Chapter: Automation, Production Systems, and Computer Integrated Manufacturing : Numerical Control

Part Programming with APT

It is a three dimensional NC part programming system that was developed in the late 19508 and early 60s Today it remains an important and widely used language in the United States and around the world.

          Part Programming  with APT


In this section, we present some of the basic principles and vocabulary of the APT language. APT is an acronym that stands for Automatically Programmed Tooling. It is a three dimensional NC part programming system that was developed in the late 19508 and early 60s (Historical Note 6.3). Today it remains an important and widely used language in the United States and around the world. APT is also important because many of the concepts incorporated into it termed the basis for other subsequently developed languages. APT was originally intended as a contouring language. but modern versions can be used for both point to point and contouring operations in up to five axes. Our discussion will be limited to the three linear axes, x, y, and z.APT can be used for a variety of machining operations.


Our coverage will concentrate on drilling (point to point) and milling (contouring) operations. There are more than 500 words in the APT vocabulary. Only a small (but important) fraction of the total lexicon will be covered here. The Appendix to this chapter lists some of these important APT words.

The reader must remember that the work described in this historical note was started in the 1950s,a lime when digital computer technology was in its infancy, and so were the associated computer programming languages and methods. The APT project was a pioneering effort, not only io the development of NC technology, but also in computer programming concepts, computer graphics, and computeraided design (CAD).

It was recognized early in the NC development research at MIT that part programming would be a timeconsuming task in the application of the new technology, and that there were opportunities to reduce the programming time by delegating portions of the task to a genetaI_purpo.e computer. In June 1951, even before the first experimemal NC machine was operating, a study was undertaken to explore how the digital computer might be used as a programming aid. The result of this study was a recommendation that a set of computer programs be developed to perform the mathematical computations that otherwise would have to be accomplished by the part programmer. In hindsight, the drawback of this approach was that, while it automated certain steps in the part programming task, the basic manual programming procedure was preserved


The significant breakthrough in computerassisted part programming was the development of the automatically programmed 1001 system (APT) during the years 19561959. It was the brainchild of mathematician Douglas Ross, who worked in the MIT Servomechanisms Lab at the time. Ross envisioned a part programming system in which (1) the user would prepare instructions for operating the machine tool using Englishlike words, (2) the digital computer would translate these instructions into a language that the computer could understand and process. (3) the computer would carry out the arithmetic and geometric calculations needed to execute the instructions. and (4) the computer would further process (postprocess) the instructiOn! 'u that they could be interpreted by the machine tool controller. He further recognized that the programming system should be expandable for applications beyond those considered in the immediate research (milling applications). 

Around this time, the Aircraft Industries Association (AlA, renamed the Aerospace Industries Assoclation in 1959) was attempting to deal with NC part programming issues through its Subcommittee on Numerical Control (SNC). Ross was invited 10 attend a meeting of the SNC in January 1957 to present his views on computerassisted part programming. The result 01 this meeting was that Rosa's work at MIT was established as a focal poinl for NC programming within thc AlA, A project was initiated in Apri11957to develop a twodimensionill version of APT, with nine alrcrafr companies plus IBM Corporation participating in the joint effort ami MIT as project coordinator. The 2DAPT system was ready for field evaluation at plant, of participating companies in April 1958.Testing, debugging, am] refining the programming system took approximately three years.during which time the AlA assumed responsibllity for further APT development. In 1961, the Illinois Institute of Technology Rcsearch lnslituto; (lITRl) was selected by the AlA to become the agency responsible for longrange maintenance and upgrading of Al'T'In 1962,IITRI announced completion of APTIII. a commercial version 01APT for threedimensional part programming. In 1974,APT was accepted as the U.S.standard for programming NC metal cutting machine tools. In 1978,it was accepted by the ISO as the international standard.


One of the initial problems with APT when it was released in the early 1960swas that a very large computer was required 10execute it, thereby limiting the number of companies that could use it Several part programming languages based directly on APT were developed to address this problem. Two of the more important APTbased languages were ADAPT and EXAPT. ADAPT (ADaptation of APT) was developed by IBM under Air Force contract to include many of the features of APT bUI required a much smaller computer. ADAPT can be used for both pointtopoint and contouring jobs. EXAPT (EXtended subset of APT) was anolher NC part programming language based on APT. EXAPT was developed in Germany around 1964in three versions',(1) EXAPT I was designed for pointtopoint applications, such as drilling and straight milling; (2) EXAPT 11was developed for turning operations; and

(3) EXAPT III was capable of limited contouring for milling.


