Monday 19 August 2013

CNC Tooling

CNC TOOLING

INTRODUCTION                   
            
The greatest metal working advance during this century is the development of numerically controlled machine tools &computerized numerically controlled i.e. CNC machine tools. These machine are a major step towards job shop automation &the progressive machining industries are making good use of it.


NEED FOR THE CNC:
           
All types of machine tools require some type of the control system for their operations and feed movement. The most versatile control system has been man. But he is far from being the most reliable.
           
He has a limited memory and may be readily influenced by his surrounding environment. His speed is slow and erratic and his ability to repeat dimension and co – ordinate more than one dimension is extremely limited.
           
Therefore the computerized numerical system is developed to overcome the many shortcoming of man as a control system. In this system of control, the instructions are give to the machine concerning the feed movement positioning, cycling and sequencing of operations through the computer programming. However, the expense of developing and constructing such a system prohibits its used for small lot production. For example it is excellent for the automotive industry where thousand of identical parts are produced but it is not suitable for aircraft industry, where in comparison to the autoindustry , relatively few parts are produced.


TOOLING FOR THE CNC MACHINE TOOLS

OBJECTIVE:
           
In this topic we are going to understand.
1)      Special tooling requirements of CNC machine tools.
2)      Meaning and usage of pre – set and qualified tools.
3)      Advantages of using automatic tool changers and multipallets on CNC machine.
4)      Different types of advanced tools.
5)      Utilization of CNC machine tools
6)      Maintenance of CNC machines.
7)      Economics of manufacturing using CNC machines.


CUTTING TOOLS

           
To minimum the tool changes and tool settings time, the CNC machine use preset and qualified tools.

TOOL PRESETTING

           
Generally normal tools used for basic machining operations in the general purpose machines are suitable for NC machine tools also. However preset and qualified tools have emerged to reduce the nonproductive time to the minimum.
           
A presetting device is used to preset axial and radial positions of the tool tip on the tool holder. Once this is done, the tool holder is ready to be mounted on the machine or tool magazine and produce a known dimension. Presetting devices to various level s of sophistication are available. Tool offset, tool length and tool diameter compensation facilities available in the present day CNC machines have brought down the importance of presetting. However the amount of compensation can be quickly and accurately decided the presetting devices.
           
For the presetting of the tools for workpieces machined on CNC or specific system oftooling some of the presetting fixtures resembles the tool positioners of universal type which can be used on different machines.


QUALIFIED TOOLS


            These are the tools in which the cutting teeth or edge is maintained at a fixed distance within a tolerance (±0.05) with respect to the reference surfaces of the holder. They should be selected wherever possible because of the following advantages.

  1. The tools need not be measured individually.
  2. Roughing cuts can be taken without a trial cut. Control dimensions of the tool are nominal and fixed. Hence programming is easy.
  3. No presetting device is used. Therefore investment is less.
  4. Setup time is reduced as elaborate organizing and planning as in preset tool is not needed.

The cutting tools should satisfy the following additional requirements for CNC machine tools.

1.            Higher control should be exercised on the run out of the multiteeth cutter for better size control.
2.Higher control on resharpening and inspection should be practiced for reliable and repeatable performance.
3.Chipbreaking facilities should be incarporated for easy chip disposal and better surface finish.
4.Cutting tools should be selected from improved designs for improved quality of workpiece and spade drills, carbide drills, core drills with carbide inserts, rimmers, coated inserts.

 

 

INTRODUCTION TO CNC TOOLING


            The special design features of CNC machine have resulted in use of higher cutting speeds and feeds leading to considerable saving in the cycle time. To fully exploit the higher metal removal rates of CNC machine the tooling used should be able to withstand the higher cutting forces in the process and help to reduces the downtime to minimum possible.

            The tooling used on CNC machines should be
  1. Rigid to withstand high metal removal rates.
  2. Capable of being preset and reset in the shortest possible time to keep the downtime minimum.
  3. Accurate enough to produces repeative accuracy of the job.

In conventional machine the cutting tools cut metal for about 25% of the total machining time whereas the CNC machine tools are expected to cut metal for 70 – 80% of the time. Since CNC machine are costly be reduced to minimum. The tooling for CNC machine tools includes the cutting tools and tool holders and work holding devices.



INDEXABLE INSERT

           
At the high cutting speeds and feeds used on CNC machines, the brazed tool are normally not used. Indexable carbide inserts are used to take up higher cutting speed and minimize the tool change time since the actual cutting time is more in CNC machines that is upto 70 – 80% of the total production time the consumption of cutting tool is also more.

The tool change time with brazed tool is many times more compressed to tool change time with indexable insert is taken out and just indexed. Harder and special grade carbides should be used to achieve faster metal removal rate. The indexable insert of tungsten carbide are further coated with wear resistant layer of titanium carbide or titanium nitride for better results.
           
