Machine Coordinate System. Coordinate Transformation. Work Coordinate System

Transcription

1 Outlne MC Programmng g II Coordnate Systems MCS vs. WCS Coordnate Transformaton Return to Home Poston Tool Change Commands Cutter Length Offset Compensaton General Structure of NC program Intalzaton Tool change Program endng Subroutnes and Subprograms Tool Radus Compensaton Machne Cycle Operatons Machne Coordnate System CNC unt mplctly keeps track of the coordnates (appearng as negatve quanttes) for the Spndle Reference Pont (SRP) wrt. a (fxed) global coordnate system called Machne Coordnate System (MCS). Poston sensors are arranged to measure wrt the MCS. Snce the programmer s nterested t n the endpont of the cuttng tool wth respect to the part, workng drectly wth the MCS turns out to be a challengng task as one needs to take nto consderaton: Fxture offsets, Effectve length of the tool used at a partcular moment. Chapter 5b ME 44 2 Chapter 5b ME 44 3 Work Coordnate System Coordnate Transformaton To overcome ths problem, most CNC systems allow users to defne a number (up to 15) of local coordnate systems called Work Coordnate Systems (WCSs). Programmer defnes these WCSs va enterng the correspondng fxture offsets (manly, FOX, FOY, FOZ) through the CNC nterface (HMI). Once the WCSs are defned, the programmer can select any of the sx (basc) coordnate systems usng G54 G59. All subsequent ent references to the axes postons wll be recognzed n the new coordnate system beng selected. Work Coordnate System # Spndle Ref. Pont Tool Ref. Pont L t z y x Workpece Tool Machne Coordnate System (Fxed) Chapter 5b ME 44 4 Chapter 5b ME 44 5

2 Transformaton (Cont d) Return to Home Poston* Coordnates of the Spndle Reference Pont n MCS x y = = x y FOX FOY z = z FOZ + Local Coordnates of the Tool Reference Pont n WCS Offsets for the Orgn of WCS wrt. MCS L t Effectve Tool Length (f Tool Length Offset Compensaton s Enabled) G28 code s used to return (rapdly) to the home poston on all axs. If an X, Y, or Z s specfed n the same block, only that axs wll return to the home and the correspondng movement wll be thru that ntermedate pont specfed. Chapter 5b ME 44 6 Chapter 5b ME 44 7 G28 Return Home n X Table retracts to the far left to ts home poston! G28 Return Home n Z Carrer moves up to ts home ( tool change ) poston! The spndle s algned wth the ATC at ths locaton. Tool Changer FOX Z MCS X Tool Changer Tool Spndle Z G28 G91 Z MCS X FOZ FOZ Z Spndle Spndle Ref. Pont Tool Z WCS Workpece X WCS Workpece X Table Table Chapter 5b ME 44 8 Chapter 5b ME 44 9

3 Home Poston When the machne s at ts home poston, the orgn of MCS concdes d wth the SRP. Z MCS Tool Changer X FOX Z WCS Spndle Spndle Ref. Pont Tool Workpece X Table Modfyng WCS If desred, the user can drectly modfy the WCS nsde an NC program va G92: (G9) G92 Xx Yy Zz Current coordnates of the tool are swapped wth the ones specfed. ed Orgn of the current WCS s effectvely shfted. A WCS (va G54 G59) should be selected before. If n effect, the tool length compensaton s automatcally performed by ths command s executed. Chapter 5b ME 44 1 Chapter 5b ME Example for G92 Tool Change Commands Assume that the tool n operaton wth G54 s postoned at (19, 15) WCS s shfted when G92 X9. Y9. s executed. T## Call up tool ## M6 Carry out tool change G43 Load the length offset H## Offset number ## G49 Cancel length offset Changng tools s very machne specfc, so be sure you know your machne. Generally, the fve codes shown n the table load the tool and the length offset. Chapter 5b ME Chapter 5b ME 44 13

