Chapter 15

INTERFERENCE CHECKING

The SSV3 version of the APT4 + Sculptured Surfaces System has a new facility for checking for possible interference of ball-ended, corner radius and flat-ended cutters with a parametric surface during regional milling and taking appropriate action to avoid gouging. In addition an APT3 type GOUGCK facility similar to that provided in the Arelem has been added to regional milling. Note that these facilities are currently only available when using the regional milling commands SCON and SMIL and not with the isoparametric milling command GOLOFT. Also the full APT seven segment cutter has been implemented in regional milling and additional tool axis control provided with SCON/AXIS. The interference checking process is a two stage algorithm that is performed each time a new cutter position is computed. The first step is to identify all patches of the surface where there is a possibilty of penetration occuring, this is a non-iterative process; the second step is to check for actual interference with each of these candidate patches, perforce this is an iterative procedure. Every effort has been made to speed up the iteration, for example by taking the results of the iterative process at the previous cutter location as start values for the current position.

15.1 Selecting and cancelling Interference Checking

Interference checking in regional milling is selected or cancelled by using the command: n CHKSRF/ › PS,ON ! › ,surface_name,ON ! OFF 1 OFF where, PS check for interference with current part surface. surface_name check for interference with a named surface. ON switches interference checking on. OFF cancels interference checking. Note that only CHKSRF/PS,... is currently implemented. The additional couplets have been provided for possible future extension to check for interference with adjacent surfaces. o CHKSRF/PS,ON will initiate interference checking. All subsequent cutter locations within regional milling will be tested and should penetration occur the maximum will be located and documented on the verification listing as follows: INTERFERENCE DETECTED TE = x , y , z TA = i , j , k WORST PENETRATION = penmax AT PATCH = pnum U = u V = v where, TE and TA are the tool end coordinates and tool axis vector; penmax is the worst penetration detected; pnum , u and v are the ppatch number, u and v parameters of the point on the surface at which the worst penetration occured; If an avoidance strategy has been specified (see section 15.2), appropriate action will be taken to avoid the penetration and a further check made to ensure no new penetration has occured. If the interference is successfully avoided the message 'INTERFERENCE AVOIDED' will be output, if not warning no: 3555 'UNABLE TO AVOID INTERFERENCE WITH CURRENT PARAMETERS' will be output to the verification listing. Optionally, details of the avoidance action taken or attempted may also be printed on the verification listing (see section 15.3). For example, INTERFERENCE AVOIDED BY LIFTING OFF BY dist PARALLEL TO TA INTERFERENCE AVOIDED BY PITCHING THROUGH n DEGREES AND LIFTING OFF BY dist NORMAL TO PS UNABLE TO FIND TOOL AXIS ORIENTATION WITHIN SELECTED RANGE WHERE TOOL CAN BE POSITION IN NON_INTERFERING CONTACT WITH SURFACE R = x , y , z NPAT = pnum U = u V = u ( details of surface point) TOOL RADIUS GT RADIUS OF CURVATURE AT ...... details of surface point ..... as can be seen the message output have been designed to give as much useful information as possible. Occasionally, when the curvature of the surface around the point of penetration is almost the same as the curvature of the corresponding point on the tool, the iterative procedure will not converge within the limits set. In this case the following message will be printed on the verification listing, PENETRATION DETECTED WITHOUT CONVERGENCE PENMAX = penmax After all the candidate patches have been examined, the worst penetration will be selected, from all those detected including those where convergence did not occur. In most cases this will result in the avoidance procedures being able to successfully avoid the interference, however if penetration still occurs then warning number 3555 will be output, preceded by a message indicating what action was attempted if the optional avoidance action logging has been requested. o CHKSRF/PS,OFF will cancel interference checking. *********************************************************************** ***** Note that invoking interference checking will significantly ***** ***** increase the processing time. ***** ***********************************************************************

15.2 Definition of Avoidance Strategies

The strategy(ies) to be employed to avoid interference if penetration of the tool into the part surface is detected may be selected by the command AVCTRL. General form: AVCTRL/›CUTANG,CONST,g,RANGE,b1,b2! $ ›,(NORMPS)! ›,LIMIT,distance! › (AXIS )! AVCTRL/( ON ) ( OFF ) where, CUTANG indicates that automatic variation of the tool axis should be used as the primary method to avoid interference. This will be applied as either a 'roll' or 'pitch' dependent on the location of the point of maximum penetration with respect to the tool contact point. A 'pitch' angle will be in the plane defined by the programmed tool axis and the local forward motion direction, positive being in the direction of forward motion. A 'roll' angle will be in the orthogonal plane through the tool axis, positive being in the direction of the vector = TA x FWD CONST,g defines a fixed 'technological' angle which will be added to the computed angular variation of the tool axis in order to clear the surface. RANGE,bmin,bmax defines the algebraic limits within which the tool axis can be varied from the programmed (i.e. ideal) orientation. NORMPS specifies lift off normal to the part surface. AXIS specifies lift off parallel to the tool axis. ›This is the default lift off and will be applied if no secondary action is specified and tilting is unsucessful in avoiding penetration! LIMIT,distance specifies a limit to the permitted lift off. ›Default = MAXDP! ON either reinstates an avoidance strategy temporarily disabled or invokes the default lift off AXIS. OFF disables the avoidance strategy but interference data will still be logged on the listing.

