How to Design CNC Parts: A Step-by-Step Guide

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  Key takeaways
• 2
  How to design CNC parts around the machining process
• 3
  Choose the material before you finalize geometry
• 4
  Geometry rules that keep a part machinable
• 5
  Set tolerances and surface finish with purpose
• 6
  Model in CAD so the shop can read your intent
• 7
  Run a DFM review before you release the drawing
• 8
  Prototype, inspect, then scale the design
• 9
  Step-by-step design workflow
• 10
  Feature design rules and their limits

A CNC machine removes material with a spinning cutter that has a fixed diameter, a fixed length, and a limited reach. That single fact shapes every good design. If you draw a pocket with a 2 mm internal corner, a Ø6 mm end mill cannot produce it without leaving a radius. The part will come off the machine with a corner the drawing never showed, and someone will have to explain the deviation.

So the first move is to think about the cutter before the geometry. What diameter will reach the deepest feature? How long does the tool shank need to be? A long tool deflects, and deflection shows up as taper and chatter. We often see parts that are perfect on screen but need a 4:1 length-to-diameter ratio to cut, which pushes surface finish from Ra 0.8–1.6 μm out to Ra 3.2 μm or worse.

This is also where you decide whether the part should be milled at all. Long, slender shafts with a single turned diameter belong on a lathe. Flat plates with holes and slots belong on a mill. A part with features on five faces may need a 5-axis setup or three separate fixtures. Each extra setup adds a tolerance stack and a handling step.

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  Check tool reach firstMeasure the deepest pocket and confirm a standard cutter can reach it without excessive overhang.
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  Design for one setup where possibleFewer setups mean tighter position tolerance between features.
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  Note the machining directionMark which face is the datum so the shop can plan the first operation.

Material choice changes the wall thickness you can hold, the finish you can reach, and the tooling the shop needs. Aluminum 6061-T6 machines fast and holds ±0.005 mm on stable geometry. It also anodizes well, which is why so many housings and brackets start there. 7075 offers higher strength but cuts with more tool wear and is harder to anodize evenly.

Stainless 303 is the free-machining grade and works well for small turned parts. 304 and 316 resist corrosion but work-harden, so light cuts and rigid setups matter. If a design calls for 316L and a 0.8 mm wall, expect chatter unless the part is supported or the wall is thickened. Titanium TC4 and Inconel cut slowly and need more passes, which raises cost without changing the drawing.

Plastics behave differently again. POM and ABS cut cleanly, but PEEK needs sharp tooling and careful cooling. A material list is not a shopping list. Match the grade to the load, the environment, and the finish you actually need.

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  Aluminum 6061-T6General purpose, good finish, easy to anodize.
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  Stainless 303 / 304 / 316LCorrosion resistance, but watch work hardening on thin sections.
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  Titanium TC4 and InconelHigh strength at temperature, slower cutting, higher cost.
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  POM, ABS, PEEKLight weight and chemical resistance, but lower stiffness than metals.

Internal corners must carry a radius at least equal to the cutter radius. A common rule is to make the corner radius slightly larger than the tool, so the cutter can clear the corner without rubbing. If a pocket is 20 mm deep and you want a 3 mm corner, the shop needs a Ø6 mm cutter with enough flute length, which may force a smaller tool and more passes.

Wall thickness is the next limit. For aluminum, 1 mm is a safe minimum for a wall that will be machined on both sides. For stainless, start at 1.5 mm. Below these values, the wall deflects under cutting force and the finished thickness varies along the length. If the design truly needs a thin wall, plan a finishing pass with light depth of cut and accept a slower cycle.

Through holes are cheaper than blind holes. A through hole can be drilled in one pass with a standard bit and cleared of chips easily. A blind hole needs a flat or conical bottom, a depth callout, and often a smaller pilot drill. Thread depth should be at least 1.5 times the nominal diameter for reliable engagement in aluminum and steel.

