AUTOMATIC PLANNING OF MACHINING FIXTURE LAYOUTS THROUGH GEOMETRIC REASONING OF CAD PART MODEL.pdf

automatic planning of machining fixture layouts through geometric reasoning of cad part model m s kale s s pande computer aided manufacturing laboratory mechanical engineering department indian institute of technology bombay powai mumbai 400 076 india abstract fixture layout planning refers to the task of identifying the locating and clamping positions on part faces this spatial placement directly governs the workpiece accuracy and stability under machining loads this paper reports the design and implementation of an intelligent system intfix to accomplish this task it describes the algorithms designed and implemented for automatic generation of fixture layout plans for machining setups it details various strategies devised to determine the most suitable locating and clamping positions for fixturing the part the paper concludes by illustrating typical example to demonstrate the proposed concepts introduction fixture design is an important activity in manufacturing which has a significant bearing upon productivity and product quality traditional fixture design activity draws heavily from human efforts expertise skill and heuristic knowledge and therefore requires longer lead times 1 a fixture layout represents the locating and clamping positions in the workpiece co ordinate system against which the part is fixtured for carrying out machining operations in the setup literature documents that scant research has been carried out for automatic generation of fixture layouts from cad models which is mostly focussed on simple cuboidal prismatic parts located using the 3 2 1 principle complex part shapes however need intensive computational efforts to evaluate large number of possibilities arising due to multiple candidate faces of the cad part model with different geometric and topological properties it is therefore a complex task to suggest the plan for optimal placement of fixturing elements a need therefore exists to develop rigorous algorithms for automatic generation of fixture layout plan from cad part model and process planning information the present research work is an attempt in this direction research scholar email msk me iitb ernet in professor email sspande me iitb ernet in and corresponding author literature review literature documents that the research in this field has been centred around generating a fixturing solution employing standard 3 2 1 location scheme for relatively simple component geometries for the chosen setup configuration the early attempts of fixture planning have primarily incorporated interactive and gt based techniques 2 3 the generative systems have incorporated various heuristic approaches which mainly reason the nominal geometry of the part to obtain the locating and clamping positions 1 4 8 factors such as the direction of cutting force and tool motion and relative orientation of candidate faces have also been considered 9 it is however observed that majority of the systems focus upon generating the fixture layout based on the nominal geometry of the part disregarding the part design specifications the approaches tend to be computationally intensive in case of complex part shapes clamping positions are obtained by using a relatively simple strategy of projecting the respective locating points on the opposite faces 5 8 and no explicit strategies seem to have been incorporated to search alternate positions in case of infeasible outcomes this paper reports the design and implementation of an intelligent system intfix which automatically determines the locating and clamping positions for fixturing the part for machining by reasoning the geometry of the cad part model figure 1 shows the modular structure of intfix the setup information generated by the automatic setup planning module provides the details of features to be machined in the present setup and the reference datums that are used as locating faces 10 fixture layout planning is accomplished by systematically interrogating and reasoning out the geometric and topological information of the cad part model based on this information the tasks of stability analysis fixture building etc can be taken up intfix has been developed in object oriented programming structure using c around the feature based part modeler fbm indigenously developed at cam laboratory iit bombay and the autocad graphical core specific objectives of the present research are outlined as under to build a cad part model representing the inprocess state of the workpiece to determine the locating positions in accordance with the 4 2 1 location configuration on primary secondary and tertiary datums of the part such that they are as far apart as possible to determine the clamping positions on appropriate clamping faces such that they are placed directly opposite to the respective locators as far as possible or within the nearest feasible region in order to minimize workpiece deformation due to clamping forces the approaches and the algorithms designed to implement the above tasks of fixture planning are elaborated in the sections to follow autocad solid modeler feature based part modeler application programming interface automatic setup planning module automatic fixture layout planning module determination of locating positions determination of clamping positions fixture analysis module geometric solid model feature based cad part model setup and fixture planning knowledge setup plans fixture layout plans user graphical user interface feature library inprocess state part model figure 1 modular diagram of intfix fixture layout planning intfix generates the fixture layout plan by first determining the suitable locating positions on the datum faces followed by corresponding clamping positions on appropriate clamping faces due to variety of possibilities of part shapes and face geometries as well as large number of feasible candidate positions it becomes difficult to find a unique