Category 4: Organizational commitment
6. THEORETICAL DISCUSSION
In manual part programming, very simple parts are machined, since, it requires a very few number of instruction (or) sequence .of operations. But, most of the complex parts can not be machined by manual part programming since they require lengthy and tedious calculations. So it is necessary to make use of computer for part programming repetitive and complex calculations involved in mathematically defined curves and other complicated geometrical shapes. So the complex programs can be generated by computer Assisted Programming (CAP). The part programmer need not learn about the specific coding formats used in different NC machine tools. Instead, he can learn only high level programming languages like APT, ADAPT, AU.TOMAP, EXAPT, PROMPT. etc which are all ‘English like statements’.
The reliability of program is enhanced since the computer makes all calculations. Besides, the computers have facilities for error detection to assist part programmer to produce better part program. The part program thing time ‘is greatly reduced by as much as 75%.
With the arrival of Computer Assisted Programming (CAP), the programmer is relieved off great burden and he has to do only the following activities.
(i) Define the geometry of the work piece from part drawing (ii) Define the sequence of operations and tool path
(iii) Write APT program [ = Automatically Programmed Tools]
(iv) Feed (Type) the program to the computer The computer’s in CAP consists of the following:
1. Input translation 2. Arithmetic Calculation 3. Cutter offset compensation 4. Post processor.
The important Computer Assisted Programming (CAP) languages are given below.
1. APT [Programmed Tools]
APT is a product of MIT (Massachusetts Institute of Technology in US) developmental work. It is’ the most widely used language. It can be used for both positioning as well as contouring programming in up to 5 axes. The different version of APT is APTURN for turning operations, APTMIL for milling and drilling operations and APTPOINT for Point to Point operations.
2. ADAPT
It is n ‘Adoption of APT’. Most of these programming languages are based directly on the APT program.
This can be used in smaller computer. It was developed by IBM under Air Force contract. It is not as much powerful as APT.
3. EXAPT
It is the ‘Extended subset of APT’. It is also based on APT and was developed in Germany. EXAPT I is designed for drilling and straight cut milling operations.
EXAPT II is designed for turning operations.
EXAPT III is designed for limited contouring operations.
It has facility to compute optimum feeds and speeds automatically.
Similarly the other software’s are given below. UNIAPT, SPLIT, COMPACT II PROMF and CINTURN II But, most widely used CNC part programming language is APT wit its derivatives ADAPT, EXAPT, UNIAPT etc
APT Language is the language for computer assisted part programming. APT is like English statements.
APT commands the cutting tool through a sequence of machining operations. ft performs all calculations to generate cutter positions. It is used to control up to five axes.
APT can be used to control a variety of different machining operations. APT uses more than 400 words.
There are tour types of APT statements 1. Geometry Statements
-Geometry statements define the geometric pattern of the work part These are also called Definitions statements.
2. Motion Statements
Motion statements are used to define the path taken by the cutting tool.
3. Post processor statements
Post processor statements are used in pacific machine tool and control system. They are used to provide data for feeds and speeds. They are also used to actuate other features of the machine.
4. Aux Statements
Auxiliary statements are used to identify’ the part and tool to specify the tolerances and to operate coolant N (or) OFF and so on. These are also known as miscellaneous
Statements
Let us see all the above statements one by one in detail.
Geometry statement
First of all, the component geometry must be defined to program in APT. The geometry statements define the path and locate the points through which the tool has to trace. The geometry statement should be given before the motion statement.
The format for geometry statement is Symbol = GebmetryType/Descriptive Data Example = Point / 2, 4, 7
Explanation: The symbol. P1 is defined as ‘POINT’ having coordinates X=2;Y=4andZ=7
So the geometric statements comprise of three sections the first section is symbol. The symbol is used to identify the geometric element. A symbol is combination
of alphabets and numerical. The maximum character should be six. At least one of the six should be an alphabet. Very important thing is the symbol should not be one of the APT vocabulary words.
3. By two points and perpendicular to other plane PL5 = PLANE/PERPTO, PL4,P1, P2
PL5 is a plane perpendicular to plane PL4 passing through P1 and P2.
