Research On Key Technologies Of Point Cloud Numerical Control Tool Path Generation

In Reverse Engineering, the method of point clouds NC tool path generation usually fits pointclouds to surface or triangular mesh surface, and generates tool path based surface or triangular meshsurface. However, surface fitting is a complex, time-consuming procedure. Directly generating toolpath based on point clouds avoids surface fitting and greatly shortens the NC tool path computingtime. So it has been one of the popular focuses of international research.After reviewing the domestic and foreign research achievement on point clouds tool pathgeneration, this paper conducts in-depth research into the technologies of point clouds tool pathgeneration, and proposes new algorithms about roughing tool path generation based layer cuttingmethod, constant scallop-height finishing tool path generation, equal-error finishing tool pathgeneration with curves of cross sections and tool path regeneration for point clouds designmodification. The main research results and innovation are summarized as follows.A new algorithm of directly generating roughing tool path based on stratifying method for pointclouds is proposed. Layered milling is employed to plan roughing tool path. Firstly, the point cloud isdivided into3D cell grids. Inverse tool offset method (ITO) is used to compute CL(Cutter-Location)point enveloping surface formed by cell grids, and flat-end cutter direction-parallel tool path in eachcutting layer is obtained. Based on the projection and the distance between tool paths, a new tool pathlinking algorithm is proposed to reduce the number of tool retraction motions. In the algorithm, themachining region boundary and the relationship between tool paths are not needed. When the cut areaof a tool path’s CL point connects with current tool path, their CL points will be linked directly. Toreduce the remaining step material of flat-end cutter machining, roughing tool path generation usingball-end cutter is proposed and an algorithm of optimize tool path is given.A new algorithm of directly generating constant scallop-height finishing tool path for pointclouds is proposed. The algorithm generates three-coordinate NC tool path for ball-end cutter. A localcoordinate system centered at each CL point of the current path is built to calculate the correspondingscallop point. Intersection points of CL point tangent plane and data points located in tool projectionarea are obtained. Circles centered at intersection points are created with tool radius to substitutescallop-height surface. The circles intersect the tool circle centered at CL point, and the intersectionpoint with the maximum coordinate is the wanted scallop point. A similar local coordinate system is built at each scallop point to calculate the wanted CL point of the next tool path. The interference-freeCL point located on the tool circle centered at scallop point is calculated, which is the wanted CLpoint. Point clouds (especially discrete point clouds) are hard to obtain offset point clouds so that it ishard to generate constant scallop-height tool path for point clouds. However, in this algorithm, offsetpoint clouds are not needed and the problem is solved.A new algorithm of generating equal-error finishing tool path generation with curves of crosssections is proposed. For dense CL points on tool path, circles centered at CL points are created withmaximum allowed step error value. The first CL point is obtained as the first equal-error CL point.Tangent lines from the equal-error CL point to the circles are obtained to compute the intersection ofthe areas between tangent lines. When the intersection area is empty, the minimum non-emptyintersection area can be calculated. Tool path computed in the non-empty intersection area is tangentto or intersected with the traversed CL points’ step error circles, which meets step error requirement.Of all intersection points of the intersection area’s boundaries and lines of CL points, the point farthestaway from the equal-error CL point is the wanted next equal-error CL point. Similarly, all equal-errorCL points can be calculated. To improve tool path fairness, an arc spline tool path interpolationalgorithm is proposed to generate G1arc tool path.A new algorithm of tool path regeneration for point cloud design modification is proposed. Thepoint cloud is divided into3D cell grids. The modified region is expressed by cell grids. Tool path outof the modified region is retained. Only CL points in the modified region are identified andrecalculated, which reuses existing mature NC tool paths as many as possible. Firstly, roughing toolpath is identified and recalculated by proposed roughing tool path generation algorithm. Finishing toolpath is identified and line-by-line recalculated, and new CL points are added to meet tolerancerequirement. The scallop height values for the modified region are calculated to judge if it is greaterthan the allowable value. New lines of CL points are added to maintain the required surface finish.The methods and algorithms proposed in this paper have been implemented in theOpenCASCADE and Visual C++6.0platforms. And practical examples are given to show thevalidity and effectiveness.