Indexable inserts are available in various shapes and with varying geometry. The chip breakers can be formed on the inserts either by shaped grooves in the inserts itself or by way of clamping arrangement. In case the solid HSS tooling is employed in some machining applications, proper cutting fluid should be used to get improved surface finish, increased tool life and accurate dimensions. The overhang of the tool should be kept minimum. As the cutting speeds and feed rate are high in case of CNC machines, the temperature of the cutting edge of the cutting tool is also very high. To ensure proper cooling, coolant is fed to the cutting edge through centrally drilled system so that if the coolant supply is not there, the machine will either give signal or may stop.



CUTTING TOOLS MATERIAL


1] UNCOATED HARD METALS (ISO GROUP - HW)

            These are used for operations involving light cuts requiring sharp cutting edges and operations demanding very high toughness. These are also used on non ferrous metals and known metallic materials.

            These have less longer life and also less cutting speeds than coated hard metals.




2] COATED HARD METALS (ISO GROUP – HC)

            These materials have the highest wear resistance and good toughness. Coating of hard metals is done by chemical vapour deposition.

            The coating of TIC or TIN or Al2O3 and very hard and thin layer of this is apply on the hard metal.

            These materials posses the much longer life and higher cutting speed.




3] CERMETS (ISO GROUP – HT)

            These are hard metals based on titanium carbonitrides (TiC, TiN, CO, Ni, Binder). They offer outstanding cutting edges strength and are particular suitable for finishing application at high cutting speeds producing excellent surface finish.



4] CERAMIC CUTTING MATERIALS (ISO GROUP - C)

            These materials have good hot hardness and high compressive strength at elevated temperature permitting very high cutting speed.

            Main application of these materials is turning of grey cast iron. These are used only for steel and due to lack of suitable chip control geometries, chip breakage problems can occur.






5] POLYCRYSTALLINE CUTTING MATERIALS

            These includes polycrystalline cubic boron nitride and polycrystalline cubic  boron nitrate and polycrystalline diamond. There application are very limited and used in specialized applications.




6] CUBIC BORON NITRIDE (ISO GROUP - BN)
            This is the hardest materials after diamond posses great wear resistance even at cutting edges temperature over 1000oC used on hardened materials chilled out cast iron and high temperature, Ni, CO alloys.





7] POLYCRYSTALLINE DIAMOND (ISO GROUPS DP)

            It is completelyt isotropic and thus less succeptible to breakage. Fine grain structure makes it possible to produce very sharp burr free cutting edges.

            This is used on all non ferrous metals particularly cast aluminium alloys at high cutting speeds and excellent surface finish.





INDEXABLE INSERTS SELECTION CHART

STEPS
PLEASE ENSURE
1. Depending on material to be removed and geometrical accuracies desired on the component determined insert shapes, nose radius, depth of cut and operating feed.
1.      For the nose radius selected ensure that operating feed does not exceeds the maximum permissible feed rate


2. Considering the maximum allowable depth of cut for different insert shapes size select the insert size or the depth of cut desired.

3. For depth of cut and feed rate combination select the appropriate chip breaker. In case of clean cut or light inturrption select chip breakers with least strength and for heavy interruption select the chip breaker with greater edge strength. Select the grade of turning insert such that midpoint of the range of cutting speed of that range of cutting speed of that grade is closest to the operating cutting speed.
2. Ensure that the effective cutting edge length for the insert shape or size chip breakers combination does not exceed the operating depth of cut.
4. For roughing cut or light interruption select chip breakers and grade combination. For clean and continous out use the next higher wear resistance grade. For forging skills and heavy interruptions use next tougher grade. Thus we can select optimized insert with the help of this chart.




INSERT FAILURE AND REMEDIES


1] FLANK WEAR
Remedies
  1. Select more wear resistant grade.
  2. Reduce the cutting speed.

2] NOTCH WEAR
Remedies
  1. Strengthen the cutting edge.
  2. Select smaller approach angle.
  3. Reduce feed.

3] CARTER WEAR
            Wear on rake face characterized by cutter depth.
Remedies
  1. Use coated hardmetal grade.
  2. Select positive insert geometries.
  3. Reduce cutting speed and chip cross.

4] EDGE CHIPPING
            Result of chip impact due unfavorable chip information.
Remedies
  1. Select tougher grade.
  2. Use insert with stronger cutting edge geometry.
  3. Vary feed.
  4. Change approach angle and chip breaker geometry.



5] INSERT BREAK EDGE
Remedies
  1. Select the tougher grade.
  2. Use stronger insert with larger corner radius.
  3. Select break geometry for heavier chip selection.
  4. Reduce feed and possibly also depth of cut.


6] BUILT UP EDGE
Remedies
  1. Increase cutting speed
  2. Use coated hard metals
  3. Use cutting fluid
  4. Select positive cutting edge geometry


SELECTION OF CHIP BREAKER

           
A part from actually cutting application indexable insert with chip breaker groove having two functions shaping and breaking the chip. The following factors are to be kept in mid while selecting the chip control groove.

