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Work at the Scale of the Model

When you start to apply toolpaths and select tools in Mastercam, all of the settings will be at the scale of actual machine motion and physical tooling. So, it's important that you work with geometry that is at this same scale and in the correct units for the machine.

Size

Prior to working in Mastercam, your geometry must be scaled to the size your final model will be (1:1). If you will be cutting the model from 3" foam, make sure all of the geometry is scaled to fit within the 3" of working height. Milling isn't a simple printing operation where you can scale everything down to fit on a page while sending it to the machine.

Sometimes scaling causes data loss issues from Rhino to Mastercam. This is especially true when the original Rhino file is in large units, such as kilometers, or miles, and the final model is very small. It is helpful in this case to change units without scaling (i.e. if the bounding box of the model is 14 miles on one side, make that 14 inches by changing units rather than using the "scale" function. However, be careful so that you don't accidentally scale it to 887040 inches). Then, once the units are correct, scale the geometry up or down by the necessary factor to make it the correct size. (i.e. for our previous example, multiply by 12/14 to change from 14 inches to a 12 inch model.

It can also help to set the Rhino Absolute Tolerance to 0.0001 units so that they are analogous to those used in Mastercam.

Units

Units should be set to inches for the routers, knee mill and desktop mill.

For the robots, units must be in millimeters.

Work Near the Origin

When it comes down to it, modeling in a 3 dimensional CAD program is a process based on math and numbers. So, storage of all of that data becomes something with which to contend. When you work far from the software's spatial origin, you are working with larger numbers. These large numbers are more difficult to save, store and translate within and between programs. Strange things can happen if the geometry you are working with is located far from the origin, so it's best to work as close to the origin as possible. This is universally true of all applications of digital fabrication, including 3D printing and the generation of STL files or meshes from surfaces.

In Rhino the origin that matters is the origin of the World Top Plane. Make sure the CPlane that you are assuming represents this origin is actually located at the World Top Origin.

Geometry Types

As mentioned earlier, different tool path motions are defined by different types of geometry. Some toolpaths are best described by a single line segment, others by a closed planar curve, and others by an untrimmed surface. Depending on the toolpaths that you wish to use to remove the material around your part, you will need to provide Mastercam certain geometry types to define your part through the tool path operations. If you will be using the basic Template File, consult the "Assign Geometry" section of Setting up a Mastercam File to determine what geometry to include from your own model. Make a file specifically for the milling process, creating and including only the geometry that you will be using in Mastercam.

Mastercam generates toolpaths by applying parameters to selected geometric entities. For example, if you only need 3 surfaces to define a feature or part, it is generally best to select only those 3 surfaces, and not the 3,000 other ones that you don't need. A considerable amount of processing time can be saved this way, reducing the likelihood that Mastercam will crash.

The same argument can be made for meshes. Using resolution settings that result in fewer polygons and produce adequate surface definition are better than those with excessively high polygon counts. Vertices that are too close or faces that are too small will significantly increase processing times, increasing the likelihood that Mastercam will crash, without necessarily producing a better toolpath or model in the end. Understand the physical implication of the size of the mesh faces with regards to the constitution of the material you will be using and of the diameter of the endmills employed.  

Surfaces

Most applications of Mastercam at the GSD make use of surface-based toolpaths. These toolpaths can typically be applied to both surfaces and meshes, but surfaces offer some advantages over meshes in Mastercam as they can more easily be selected either as individual entities or as groups of surfaces, allowing us to better control how material is removed from particular features of the part.

