Work piece alignment is a key issue for hybrid laser aided part repair, a process utilizing
both machining and laser deposition. Proper alignment can greatly improve the accuracy of the
repair process. This paper introduces a method for aligning a physical work piece and a CAD
model using a Renishaw touch probe and software tools. Also discussed is a model for
computing 5-axis CNC positions based on a desired work piece orientation.Mechanical Engineerin

Liou, Frank

Padathu, Ajay Panackal

Sparks, Todd

English

Texas ScholarWorks

WORKPIECE ALIGNMENT FOR HYBRID LASER AIDED PART REPAIR PROCESSAjay Panackal Padathu, Todd Sparks, and Frank LiouUniversity Of Missouri - RollaRolla, Mo 65409-1350AbstractWork piece alignment is a key issue for hybrid laser aided part repair, a process utilizingboth machining and laser deposition.  Proper alignment can greatly improve the accuracy of therepair process.  This paper introduces a method for aligning a physical work piece and a CADmodel using a Renishaw touch probe and software tools.  Also discussed is a model forcomputing 5-axis CNC positions based on a desired work piece orientation.IntroductionLaser Aided Manufacturing Process (LAMP) part repair uses laser deposition process andmachining to restore a damaged metal part to near-original condition.  The LAMP repair processis a hybrid repair process. The damaged portion of the work piece is first machined, both toremove damage and to make a surface suitable for laser deposition. Then metal powder is laserdeposited at the damaged location.  Finally, the work piece is finish machined back to its originalcondition. Alignment of the work piece becomes highly critical in this application.  Poor alignmentmight result in deposition or machining at the wrong location or even damage to the depositionsystem itself.  The strategy for alignment is as follows.  A Renishaw touch probe is used to getthe point cloud data from the work piece.  This data is then used to orient the work piece in adirection we want before machining away the damaged portion. After machining, the touchprobe is used again to calculate the orientation of the work piece. Then, The LAMP system isused to repair the part.  The work piece is again probed to make sure that the finished part has therequired accuracy.418Figure 1 – Repair ProcessEquipment OverviewLaser Metal Deposition (LMD) is a layered manufacturing process where metal powder isfocused into a melt pool created by a laser incident on a substrate.  The advantage of this processis that complex geometries can be constructed with near net shape.  The LAMP system at theUniversity of Missouri Rolla is comprised of a 1.4 KW Nuvyonx diode laser (808 nm) withintegrated 5-Axis FADAL CNC with a maximum spindle speed of 7500 RPM.  Powder isdelivered by a Bay State thermal spray powder feeder419Figure 2 – LAMP ProcessThe 5-axis CNC has the capability of producing overhang parts and does not use supportmaterial.  Use of support increases the build time of the part and requires a time consuming post-processing.  With a five-axis deposition integrated with five-axis machining, these obstacles areremoved Alignment MethodologyThe FADAL CNC is a five-axis one with three linear axes, X, Y, and Z and two rotaryaxes,  A and B.  In laser deposition the flatness of the substrate is a priority for good deposition.The A and B axes of motion are combined together to get a C axis of motion which makes thesurface for deposition level.  Data obtained by probing the surface to be deposited is used tocalculate the A & B angles needed.  The calculations for getting the A and B angles are shownbelow in Figure 3.420Figure 3 – Surface LevelingThe solution obtained for leveling a flat surface from a combination of A and B axesmotion is a unique solution.  Any further rotary motion in either A or B directions will affect thelevel of the substrate.The process planning of LAMP repair process is based on the CAD model of theworkpiece.  The aim of alignment is to make sure that the coordinate system of the CAD modeland workpiece match, so that the position and orientation of the workpiece is known.After affixing the work piece in the vise, the top surface is probed and the data is used tolevel the surface using a combination of A and B rotary motions.  