CHAPTER 7

TOOL PATH CONTROL USING SCULPTURED SURFACES


The principal purpose of the APT processor is to prepare numerical control data. Sculptured Surfaces can be used in the same manner as conventional APT surfaces for tool path control. That is all the usual techniques for producing tool motion are valid for sculptured surfaces. (Refer to Volume 1, Chapters 3 and 10.) In addition, a special type of tool axis control and an optional arithmetic element (ARELEM) which performs tool offset calculations more efficiently for sculptured surfaces are available. Also, the various geometric construction formats can be used to produce tool position data, which can provide a useful backup for the standard ARELEM and may at times be a more convenient way of generating tool path control and inspection data. Finally, a facility for the regional milling of sculptured surfaces is provided and is described in Chapter 8 of this volume.

7.1 SCULPTURED SURFACES USED ON APT CONTROL SURFACES

Sculptured surfaces are ordinary control surfaces for the purpose of tool path generation in APT. They can serve as part, drive or check surfaces and all the usual techniques for tool path control in APT are available. Note that sculptured surfaces are by definition bounded surfaces unlike APT surfaces which are unbounded. As an aid to tool path control the APT system provides ruled surface extensions for every boundary patch. (Refer to Section 7.1.5.) Sculptured surfaces can be intermixed with other APT control surfaces, including TABCYL, PARSRF AND RLDSRF surfaces. (Refer to Section 7.1.6.) NOTE: Synthetic curves cannot be used as APT control surfaces directly since they are not surfaces but space curves. Examples of the use of sculptured surfaces as conventional APT tool path control surfaces are given in Sections 7.1.1, 7.1.2 and 7.1.3. Section 7.1.4 shows an example of the use of THICK with sculptured surfaces. 181
Figure 7.1 182

7.1.1 SCULPTURED SURFACE USED AS A PART SURFACE

The following part program demonstrates the use of a sculptured surface as a part surface (PS1). In this example the drive and check surfaces (DS1, CS1, and CS2) are planes. PARTNO/'SSURF AS PART SURFACE' UNITS/MM NOPOST CLPRNT $$ P1 = POINT/0,0,0 P2 = POINT/25,0,8 P3 = POINT/50,0,0 $$ SC1 = SCURV/CURSEG,P1,P2,P3 $$ PS1 = SSURF/REVOLV,SC1,AXIS,(POINT/0,0,-50),$ (VECTOR/1,0,0),CLW,60,-30 $$ DS1 = PLANE/0,1,0,0 CS1 = PLANE/1,0,0,0 CS2 = PLANE/1,0,0,50 $$ CUTTER/10,5 $$ BALL ENDED CUTTER $$ FROM/(STPT = POINT/10,-50,50) GO/TO,DS1,TO,PS1,TO,CS1 GORGT/DS1,TO,CS2 FINI The 10mm diameter ball ended cutter, initially at the point STPT is brought into contact with the drive, part and initial check surface by the conventional three surface start up statement, GO/TO, DS1, TO, PS1, TO, CS1 then constrained to move to the right in contact with the drive and part surfaces until the final check surface is reached by the motion command, GORGT/DS1,TO,CS2 The resultant tool path is shown in Figure 7.1 and listed overleaf. 183
PARTNO SSURF AS PART SURFACE UNITS/MM CUTTER/ 10.0000, 5.0000 FROM / STPT ( 0) X Y Z 10.0000000 -50.0000000 50.0000000 GO / DS1 ( 0) 5.0000000 -5.0000000 3.4401681 GORGT/DS1 ( 0) 6.4061789 -5.0000000 4.0587469 8.4586358 -5.0000000 4.8675892 10.5461574 -5.0000000 5.5811554 12.6646244 -5.0000000 6.1979933 14.8095361 -5.0000000 6.7167490 16.9763749 -5.0000000 7.1362762 19.1605621 -5.0000000 7.4556378 21.3574780 -5.0000000 7.6741080 23.5624717 -5.0000000 7.7911745 25.7708714 -5.0000000 7.8065397 27.9779944 -5.0000000 7.7201214 30.1791570 -5.0000000 7.5320534 32.3696850 -5.0000000 7.2426853 34.5449238 -5.0000000 6.8525818 36.7002481 -5.0000000 6.3625218 38.8310724 -5.0000000 5.7734966 40.9328609 -5.0000000 5.0867078 42.1789912 -5.0000000 4.6279559 43.6758256 -5.0000000 4.0241402 45.0000000 -5.0000000 3.4401651 FINI 184