optimal solution in the present work heuristic techniques are designed and implemented which attempt to determine the most suitable fixturing positions locating and clamping as far as possible following section presents the details of various strategies incorporated cad part model representing the inprocess workpiece state the inprocess part model provides the information of the fixturing regions of the part which would be available for the setup under consideration for this purpose intfix builds a temporary feature based cad part model from the original model representing the finished workpiece however the temporary model consists of only those features which would be produced after the machining operations from the first setup to the that under consideration this model is interrogated and reasoned in order to generate fixture layout plans following section presents the algorithm developed for determining the locating positions determination of locating positions intfix focusses upon planning a 4 2 1 location configuration which is basically a variant of 3 2 1 location scheme consisting of three fixed locators and one floating locator acting as a support on the primary locating face it offers distinct advantages over 3 2 1 location scheme such as wider support area greater part stability and improved machining accuracy etc locating positions on primary datum face intfix determines four locating positions on primary datum face through following steps refer figure 2 vr1 vf1 b1 vf2 b2 pl2 pl1 pl4 pl3 b3 vf3 b4 vr3 vr4 vr1 vr4 vertices of extents rectangle vf1 vf4 nearest vertices of edges b1 b4 boundary points pl1 pl4 locating points extents rectangle of the face vr2 e2 e1 e3 e4 e5 e7 vf4 e8 e9 e10 boundary of primary face figure 2 determination of primary locating positions 1 primary locating face is enclosed in the rectangle formed by the extents of the face and corresponding to each of its vertex the nearest vertex of the face edge is selected these are shown as points vf1 to vf4 in figure 2 these are referred as boundary points corresponding to which the locating positions inside the face are determined 2 if the face edge containing the boundary point overlaps the edge of the extents rectangle by less than half of its length then the boundary point is shifted to the midpoint of the face edge in order to have more balanced placement of the locator referring to figure 2 boundary points vf3 and vf4 are shifted to midpoints of the edges e3 and e6 respectively the final boundary points are thus shown as points b1 to b4 3 unit vectors referred as search vectors are established from each boundary point directing inside the face the vectors are along the bisector of the face edges sharing the boundary point or along the normal of the edge if the boundary point corresponds to the midpoint of the edge 4 along each search vector points are successively computed at incremental distance and the first feasible point lying on the face is chosen the chosen point is confirmed as the locating point if two additional points at small distance on its either side also lie on face if any of the three points is found to lie on void region of the face corresponding to feature openings etc then next point at the incremental distance along the search vector is computed and validated as mentioned above the procedure is schematically shown in figure 3 figure 2 shows the locating positions pl1 through pl4 on the primary datum face obtained by applying the above algorithm search vector p1 p2 p3 p2 p3 search vector along bisector of the edges p1 search vector along normal to the edge b boundary point b b presence of holes on either side search for feasible positions at incremental distance boundary point p1 p1p2 p3 b face boundary figure 3 searching and validation of locating position by vectored search determination of side locating positions side location is achieved by two points on secondary and one point tertiary locating face intfix determines the secondary locating positions preferably at the midheight of the secondary locating face provided the face width at this height is sufficient compared to maximum width of the face the measure of sufficiency is defined by the user as a percentage of maximum face width if the width is found insufficient then the face is first scanned in the lower region and then in the upper region at successive heights till the sufficient face width is detected the tertiary locating position is determined preferably at the same height as that of the secondary locating positions and at the farthest possible distance from the secondary locating face the side locating positions are determined along the selected widths of the faces with the help of search vectors which originate from the extreme boundary points the search vectors on secondary locating face direct towards each other while that on tertiary locating face directs towards the secondary locating face as shown in figure 4 the candidate points are successively computed along the search vectors at the incremental distance and the first valid points encountered are selected as the side locating positions figure 4 shows points sl1 sl2 and tl as the side locating points determined by intfix h 2 h secondary locating face v1 v2 v3 sl1 sl2 tl tertiary locating face v1 v2 v3 search vectorssl1 sl2 secondary locating positions tl tertiary locating position figure 4 determination of side locating positions the approach followed in the present algorithm is to determine the points on the face boundary corresponding to which nearest feasible locating positions inside the face are searched these positions are therefore also ensured to be widely spaced the algorithm requires limited number of computations as locating positions are searched from only a few boundary points irrespective of the irregularities in the shape of the face thus the need of generating the locus of the candidate points inside or on the face boundary 5 and assess the feasibility of every candidate point for the