Any further rotary motion willchange the orientation of the surface.  So, for the CAD model to match the work pieceorientation, the CAD model has to be rotated and not the work piece.  The YZ plane of the workpiece is then probed.  The data obtained is used to calculate the equation of the plane. Thecorresponding plane is then identified in the CAD model,, as highlighted in Figure 5.  The anglebetween the two planes is calculated via Equation 1.  It doesn't  matter if the XZ plane is probedinstead of the YZ plane as the workpiece is a rigid body.421If n1 and n2 are the normal vectors of two planes, the angle between them is given by:cos =n1°n2∣n1∣⋅∣n2∣(1)                                                       Figure 4 - CAD Model                                       Figure 5 - CAD Model with the YZ plane highlightedThe CAD model is then rotated to correct the misalignment.  The angle between the twoplanes is again calculated to make sure that the CAD model and the work piece orientationmatch.      This algorithm can be used only for parts  with at  least  two non-parallel  flat  surfacesaccessible  to  the  touch  probe.   Parts  not  meeting  this  criteria  require  a  more  complex  andcomputationally expensive surface fitting algorithm, which will be a part of a future work. 422The levelness of the substrate is important in this algorithm. The solution for attainingthat objective  is a unique one.  Hence, a coordinate transformation is imperative for processplanning.  For example, if a 1in square patch is needed on the surface of a part, but the part isrotated by 45 degrees in both the A and B axes, then the transformation would provide thecoordinates of the patch in the tranformed state (Figure 6 and Table 1). The Octave code for thistransformation is attached in Appendix 1                                                     Figure 6 -Transformation example423Table 1 – Transformation Coordinates                                                   ConclusionsAlignment is a critical for a viable part-repair  process.  Using touch probe data, it  ispossible to align a part with atleast two non-parallel flat surfaces.  The levelness of the substrateis important to the deposition process and critical for the machining process.  The algorithmdiscussed in the paper takes this into account. Without precision alignment of workpiece, anefficient part-repair process becomes impossible. The coordinate transformation discussed in thelast part of the paper is imperative for successful process planning.AcknowledgmentsThis research was supported by the National Science Foundation Grant Number DMI-9871185,  Army  Research  Office,  and  Air  Force  Research  Laboratory and  UMR  IntelligentSystems Center.     Their support is appreciated.References[1] Pettofrezzo,  Anthony J.   Vectors and Their Applications.  Englewood Cliffs, NJ: Prentice-Hall, 1966[2] Oakley, Cletus O. Analytic geometry problems. New York: Barnes & Noble, 1958[3] Spong, Mark.W. and M.Vidyasagar. Robot dynamics and control. New York: John Wiley andSons, 1989Appendix 1Octave code for coordinate transformationfunction W=fiveaxiscoords(Xp,Yp,Zp,A,B,Pzerox,Pzeroy,Pzeroz)phi=A/180*pi();theta=B/180*pi();RA=[cos(phi) -sin(phi) 0 0;sin(phi) cos(phi) 0 0; 0 0 1 0; 0 0 0 1];RB=[cos(theta) 0 sin(theta) 0 ;0 1 0 0; -sin(theta) 0 cos(phi) 0; 0 0 0 1];Pzero=[Pzerox;Pzeroy;Pzeroz;1];Tpc=[1 0 0 Xp;0 1 0 Yp;0 0 1 Zp;0 0 0 1];Wzero=[1 0 0 -Pzerox;0 1 0 -Pzeroy ;0 0 1 -Pzeroz;0 0 0 1];W=Wzero*RB*RA*Tpc*Pzero;424

Workpiece Alignment for Hybrid Laser Aided Part Repair Process

Work piece alignment is a key issue for hybrid laser aided part repair,a process utilizing both machining and laser deposition.Proper alignment can greatly improve the accuracy of the repair process.This paper introduces a method for aligning a physical work piece and a CAD model using a Renis aw touch probe and software tools. Also discussed is a model for computing 5-axis CNC positions based on a desired work piece orientation

Sparks, Todd E.

Liou, Frank W.

Missouri University of Science and Technology (Missouri S&T): Scholars' Mine

https://repositories.lib.utexas.edu/bitstream/2152/80078/2/2005-37-Padathu.pdf

Workpiece Alignment for Hybrid Laser Aided Part Repair Process

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