Motion statements
Motion statements have a general format as given below Motion command Descriptive Data
GOTO / P2
The above statement consists of two sections.
The first section (GOTO) tells tool what to do. The second section (P2) tells, tool where to go. By this motion statement, the tool is ordered to go to point 2 (P2) which should have defined already in the geometry statement.
The following are three types of motion commands 1. Setup commands
2. Point-to-Point motion commands Setup command
The tool must be given a starting point at the very beginning of the motion.
The starting point is- known as target point where the operator positions the too at the a part of the job.
The setup motion command is FROM / TARG
The ‘TARG’ is the target (or) starting point from which others will be referred.
FROM / -4, -3, 0
I.e. The tool should start from X=—4; Y—3; Z=O. Another way to make statement - FROM / SETPT where SETPT is the Starting point.
3. Continuous path (contouring) motion commands Another way to make this statement is
Point—to—Point Motion Command (1) (XJTOIPJ
The motion statement is used to position the tool at a particular point P1 (Eg positions the drill above a hole to be drilled)
(ii) GOTOI5, 6, 3
This motion statements tells the tool to go to point X = 5 Y = 6 and Z = 3 (iii) GODJ
This motion statement gives incremental instruction to move the tool in specified direction (in X, on Y (or) Z direction) from its current position. The GODLTA command specifies an incremental move for the tool.
In the following example.
GODLTA/4, 8, 0,
The tool is ordered to move from its current position 4 mm in X direction, 8 mm in Y direction and no change in Z direction.
.The GODLTA motion command is very much useful in drilling and related operations So, GOTO statement is used to direct the tool to a particular hole location.
And GODLTA statement is used to drill the hole as given in the following example.
Contouring Motion Statement
Contouring motion commands are more complicated because the tool’s position should be continuously controlled throughout its motion. ‘To accomplish this control, the tool is directed along two intersecting surfaces namely ‘Drive Surface’ and ‘Part Surface’ and the tool motion is stopped by the surface namely
‘check surface’ as shown in fig.
Drive Surface
Drive surface is used to guide the side of the cutter.
Part Surface: The bottom of the cutter rides on the part surface. The part surface may or may not be the actual surface of the workpart. The part surface should be defined along with the drive surface to maintain the continuous path control of the tool.
Check Surface
This check surface stops the movement of the tool in its current direction. i.e. The forward movement of the tool is stopped by this surface.
The APT contour motion statement commands the tool to move along the drive surface and over the part surface and the movement ends when the tool is at the äheck surface.
The six contour motion commands are given below
TO ON PAST and figure.
These six commands are mostly used along with one of the four modifiers to define the check surface, drive surface (or) part surface. The foig modifiers are given below
TANTO
The usage of four modifiers with respect to check surface is shown in following
‘TO’ moves the tool until the tool touches the check surface. ‘ON’ moves the tool until the tool centre is on check surface. ‘PAST’ moves the tool just beyond the check surface. ‘TANTO’ moves the tool upto the point of tangency between two surfaces, atleast one of which is circular.
‘TANTO’ is used for check surface being tangent to the Drive Surface. The format for using modifiers is given below –
Any of the surface can be omitted and it is optional.
Motion word!Drive surface, modifier, check surface Example: GORGTJL1, PAST, L2
Meaning: Move on the ri along Li until Past L2 (i.e. until L2 is passed) feed,
given
Postprocessor Statements
-These postprocessor statements are used to control the operation of the spindle, the and qther features of the machine tool. Some of the postprocessor statements are below.
COOLNTION and COOLNTIOFF for switching coolant. ON and OFF.
RAPID for rapid traverse for positioning the tool ENI to shut down the CNC. machine.
FEDRAT—It is used for giving feed rate.
SPINDJJ2OdO, CLW means spindile speed is 2000 rpm clockwise.
Auxiliary statement Auxiliary statements aie used for cutter size definition, part identification and tool change etc Some of the auxiliary statements are given below
CUflERJ13 means the diameter of the cutter is 13 mm.
FINI means it is adyising the computer to terminate the program.
PARTNO is used to identify the workpart.
Example 14: Write a program in APT for the finished pan shown in fig.