purpose of location is eliminated following section presents the algorithms to determine the appropriate clamping positions determination of clamping positions intfix adopts following strategy for selection of appropriate clamping faces 1 part faces that are parallel and oppositely oriented to the respective locating faces are selected 2 the face for which the projection of the locating point on the face lies within the face boundary are preferred this enables direct opposite placement of clamp with respect to the locators and therefore requires minimum clamping force to counteract the negative reaction at the locator 3 if no face is available for directly opposite placement of clamp then the face corresponding to minimum distance between the projection of the locating point and the face boundary is selected 4 in case of multiple candidate faces the tie is broken as follows for top clamping the clamping face at the lowest height from the primary face is selected so as to keep minimum height of the clamping tower for side clamping the clamping faces at the farthest distance from the respective side locating faces are selected so as to have placement of the clamping tower outside the workpiece enevelope determination of top clamping positions the steps of the algorithm to determine top clamping positions are enumerated below refer figure 5 1 the locating point is projected on the plane of chosen clamping face point p in figure 2 if the projection lies within the face the corresponding point is chosen as candidate clamping point 3 if the projection lies outside the face then the projected point is moved to the face boundary along the search vector selected from three candidate vectors originating from the point of projection which corresponds to the minimum distance of travel the directions of two of these vectors are taken as from the corresponding locator to its previous and next locating points respectively the third vector is directed towards the face boundary and along the bisector of the first two vectors this is shown as vectors v1 v2 and v3 respectively in figure 5 4 the clamping position inside the face is determined by computing the points at incremental distance along the selected search vector and selecting the first feasible point thus point c1 is obtained by intfix as the feasible clamping point on the face as shown in figure 5 v1 v2 v3 offset distance l1 l2 l3 l4 c1 l1 l4 locating points v1 v3 search vectors c1 candidate clamping position top view front view projection of locating point chosen clamping face step to be machined p p l1 l4l2 l3 figure 5 determination of top clamping positions determination of side clamping positions if top clamping exists one clamping position each against side locating faces is determined as top clamping has a major contribution in workholding if the top clamping is not possible due to machining operation on entire face then one clamping position against each side locator is determined the algorithm for obtaining the side clamping positions is similar to that explained for clamping against primary locating face except for the orientation of search vectors which is as shown in figure 6 it also shows points sc1 sc2 and tc as side clamping positions determined by intfix the strategy of determining a single clamping position against the secondary face is depicted in figure 7 if no suitable clamping positions are obtained then the search is carried out in the lower region of the face at successively decremented heights sl1 sl2 tl sc1 sc2 tc sl1 sl2 tl side locating positions sc1 sc2 tc side clamping positions search vectors figure 6 determination of side clamping positions sl1 sl2 sc sl1 sl2 secondary locating points sc clamping point against secondary locating face search vectors figure 7 determination of single clamping position against secondary locating face intfix thus attempts to place the clamps directly opposite to the corresponding locators as far as possible or in the nearest feasible region by adopting vectored search strategy this enables reduced requirements of clamping forces it also ensures the clamp placements to be inside or on the boundary of the span of the locating points which avoids tilting of the workpiece during clamp actuation this provides a better approach over majority of the reported systems which adopt a relatively simple strategy of projecting the locating points on the opposite faces example figure 8 shows the typical component drawing with relevant dimensions and the fixture layout generated by intfix around it the locating and clamping clamping positions generated by intfix are symbolically shown on the workpiece faces it would be seen that intfix detects the insufficient width of the secondary locating face f2 at the midheight level compared to the maximum width of the face the locating positions are therefore determined in the lower region of the face at height 40mm where the width of the face is found sufficient the system also detects that the clamps c1 and c4 can not be placed directly opposite to l1 and l4 respectively due to presence of inclined face the clamping positions therefore are moved so that they lie within face b and close to its boundary similarly clamp c5 has been shifted to its left so as to avoid the open face of the slot all dimensions in mm features machined face d holes sh1 sh2 on three axis vertical machining centre setup datums primary face f1 secondary face f2 tertiary face f3 locating positions clamping positions l1 196 46 3 54 0 c1 185 3 54 50 on face a l2 3 54 3 54 0 c2 3 54 3 54 50 on face a l3 3 54 96 46 0 c3 3 54 96 46 50 on face a primary l4 196 46 96 46 0 c4 185 96 46 50 on face a l5 5 0 40 secondary l6 195 0 40 c5 110 100 40 on face b tertiary l7 0 95 40 c7 200 95 40 on face c figure 8 fixture layout generated by intfix for example component conclusions this paper has presented the algorithms for automatic fixture layout planning for machining setups focussing upon determining the most suitable locating and clam