7.1.2 SCULPTURED SURFACES USED AS DRIVE AND CHECK SURFACES

This sample part program demonstrates the use of sculptured surfaces as drive and check surfaces (DS1 and CS1). The part surface and the initial and final check surfaces (PS1, CS0 and CS2) are planes. PARTNO/'SCULPTURED SURFACES AS DRIVE AND CHECK SURFACES' UNITS/MM NOPOST CLPRNT $$ P4 = POINT/-30,20,0 P5 = POINT/-10,28,0 P6 = POINT/10,21.5,0 P7 = POINT/30,11.5,0 P8 = POINT/50,16,0 P9 = POINT/70,30,0 $$ SC2 = SCURV/SPLINE,P4,P5,P6,P7,P8,P9 $$ DS1 = SSURF/RULED,SC2,AXIS,(VECTOR/0,0,1) $$ DRIVE SURFACE $$ P10 = POINT/60,-48,0 P11 = POINT/55,-36,0 P12 = POINT/50,0,0 P13 = POINT/55,36,0 $$ SC3 = SCURV/CURSEG,P10,P11,P12,P13 $$ CS1 = SSURF/RULED,SC3,AXIS,(VECTOR/0,0,1) $$ CHECK SURFACE $$ PS2 = PLANE/0,0,1,0 $$ XY PLANE IS PART SURFACE $$ CS0 = PLANE/1,0,0,-20 $$ INITIAL CHECK SURFACE CS2 = PLANE/0,1,0,-40 $$ FINAL CHECK SURFACE $$ CUTTER/10 $$ SLOT DRILL $$ TOLER/0.1 $$ FROM(STPT=POINT/10,-30,30) $$ GO/TO,DS1,TO,PS2,TO,CS0 GORGT/DS1,TO,CS1 GORGT/CS1,ON,CS2 $$ FINI 185
Figure 7.2 186
In this example, a 10mm slot drill, initially positioned at the point STPT is brought into contact with the drive, part and initial check surfaces by the three surface start up command, GO/TO,DS1,TO,PS2,TO,CS0 then constrained to move in contact with the part and drive surfaces until the check surface, CS1 is reached by the motion command, GORGT/DS1,TO,CS1 The check surface CS1 then becomes the next drive surface and the cutter is constrained to move in contact with the current part surface and the new drive surface until the final check surface, CS2 is reached GORGT/CS1,TO,CS2 The resultant tool path is shown in Figure 7.2 and listed overleaf. 187
PARTNO SCULPTURED SURFACES AS DRIVE AND CHECK SURFACES UNITS/MM CUTTER/ 10.0000 TOLER/ 0.1000 FROM / STPT ( 0) X Y Z 10.0000000 -30.0000000 30.0000000 GO / DS1 ( 0) -15.0000000 22.2378771 0.0000000 GORGT/ DS1 ( 0) -10.8515320 22.9438549 0.0000000 -7.3029020 22.8762647 0.0000000 -3.4704797 22.1223195 0.0000000 0.7825144 20.6209792 0.0000000 5.6230625 18.2390228 0.0000000 11.2784681 14.8143564 0.0000000 14.8623411 12.4680245 0.0000000 19.5532437 9.7908538 0.0000000 22.8586772 8.2591325 0.0000000 26.5410151 7.0196236 0.0000000 30.4469384 6.3212005 0.0000000 34.8267893 6.2621592 0.0000000 39.4874859 6.9062830 0.0000000 45.2996965 8.5453333 0.0000000 GORGT/ CS1 ( 0) 44.8968020 -1.0197214 0.0000000 45.1281187 -10.1742884 0.0000000 45.9629283 -18.8827946 0.0000000 47.3682623 -27.1125827 0.0000000 48.6911923 -32.6425510 0.0000000 50.9912576 -40.0000000 0.0000000 FINI 188