4 Tool Change for VMCs Cutter Length Offset Compensaton Tool change command (M6) along wth a tool number (T##) wll execute a tool change for that partcular tool. Ths command wll automatcally stop the spndle whle the Z-axs wll move up to the home (tool- change) poston. The ATC wll remove the current tool and the selected one wll be put n the spndle. The coolant pump p wll be shut off before the executon of the tool change. SRP Whenever a new tool s loaded to the spndle, the CNC unt must also take nto account the effectve length of the current tool to carry out the automatc coordnate transformaton between the WCS and MCS (see Slde 6). Chapter 5b ME Chapter 5b ME Cutter Length Offset (Cont d) Cutter Length Offset (Cont d) As outlned earler, the cutter length compensaton can be regarded as the subtracton of the tool length from the dstance between the MCS and WCS n the Z drecton. Hence, the programmer can effortlessly program the end- center pont of the current cuttng tool wrt the selected WCS. Chapter 5b ME Chapter 5b ME 44 17

5 Cutter Length Offset (Cont d) Tool Change Command Sequence Even wth the length compensaton, the user stll needs to nsure that The flutes are long enough to cover the maxmum depth of cut. Toolholder does not nterfere wth ether the workpece or the fxture. Tool lchanger Spndle Tool #1 L 1 = X: +1. Y: Z: + 8. Spndle Ref. Pont Tool Ref. Pont Orgn of WCS Chapter 5b ME Chapter 5b ME Tool Change Sequence G49 Tool Change Sequence G28 Tool Changer G49: Cancel CLO Compensaton Spndle X: +1. Y: Z: SRP & TRP now overlap! Chapter 5b ME 44 2 Chapter 5b ME 44 21

6 Tool Change Sequence T3 Tool Change Sequence G54 Tool Changer Tool Changer L G9 T3 M6: 3 = 32 Change Tool Tool #1 Tool #3 X: +1. Y: Z: +5. G54 G X2. Y2.: Rapd Move n WCS#1 X: +2. Y: + 2. Z: +5. Chapter 5b ME Chapter 5b ME Tool Change Sequence G43 Offset Entry* Chapter 5b ME WCS and tool offsets can be convenently entered thru graphcal user nterfaces of modern CNC systems (lke Semens 84DI*). For specfc detals, one should refer to operator/user manuals before programmng the machne. Chapter 5b ME 44 25

7 Structure of a General NC Program The structure of a generc NC program can be gven as Program ntalzaton Tool change Machnng functons Program end Except for machnng functons, the remanng gportons are smlar n all programs. Chapter 5b ME Intalzaton: N G21 G4 G49 G8 G99 N G9 Structure (Cont d) ; METRIC / CANCEL ALL CYCLES ; ABSOLUTE COORDINATE MODE Tool Change: N M9 ; COOLANT OFF N M5 ; SPDL OFF N G49 ; CANCEL TOOL LENGTH COMPENSATION N G91 G28 Z ; INCREMENTALLY GO HOME IN Z DIRECTION N G9 ; ABSOLUTE MODE N T## M6 ; CALL TOOL ## AND DO THE TOOLCHANGE N Sssss M3 ; SET SPDL SPEED TO ssss RPM / CW N G54 G XX YY ; GO TO FIRST X, Y POSITION IN THE WCS N G43 H## Z M8 ; LOAD OFFSET / MOVE TO Z / COOLANT ON End: N M9 ; COOLANT OFF N M5 ; SPINDLE OFF N G49 ; CANCEL TOOL LENGTH COMPENSATION N G91 G28 Z ; INCREMENTALLY GO HOME IN Z DIRECTION N G28 Y ; HOME IN Y TO MAKE UNLOADING PART EASIER N G9 ; ABSOLUTE MODE N M3 ; END OF PROGRAM Chapter 5b ME Subroutnes In Snumerk 82D SL, one can call subroutnes (subprograms, to be exact!) nsde the man NC program. Subroutnes allow the CNC programmer defne a seres of command that are repeated several tmes thoughout the program. Instead of repeatng them, they are called up when needed. M98 P nnnn mmmm statement calls a subroutne: Number of repettons: nnnn (max. 4 dgts) Program number: mmmm (max. 4 dgts) For example, M98 P59 ; subroutne n fle 9.mpf s executed 5 tmes M98 P41 ; subroutne n fle 41.mpf s executed once An alarm s ssued n case the specfed program s mssng. End of Subroutnes Subroutnes must end wth M99 Pnnnn where nnnn refers to the block number of the return (resumpton) pont n the man program. For example, suppose that a subroutne s termnated wth M99 P125: As soon as the subroutne ends, the control s transferred to the block startng wth N125 n the man (host) program. If Pnnnn s omtted, the man program s executed rght from the start. Chapter 5b ME Chapter 5b ME 44 29