APT is not only a language; it is also the computer program that processes the APT statements to calculate the corresponding cutter positions and generate the machine tool control commands. To program in APT. the part geometry must first be defined. Then the tool is directed to various point locations and along surfaces of the workpart to accomplish the required machining operations. The viewpoint of the programmer is that the workpiece remains stationary and the tool is instructed to move relative to the part. To complete the program, speeds and feeds must be specified, tools must be called, tolerances must be given for circular interpolation, and so forth. Thus, there are four basic types of statements in the APT language:


    Geometry statements, also called definition statements, are used to define the geometry elements that comprise the part.


    Motion  commands  are used to specify  the tool path.


    Post-processor statements control the machine tool operation, for example, to specify speeds and feeds, set tolerance values for circular interpolation, and actuate other capabilities of the machine tool.


Auxiliary statements, a group of miscellaneous statements used to name the part program, insert comments in the program and accomplish similar functions.

These statements arc constructed of APT vocabulary words, symbols. and numbers, all arranged using appropriate punctuation. APT vocabulary words consist of six or fewer characters. 1 he characters are almost always letters of the alphabet. Only a very few APT vocabulary words contain numerical digitsso few in fact that we will not encounter any of them in our treatment of APT in this chapter. Most APT statements include a slash (I) as part of the punctuation. APT vocabulary words that immediately precede the slash arc called major words. whereas those that follow the slash are called minor words.


Geometry Statements. The geometry of the part must be defined to identify the surfaces and features that are to be machined. Accordingly, the points, lines, and surfaces must be defined in the program prior to specifying the motion statements. The general form of an APT geometry statement is the following:


An APT geometry statement consists of three sections. The first is the symbol used to identify the geometry element. A symbol can be any combination of six or fewer al phabetical and numerical characters, at least one of which must be alphabetical. Also, the symbol cannot be an APT vocabulary word. Some examples are presented in Table 6.12 to illustrate what is permissible as a symbol and what is not. The second section of the APT geometry statement is an APT major word that identifies the type of geometry element. Examples are POINT, LINE, CIRCLE, and PLANE. The third section of the APT geometry statement provides the descriptive data that define the element precisely, completely, and uniquely. These data may include numerical values to specify dimensional and position data, previously defined geometry dements, and APT minor words.


Punctuation in an APT geometry statement is indicated in Eq. (6.3). The definition statement is written as an equation, the symbol being equated to the geometry element type, followed by a slash with descriptive data to the right of the slash. Commas are used


to separate           the words and numerical         values  in the descriptive         data.


There arc a variety of ways lu spectry the various geometry elements. The Appendix to this chapter presents a sampling of statements for defining the geometry elements we

will be using in our treatment of APT: poiuts.Iines, planes, and circles. The reader may benefit from a few examples:




PI "' POINI!2IJ.U,40.U,60.0


where the descriptive data following the slash indicate the X·, yo, and zcoordinates. The specification can be done in either inches or millimeters (metric). We use metric values in our examples. As an alternative, a point can be defined as the intersection of two mtersecrmg lines, as in the following


P2 =  POINTfINTOF,    Lt. L2


where the APT word INTOF in the descriptive data stands for "intersection of." Other methods of defining points are given in the Appendix under POINT


Lines. A line defined in APT is considered to be of infinite lengtb in hoth directions. Also, APT treats a line as a vertical plane that is perpendicular to the .rj' plane. The easiCStway to specify a line is by two points through which it passes:


L3 = LINE/P3.   P4


In some situations, the part programmer may find it more convenient to define a new line as being parallel to another line that has been previously defined; for example,


L4 =  LlNEIP5,    PARLEL,    L3


where PARLEL is APT's way of spelling "parallel."The statement indicate, line l4 passes through point PS and is parallel to line L3.


Planes. A plaue can be defined by specifying three points through which the plane passevasin the following'


PLl             = PLANE/Pl.P2,P3


Of course. the three points must he noncollinear. A plane can also be defined as being parallel to another plane that has been previously defined; for instance,


PL2 =  PLANE!P2,        PARLEL,    PLl


which states that plane PL2 passes tbrough point P2 and is parallel to plane PLI. In APT, a plane extends indefinitely.