7.1.3 SCULPTURED SURFACES USED AS PART, DRIVE AND
CHECK SURFACES

This next example illustrates the use of sculptured surfaces as part, drive and check surfaces. PARTNO/'SCULPTURED SURFACES AS PART, DRIVE AND CHECK SURFACES' UNITS/MM NOPOST CLPRNT $$ P1 = POINT/-30,0,0 P2 = POINT/15,0,10 P3 = POINT/60,0,0 $$ SC1 = SCURV/CURSEG,P1,P2,P3 $$ PS1 = SSURF/REVOLV,SC1,AXIS,(POINT/0,0,-50),$ (VECTOR/1,0,0),CLW,60,-35 $$ P4 = POINT/-30,20,0 P5 = POINT/-10,28,0 P6 = POINT/10,21.5,0 P7 = POINT/30,11.5,0 P8 = POINT/50,16,0 P9 = POINT/70,30,0 $$ SC2 = SCURV/SPLINE,P4,P5,P6,P7,P8,P9 $$ DS1 = SSURF/RULED,SC2,AXIS,(VECTOR/0,0,1) $$ DRIVE SURFACE $$ P10 = POINT/60,-48,0 P11 = POINT/55,-36,0 P12 = POINT/50,0,0 P13 = POINT/55,36,0 $$ SC3 = SCURV/CURSEG,P10,P11,P12,P13 $$ CS1 = SSURF/RULED,SC3,AXIS,(VECTOR/0,0,1) $$ CHECK SURFACE CS0 = PLANE/1,0,0,-20 $$ INITIAL CHECK SURFACE CS2 = PLANE/0,1,0,-40 $$ FINAL CHECK SURFACE $$ CUTTER/10,5 $$ BALL ENDED CUTTER TOLER/0.1 $$ FROM/(STPT=POINT/10,-30,30) GO/TO,DS1,TO,PS1,TO,CS0 GORGT/DS1,TO,CS1 GORGT/CS1,ON,CS2 FINI 189
Figure 7.3 190
Again, conventional APT motion statements are used to control to tool path, which is listed below and shown in Figure 7.3. PARTNO SCULPTURED SURFACES AS PART, DRIVE AND CHECK SURFACES UNITS/MM CUTTER / 10.0000, 5.0000 TOLER / 0.1000 FROM / STPT ( 0) X Y Z 10.0000000 -30.0000000 30.0000000 GO / DS1 ( 0) -15.0000000 22.2378531 1.7723811 GORGT / DS1 ( 0) -10.8518962 22.9438306 2.6504667 -7.3026864 22.8762462 3.5219662 -3.4880253 22.1271511 4.5681619 0.7633875 20.6290711 5.7707848 5.6304601 18.2349187 7.0726088 11.3370644 14.7772050 8.3288236 17.2098310 11.0701046 9.1194902 20.5087939 9.3119675 9.2827971 24.0688607 7.7933190 9.2494316 27.8001274 6.7226293 9.0002615 32.2783531 6.2105348 8.4393663 36.7060855 6.4412797 7.6242914 40.4538403 7.1195617 6.7325833 45.2995941 8.5452564 5.2856223 GORGT / CS1 ( 0) 44.9420782 1.5326482 5.9694832 44.9320357 -5.5298437 5.7404043 45.2888194 -12.4886938 4.5969388 46.0074631 -19.2152197 2.5631796 47.0593640 -25.5902898 -0.3166373 48.3932545 -31.5101353 -3.9816452 49.3289745 -34.9045138 -6.6092454 50.9912677 -40.0000000 -11.4759741 FINI 191
Figure 7.4 Figure 7.5 192