8 Tool Radus Compensaton Radus Compensaton (Cont d) In prevous NC programmng exercses, the path of the cutter s defned consderng the center lne of the cuttng tool. Ths s a major nconvenence for the user as one needs to shft the cutter outsde the part profle by the tool radus at a partcular nstant of tme. On modern CNC systems, specal radus compensaton codes (G4, G41, G42) are provded to make that shft automatcally. In an NC program, a G41 (left) or G42 (rght) code should be specfed at the start of a contourng moton. A radus offset code Dd (d: tool number) must be programmed to select the tool radus from the correspondng offset table. G4 s used to cancel the compensaton. Chapter 5b ME 44 3 Chapter 5b ME Left / Rght Compensaton G41 Cutter Compensaton Left G42 Cutter Compensaton Rght Note that the tool s ether on the LEFT- or the RIGHT sde of the part contour as the tool goes along ts path. Cardnal Rules n Radus Compensaton Always select the start poston of the cutter away from the contour, n the clear area. Always apply the cutter radus offset together wth a tool moton command (G or G1). Never start or cancel the radus offset n an arc cuttng mode (G2 or G3). Between the startup block and the cancel block, arc commands are allowed. Make sure the cutter radus s always smaller than the smallest nsde d radus of the part contour. Cancel cutter radus offset wth the G4 command, along wth tool moton command d(g or G1) only, preferably a sngle-axs moton. In G4 mode, move the cutter to a clear area. Always consder the cutter radus as well as all reasonable clearances. If possble, retract the tool along the Z axs only after the radus offset has been cancelled. Chapter 5b ME Chapter 5b ME 44 33

9 Programmng Example 3 Programmable Data Input G1 PROG_3 N1 G21 G9 S9 M3 N2 G42 G1 X4. Y4. Z-4. D1 F25. N3 X96. ; MOVE TO POINT 2 N4 Y96. ; MOVE TO POINT 3 N5 X4. ; MOVE TO POINT 4 N6 Y4. ; MOVE TO POINT 5 N7 G4 X-3. Y Z M5 ; GO BACK N8 M3 Snumerk 82D allows users to modfy tool (or fxture) offsets nsde an NC program va G1 functon. Offsets could be changed as f they were entered through the CNC pendant. Exstng offsets can be overwrtten va G1. Not possble to create new tool offsets! Command formats are as follows: G1 L1 Pp Rr ; Tool length compensaton, geometry G1 L11 Pp Rr ; Tool length compensaton, wear and tear G1 L12 Pp Rr ; Tool radus compensaton, geometry G1 L13 Pp Rr ; Tool radus compensaton, wear/tear Here, p s the tool (memory) ndex; r s the value. Chapter 5b ME Chapter 5b ME Y Radus Compensaton for Multple Passes Allowance for Roughng Cut Allowance for Fnshng Cut In machnng, multple cuts are necessary to produce the desred workpece geometry: Roughng passes Fnshng passes Geometry of the part must be revsed to accommodate these machnng allowances: X In the frst glance, the radus compensaton does not appear to help!.5 mm Multple Passes (Cont d) Fnal Contour Fnshng Pass Roughng Pass 2 Roughng Pass 1 Soluton s the radus compensaton wth fcttous tool offsets: Machnng s performed by the real tools usng fcttous offsets: Ths accommodates all machnng allowances. Programmer only deals wth the orgnal contours of the part. No need to rework the geometry for each cut! To llustrate the applcaton, let us consder an example: Two roughng passes ( Sde cut) Wdth of cut: 2.5 mm Fnshng cut Wdth of cut:.5 mm Only one tool (wth a dameter of 1 mm) s to be used for all these operatons. Chapter 5b ME Chapter 5b ME 44 37