Circles.          In APT, a circle is considered        to be a cylindrical surface        that is pcrpendic


ular to the xj plane and extends to infinity in the zdircction. The easiest way to define a circle i~ by its center and radius. as in the following




By convention. the circle is located in the .e.y plane. An alternative way of defining a circle i~to specify thur it passes through three points; for example,


C2 = CIRCLE/P4,       P5, P6


where the three points must not he collinear There are m;'lny other ways to define a ciretc, several of which are listed in the Appendix under CIRCLE

Certain     ground    rules must  he obeyed when formulating APT  geometry     statements.


Following   are  four     important   APT  rules:


t.  Coordinate  data must be specified in the order x, then y, then z, because the statement


PI  =  POINT/20.5,40,O,60.0


is interpreted           to mean  x =  20.5 mm,y       = 40.0 mm. and    e =  60.0 mm


    Any symbols used as descriptive data must have been previously defined; for example.In the statement




the two lines Ll and L2 must have been previously defined. In setting up the list of geometry statements, the APT programmer must be sure to define symbols before using them in subsequent statements.


    A symbol can be used to define only one geometry element. The same symbol cannot be used to define two different elements. For example. the following statements would be incorrect if they were included in the same program:


Pl  = POINTI20,40,60


PI  =  POINT 130,50,70


    Only one symbol can be used to define any given element. For example, the following two statements in the same r"rt program would be incorrect;


PI =  POINTI20,40,60


P2 =  POINT/20,40,60


EXAMPLE       6.3       Part Geometry     Using APT


Let us construct the geometry of our sample part in Figure 6.15. The geometry dements of the part to be defined in APT are labeled in Figure 6.18. Reference is also made to Figure 6.16, which shows the coordinate values of the points used to dimension the part. Only the geometry statements are given in the APT sequence that follows:


PI  = POINT 10,0,0


P2 = POiNT/160.0,O,O

P3 = PUINT /,0

P4 =  POINT/


P5  =  POINT/


P6 = POINT/,0


P7 =  POINT /,0


P8 =  POINT  /,0


L1 = LINE/Pl,P2


L2 = LINE/P2,      P3


Cl  = CIRCLE/CENTER,        P8, RADIUS.       30.0


L3 = L1NE/1'4,  PARLEL,      L1


L4 = LINE/P4,      PI


Motion Commands. All APT motion statements follow a common fonnat,just as geometry statements have their own format. The format of an APT motion command is'


MOTION  COMMAND/descriptive data   (6.4)


An example  of an APT  motion       statement    is




I'he statement consists of two sections separated by a slash. The first section is the basic command that indicates what move the tool should make. The descriptive data following the slash tell the tool where to go. In the above example, the tool is directed to go to (GOTO) point PI, which has been defined in a previous geometry statement.


At the beginning of the sequence of motion statements, the tool must be given II starting point. This is likely to be the target point. the location where the operator has positioned the tool at the start of the job.The part programmer keys into this starting position with the following statement:

fROM/PTARG  {651

where FROM is anAPT vocabulary word indicating that this is the initial point from which all others will be referenced; and PTARO is the symbol assigned to the starting point. Another way to make this statement is the following'


FROMj20.0.    20.0,0


where the descriptive data in this case arc the X, y, and zcoordinates of the starting point. The FROM statement occurs only at the start of the motion sequence.


In our discussion of APT motion statements, it is appropriate to distinguish between pointtopoint motions and contouring motions. For potnrtopoiru motions, there are onl two commands: GOTQ and GODLTA.The GOTO statement instructs the tool to go to a particular point location specified in the descriptive data. TWo examples are:


 GOTOIP2         (6.6,)

001'0/25.0,40,0,0        (6.6b)


In the first command, P2 is the destination or the too] point. In the second command, the 1001 has been instructed to go to the location whose coordinates are x = 25.0, Y = 40.0, and z = O.


