What File Format for BobCAD CNC Machine: How Geometry Survives the Trip

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  Why the File Format for BobCAD CNC Machine Decides Your Tolerance
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  Three Families of Format, and What Each One Keeps
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  When DXF Is the Right Answer and When It Is a Trap
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  STEP, IGES and Parasolid on Contoured Parts
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  What Actually Breaks, and How to Catch It Before Cutting
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  How the File Meets the Machine at GreatLight
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  A Practical Export and Handoff Sequence
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  Format Comparison for BobCAD Toolpath Work

BobCAD-CAM is a CAM system. It does not machine your part. It reads geometry, lets a programmer attach toolpaths, and posts G-code to a controller. Every step in that chain is a translation, and every translation can move a surface. When a part is held to ±0.005 mm, a translation error of 0.02 mm is not noise. It is scrap.

Most format problems are not bugs. They are structural. A solid format carries surfaces and volume. A wireframe format carries lines and arcs. A mesh format carries triangles. BobCAD can work with all three, but it can only machine what it can measure, and you cannot put a Ø8 mm tool into a triangle soup and expect a true radius.

The practical question is not which format BobCAD opens. It opens many. The question is which format preserves the features your toolpath depends on: arc centers, hole axes, fillet radii, and the difference between a flat face and a drafted wall.

We see this daily at our Dongguan and Singapore plants. Parts arrive as STEP, DXF, STL, native BobCAD files, and occasionally a PDF that someone hoped would work. The ones that machine cleanly share one trait: the file was chosen to match the job, not to match the sender's CAD seat.

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  Solids for 3D contoursUse a solid or surface format when the toolpath follows a curved or drafted face.
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  Wireframe for 2D profilesDXF is enough when the part is prismatic and every cut is a straight line or an arc.
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  Mesh only for referenceSTL has no arc data, so hole diameters drift.

Native BobCAD files carry the most context inside BobCAD itself. Layers, construction geometry, toolpath settings and stock definitions all survive a round trip. The catch is portability. A .bbcd file is not useful to a shop running a different CAM package, and it does not travel well across BobCAD versions.

Neutral CAD exchange formats are the working standard. STEP and IGES both carry true curves, and STEP in particular stores analytic surfaces as NURBS rather than tessellated approximations. That matters when a toolpath needs to follow a radius. IGES is older and tolerates sloppier exports, which is sometimes an advantage and sometimes a warning sign.

Mesh formats are the third family. STL and its relatives describe a surface as triangles. A 10 mm hole tessellated at a coarse chord tolerance is no longer round. The controller sees a polygon, the tool follows a polygon, and the hole measures out of tolerance on a CMM.

Drawing formats sit slightly outside these families. DXF and DWG are 2D-first. They are excellent for profile milling, laser and waterjet work, and they are a poor carrier for anything with a contoured third axis.

DXF earns its place on prismatic parts. A bracket, a plate, a manifold block with drilled holes: these are line-and-arc problems. DXF carries arcs exactly, so a Ø6 mm hole stays Ø6 mm and the CAM programmer can select the arc center directly instead of hunting for a centroid.

The trap is splines. Many CAD packages export a spline as a polyline with hundreds of short segments. BobCAD reads it, but every segment junction becomes a node. Toolpath smoothing degrades, the post processor emits far more blocks, and the machine may stutter at each node. If your profile is a true spline, export a STEP file instead.

DXF also has no concept of a solid. If the part has a drafted wall, a fillet that blends into a boss, or any surface that is not perpendicular to the tool axis, DXF cannot describe it. You will get a silhouette, and the silhouette is not the part.

One more detail: layer naming. A clean DXF uses one layer per feature family, such as OUTLINE, HOLES, POCKET. A messy DXF puts a drawing border, a title block and a revision cloud on the same layer as the profile. The programmer then spends an hour deleting geometry instead of cutting metal.

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  Keep arcs as arcsSet the exporter to output true arcs, not polylines, whenever the option exists.
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  Delete dimensions and hatchingThey are not geometry and they clutter the CAM tree.
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  One layer per featureIt cuts setup time and reduces selection mistakes.

STEP is the default recommendation for anything with a curved or angled surface. A modern STEP export stores holes as analytic cylinders, fillets as true toroidal or cylindrical surfaces, and freeform faces as NURBS. When BobCAD builds a toolpath along one of those faces, the path follows the math, not a polygon approximation.

Version matters more than people expect. STEP AP203 and AP214 differ in what metadata they carry, and some exporters write AP242 by default. For machining, the geometry is usually equivalent. For assembly context and color, it is not. If a shop returns your file asking for a re-export, ask which application protocol they want before you guess.

IGES is the fallback. It predates STEP and its tolerance handling is looser, which means a sloppy export can still produce a watertight-looking model that has hairline gaps at trimmed edges. BobCAD can often heal these, but healing a surface is a manual step. On a 16-cavity part, that step can cost a shift.

Parasolid is the tightest of the three. It is a B-rep kernel format, so surfaces are stitched into a closed solid with a defined tolerance. For hard-surface mechanical parts with tight wall thickness, that closure is worth having. The trade-off is licensing: not every shop runs a Parasolid-capable seat, so confirm before you send.

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  Export AP214 for general workIt carries geometry plus common assembly metadata.
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  Check for gaps after IGESRun a stitch or heal check before programming.
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  Parasolid for closed solidsBest when wall thickness and closure are critical.

The most common failure is unit scaling. A file modeled in inches imports into a millimeter document, and the part comes out 25.4 times too large. It sounds obvious. It still happens, and it is caught at the machine only if the operator reads the first rapid move instead of pressing cycle start.

The second is the tessellation drift we described above. A hole exported as a mesh measures undersized because the inscribed polygon sits inside the true circle. On a Ø10 mm hole at a 0.05 mm chord tolerance, the error is small. At a 0.5 mm tolerance, it is not.

The third is duplicate or overlapping geometry. Two coincident lines look like one line on screen. The CAM system may pick either, and the resulting offset may land on the wrong side of the profile. A quick geometry check catches this in seconds.

The fourth is the missing feature. A file exports the outer shape but drops an internal pocket because the pocket was on a suppressed layer. Nothing in the CAM tree flags it. The part machines correctly and is still wrong. A drawing cross-check is the only defense.

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  Confirm units firstCheck the import dialog before anything else.
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  Inspect hole diametersMeasure a known hole in the CAM system, not just on screen.
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  Overlay against the drawingA 30-second visual check beats a scrapped run.

Our plants run 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. Maximum processing size is 4,000 mm, with a Ø400 mm rotary table for parts that need to rotate around a bore. That range means we see almost every file format a customer can produce.

Whatever arrives, the geometry is rebuilt into our own CAM environment before a tool touches metal. If a STEP file has a surface gap, we heal it. If a DXF has exploded splines, we ask for a clean STEP instead. If an STL is the only thing that exists, we will say so plainly: it is good for a form check, not for a ±0.005 mm bore.

Inspection follows the same logic. Raw material is checked on receipt, dimensions are monitored in process, and every part is inspected before shipment, with reports available on request. Our qualification rate runs at 99.99%, and historical late-delivery probability is below 2%. Those numbers come from controlling the input, not from inspecting harder at the end.

We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The last one matters for file handling: uploads are secure and confidential, and an NDA is available on request before you send anything sensitive.

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  No minimum order quantityFrom one prototype to 10,000+ part runs.
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  Finishes in houseAnodizing, plating, powder coating, bead blasting, laser marking.
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  Materials on hand6061-T6, 7075, 17-4PH, Ti-6Al-4V, Inconel, PEEK and more.