15.3 Optional Logging of Avoidance Action

Details of the avoidance action taken or attempted can be optionally printed or inhibited by the command, PRINT/AVPRT,(ON (OFF This is an extension to the existing PRINT command and may be used as an additional couplet in a manner similar to SSPRT and SSTEST. AVPRT/ON details of avoidance action will be listed on the verification listing, following the notification of interference. AVPRT/OFF inhibits the printing of the avoidance action details but the messages indicating if interference has been avoided or not will still be printed.

15.4 Example Demonstrating Interference Checking

PARTNO/'SIMPLE EXAMPLE DEMONSTRATING INTERFERENCE CHECKING' MULTAX/ON UNITS/MM CLPRNT NOPOST P1=POINT/0,100 QC=POINT/50,60 P5=POINT/100,100 C1=CIRCLE/CENTER,QC,RADIUS,30 L1=LINE/P1,RIGHT,TANTO,C1 L2=LINE/P5,LEFT,TANTO,C1 P2=POINT/XLARGE,INTOF,L1,C1 P3=POINT/C1,ATANGL,-90 P4=POINT/XLARGE,INTOF,L2,C1 SC1=SCURV/CURSEG,P1,P2 SC2=SCURV/CURSEG,P2,P3,P4 SC3=SCURV/CURSEG,P4,P5 M1=MATRIX/YZROT,90 SC4=SCURV/COMBIN,SC1,SC2,SC3,TRFORM,M1 V1=VECTOR/0,1,0 INTSS1=SSURF/RULED,SC4,AXIS,V1 R1=POINT/47,-20,50 R2=POINT/47,20,50 DC1=SCURV/CURSEG,R1,R2 PP=POINT/50,0,100 VP=VECTOR/0,0,-1 TA=VECTOR/0,0,1 CUTTER/10,2 $$ CORNER RADIUS CUTTER TOLER/0.01 GOTO/PP $$ CHKSRF/PS,ON $$ INVOKE INTERFERENCE CHECKING $$ AVCTRL/CUTANG,CONST,2,RANGE,-90,90,AXIS $$ SPECIFY AVOIDANCE ACTION $$ 1) TILT TOOL $$ WITH 2 DEG. CLEARANCE $$ WITHIN + 90 DEG OF TA $$ ( TA = VECTOR/0,0,1 ) $$ 2) UNLIMITED LIFT OFF $$ PARALLEL TO TOOL AXIS $$ PRINT/AVPRT,ON $$ DOCUMENT AVOIDANCE ACTION $$ TAKEN SCON/INIT,ALL SCON/DS,DC1,PARAM,0,1,AT,VP $$ GUIDE CURVE SCON/PS,TO,INTSS1,PLUS,0 $$ PART SURFACE SCON/AXIS,TA SCON/FEED,200,400,50,2000 $$ SMIL/PATH,DS,PARAM,0,0,TANSPL,PLUS $$ FINI

15.5 APT 7 segment cutter in Regional milling

The full APT 7 segment cutter has been implemented in regional milling and has the following effects. The diagrams below show the tool end point TE that is calculated given the various combinations of SCON/PS,... and SCON/DS,... and tool axis vector TA. For the TLOFPS, TLAT condition, SCON/PS,TO,... SCON/DS,...,AT,.. For the TLOFPS,TLON condition, SCON/PS,TO,... SCON/DS,...,ON,... Tool control point is TEB the effective tool end of a spherical cutter with radius equal to the corner radius of the actual cutter. For the TLONPS,TLON and TLONPS,TLAT conditions, SCON/PS,ON,... SCON/DS,...,ON,... and SCON/DS,...,AT,...

15.6 Additional Tool Axis Control in Regional Milling

Additional tool axis control has been provided in regional milling, still programmed using the SCON/AXIS command. Existing formats: SCON/AXIS,vector SCON/AXIS,NORMPS, PLUS MINUS Additional Formats: SCON/AXIS,ATANGL, DS,a,CUTANG, ›, LEAD ! ,b PS › LAG ! where, DS indicates that the tool axis is controlled with respect to the guide surface. PS indicates that the tool axis is controlled with respect to the part surface. ATANGL,...,a indicates the 'roll' angle a (in degrees) that is to be maintained between the tool axis and the controlling surface normal. In the case of PS control this will be in the plane perpendicular to the motion direction FWD, (where FWD is projected onto the part surface tangent plane). A positive angle will be CCLW about FWD. Note: If ZIGZAG motion is operative the 'roll' will always be applied in the same sense as on the first pass, i.e. a will change from being CCLW to CLW about the motion direction on alternate passes. In the case of DS control this will be in the plane defined by the tool axis and the guide surface step-over direction or guide curve tangent direction. Values of a with absolute values greater than 90 will have the effect of 'inverting' the surface normal. CUTANG,b indicates the 'pitch' angle b (in degrees) that is to be maintained between the tool axis and the controlling surface normal in the direction of motion. Positive will be in the direction of motion. LEAD tilt tool axis in the motion direction for a positive b. LAG tilt the tool axis backwards for a positive b.

15.7 GOUGCK facility in Regional Milling

An APT3 type GOUGCK facility has been added to regional milling. This causes a check to be made on each cut vector computed to ensure that gouging with the point of contact does not occur because either there is a very rapid change in the surface curvature and thus the computed stepout although OK for curvature at initial point is too large, or where the point of contact on the tool changes significantly caused by a change of sign of the cross product of the surface normal and the tool axis. The effect will be to generate addition cut vectors. GOUGCK/ON will invoke gouge checking in both the Arelem and Regional Milling. GOUGCK/OFF will cancel all gouge checking.