How to Make a 3D Graphic for CNC Machine Work

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  Key takeaways
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  What a 3D graphic for CNC machine work actually is
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  Requirements and tolerances before modeling
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  CAD setup that survives the CAM handoff
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  Validate the model against real tooling
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  Common errors that send a model back
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  When to send the job to a machining partner
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  Step by step: from sketch to first chip
• 9
  Choosing the file format and machine setup

A 3D graphic for CNC machine work is a solid model that carries geometry and enough manufacturing information to generate toolpaths. It is not a rendering or a display mesh. The file must describe every face, fillet and hole so CAM software can offset a cutter around it without guessing. That is the difference between a model that looks right and one that machines right.

Most CNC shops in the West work from STEP AP214 or AP203 files. STL is accepted for 3D printing and rough checks, but it stores triangles, not true surfaces. A 25 mm bore in STL may come in at 24.94 mm once the tessellation is applied. If the drawing calls a bore with an H7 fit, send a STEP file and state the tolerance in the purchase order.

The model also has to match the machine. A three-axis mill cuts from one direction plus the side faces; a five-axis center can tilt the tool and reach undercuts. If your graphic is designed for five-axis access but the job is quoted on three-axis machines, the programmer either adds setups or rejects the design. Decide the machine type before you finish the model.

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  Solid body, not surfacesA closed solid gives CAM a clear inside and outside.
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  True arcs, not polygonsKeep holes and radii as analytic geometry where possible.
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  One coordinate systemModel at the datum; do not nest parts at odd offsets.

Write down the functional faces first. On a bracket, that is usually the mounting face, the two bores and the boss height. Those get tight tolerances. Everything else can sit at general tolerance, typically ±0.1 mm on machined aluminum or ±0.05 mm on ground steel. Putting ±0.005 mm on a non-functional edge adds cost with no benefit, because the shop has to slow the roughing pass and scrap more parts.

Next, pick the stock. A part cut from a 6061-T6 block behaves differently from one cut from a casting. Forgings and castings carry draft angles, parting lines and hard spots that the model must reflect. If you model a cast part as a clean prismatic solid, the first toolpath will cut into a surface that is not there. Ask the foundry or the shop for the as-cast model before you design the finish geometry.

Material choice also sets the surface finish target. Ra 0.8–1.6 μm is a normal machined finish on aluminum and stainless. Ra 0.2–0.8 μm needs a finishing pass with a small stepover, a sharp tool and often a polished insert. Titanium and Inconel fight back: Ti-6Al-4V and Inconel cut hotter, so a model with deep thin ribs may chatter no matter how good the graphic is.

Finally, confirm the order quantity. A one-off prototype can be machined from a block with plenty of stock. A 10,000-part run usually gets its own casting or forging, and the model must include draft, fillets and machining allowance. Design decisions made for one unit often have to be redone for volume.

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  Tolerance mapList tight faces separately from general faces.
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  Stock formBlock, plate, casting or forging changes the model.
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  Finish calloutRa 0.8–1.6 μm is standard; finer needs a reason.

Set the model origin on the datum that the shop will touch off. On a plate part, that is often the corner of the finished face plus one edge. If the origin floats in space, the programmer has to move the model, and every re-import risks a shift. Model in millimeters and keep one unit system through the whole file. Mixed units are a common source of a part that comes out 25.4 times too big.

Build the part as a single solid where possible. Boolean everything together and check that the body is watertight. Most CAD packages have a body-check or stitch tool that reports free edges and sliver faces. Fix those before export. A 0.01 mm sliver may look harmless on screen but can make CAM fail to offset the cutter around a pocket.

Add the manufacturing features deliberately. Model chamfers and fillets with real radii, not cosmetic edges. A 0.5 mm corner radius is fine on a three-axis mill with a 1 mm cutter, but a 0.2 mm internal corner in a 30 mm deep pocket is not machinable with standard tooling. If the design needs it, note it as EDM or leave it sharp and let the shop decide.

Keep cosmetic detail out of the functional model. Engraved logos, texture and knurling add file size and toolpath time. Laser marking on GreatLight parts has a minimum character height of 1.5 mm, so a 0.5 mm logo will not reproduce. Model the marking area as a flat face and specify the artwork separately.

