anderswallin.net » CNC

Shaped cutters

Administrator — Thu, 02 Sep 2010 09:18:16 +0000

For making 60-degree dovetails, and R=2mm rounded edges.

Equatorial wedge – top plate

Administrator — Sun, 29 Aug 2010 19:33:35 +0000

This is the top plate of the wedge which will be tilted so that the alt/az mount which is bolted to it has its azimuth axis pointing towards the celestial north pole. The side-plates will have to wait until next weekend.

Equatorial wedge – base plate

Administrator — Sat, 28 Aug 2010 19:26:17 +0000

I’m building an equatorial wedge which will transform my new alt/az goto-mount into an equatorial mount. This is the first part which attaches to the tripod. Two smaller side plates will be bolted to this plate and support a similarly shaped tilted plate to which the alt/az head attaches. The design is similar to the “mega wedge pro” (how about that for a product name!).

AXIS with PyVCP jogwheel

Administrator — Fri, 27 Aug 2010 15:12:55 +0000

By popular demand, an example where a PyVCP jogwheel is hooked up to AXIS:

The files needed to make this work are in here: axis_jogwheel.tar

Radial tool projection

Administrator — Fri, 27 Aug 2010 11:53:23 +0000

The basic operation when producing waterline-paths is to push the cutter along either the X or Y axis (red and green arrows) into contact with a triangle (cyan lines). That’s done through three different functions, one each for the vertices, facet, and edges of the triangle. The vertex test (results shown as red dots) and the facet test (green dots) are straightforward to implement. The edge-test (blue dots) is more involved. The above figure is for a CylCutter where the edge-test is implemented through the vertex-test (thus red dots along the edges), but this figure for a BallCutter shows the colors better:

Blue dots show edge-contacts with the spherical part of the cutter, light-blue dots show edge-contacts with the cylindrical shaft of the cutter.

The vertex test requires only a radius(h) function that returns the radius of the cutter at height h. The facet test needs three parameters (n, nxy, c) for each cutter, which define where the cutter should be located relative to a point which lies on the facet. From the CC-point on the facet we go a distance n along the 3D unit-normal, then a distance nxy along a unit-normal in the XY-plane to find the cutter center. From the cutter center we go down along the z-axis by a distance c to find the CL-point.

Figuring out the (n,nxy,c) parameters for CylCutter and BallCutter is left as an exercise for the reader.

These points are then hooked up in the correct order to produce waterlines like this (CylCutter on the left, BallCutter on the right).

The edge-test for CylCutter reduces to a 2D problem of line-line intersections, while the edge-test for BallCutter can be done by intersecting a cylinder/tube around the edge with a line. The filleted/toroidal/BullCutter edge-test is much harder. Here I’ve just implemented the special case where the edge is horizontal and the solution is easy to find analytically. The general case where the edge slopes up or down requires an iterative solution to either a quartic or the offset-ellipse problem.

BallCutter(d=6) and BallCutter(d=8)

Administrator — Thu, 19 Aug 2010 12:41:55 +0000

Octree animation

Administrator — Wed, 18 Aug 2010 21:15:34 +0000

Update: this figure shows the numbering of vertices(red), edges(green), and faces(blue). The arrows show the direction of the X-(red), Y-(green), and Z-axes(blue).

Here the sides of the cube are not generated with Marching-Cubes, they are just extracted directly from the octree. Nodes are subdivided whenever the signed distance-field of the cutter indicates that the surface is contained within the node, i.e. the distance-field evaluates to both positive and negative at the eight ~~four~~ vertices of a node. This apparently leads to transient holes in the surface when the cutter is just about to enter a coarse node which hasn’t been subdivided very far yet. It should be possible to adjust the subdivision criterion so that the octree ‘anticipates’ the cutter slightly and subdivides ahead of the actual cutter surface.

Holes in plastic

Administrator — Tue, 17 Aug 2010 09:14:05 +0000

CNC-milled three hemispherical (ca 50mm diameter) holes into these bits of plastic yesterday. To fit a steel cup (far left), attached by gluing.

Cutsim progress

Administrator — Mon, 16 Aug 2010 06:37:44 +0000

The speed of the new cutting-simulation code makes it possible to run it at a higher resolution than before. That makes the surfaces look smooth and nice. Alas, some problems still remain with holes in the fabric of reality mystically appearing and disappearing .

There is an edge-flipping paper by Kobbelt et al. from 2001 which improves the jagged/aliased look of sharp edges.

Update: Kobbelt provides a LGPLv2 licensed sample-implementation of the algorithm here: http://www-i8.informatik.rwth-aachen.de/index.php?id=17

OpenCAMLib machining simulation, v.2

Administrator — Sat, 14 Aug 2010 07:23:32 +0000

This is my second attempt at a machining simulation where a moving milling tool cuts away voxels from the stock material. To save space an octree data structure is used to store the voxels, and to produce a nice looking surface you store the signed distance to the exact surface in each vertex of the octree. You then use marching-cubes to extract triangles for a distance=0 isosurface in order to draw the stock.

Unlike my first attempt, this works well enough to warrant further experiments (on the to-do list are: differently shaped tools, colouring triangles based on which tool cut the voxel, lathe operations, material removal-rate, etc.). It should be straightforward to hook this up to the EMC2 G-code interpreter so that any G-code, not just densely sampled CL-points from OCL, can be simulated. You could also flip the sign of all the numbers, and simulate an additive process, like 3D printing (reprap / makerbot).

This approach to machining simulation is described in a 2005 paper by Yau, Tsou, and Tong.