The GODLTA command specifies an incremental move for the tool.To illustrate, the following statement instructs the tool to move from its present position by a distance of 50.0 mm in the xdirection, 120.0 mm in the ydirection, and 40 rum in the zdirection


GODLTA!50.0, 120.0, 40.0


The GODLY A statement is useful in drilling and related machining operations. The tool can be directed to go to a given hole location; then the GODLTA command can be used to drill the hole, as in the following sequence:




GODLTA/[),      0, 50.0




Contouring motion commands are more complicated than PTP commands are because the tool's position must be continuously controlled throughout the move. To exercise this control. the tool is directed along two intersecting surfaces until it reaches a third surface, as shown in Figure 6.20. These three surfaces have specific names in APT; they are:


       Drive surface. This is the surface that guides the side of the cutler. It is pictured as II plane in our figure.


       Part surface. This is the surface, again pictured as a plane, on which the bottom or nose of the tool is guided.

    Check; surface. This is the surface that stops the forward motion of the tool in the execution of the current command. One might say that this surface "checks" the advance of the tool.


It should be noted here that the "part surface" mayor may not be an actual surface of the part The part programmer may elect to use an actual part surface or some other previously defined surtace for the purpose of maintaining continuous path control of the tool. The same qualification goes for the drive surface and check surface.

There are several ways in which the check surface can be used. Ifus is determined by using any of four APT modifier words in the descriptive data of the motion statemen!. The four modifier words arc TO, ON, PAST. and TANTO. As depicted in Figure 0.21, the word TO positions the leading edge of the tool in contact with the check surface; ON positions the center of the tool on the check surface; and PAST puts the tool beyond the check surface. so that its trailing edge is in contact with the check surface. The fourth modifier word TANTO is used when the drive surface is tangent to a circular check surface, as in Figure 6.22. TANTO moves the cutting tool to the point of tangency with the circular surface

An APT contouring motion command causes the cutter to proceed along a trajectory defined by the drive surface and part surface; when the tool reaches the check surface it stops according to one of the modifier words TO, ON, PAST, or TANTO. In writing a

Figure  6.23  Use of the APT motion words. The tool has moved  from a previous  position  10 its present  position. The direction  of the next move is determined   by one of the APT motion words GOLFT.  GORGT,  GOFWD, GOBACK,  GDUP,  or GODOWN.

motion statement, the part programmer must keep in mind the direction from which the tool is coming in the preceding motion command. The programmer must pretend to be riding on top of the tool, as if driving a car. After the tool reaches the check surface in the preceding move, does the next move involve a right tum or left tum or what? The answer to this question is determined by one of the following six motion words, whose interpretations are illustrated in Figure 6.23:


   GOLIT  commands  the tool to make  a left tum relative  to tbe last move.


   GORGT  commands  the tool to make  a right tum  relative  to the last move.


    GOFWD  commands  the tool to move  forward  relative  to the last move.


    GOBACK  commands  the tool to reverse  direction  relative  to the last move.


    GOUP  commands  the tool to move upward  relative  to the last move


   GODOWN  commands  the tool to move down relative  to the last move.


In many cases, the next move will be in a direction that is a combination of two pure directions. Forexample, the direction might be somewhere between go forward and go right. In these cases, the proper motion command would designate the largest direction component among the choices available.


To begin the sequence of motion commands, the FROM statement,Eq. (6.5) is used in the same manner as for pointtopotnt moves. The statement following the FROM command defines the initial drive surface. part surface, and check surface. With reference to Figure 6.24. the sequence takes the following form:


oo/ro, flLl, TO. PL2, TO PL3         (6.7)


The symbol PTAR.G represents the target point w~ere the ?perator has. set up the tool. The GO command Instructs the tool to move to the mtersecnon of the drive surface (PL1), the part surface (PL2), and the check surface (PL3). Because the modifier word TO has been used for each of the three surfaces, the circumference of the cutter is tangent to PLI and PL3. and the bottom of the cutler is on PL2. The three surfaces included in the GO statement must be specified in the order: (I) drive surface, (2) part surface, and (3)checksurlacc


Note that GO/TO is not the same as the 001'0 command. Eq. (6.6). 001'0 is used only for JYrp motions. The GO/ command is used to initialize a sequence of contouring motions and may take alternative forms such as GO/ON, GorrO, or GOIPAST.