7.1.4 USE OF THICK WITH SCULPTURED SURFACES

The THICK facility in APT allows the part programmer to apply a uniform offset to any of the control surfaces. Different offsets can be applied to each of the controlling surfaces. (Refer to Volume 1, Section 10.8.) This facility is also available when machining sculptured surfaces and is particularly useful when generating roughing cuts. If the motion sequence in the previous example is replaced by FROM/(STPT=POINT/10,-30,30) $$ THICK/2,1,0 $$ 2mm on PS1, 1mm on DS1 and 0mm on CS0 GO/TO,DS1,TO,PS1,TO,CS0 THICK/2,1,.5 $$ 0.5mm on CS1 GORGT/DS1,TO,CS1 THICK/2,0.5,0 $$ 0mm on CS2 GORGT/CS1,ON,CS2 then a 2mm offset normal to the surface PS1 will be applied together with a 1mm offset to the surface DS1 and 0.5 mm offset to CS1, as illustrated in Figures 7.4 and 7.5.

7.1.5 EXTENSIONS TO SCULPTURED SURFACES

Scupltured surfaces are by definition bounded surfaces whereas general APT surfaces are unbounded. To assist in tool path control, the APT system provides surface extensions for every boundary patch of a sculptured surface. These are ruled extensions, the rulings being the result of extrapolating the surface tangents across the boundary. A doubly ruled extension is provided at the corner of a sculptured surface, generated by extrapolating the two direction tangents at the corner point. Note that if a check surface is missed when the part or drive surface is a sculptured surface, then the tool will move out on the extension until the present APT4 maximum length is violated. 193
Figure 7.6 Figure 7.7 194
For example, if in the part program given in Section 7.1.1, the second check surface had been erroneously specified at x = -50, then the motion command GORGT/DS1,TO,CS2 would fail, because the system would be unable to find the check surface, CS2, resulting in the ARELEM restart diagnostic 24005, as soon as the maximum cut sequence length has been exceeded. In the example, shown in Figure 7.6, the maximum cut length was restricted to 75mm by programming MAXDP/10,75. The part programmer, knowing the properties of these extensions, may find that they actually describe adjacent areas of the part, in which case they may be used for machining as shown in Figure 7.7, where the first surface has been specified at x = -20 and the second at x = 65. Care should be exercised when various extensions intersect each other since motion failures may occur.

7.1.6 FACTORS AFFECTING EFFICIENCY

The part programmer should be aware of factors which may affect the efficiency of tool path calculation when using sculptured surfaces and control surfaces. Dependent on the system configuration, particularly those system releases which are overlaid, the use of PARSRF and RLDSRF together with sculptured surfaces may be very inefficient since the relevant ARELEM calculations for PARSRF and RLDSRF may be performed in different overlays from those associated with sculptured surfaces. A further loss of efficiency may occur if more than one large data array type surface (e.g. TABCYL and SSURF) are referenced in a single motion statrement and there is insufficient memory space to hold the canonical forms of all the relevant surfaces at the same time. Should this occur the surfaces will be repeatedly re-read over each other during the ARELEM calculations causing excessive transfers between external files and memory. Therefore it is important that the user is aware of the data sizes of the surfaces (See Section 5.1.3) and the memory space available in the APT system implementation being used. 195
Figure 7.8 Figure 7.9 196

7.2 SPECIAL ARELEM

In order to improve the efficiency and reliability of tool path computation with respect to sculptured surfaces, a special control mode for ball ended cutters has been implemented and is the default for the system. The cutter is treated as a complete sphere as far as sculptured surfaces are concerned but continues to look like the defined cutter to all other APT surfaces, that is if the h parameter is not specified a default height of 5 inches is assumed. This could cause gouging problems (see Figure 7.8) therefore the programmer must take special care and be aware of the true shape of the cutter and its relationship to the controlling surfaces. This mode which also applies to a point cutter can be removed by the use of the command, MAXDP/-3 in which case the normal APT cutter representation and ARELEM will be used, and the gouging problem will no longer occur as shown in Figure 7.9. However, the computation time for the execution phase will be significantly increased, typically by 30 per cent. The special Arelem can be restored by the command. MAXDP/-2 197