Solution: The geometry statements &e given beIos by referring the diagram.
$$ All points are th here.
$$ Define the top and bottom surfaces.
TOPSRF = PLMTE/P1, P2, P5
BOtSRF = PLANEIPARLEL, TOPSRF, Z LARGE, —25
$ [ is a surface parallel to top surface parallel to top surface and bn the lar side of 2 y 25 downward)]
$$ Define the circles and lines
CR1 = CIRCLE/CENTER, PS, RADIUS, 30 L2 LINEIP2, LEfl, TPsNTO, CR1 1 CR2]
$$ [ The line L2 passes from point P2 and is tangent on the lef*side of arc e L3 = LINE/P4, RIGHT, TANTO, CR1
$$ [ The line L3 passes from point P4 and is tangent on the right side of circle CR1]
L4 = LIE/P4,P5 L5 = LINE/P5,P1
$$ Define the tool, feed and speed.
LOADTL/i $$ [ tool No. 1]
CUTPERJ2O
$$ CU?YER dia is 20 mm
FEDRAT/60, MMPM $$ [ rate is 60 thm/min]
SPINDL/2500, CLW $$ [ Spindle speed is 2500 rpm cloekwise]
$$ Give the motion statements.
FROM/SETPT [ Position the tool to initial position]
GO/TO, Li, TO, BOTSRF, TO L5
$$ [ is the Drive surface along which the tool moves. BOTSRF is the part surface at which the tool end face will be placed throughout the operation and Lfr is the Check surface for this position]
GOLFTIL1,PAST,L2
[ Move the tool in the left along the line Li until the line L2 is passed.
GO RGT/L2, TANTO, C
$$ Move the tool in the right direction along the line L2 until it touches the point of tangency of circle CR1.
GO FWD/CR1, TANTO, L3
$$ Move the tool in forward direction (in the direction of motion) along, the circle CR1 until the line L3 becomes tangent to’ it.
GO FWDIL3, PAST, I
$$ Move the tool along the line L3 until it passes line L4 GO RGT/L4, PAST, L
GO RGT/L5, PAST, Li GOTO/SETPT FINI
END.
Example 15: Write a part program for the finished part shown in figure
* Use the milling cuter alid drill. Assume the thickness of the plate is 20 mm. Setpt is at (0, 20, 3) and Z = 0 at the surface of the job.
Solution: The dlre of motion is defined in the figure and also all points and lines are shown here.
$$ Defining• all points:
BOTSEF = PLANE/PAIRLEL, TOPSRF, Z LARGE, 20
$$ Defining the three part of circles:
CR1 = CIRCLE/CENTER, P1, RADIUS, 10 CR2 = CIRCLE/CENTER,P2, RADIUS, 10 CR3 = CIRCLE/CENTER,P3, RADIUS, 10
$ Defining lines Li, L2, and L3
Li = LINE/LEFT,TANTO, CR1, LEFT, TANTO, CR2 L2 LINEIRIGHT,TANTO CR2, RIGHT, TANTO, CR3 L3 = LINEILEFT,TANTO, CR3, RIGHT, TANTO, CR1
$$ Giving Feed and Speed statements:
LOADTIJ1
$$ Define Line Li as Drive surface, BOTTOM SURFACE as pa*tsurface GO/TO, Li, TO, BOTSRF, TANTO, CR1
GO LVr/L1, TANTO, CR2
SPINDL(2500, CLW
$$ Drilling first hole GOTO/Pi
GO DOWN/PAST, BOTSRF GO UP/PAST, TOPSRF
$$ Drilling second hole GO TO/P2
GO DOWN/PAST, EOTSRF GO UP/PAST, TOPSRF
$$ Drilling third hole GO TO/PS
GO DOWN/PAST, BOTSRF GO UP/PAST, TOPSRF
$$ End of job GOTO/SETPT FIN!
END.
MACRO STATEMENT IN APT
The sequence of similar (or) identical statements which have to be repeated more often in a part program are best referred by a MACRO Statement in APT so that the lengthy part program is reduced. It is sImilar to a SUBROUTINE in FORTRAN and other Computer Programming languages.