After initia'ization, the tool is directed along its path by one of the six motion command words. It is [Jot necessary to redefine the part surface in every motion command after it has been initially defined as long as it remains the same in subsequent commands. In the preceding motion command, Pq. (6.7), the cutter has been directed from P"IARG to lhe interscction of surfaces PLI, PL2, and PL3. Suppose it is now desired to move the tool along planc PL3 in Figure 6.24, with PL2 lCllw.i"ing a~ the pan surface. The following command would accomplish this motion:


GORGT!PL3,           PAST,PL4


Note that PL2 is not mentioned in this new command. PD, which was the check surface in the preceding command, Eq. (6.7), is the drive surface in the new command. And the new check surface is PL4. Although the part surface may remain the same throughout the motion sequence. the drive surface and check surface must be redefined in each new contouring motion command.


There arc many parts whose features can all be defined in two axes, x and y. Although such parts certainly possess a third dimension, there are no features to be machined in this direction. Our sample part is a case in point. In the engineering drawing, Figure 6.15, the sides of the part appear as lines, although they lire threedimensional surfaetls on the physical part. In cases like this, it is more convenient for the programmer to define the pan profile in terms of lines and circles rather than planes and cylinders. Fortunately, the APT language system allows this because in APT, lines me treated as planes and circles are treat. ed as cylinders, which are both perpendicular to the ry plane. Hence, the planes around the part outline in Figure 6.15 can be replaced by lines (call them Ll, L2, L3, and L4), and the APT commands in Bqs (6.7) and (6.8) can be replaced by the following:




oo/ro. Ll, TO, PL2, TO L3


GORGT/L3,             PAST,L4


Substitution of lines and circles for planes and cylinders in APT is allowed onlv when the sides of the pall are perpendicular to the xy plane. Note that plane PL2 has not been converted to a line. As the "part surface" in the motion statement, it must maintain its status as a plane parallel to the .r and yaxes,

EXAMPLE        6.4     APT  Contouring Molion        Commands


Let U~write the AM' motion commands to profile mill the outside edges of our sample workpart. The geometry clements are labeled in Figure 6,18, and the too! path is shown in Figure 6.17. The tool begins its motion sequence from a target point PTARG located at x = 0, y = 50 mm and l = 10 rnm. We also assume that "part surface" PL2 has been defined as a plane parallel to the xy plane and located 25 mm below the top surface of the part (Figure 6.16), The reason for defining it this way is to ensure that the cutter will machine the entire thickness of the part.




GO/TO.    L1, TO, PL2. ON,L4


GORGT/Lt,       PAST, L2

GOLFTfL2,       TANTO,  Cl


GOFWD/L3,      PAST, L4

GOLFf/L4,         PAST, L1



Postprocessor    and  Auxiliary      Statements. A complete APT part  program        must


include functions not accomplished by geometry statements and motion commands. These additional functions are implemented by postprocessor statements and auxiliary statements.

Postprocessor s[{]/emenls control the operation of the machine tool and playa supporting role in generating the tool path. Such statements are used to define cutter size,

specify speeds   and feeds, turn coolant  flow on and off, and control  other  features         of the     par

ticular  machine  tool on which the machining  job     will be performed.          The general  form  of a

postprocessor    statement  is the following:                                 

       POSTPROCESSOR      COMMAND/descriptive data (6.9)


where the POSTPROCESSOR COMMAND i~an APT major word indicating the type of function or action to be accomplished, and the descriptive data consists of APT minor words and numerical values. In some commands. the descriptive data is omitted. Some exarnples of postprocessor statements that appear in the Appendix at the end of the chapter arc the following:


       llNITSiMM  indicates  that the specified  units used in the program  an: INCHES  or Mlv1.


       INTOL/O.02  specifies  inward  tolerance  for circular  interpolation.


       OUTTOLjO.02   specifies  outward  tolerance  for circular  interpolation.


       eUTTERj20.0 defines cutter diameter for tool path offset calculations; the length and other dimensions of the tool can also be specified, if necessary, for threedimension, at machining.


       SPINDLjlOOO, CLW specifies spindle rotation speed in revolutions per minute. Either CLW (clockwise) or eeLW (counterclockwise) can be specified

   SPINDLIOFF   stops  spindle  rotation.


   FED RAT / 40, IPM specifies feed rate in millimeters per minute or inches per minute. Minor words IPM or IPR are used to indicate whether the feed rate is units per minute or units per revolution of the cutter, where the units are specified as inches or millimeters in a preceding UNITS statement.