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  Origin on the datumThe shop touches off the same corner you modeled.
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  Watertight solidRun a body check; fix free edges and slivers.
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  Real tool radiiInternal corners need a cutter that can reach them.

Before export, measure the smallest internal radius and compare it to the cutter list. A standard 6 mm end mill leaves roughly a 3 mm corner radius. If the pocket corner is 1 mm, the programmer needs a 2 mm cutter, which is longer to run and easier to break. On deep pockets, the tool length-to-diameter ratio matters more than the radius. Anything past 5:1 starts to deflect and chatter.

Check wall thickness. Thin aluminum walls under 0.8 mm will move during and after cutting. A 0.5 mm wall on a 100 mm long part will bow even with light finishing passes. Either thicken the wall or accept that the shop will add support material and a stress-relief step. On titanium, this limit is higher, closer to 1.5 mm.

Look at undercuts and re-entrant angles. A dovetail slot or an internal groove needs a T-slot cutter, a lollipop cutter or a five-axis tilt. If the design has a hidden undercut that no tool can reach, the model is not machinable as drawn. Redesign to split the part, add a removable cover, or move the feature to a secondary operation.

Run a basic clash check between the tool axis and the part. On three-axis work, the tool comes straight down. Any feature that requires the tool to come in from the side means either a fourth-axis setup or a redesign. Catching this in CAD costs minutes; catching it on the machine costs a fixture and a day.

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  Corner radius vs cutter6 mm cutter leaves about 3 mm radius.
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  Wall thicknessKeep aluminum walls above 0.8 mm.
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  Undercut accessIf no tool reaches it, the model is not machinable.

The most frequent problem is a model that is not watertight. Free edges, duplicate faces and zero-thickness walls all break the offset calculation in CAM. The fix is simple: run the CAD body check, knit the surfaces, and delete the sliver faces. Do this before export, not after the shop complains.

The second problem is tolerance stacking. A designer dimensions a hole to an edge, the edge to another edge, and the chain adds up past the functional limit. Dimension from the datum instead. On a part with a ±0.005 mm bore position, chained dimensions are almost guaranteed to fail inspection.

The third problem is ignoring the tool. A pocket that is 40 mm deep with a 2 mm corner radius cannot be cut with a 4 mm cutter at that depth without chatter. Either open the corner radius, reduce the depth, or accept a slower EDM operation. Catching this in CAD saves a setup and a scrapped blank.

The last problem is a missing revision. When a model changes after the first article, the shop needs a clear revision letter and a note on what changed. A file named final-v3-really.step tells nobody anything. Use part number, revision and date.

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  Not watertightKnit surfaces and remove sliver faces.
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  Chained dimensionsDimension from the datum, not edge to edge.
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  Unreachable cornersMatch internal radius to real cutter sizes.

If the part needs five-axis access, tight position tolerances or a surface finish below Ra 0.8 μm, the modeling work is only half the job. The shop has to plan fixtures, choose cutters and prove the process. GreatLight runs 16 simultaneous five-axis centers, 12 four-axis mills and 27 three-axis machines, with a maximum processing size of 4,000 mm. That range covers most bracket, housing and impeller work.

A DFM review catches problems that CAD alone will not. At GreatLight, quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours once the model is released. Parts typically ship in 3 to 5 days. There is no minimum order quantity, so a single prototype and a 10,000-part run go through the same review.

The shop also handles the finishing steps that affect the final graphic-to-part match. Anodizing, plating, powder coating, bead blasting and laser marking all change dimensions slightly. If a bore has a tight fit and will be anodized, the model should note the pre-plate dimension. Hardcoat anodizing can add 0.025 to 0.05 mm per surface, which matters on a ±0.005 mm feature.

Send the STEP file, the tolerance drawing and the finish callout together. Include the material grade and the quantity. Uploads are kept confidential and an NDA is available on request. The clearer the input, the fewer questions come back before the first cut.

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  Five-axis accessUndercuts and contoured faces need tilt.
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  Finish allowanceAnodizing and plating change dimensions.
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  Send model plus drawingTolerances live on the drawing, not the STEP.