waterline with bullcutter

Update: Here is another example with the CL-points coloured differently. At each z-height the innermost loop is with the ball-cutter, next is the bullcutter, and the outermost loop is calculated for a cylindrical cutter. The points are coloured based on which test (vertex, facet, edge) produced them. Vertex-test points are red. Facet-test points are green. The edge-test is further subdivided into (1) a test for horizontal edges (orange), (2) a test for contact with the cylindrical shaft of the cutter (magenta), and (3) the general edge-push function (light blue for ball/bull, pink for cyl). If/when I get the cone-cutter done the cutter-location algorithms in opencamlib should be complete (at least for the moment...), and I can move on to more interesting high-level algorithms.



This figure shows one of the first times I got the push-cutter/waterline algorithm working for bullcutter (filleted endmill, bull-nose cutter, toroidal cutter, a dear child has many names...).

The thin cyan lines are edges of a triangle. The outer cyan spheres are valid cutter locations (CL-points) for a cylindrical endmill. The innermost yellow CL-points are for a spherical (or ball-nose) endmill. Between these two point-sets the new development is the magenta points, which are CL-points for a bull-nose cutter.

The algorithm works by pushing the cutter at a specified Z-height along either the X-axis or the Y-axis into contact with the triangle. There are three sub-functions for handling the case where the cutter makes contact with a vertex, the triangle facet, and an edge. The edge-contact case is the non-trivial (read "hard") one. The approach I am using is based on the offset-ellipse, courtesy of the freesteel blog. Pushing a toroid into contact with an edge/line is equivalent to pushing the cylindrical "core" of the bullcutter into contact with an edge that has been 'inflated' to a cylinder with a radius equal to the bullcutter corner radius. Slicing this cylinder/tube with a z-plane gives us an ellipse, and the sought cutter-location lies on the offset of this ellipse. I should make some diagrams and post longer/better explanation later (I wonder if anyone reads these 🙂 ).

The bullcutter is important not only in itself, but also because it is the offset of a cylindrical cutter. When we want to do z-terrace roughing with a cylindrical cutter, and specify a stock-to-leave value, we do it by calculating the toolpath with cylcutter->offsetCutter() which is a bullcutter, and then actually machining with the cylindrical cutter. That will achieve the desired stock to leave (to be removed later by a finish operation).

Offset ellipse

Contacting a toroidal cutter (not shown) against an edge (cyan line), is equivalent to dropping down a cylindrical cutter (lower edge shown as yellow circle) against a cylinder (yellow tube) around the edge, with a radius equal to the tube-radius of the original toroidal cutter.

The plane of the tip of the cylindrical cutter slices through the yellow tube and produces an ellipse (inner green and red points). The way this example was rotated it is  obvious where the center of the ellipse along the Y-coordinate (along the green arrow) should lie. But the X-coordinate (along the red arrow) is unknown. One way of finding out is to realise that the center of the original toroidal cutter (white point) must lie on an offset-ellipse (outer green/red points). Once the X and Y coordinates are known it is fairly straightforward to find out the cutter-contact point between the cylindrical cutter and the tube, and from that the cutter-contact point between the toroid and the edge. Finally from that the cutter-location can be found.

Something to implement in opencamlib soon...