CNC MILLINGSUBJEK: AMALAN KEJURUTERAAN MEKANIKAL III KOD SUBJEK : BDA 2801

1.1 Learning OutcomeAt the end of this module, the student should be able to: Apply and practice workshop safety regulations during working in CNC workshop. Apply the concept of CNC metal cutting operation correctly in milling process. Carry out CNC mill machining procedures systematically. Utilize the important parameters in CNC mill machining operation effectively according to a given task

1.2 Introduction Milling is a cutting operation with a geometrically specified cutting edge in which the tool makes the rotating main movement, and the feed as well as the infeed movement are generally made by the work part. Milling operations are classified according to the position of the milling axis towards the work part, i.e. between face milling and peripheral milling. In case of face milling, the milling axis is located vertically to the machining. The work part surface is machined by the main cutting edges. Also, the work part surface is further finished with auxiliary cutting edges

Figure 1.1 : Milling Cutting Operation

Figure 1.2 : End Milling

Figure 1.3 : Plain Milling

synchronous and conventional milling (Figure 1.4 and 1.5) are differentiated.

In case of conventional milling the rotationdirection of the milling tool is opposite to the feed direction of the work part. The milling tool chamfer edge starts with chip thickness zero. The milling tool cutting edge slides in front of the chip chamfer edge until the required minimum chip thickness has been achieved for chip building.

Figure 1.4 : Conventional Milling

For synchronous milling the rotation direction of the milling tool and the feed movement of the work part are parallel. The tooth of the milling cutter immediately penetrates into the work part. Since the milling tool cutting edge is exposed to impact forces the feed drive needs to be play free. Several cutters should always be in operation. The surface quality is flatter and duller when synchronous milling is used. Compared with conventional milling higher feed movements and cutting speeds within the same cutting edge life can be achieved.

Figure 1.5 : Synchronous Milling

Due to the cutting path comma-form chips are cut with a changing chip thickness

Figure 2.6 : Milling Plan

1.3 Theory of CNC Milling Machine1.3.1 Characteristics of CNC Milling Machine Tools Work part machining on CNC machine tools requires controllable and adjustable infeed axes which are run by the servo motors independent of each other. CNC- milling machines (Figure 1.7) on the other hand have at least 3 controllable or adjustable feed axes marked as X, Y, Z.

Figure 2.7 : Controllable NC Axes on a Milling Machine

1.3.2 Basic Geometry for CNC Millinga) Coordinate Systems On CNC Machine Tools Coordinate systems enable the exact description of all points on a work plane or room. Basically there are two types of coordinate systems

Cartesian coordinate system A Cartesian coordinate system, also called rectangular coordinate system includes for the exact description of the points Two coordinate axes (two-dimensional Cartesian coordinate system) or also. Three coordinate axes (three-dimensional Cartesian coordinate system), located vertically to each other

Figure 1.8 : Cartesian Coordinate System

Polar Coordinate System In the Cartesian coordinate system a point is described, for instance, by its X and Y coordinates. For rotation symmetrical contours, such as circular boring patterns, calculating the needed coordinates requires extensive computing. In the polar coordinate system a point is specified by its distance (radius r) to the point of origin and its angle () to a specified axis. The angle () refers to the X axis in the X,Y coordinate system. The angle is positive, if it is measured counterclockwise starting from the positive X axis (Figure 1.9)

Figure 1.9 : Polar Coordinate System (Positive Angle )

Figure 1.10 : Right-Hand-Rule The specifications of the three axes as well as the three coordinates are done as a so-called clockwise-rotating system and follow the righthand-rule (Figure 1.10). The fingers of the right hand always show to the positive direction of each axis. This system is also called the clockwise-rotating coordinate system.

b) CNC Milling Machine Coordinate System Machine Coordinate System The machine coordinate system of the CNC machine tool is defined by the manufacturer and cannot be changed. The point of origin for this machine coordinate system, also called machine zero point M, cannot be shifted in its location.

Work Part Coordinate System The work part coordinate system is defined by the programmer and can be changed. The location of the point of origin for the work part coordinate system, also called work part zero point W, can be specified as desired.