7.3 MULTI-AXIS MACHINING OF SCULPTURED SURFACES

All allowable forms of tool axis control provided in APT are available when machining sculptured surfaces. A detailed description of these together with a discussion of general points to watch when programming multi-axis machining is given in Volume 1, Chapter 10, Section 10.13. In addition, a special type of multi-axis 'swarf' cutting is available on a sculptured surface.

7.3.1 CONVENTIONAL TOOL AXIS CONTROL

There are five basic formats for the TLAXIS statement all of which can be used when performing multi-axis machining of sculptured surfaces. TLAXIS/1 tool axis remains fixed according to immediately prior orientation. TLAXIS/I,J,K tool axis defined by its direction cosines. TLAXIS/ NORMPS tool axis maintained normal NORMDS to part or drive surface, as specified. TLAXIS/PARLEL,A ,R,H! tool axis aligned parallel to surface rulings A = 1 for part surface = 2 for drive surface Optional parameters: R = radius of contact disc H = height of contact disc from tool tip. 198
TLAXIS/A,B,R,H,A1, X,Y,Z ,A2 Full tool axis statement. I,J,K A = 1 for 3 axis = 2 for part surface control = 3 for drive surface control B = 0 for 4 axis = 1 for 5 axis = 2 for setting tool axis parallel to surface rulings. = 3 for 'new surface' = 4 for pivot point. R and H define contact disc A1 inclination of tool axis measured from the controlling surface normal X,Y,Z coordinates of pivot - point I,J,K direction cosines of a vector A2 lead or lag angle - inclination of tool axis to a plane perpendicular to the forward motion of the tool. 199
Figure 7.10 200
For example, TLAXIS/3,1,5,15,85,0,0,0,-10 GOFWD/DS2,ON/CS2 would specify that the tool axis is to be controlled by the drive surface (A = 3) and 5 axis motion is required (B = 1). The contact disc being 5 mm radius and 15 mm from the tool tip. The tool axis is also required to be inclined at an angle 85 degrees to the surface normal at the point of contact and to have a lag angle of 10 degrees. Figure 7.10 shows a multi-axis cut of this type where the tool axis is controlled by the sculptured drive surface, DS . Note that dummy values for a pivot point (0,0,0) have been included to satisfy the requirements of the full TLAXIS statement and permit the inclusion of a lag angle. As for all multi-axis machining, the MULTAX statement causes the APT system to store the resultant cutter locations and tool axis vectors on the CLFILE, as shown below. GOFWD/DS2 ( 0) X Y Z I J K 20.9631363 -35.0755806 0.0386247 -0.0661941 0.1049216 0.9922751 22.9408154 -35.9867036 0.0352702 -0.0678142 0.0972602 0.9929460 24.8723850 -32.8396007 0.0327109 -0.0694752 0.0906352 0.9934578 26.7546732 -31.6307388 0.0306606 -0.0711003 0.0846841 0.9938679 28.8003259 -30.2000606 0.0287017 -0.0727554 0.0784499 0.9942597 30.5810060 -28.8810108 0.0281215 -0.0751501 0.0746288 0.9943757 201
Figure 7.11 Figure 7.12 202