   RAPID  engages  rapid  traverse  (high feed rate)  ror nexc movers}.


  COOLNT/FLOOD  turns  CUlling fluid  on


   LOADTL/Ol used with automatic toolchangers to identify which cutting tool should be loaded into the spindle


    DELAY /30 temporarily  stops  the machine  {Qol [or a period  specified  in seconds.


Auxiliary slatfment.~ are used to identify the part program, specify which postprocessor to usc. insert remarks into the program, and so on. Auxiliary statements have no effect on the generation of tool path. The following APT words used in auxiliary statements are defined in the Appendix:


   PARTNO is the first statement in an APT program, used to identify Ihe program; fUI example,




   MACHIN! permits the part programmer to specify the postprocessor, which in effect specifies the machine tool.


   CLPRJ\T stands for "cutter location print," which is used to print out the cutter location sequence


   REMARK is used to insert explanatory comments into the program that are not interpreted or processed by the APT processor.


   FINl  indicates  the end of an APT  program.


The major word MACHIN reqnires a slash (I) as indicated in our list above, with descriptive data that idemify the postprocessor to be used. Words such as CLPRNT and FINI are complete without descriptive data. PAKTNO and REW.ARK have a format that is an exception to the normal APT statement structure. These are words that are followed by descriptive data, but without a slash separating the APT word from the descriptive data. PARTNO ts used at the very beginning of the part program and is followed by a series of alphanumeric characters that label the program. REMARK permits the programmer to insert comments that the APT processor does not process,


Some APT Part Programming Examples. As examples of APT, we will prepare two part programs for our sample part, one to drill the three holes and the second to profile mill the outside edges. As in our example programs in Section 6.5.2, the starting workpiece is an aluminum plate of the desired thickness, and its perimeter has been rough cut slightly oversized in anucipauon uf the profile millillg operation. Tn effect, these APT programs will accomplish the same operations as previous Examples 6.1 and 6.2 in which manual part programming was used.

EXAMPLE          6.5     Drilling  Sequence  in APT


let us write the APT program to perform the drilling sequence for our sample part in Figure 6.15. We will show the APT geometry statements only for the three hole locations, saving the remaining elements of geometry for Example 6.6










REMARK    Part geometry.     Points  are defined         10 mm above  part surface.


PTARG         POINT/0,50.0,        10.0


P5 =  POINT /70.0,30.0,10.0


P6   = POTNT/120.0,3D.O,  10.0


P7 = POINT/70.0,60J1, 10.0


REMARK Drill bit motion statcmenh












GODLlA/O,O,      25


GODLTAjO,0,     25










GODLTAIO,O,     25


GODLTA/o,0,      25






SPTNDL/l000,     CLW


FED RAT 10.05, IPR



GODLTA/O,         0, 25









EXAMPLE 6.6     TwoAxis  Profile  Milling  in APT


The three holes drilled in Example 6.5 will be used for locating and holding the work part for milling the outside edges. Axis coordinates are given in Figure 6.i6. The top surface of the part is 40 mm above the surface of the machine table. A 20mm diameter end mill with four teeth and a side tooth engagement of 40 mm will be used. The bottom tip of the cutter will be positioned 25 mm below the top surface during machining, thus ensuring that the side cutting edges of the cutter will cut the full thickness of the part. Spindle speed « 1000 rev/min and feed rate = 50 mm/min. The tool path, shown in Figure 6.17, is the same as that followed in Example 6.2.












REMARK Part geometry. Points and lines are defined 25 nun below part top surface




PI  = POINT /0,0,  25


P2 = POINT/I60,0,25


P3 =  POINT /160,60,25


P4        POINT/35,90,25


P8 =  POINT/130.60,25


Ll        LINE/PI,         P2


L2 =  LINE/P2,     P3


CI  =  CIRCLE/CENTER,       P8. RADIUS,       30




L4 = LINEIP4,     P1


PLl       PLANE/PI,    P2. P4


REMARK      Milling  cutter   motion        statements.




SPINDL/1000,    CLW


FFDRAT         /50, IPM


GO/TO.L1,TO,PLl,        ON,L4


GORGT/Ll,        PAST. L2


GOLFr/L2.       TANTO, (:1


GOFWDiC        I, PAST, U


GOF\vO/Ll,        PAST, L4


GOLFTIL4,       PAST, L1










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