Figure 2.11 : Machine Coordinate System

Figure 2.12 : Work Part Coordinate System

The design of the CNC machine specifies the definition of the respective coordinate system. Correspondingly, the Z axis is specified as the working spindle (tool carrier) in CNC milling machines (Figure 1.13), whereby the positive Z direction runs from the work part upwards to the tool. The X axis and the Y axis are usually parallel to the clamping plane of the work part. When standing in front of the machine, the positive X direction runs to the right and the Y axis is away from the viewer. The zero point of the coordinate system is recommended to be placed on the outer edge of the work part.

Figure 1.13 : Milling Part In Three-Dimensional Cartesian Coordinate System

1.3.3 Zero And Reference Points On CNC Machine Toolsa) Types Of Zero And Reference PointsTable 1.1 : Types of zero and references points

b) Machine Zero Point M Each numerically controlled machine tool works with a machine coordinate system. The machine zero point is the origin of the machine-referenced coordinate system. It is specified by the machine manufacturer and its position cannot be changed. In general, the machine zero point M is located in the center of the work spindle nose for CNC lathes and above the left corner edge of the work part carrier for CNC vertical milling machines.

c) Reference Point R A machine tool with an incremental travel path measuring system needs a calibration point which also serves for controlling the tool and work part movements. This calibration point is called the reference point R. Its location is set exactly by a limit switch on each travel axis. The coordinates of the reference point, with reference to the machine zero point, always have the same value. This value has a set adjustment in the CNC control.

Figure 1.14 : Location Of The Zero And Reference Point For Milling

1.3.4 Structure of a NC Milling Programminga) Structure of an NC-Block (Format) Unlike the conventional milling machine, a modern machine tool will be equipped with a numerical control system. The machining of a work part can be executed automatically, provided that each machining cycle has been described in a "language" (code) which can be read by the control system. The total of coded descriptions relating to a work part is called an NC-program.

* Blocks Each NC-program consists of a number of so-called blocks, which contain the commands to be executed. The blocks are consecutively numbered; each block number consisting of a letter "N" plus a (e.g. three-digit) numeral. Block numbers appear at the beginning of each program line * Words Address, Value As a rule an NC block is comprised of several words. Each word consists of an address (letter) and a value or code (numerals).

A numeral can either represent a code (e.g. G01: Linear feed motion) or a real value (e.g. X+60 : Approaching the target coordinate X=60). Example of part program:

N110 F95 S850 M03 N115 G00 X+25 Y+30 N120 G01 Z-8 N125 X+105 N130 Y+80

Explanation:Block-No.

N110 A feed rate of 95 mm/min and a spindle speed of 850 U/min is programmed. N115 The tool is moved in the rapid traverse motion from its current position to the starting point (X+25 Y+30)t N120 Infeed in the Z-axis at the programmed feed rate (G01) N125 Because G01 is a modal command, the tool will continue to move at the programmed feed rate on a straight line to the target position X=105 N130 The tool moves in the Y-axis to the target position Y=80. The technology data programmed in block N110 (feed rate, speed and sense of cutter rotation) will be retentive and take effect through blocks N120 to N130.

Figure 1.15 : Tool motions effected by modal commands (G01)

b) List Of G Codes G00 Rapid move G0 X# Y# Z# up to 6 axis or G0 Z# X# G01 Linear feedrate move G1 X# Y# Z# up to 6 axis or G1 Z# X# G02 Clockwise move G03 Counter clockwise move G04 Dwell time G08 Spline smoothing on, optional L# number of blocks to buffer G09 Exact stop check, spline smoothing Off G10 Linear feedrate move with decelerated stop G11 Controlled Decel stop G17 X Y Plane G18 X Z Plane G19 Y Z Plane G28 move to position relative to machine zero G53 Cancel fixture coordinate offsets G54-G59 fixture coordinate offsets 1 through 6

Cont G70 Inch mode G71 Millimeter mode G80 Cancels canned cycles and modal cycles G81 Drill cycle G82 Dwell cycle G83 Peck cycle G84 Tapping cycle G85 Boring cycle 1 bore down, feed out G86 Boring cycle 2 bore down, dwell, feed out G88 Boring cycle 3 bore down, spindle stop, dwell, feed out G89 Boring cycle 4 bore down, spindle stop, dwell, rapid out G90 Absolute mode G91 Incremental mode G92 Home coordinate reset G93 cancel home offsets G98 - G199 User-definable G codes

c) Additional Functions (M-Functions) With each NC-block a number of additional functions (M-Functions) can be programmed, such as machine functions and switches, e.g. to specify the feed rate, the spindle speed and the tool change. List of M Codes