  1. Cutting conditions such as speed, feed, depth of cut
  2. Nature of work material such as alloying elements hardness and conditions of heat treatments.
  3. Profile of the tool such as nose radius and approach angle
  4. Shape of chip breaker groove such as width, depth position of groove relative to cutting edge.
  5. Friction at the groove and cutting fluid used.
  6. Material to be cut, mounting of insert in holder and machining method.

 

 

 

TOOL HOLDERS


    • Work holding devices
    • Automatic tool changers and tool positioners
    • Drills
    • Milling cutters
    • Reamers

 




TOOL HOLDER


            Tool holder used on CNC machine tool should be of quick changing type and should be capable of being preset. Since in the modern CNC machines, there is a provision for automatic tool changing, the quick changing type tool holders can reduce the tool change time about 3 to 5 seconds only.

QUICK CHANGE TOOLING


            The modern throw away insert tools have brought about considerable reductions in machine down time for replacement of dull tools. It is specially used on CNC lathes and boring machine. When the cutting edge become dull, only the insert is indexed to the new cutting edge within few seconds. More important the new cutting edge is in the same relative position to the workpiece and the operator does not have to read just the tool to the original reference point.

            Also the spindle nose tool hold are available for the quick tool changing.

 

 

 

WORKING HOLDING DEVICES


            The numerically controlled machine are capable of performing a number of operations using different tools, on different faces of a component in a single setting. This requires that the component should be accusable from different sides without changing of clamps or repositioning of component. The work holding devices have to bear multi – directional cutting forces. So additional demands are made on work holding devices in numerically controlled machine tool. To reduce the clamping and unclamping time, hydraulic and pneumatic actuation is widely used in work holding device. In general, a work holding device for CNC applications should following features.



  1. It should restrict the linear and rotary motion of the component.
  2. The component should not distort or deflect due to cutting forces in the process.
  3. It should facilitate quick loading and unloading of component.
  4. It should be fool proof.
  5. It should not interfere with the cutting tools.
  6. It should permit number of operations on different planes in a single setting.
  7. It should provide for easy removal of chips.
  8. It should be adaptable to automated loading/ unloading of components.
  9. It should be safe.




 

 

AUTOMATIC TOOL CHANGERS

           
The CNC machine are designed to perform a number of operations in a single setting of the workpiece. To reduce the down time in change over from the one application to next, the CNC machines are equipment with automatic tool change over from the one application to next, the CNC machines are equipment with automatic tool change facility. The tool is automatically selected and changed based on the tool control function in the part programme.
           
Turning centers are available with the tool turrets containing 8 – 12 tools. As the tool change command is received by the control system, the tool turret moves a fixed tool change position and the required tool comes to the cutting position. On making centers, automatic tool changes are provided to reduce the ideal time between change over from one tool turret moved a fixed tool change position and the required tool comes to the cutting position. On making centers, automatic tool changes are provided to reduce the ideal time between change over from one operation to another.



MAINTAINANCE OF CNC MACHINES

           
Repairing CNC machines in time – consuming and costly considering the following factors
i.        A substantial amount of spares are not in digeneously available.
ii.Delay in producing spares.
iii.    Lack of adequate manpower to troubleshoot CNC systems, particularly when the systems are imported.

Breakdown maintenance, therefore is less preferred than preventive maintenance.



PREVENTIVE MAINTENANCE

           
The main objectives of preventive maintenance of CNC machines are to
  1. Maximize machine availability.
  2. Minimize downtime
  3. Maintain positioning accuracy
  4. Increase the service life of the machine.

Generally the manufacturer of the machines will give certain guidelines regarding preventive maintenance. The general emphasis of preventive maintenance is to isolate wear and malfunctions. Preventive maintenance programmers should include the following

1.      DAILY SCHEDULE

Oil checks (hydraulic oil, lubricating oil) in various locations are to be carried out daily. It is a good practice to clean the machine ways thoroughly and oil them at the end of the day’s work. The hydraulic oil pressure and pneumatic pressure (if used) should be monitored. Hydraulic pressure settings should be checked. Tool changers, turrets, tool probes etc. should be properly cleaned.

2.      WEEKLY PREVENTIVE MAINTENANCE

This emphasize on checking abnormal equipment performance, visual for oil leak, cleanliness of filters, proximity switches, tape reader heads etc. It is a good practice to set apart at least half a shift at the end of every week to clean and check the machine completely. Checks can also be made in tool change time, table indexing time, feed rate, regulations of spindle speed, working of emergency stops, limit switches etc.




3.      MONTHLY PREVENTIVE MAINTENANCE

This may include hydraulic oil cleaning, checking of clamping, hydraulic operation of chucks, tailstocks, steadies, filter changes etc. The schedules for some of the preventive maintenance actions are normally indicated by manufacturers. These includes
a)      Frequency of hydraulic oil change
b)      Checking and changing of air filters (for blowers) and oil filters
c)      Metering devices in lubricating system
d)     Change of brushes in DC servimotors and DC motors
e)      Checking of backlash in ball screws
f)       Alignment of the machine
g)      Cleaning of coolant equipment
h)      Setting of pressures in pressure switches, pressure regulators etc.
Checking the battery back – up for parameter storage.
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