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Chapter 3Computer-Assisted Programming 

Computer assisted programming methods are much faster andmore reliable than manual programming techniques. There are a varietyof forms of computer assisted programming. The common feature ofthese programs is that the part and machining paths are not defineddirectly with G-code but through English-like statements or throughinteractive graphic instructions.

In this chapter three types of computer assisted programmingmethods are discussed. The first method is called APT (AutomaticallyProgrammed Tools) and is presented in Section 3.1. It is the originalcomputer assisted method of part programming and has been widelyused over the last two decades. It consists of a series of English-likegeometry and motion statements, which are used to define the workpieceand machine tool path. An extensive (usually mainframe) computerprogram is then used to convert the APT instructions into G-codeprograms.

 This is followed, in Sections 3.2 and 3.3, by two examples ofinteractive computer graphics-based programs, which are supplied by

machine tool manufacturers. The programs, called Symbolic FAPT forlathes and Conversational Programming for mills, run directly on thecontrollers of CNC machine tools. They are fairly comprehensive andenable the part figure, blank shape, and machine tool path to bespecified graphically at the control console. They allow for input ofworkpiece material, tool shape, surface roughness etc., and enable feedsand speeds to be automatically generated and manually overridden.

 The third method, given in Section 3.4, is an interactive, PC-basedgraphics package. The program, called SmartCAM, is typical of graphics-based CAM packages in which the machine tool path is developed frompart definition generated with CAD packages such as AutoCAD orPersonal Designer.

3.1 APTAPT uses English-like language statements to define part shape

and tool motion as well as machine tool dependent data, such asfeedrates and spindle speeds. This data is contained in an APT part-program. An APT processor program is used to read these statements,interpret the meanings, and perform all the necessary calculations inorder to generate a series of cutter location points that

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72 Computer-Assisted Programming [Ch.3define the cutter path. The APT processor is a computer program whichruns on a mainframe computer, possibly at a central site with time-sharing facilities. The generalized APT output is converted to theparticular format G-code required by the CNC machine using apostprocessor program. Although G-codes are fairly well standardized,differences do exist between machine suppliers and CNC machines canbe supplied with a variety of subsets of available codes. A description of

postprocessor programs is given in Chapter 5.APT is a three-dimensional system which can be used to define

complex geometrical shapes and to control up to five axes CNCmachines. A major advantage of APT is that it has developed into anaccepted standard for machine tools. There are many versions of theAPT language available, each with particular benefits and characteristics. The prime disadvantage of APT is that it is uses English-like commandsto define geometry instead of the much more convenient graphicalmethods. Other disadvantages are that it requires extensive computingcapability and can have a slow response (particularly with programmerswho use the system intermittently and require multiple overnight runs

on time-shared facilities). The four types of statements in the APT language are:• geometry statements which define primitive elements such as

points, lines, circles, planes, cones and spheres• motion statements which describe the tool path in relation to

the part geometry• postprocessor statements which give specific machine tool code

information as well as feeds and speeds• auxiliary statements which give part and tool tolerances.

APT part programs usually list the part and the postprocessor referencenumber followed by the program statements as follows:

PARTNO___MACHIN/___Geometry statementsMotion statements and machine tool commandsFINI.

3.1.1 Geometry Statements The general form of geometry statements is:

symbol = geometry type / descriptive data

For example a point, named P1, can be defined at coordinates X = 100mm, Y = 200 mm and Z = 300 mm by the statement:P1 = POINT/100.0, 200.0, 300.0

A line, named L1, can be defined between two points, P1 and P2, by thestatement:

L1 = LINE/P1, P2or through the point, P1, and parallel to another line, L2, by thestatement:

L1 = LINE/P1, PAREL, L2

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Sec 3.1] APT  73

A plane, named PL1, can be defined between three points, P1, P2 and P3,by the statement:

PL1 = PLANE/P1, P2, P3or through the point, P1, and parallel to another plane, PL2, by thestatement:

PL1 = PLANE/P1, PAREL, PL2

A circle, named C1, can be specified by a centre position, P1, and aradius of 100 mm by the statement:

C1 = CIRCLE/CENTER, P1, RADIUS, 100.0or by a centre position, P1, and tangent to line, L1, by the statement:

C1 = CIRCLE/CENTER, P1, TANTO, L1.

3.1.2 Motion StatementsMotion statements use the defined geometry primitives in order to

define tool movement. The general form of motion statements is:

motion statement/descriptive data

 The instruction to go from an initial starting point, P1, and from whichall other points are referenced, is given by the statement:

FROM/P1 The instruction to go to the point, P2, is given by the statement:

GOTO/P2or to go to the absolute position X = 100 mm, Y = 200 mm, and Z = 300mm is given by the statement:

GOTO/100.0, 200.0, 300.0 The instruction to move by an incremental (or delta) amount from thepresent position is given by the statement:

GODLTA/100.0, 200.0, 300.0Contouring commands are used to direct the tool along two

intersecting surfaces. The drive surface guides the side of the cutter,the part surface defines the position of the bottom of the cutter, and thecheck surface defines the limit of current tool motion. Modifier words,such as TO, ON, PAST or TANTO, are used to govern the position of thetool in relation to the check surface, as shown in Figure 3.1. Motionstatements, GOLFT (go to the left), GOFWD (go forward) and GOUP (goup), are also used to control the cutter motion.

 The instruction for the tool to move forward, with the drive surface,S1, on the left hand side, and past the check surface, S2, is given by the

statement:GOLFT/S1, PAST, S2.

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74 Computer-Assisted Programming [Ch.3

Figure 3.1 APT contouring surfaces

3.1.3 Postprocessor and Auxiliary StatementsStatements which specify machine tool related functions, such as

those covered by F-, S-, T-, and M-codes, are defined in thepostprocessor statements. For example, the instruction to set thefeedrate at 100 mm per minute is given by the statement:

FEDRAT/100, MPM The instruction to set the spindle at 1500 rpm in a clockwise direction isgiven by the statement:

SPINDL/1500, RPM, CLW The instruction to set the coolant on is given by the statement:

COOLNT/ONOther auxiliary statements are used to define cutter size, part

number, and curve tolerance. The instruction to define a 50 mmdiameter cutter is given by the statement:

CUTTER/50.0