Feed Rate, F The feed rate is programmed in millimeters per minute (mm/min). Example: F080.000; Here the programmed feed rate is 80 millimeters per minute. Spindle Speed, S The spindle speed is programmed in revolutions per minute (RPM). Example: S500; Here the programmed spindle speed is 500 revolutions per minute. Tool Change, T A tool change is programmed by a four-digit number at the address T. The first two positions of that number indicate the tool position in the magazine; the last two positions indicate the tool compensation storage. Example: T0808; This command effect the loading of the tool to position No.8 of the current tool magazine and the reading-in of the corresponding compensation value storage No.8.

Cont In the CNC Simulator there is a maximum of 99 magazine positions available, as well as 99 compensation value registers. This provides the opportunity, for example, to assign the compensation value register No. 36 to the tool in the magazine position No. 12). The applicable NC-command would then be programmed as follows: T1236

1.3.5 Clamping Devices For Milling The following clamping variations can be distinguished for milling machine. Jaw Chucking Magnetic Chucking Modular Chucking The milling cutter machine table with its T-slots is the basis for work part clamping. Depending on how the work part is to be clamped, the following clamping devices can be distinguished: Mechanical clamping devices Hydraulic clamping devices Pneumatic clamping devices Electric clamping devices

Figure 1.16 : Clamping Devices

Mechanical Clamping Devices

Figure 1.17 : Clamping Iron And Clamping Bard

Shallow clamps are used for flat work parts whose surfaces need to be kept free for machining.

Figure 2.18 : Shallow Clamp

Machine Vises Machine vises are easy to use and reliable. They are used for clamping smaller work parts. Alignment is achieved with a measuring gauge.

Figure 1.19 : Machine Vise

Figure 2.20 : Power Transmission

Universal machine vises can be horizontally as well as vertically turned. Furthermore, there are also vises that pneumatically generate clamping power.

Figure 2.21 : Precision Sine Vise

Magnetic Clamping Devices Work parts made of iron can be clamped with electromagnetic devices. The work part is drawn to the clamping plate after a current is switched on. It can be easily removed after the current is switched off.

Figure 1.22 : Electromagnetic Clamping Plate

1.3.6 Cutting Values Milling is a cutting operation with a rotating tool, whereby the cutting edges are not in operation all the time. The cutting movement is caused by the rotation of the tool. Feed direction and cutting direction do not depend on each other. It is realized either by the tool or by the work part or by both of them (Figure 2.23). The Cutting Speed (Vc) and the Feed Speed (Vf) is overlap to each other and results in a continuous cutting operation.

Cutting Speed (Vc) The cutting movement is the movement between the tool and the work part, generating only one nonrecurrent chip cut during one rotation without a feed movement. Cutting speed corresponds to circumferential speed of the milling tool on the current cutting edge. It is expressed as Vc and m/min. Under consideration of the number of rotations of the spindle n the following formula is received Vc = * d * n in m/min

The cutting speed of a cutting tool depends on the number of the rotations. The direction constantly changes however during cutting operation.

Figure 1.23 : Cutting Values For Milling

Feed Speed (Vf) The feed movement together with the cutting movement enables a constant chip removal during several rotations. In milling, the feed can be indicated in three ways: Feed speed (Vf) in mm / min Feed per tooth (fz) in mm Feed per milling rotation (f ) in mm

The calculation of the feed speed Vf is based on the feed fz , i.e. the feed path per milling tooth. Under consideration on the number of rotations n and the number or teeth z the formula is as follows: Vf= fz * n * z in mm / min

The feed speed can be expressed with the following formula as well with reference to the feed per milling rotation. Vf = f * n in mm / min

Consequently, the following equivalence is valid: Vf = f * n = fz * n* z in mm / min

Calculation Examples Example 1.1Calculate the cutting speed for milling if the milling tool diameter, d = 50 mm and the number of rotations, n = 520 1/min

Example 1.2Calculate the number of rotations, n of an end mill with a diameter of d = 12 mm and cutting speed of Vc = 120m/min.

Example 1.3In plain milling with a face milling cutter a cutting speed of Vc = 180 m/min has been scheduled and the number of rotations should not exceed 400 1/min. What is the maximum diameter, d of the face milling cutter so that these values are not exceeded?