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  Corner radius ≥ cutter radiusA Ø6 mm tool leaves a 3 mm minimum internal radius.
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  Minimum wall: 1 mm aluminum, 1.5 mm stainlessThin walls deflect and lose tolerance.
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  Prefer through holesFewer operations, easier chip evacuation.
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  Thread depth 1.5 × diameterShallow threads strip under load.

Every tolerance you call out is a promise the shop has to measure and hold. General dimensions can live at ±0.1 mm. Bearing bores, dowel holes, and mating faces may need ±0.005 mm. The gap between those two numbers is where most of the cost sits, because tight tolerances require a finishing pass, a controlled temperature, and an inspection report.

Surface finish follows the same logic. As-machined surfaces sit around Ra 1.6–3.2 μm and are fine for brackets and covers. Sealing faces and sliding surfaces often need Ra 0.8–1.6 μm. Optical or medical contact surfaces may need Ra 0.2–0.8 μm, which means a separate finishing operation and sometimes a different machine.

A practical approach is to tolerance only the features that touch another part. Leave the rest at general tolerance and let the shop choose the cutting strategy. That keeps the price down without giving up function.

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  General tolerance ±0.1 mmUse for non-mating dimensions.
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  Critical tolerance ±0.005 mmReserve for fits and alignment features.
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  Finish by functionAs-machined for covers, finer for seals and slides.

A clean CAD model is more than a shape. It carries the datum structure, the thread callouts, and the notes that tell the machinist what matters. Start by defining the primary datum on the face that will sit in the fixture. Then add the secondary and tertiary datums so the inspection team can reproduce the setup.

Use standard thread and hole callouts rather than modeled threads. A modeled thread adds geometry that slows the CAM programmer and does not improve the part. For holes, use the drill size and the tolerance class, not a random diameter. If a hole is for a clearance fit, say so; the shop can then choose the drill and reamer that hit it.

Before release, run a quick check for duplicate faces, open edges, and zero-thickness geometry. These errors are common in imported models and they stop the CAM process. A five-minute cleanup saves a day of email.

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  Define datums explicitlyPrimary, secondary, tertiary, in that order.
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  Use standard calloutsThreads, hole classes, and surface symbols.
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  Check for model errorsOpen edges and slivers break CAM toolpaths.

A DFM review is a structured pass over the design to find features that will cost time or cause scrap. The reviewer checks tool access, wall thickness, corner radii, hole depth, and tolerance count. They also look at the drawing for missing information, such as which face is the datum or how a thread should be gauged.

At GreatLight, we return a quotation and a free DFM analysis within 12 hours. The report lists any feature that needs a change, with a suggested value. For example, we might note that a 0.6 mm wall in 304 stainless will chatter and recommend 1.5 mm, or that a deep pocket needs a larger corner radius to avoid a long, thin cutter.

The goal is not to redesign your part. It is to remove the small mismatches between the CAD model and the machine before the first chip is cut. Most of the time, a handful of edits turns a difficult part into a straightforward one.

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  Check tool accessCan a standard cutter reach every feature?
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  Count tight tolerancesEach one adds inspection time.
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  Confirm drawing notesDatums, threads, and finish callouts.

A prototype run proves the design before tooling or volume commitments. We machine prototypes from the same material and on the same class of machine as production, so the results transfer. After machining, the part goes through 100% inspection before shipment. That includes a raw material check, in-process monitoring, and a final dimensional report on request.

Inspection data often reveals a pattern. If a bore is consistently 0.01 mm small, the fix may be a tool offset, not a drawing change. If a wall is consistently thin in one area, the issue may be fixture support. Catching these on the first article is far cheaper than catching them on part 500.

Once the design is stable, scaling up is mostly a scheduling exercise. With no minimum order quantity, we can move from one prototype to a 10,000-part run without changing the process. Parts ship in 3–5 days, and the historical late-delivery probability is below 2%.

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  Prototype in production materialResults transfer to the volume run.
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  Inspect the first articleCatch offsets before the run grows.
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  Scale without a process changeNo MOQ, same machines.