7.3.2 SPECIAL TOOL AXIS CONTROL FOR SCULPTURED SURFACES

The formats of the tool axis statement in APT which constrain the cutter to lie parallel with the rulings of a 'RLDSRF'. TLAXIS/PARLEL,A,R,H A = 1 for Part Surface Control A = 2 for Drive Surface Control and TLAXIS/A,2,R,H ,90! A = 2 for Part Surface Control A = 3 for Drive Surface Control also apply to a ruled sculptured surface. The drive surface is usually the surface controlling the tool axis, so the forms are usually, TLAXIS/PARLEL,2,R,H and TLAXIS/3,2,R,H ,90! The parameters R and H define a disc which determines the point of contact of the cutter with the rulings. The fifth parameter which defines the angle between the tool axis and the surface normal, is optional and if included should be 90. Figure 7.11 shows a cut of this type being applied to a ruled sculptured surface, DS1, the cutter aligned with the surface rulings. The APT statements which generated this were, CUTTER/10,5 $$ BALL ENDED CUTTER TLAXIS/3,2,5,15,90 GO/TO,DS1,TO,PS0,ON,CS0 GORGT/DS1,ON,CS1 The same concept also applies for a sculptured surface which is not ruled. In this case, the tool axis is aligned with the cross spline tangent vector at the point where the controlling disc of the cutter is in contact with the sculptured surface. Figure 7.12 shows such a 'swarf' cut across a convex sculptured surface, DS2, which was the result of the following APT statements, CUTTER/10,5 $$ BALL ENDED CUTTER TLAXIS/3,2,5,15,90 GO/TO,DS2,TO,PS0,ON,CS0 GORGT/DS2,ON,CS1 203
Figure 7.13 204
As pointed out in Section 10.13.5 of Volume 1, because only a cutter disc is considered, the programmer must take care to ensure that the physical cutter does not gouge the surface being machined. If the sculptured surface is a developable surface, then a regular APT cutter can be laid along the rulings by specifying, TLAXIS/PARLEL,A A = 1 for Part Surface Control A = 2 for Drive Surface Control Figure 7.13 shows an example of this, where the surface is a portion of the surface of a cone, which is developable. In this case the following statements were programmed, CUTTER/40,10 $$ CORNER RADIUS CUTTER MAXDP/-3 $$ APT ARELEM TLAXIS/PARLEL,2 GO/TO,DS1,TO,PS0,ON,CS0 GORGT/DS1,ON,CS1

7.4 TOOL PATH GENERATION BY GEOMETRIC CONSTRUCTION

The parametric lines of a sculptured surface can frequently be used for tool path control. It is possible by repetitive calls to intrinsic geometric construction formats and direct tool offset calculations via APT language calculations to produce complete cutter location data for machining a sculptured surface. Although this technique is somewhat redundant since the inclusion of regional milling in the sculptured surface processor, is still of value if a cutter other than a spherical cutter is to be used.

7.4.1 TOOL OFFSET CALCULATION FOR A SPERICAL CUTTER

The calculation of the coordinates of the tool reference point, the tool tip, when a spherical cutter is in contact with a sculptured surface at a known surface point, defined by its parameters can be accomplished in the following manner. See Figure 7.14. 205
Figure 7.14 206
First use intrinsic geometric construction formats, see Section 9.3, to define the surface point and normal for the current parameters UU and VV in the current patch, PNUM. SP = POINT/INTOF,SS,PARAM,UU,VV,PNUM SN = VECTOR/INTOF,SS,PARAM,UU,VV,PNUM,NORMAL,UNIT Then given the cutter radius R and tool axis vector, TA, obtain the coordinates of the surface point, SP, and the components of the surface normal vector, SN, and tool axis vector, TA, as follows. OBTAIN,POINT/SP,PX,PY,PZ OBTAIN,VECTOR/SN,SNX,SNY,SNZ OBTAIN,VECTOR/TA,TAX,TAY,TAZ Finally, calculate the coordinates of the tool reference point and define, TE, as follows, TEX = PX + R* (SNX-TAX) TEY = PY + R* (SNY-TAY) TEZ = PZ + R* (SNZ-TAZ) TE = POINT/TEX,TEY,TEZ The cutter location on the CLFILE can then be generated by using the GOTO/ command, GOTO/TE If the tool axis vector is not parallel to the z axis, then the multi-axis version will be required GOTO/TE,TA By incorporating these commands in a loop or macro, varying PNUM, UU AND vv, the complete tool path for machining a sculptured surface can be generated. 207
Figure 7.15 208

