to the movements of the table.

HYDRAULIC AND PNEUMATIC EQUIPMENT MAINTENANCE:
As in the above-mentioned elements, hydraulic and pneumatic equipment should be maintained periodically. As these elements generally apply compressed force, it should be checked whether the continuous working pressures are at ideal values ​​and any malfunctions should be eliminated. Otherwise, they pose a great danger to the machine, operator and those working in the environment. Estimate that the pneumatic bench chuck does not tighten the workpiece sufficiently, that the cutters are not securely attached and that accidents that may result.

TOOL CHOICE FOR CNC MACHINES:
One of the most important features of CNC machines is that they have very high metal removal rates. It is quite unbelievable that there are tools that can perform these operations on these machines. If we add to this an adjustment time that is less than an industry and an automatic tool change job that takes about five seconds, we better understand why production engineers think that tools are the most crucial issue in numerically controlled machining.

TOOL MATERIALS:
Although high-speed steel (HSS) tools are used in operations such as small hole drilling, tapping, reaming, tack hole and wedge grooving, sinter carbide (Carbide) tools are generally used in CNC machining.

The physical properties sought in the tools to be used in these machines are the hardness and toughness of the material, which can reach up to 600 ° C. High-speed steels are tougher than sinter carbide but not as hard. Therefore, extensive research is underway to develop new types of carbides that can meet the requirements of high speed metal removal techniques. In order to understand how successful these researches are, it is sufficient to monitor CNC machines while working.

TOOL CONTROL:
The efficient use of expensive CNC machines requires a highly methodical tool handling approach. The original or replacement tool on the machine must correspond to the tool in the part program. For this reason, close cooperation should be ensured among people working in programming.

An effective tool control should provide the following functions;

Improvement including replacing worn, damaged tips, sharpening when appropriate,
Preparation including sizing, presetting,
Storage until use,
Transport,
Protection at the counter.
This concept is given schematically below;

Improving the tools with sharpening methods requires a lot of attention. The efficiency of metal removal at long speeds requires a complete tool geometry. Tool control system;

tool warehouse transport on bench / storage machine tool transport tool room transport
supply preset identification maintenance
When tools are not in use, they should be stored with identification cards on heavy-duty steel shelves. Stored tools can be private business or general purpose. No matter what, these tools have dimensional properties, applications etc. should be determined with the information that contains. It is also very useful to make a list of available tools, to reference both the part programmer and the operator. The team list is often referred to as the team library.

WORK LOADING AND WORKING AT CNC:

Any work clamping device must meet the following conditions.

He must tie the work tight,
Provide positive placement,
It should be fast and easy to use,
There are many used work clamping devices that have been tried and used in traditional benches; vise, chuck and collet are the best known examples and they are also used in numerically controlled benches. These work clamping devices can operate mechanically, hydraulically or pneumatically. Mechanical workers require manual skills in loading and tightening of the workpiece. Therefore, hydraulic and pneumatic clamping, especially the latter, is preferred. Hydraulic and pneumatic clamping is easily controlled electronically by the machine control unit, providing fast operation and uniform clamping pressure. Such conventional work clamping devices are more suitable for processing uniform shaped stock material or workpiece such as rectangular, angular, hexagonal. Irregular shapes can sometimes be adapted to traditional machining with summary designed conveniences, with pneumatic or hydraulic tightening arrangements. As a general practice, the workpiece must be positively placed so that it does not move during machining. In both cases, the workpiece is placed against fixed jaws. Movement of the workpiece in any machining process is not desired for safety reasons. There may be a problem with the movement of the workpiece, even if it is small in the numerically controlled machining process. This means that the dimensional accuracy of the workpiece is lost, since the workpiece size is not constantly monitored during processing.