• In general, a toolpath will be active only over the selected surfaces, so in order to remove material from the body of the part, it has to have surfaces to define it (insert diagram).
• Vertical Surfaces: Most applications of the CNC routers and mills are for 3-axis jobs where the tool is always held vertically in the machine. This naturally produces vertical walls between surfaces that have edges located at different heights within a part. Mastercam "looks" at selected surfaces from the TOP plane, so only those that are visible from this view will be used to generate the toolpath (insert diagram). In this instance, it would not be necessary to model the vertical wall between two surfaces nor to use it in Mastercam to define a toolpath as it doesn't have a value when viewed from the TOP plane and would only be extra information for Mastercam to try and process. Additionally, when selected together, Mastercam will recognize overlapping surfaces (insert diagram) and only mill the portion of surfaces visible from above, those not obscured by other selected surfaces.
• Surface Types: In Mastercam, surfaces are used to define a toolpath in one of two ways: as Drive Surfaces or as Check Surfaces. Drive Surfaces are the surfaces of the file that will be cut by the toolpath. Check Surfaces can be thought of as masks over the Drive Surfaces beneath them and, as surfaces that you tell the toolpath to avoid, are used to prevent the tool from cutting areas selected as Drive Surfaces (insert diagram). For each surface-based toolpath, you'll have an opportunity to tell Mastercam which surfaces to consider as the Drive Surfaces, and which to consider as the Check Surfaces. It is always necessary to select Drive Surfaces, but Check Surfaces are always optional, depending on what you want to do (and sometimes Containment Curves are a better approach than the use of Check Surfaces to limit the application of a toolpath on a Drive Surface). A surface that is used as a Drive Surface in one toolpath might then be used as a Check Surface in another.
• Small Features: When preparing a site model in a 3D modeling environment, it is easy to lose track of how big or small features of the model will actually be. Some of the features may be too small to be recognizable at the scale of the model, especially if you are working with a material that is not conducive to holding fine detail or if you are using tools whose diameter is too large to create that scale of detail. Features such as the width of roads, height of curbs, or density of pattern repetition may need to be exaggerated, simplified, or omitted considering factors of time, tool size, complexity of file preparation, and material characteristics. Eliminate the need to have many small surfaces as they may result only in increased processing time and frustration, and will not necessarily produce a better model in the end. CNC Machines are not the equivalent of electromagnetic shrink ray machines of the late 1980's; some design thinking will have to be performed in terms of how best to make a model using this process.
• Model Cleanly: When creating surfaces, define them as simply and as cleanly as possible: reduce isocurve counts, shrink trimmed surfaces, and remove duplicate surfaces. Look at your surfaces from the TOP plane and make sure there are no unintended gaps between them; an area of the model with no surface to define it may be an issue.

Curves

Some applications of Mastercam make use of only toolpaths that are defined by curves and do not need any surfaces. In general, these applications are limited to 2D contour cutouts from materials like plywood sheets, or to pocket operations that remove material contained within a closed curve. Curves can also be used to define 3D contours, where the tip of the tool follows the path of the curve through space, changing the height of the tool tip along the path. Additionally, curves can be used to supplement surfaces when defining surface-based tooplaths.

• Curves for Contour Toolpaths: It is best practice to locate curves such that their height defines the bottom of the feature they will be used to control (insert diagram/screen shot). If you wish to cut along the perimeter of a model at its base, locate that curve at Z=0, so that the tip of the tool is instructed to go to that depth. If you wish to etch a curve-based pattern at a depth that is just below a surface, locate those curves just below the surface, at the depth you wish to make the cut. Curves selected for one toolpath do not need to be located at the same depth; they can be located at different depths. In Mastercam, you will have to options with regards to depth of cut and curve-based toolpaths: to cut to an Absolute Depth or to an Incremental Depth. Absolute Depths will result in all curves being cut to the same height with respect to Z=0 of the file, regardless of where they are drawn in the model. Incremental Depths will result in all curves being cut to the same height with respect to where each is drawn in the model. For the sake of clarity, it is best to model curves at the height you would like them to be cut in the model.
• Curves for Pocket Toolpaths: In addition to the recommendations made above with regards to depth of cut and location of curves, curves defining pockets must also be closed and in a plane parallel to the TPlane in Mastercam (the TOP Plane, in general). (Screen Shot)
• Curves for Containment: Containment Curves are used when defining surface-based toolpaths. Similar to Check Surfaces, Containment Curves are used to limit or restrict the action of the toolpath over the Drive Surface. Containment Curves must be closed curves, but can be located anywhere in terms of their height. When used, a surface-based toolpath will be generated over the Drive Surface only within the area of the closed curve. It is sometimes necessary to generate Containment Curves by drawing them in Mastercam (diagram).
• Model Cleanly: When creating curves, define them as simply and as cleanly as possible: reduce point counts, rebuild as necessary, and remove duplicates. Make sure curves that need to be closed are closed end-to-end and don't intersect oddly. Curves might also need to be planar (and in a plan parallel to the TOP plane), check that this condition is met, as necessary. Be aware that joined curves in Rhino does not necessarily result in joined curves in Mastercam, but this shouldn't be a cause for great concern as Mastercam will detect when two segments have coincident endpoints and can select a number of segments as one continuous chain. Make sure that segments do have coincident end points if you intend to select them as a closed chain (and that there are not other curves who share that coincident endpoint, resulting in more than two curves with a common end point).

Points

Points are used only to define Drill Toolpaths, indicating the center of the hole to be made with a drill. The size of the hole is then controlled by choosing the appropriate diameter drill from the tool library. Similar to how the location of curves controls the depth of cut for Contour or Pocket Toolpaths, the location of the point will control the depth of the hole made. Be sure to eliminate duplicate points.

Meshes

Meshes can be used to define most surface-based toolpaths.

• Special surface-based toolpaths reference isocurves of a surface, so do not work with meshes, such as the Surface Finish Flowline Toolpath.
• Containment of Meshes: In Mastercam meshes can also be problematic along their edges, requiring the use of Containment Curves that limit the action of the toolpath to the area on top of the mesh only, and not over its outermost edges.