7.4.2 TOOL OFFSET CALCULATION FOR A CORNER RADIUS CUTTER

If a sculptured surface is to be machined using a corner radius cutter, as shown in Figure 7.15, where D is the cutter diameter and R is the corner radius, then the tool offset calculation at a surface point, SP, with surface normal, SN, and tool axis, TA, would be, in vector notation, TE = SP + R * (SN - TA) -- -- -- -- + (D/2 - R) * (SN - (SN.TA)TA) / ABS(SN -(SN.TA)TA) -- -- -- -- -- -- -- -- which could be programmed as follows S1 = DOTF(SN,TA) $$ S1 = SN.TA V1 = VECTOR/S1,TIMES,TA $$ V1 = S1-TA V2 = VECTOR/SN,MINUS,V1 $$ V2 = SN-V1 V3 = VECTOR/UNIT,V2 SV3 = VECTOR/(D/2-R),TIMES,V3 $$ SV3 = (D/2-R)*V3 V4 = VECTOR/SN,MINUS,TA $$ V4 = SN-TA SV2 = VECTOR/R,TIMES,V4 $$ SV2 = R*V4 OBTAIN,POINT/SP,PX,PY,PZ SV1 = VECTOR/PX,PY,PZ $$ SV1 = SP V5 = VECTOR/SV1,PLUS,SV2 VP = VECTOR/V5,PLUS,SV3 $$ VP = SV1+SV2+SV3 OBTAIN,VECTOR/VP,VX,VY,VZ TE = POINT/VX,VY,VZ $$ TOOL REFERENCE POINT In practice, provision would need to be made for handling the special case when the surface normal is parallel to the tool axis, since it is usual to position the corner radius at the point rather than the tool reference point. 209

7.5 ARELEM PROBLEMS AND SOLUTIONS

Although the special Arelem is more reliable than the standard APT4 Arelem, problems can still occur, particularly when performing multi-axis programming. This section describes some of these problems and suggests methods to overcome them.

7.5.1 CUTTER STARTS UP ON WRONG SIDE OF SURFACE

In order to resolve any ambiguity in the intended side of the control surface, when initially bringing the cutter into contact with a start up statement, GO/..., the APT statement defining the surface vector, SRFVCT/... should be used. See Volume 1, Section 10.1.2 for details. Note, that if the surface vector points from the surface away from the cutter, the cutter side of the surface will be the TO side and the other side the PAST side.

7.5.2 CUTTER OUT OF TOLERANCE WITH DRIVE SURFACE

AT START OF CUT

If an error message indicating that the cutter is out of tolerance with the drive surface at the start of a cut occurs when trying to move off along the drive surface, following an apparently successful start up, during multi-axis motion, or if the APT Arelem is being used, then carry out the following checks. First, is the cutter on the intended side of the surface, if not rectify as for 7.5.1. If the cutter is on the correct side then the problem may be caused by the fact that the cutter control disc is not in contact with the new drive surface. Therefore, check if the surface is concave, in which case the problem may be solved by reducing the cutter height to the height of the control disc, or reverting to the sculptured surfaces special Arelem. Alternatively, if there is a lead or lag angle operative then set it to zero for the start up and re-impose it for the subsequent motion. 210

7.5.3 PROBLEMS FINDING A SURFACE DURING START UP

This is usually caused by some ambiguity and can normally be resolved by the use of INDIRP/... or INDIRV/... as described in Volume 1, Section 10.1.1.

7.5.4 PROBLEM IN MOVING OFF IN THE CORRECT DIRECTION AFTER A

SATISFACTORY START UP TO A SCULPTURED DRIVE SURFACE

This may be overcome by the use of INDIRP/... and INDIRV/... followed by a GOFWD/... command, or by the inclusion of additional check surface(s) between the current tool position and the final check surface. Again check for any possibility that the tool control disc may not be in contact with the drive surface during multi-axis programming, as for 7.5.2.