10 Multple Passes (Cont d) Canned Cycle Operatons Operatons are to be performed by ONLY employng dynamc offsets: Roughng Cut 1: Tool 1 Wth a dameter offset of 16 mm, Roughng Cut 2: Tool 1 Wth a dameter offset of 11 mm, Fnshng Cut: Tool 1 Wth a dameter offset of 1 mm. Canned cycles are defned for the most common Z-axs repettve operatons such as drllng, tappng, and borng. There are a number of canned cycle operatons to choose from: G82: Counter-borng cycle G83: Peck drllng cycle G84/G85: Tappng/borng cycles Cycle operatons can be cancelled wth G8. Some cycles could be cancelled va G...G3. Chapter 5b ME Chapter 5b ME [*] Haas Automaton. G81 - Spot Drllng* Spot Drllng wth G98 + G9 G81 G98 Xx Yy Zz Rr Ff Rapd Travel Feed Surface (z = ) After the drllng, the tool returns to the ntal (G98) plane. x, y denote the absolute coordnates of the hole. z s the absolute coordnate of the hole bottom. r s the absolute coordnate of the R plane. f s the feedrate. Chapter 5b ME 44 4 Chapter 5b ME 44 41

11 Spot Drllng wth G99 + G9 Spot Drllng wth G98 + G91 G81 G99 Xx Yy Zz Rr Ff After the drllng, the tool returns to the R (G99) plane. x, y denote the absolute coordnates of the hole. z s the absolute coordnate of the hole bottom. r s the absolute coordnate of the R plane. f s the feedrate. G81 G98 XΔx YΔy ZΔz Rr Ff Kk After the drllng, the tool returns to the ntal (G98) plane. Rapd Travel Feed Δx, Δy denote the ncr. x, y x, y Intal coordnates of the hole. P A A' Level Δz s the ncr. coordnate r of the hole bottom. R R' R Level r s sthe ncr. coordnate cood ateof Surface the R plane. (z = ) z f f f s the feedrate. Z Level k s number of cycles to B B' executed. Chapter 5b ME Chapter 5b ME Spot Drllng wth G99 + G91 Programmng g Example 4 G81 G99 XΔx YΔy ZΔz Rr Ff Kk After the drllng, the tool returns to the R (G99) plane. Rapd Travel Feed Δx, Δy denote the ncr. x, y coordnates of the hole. Δz s the ncr. coordnate of the hole bottom. x, y r s sthe ncr. coordnate cood ateof Surface the R plane. (z = ) f s the feedrate. k s number of cycles to executed. Chapter 5b ME Chapter 5b ME 44 45

12 NC Program for Example 4 PROG_4 N1 G21 G91 S35 M3 ; INCR MODE / START SPDL N2 G99 G81 X1. Z-2. R-72. F1. K4 ; DRILL 4 HOLES ON X-AXIS N3 Y75. K2 ; DRILL HOLES #5 AND #6 N4 X-1 1. K3 ; DRILL HOLES #7, #8, #9 N5 Y-75. K1 ; DRILL HOLE #1 N6 G Z72. ; CANCEL DRILL CYCLE AND MOVE TOOL UP N7 G81 X75. Z-3. R-22. K1 ; DRILL HOLE #11 N8 X15. K1 ; DRILL HOLE #12 N9 G9 G X-1. Y Z75. M5 ; MOVE BACK / STOP SPINDLE N1 M3 Plannng One should follow a seres of steps to create a successful program: Examne the part drawng thoroughly and get a rough dea of how you want to proceed. Fgure out how to hold the raw materal so you can perform as much machnng as possble n one setup. Decde what cutters are necessary to perform the varous operatons. Ths s more crtcal on machnng centers because the holder and fxture can nterfere wth the work. Wrte down the exact sequence of operatons necessary to machne the part, one cutter at a tme. Convert your sequence of operatons nto a program and smulate the program f possble. Chapter 5b ME Chapter 5b ME 44 47