How to Prepare a CAD File for CNC Machining

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  Key takeaways
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  What a CNC shop actually needs from your CAD file
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  Set units, origin and geometry before anything else
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  Tolerances: model to nominal, dimension to reality
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  Simplify features that cost money and buy nothing
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  Export settings and what to send with the file
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  Step by step: prepare a CAD file for CNC machining
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  CAD formats and when to use each

A CAD file is a solid model, nothing more. It tells the machine where material should be. It does not say which faces matter, which surface gets seen, or how tight a bore must be. Those answers usually live in a drawing, a PDF, or a short email.

When a model arrives alone, the shop has to guess. Guessing costs time. An engineer may spend an hour rebuilding dimensions that were already known to the designer. That hour shows up in the quote.

The fastest way to prepare a cad file for cnc machining is to treat it as a package: solid model, a drawing with tolerances, a material callout, and a finish callout. That package lets a shop quote without back and forth.

File format matters less than people think. STEP, IGES, Parasolid and native SolidWorks all convert cleanly at our end. What does not convert is a mesh saved as a solid, or a surface model with gaps.

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  Solid modelWatertight, single body if possible, no zero-thickness walls.
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  2D drawing or PDFDatums, ± tolerances, GD&T, thread callouts, finish notes.
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  Material and finishAlloy grade and surface treatment, written in plain text.
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  Quantity and target dateOne prototype or a 10,000-piece run changes the process route.

Start with units. A model built in inches and imported as millimeters becomes a part 25.4 times too large. This happens every week. Put the unit in the file name: bracket_v3_mm.step. It takes four seconds and prevents a scrapped blank.

Set the origin somewhere useful. The center of a main bore, a mounting face, or the corner of the stock. A part modeled 800 mm from origin looks fine on screen and causes a setup error on a 4,000 mm mill. Keep the part near the origin.

Check wall thickness before you check anything else. Aluminium 6061 machines well down to 0.8 mm walls. A 0.5 mm wall in titanium TC4 will chatter and deflect. If a thin wall is functional, say so. If it is cosmetic, thicken it.

Remove duplicate bodies, stray surfaces and construction geometry. A STEP file with 40 hidden bodies makes a CAM programmer rebuild the model by hand. One clean solid quotes faster than a messy assembly.

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  Unit in the file namebracket_v3_mm.step or bracket_v3_in.step.
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  Origin near the partWithin a few hundred millimeters, not meters away.
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  Minimum wallAround 0.8 mm for aluminium, 1.5 mm for stainless, more for titanium.
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  Single solid bodyBoolean the model into one body unless the assembly really moves.

Model at nominal size. A 10 mm pin goes in the CAD as 10.000 mm. The tolerance tells the shop it may land between 9.990 and 10.000 mm. If you model the pin at 9.995 mm to split the tolerance, the machinist now has to hit a number you invented, and the drawing and model disagree forever.

Give every tolerance a reason. A bearing bore needs a press fit, so it gets a tight callout. A clearance hole for an M6 bolt needs ±0.2 mm at most. When every dimension carries ±0.01 mm, the shop must inspect every dimension, and the price climbs fast.

Our machines hold ±0.005 mm (±0.0002 in) when the process calls for it. That number is a capability, not a default. Reaching it means slower passes, more inspection and a higher price. Use it where a fit depends on it.

Surface finish follows the same logic. Ra 0.8–1.6 μm is a normal machined finish. Ra 0.2–0.8 μm needs a finishing pass on a good setup. Call out a finish only on the faces that touch or seal.

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  Nominal modelNo pre-shrinking, no tolerance stacks baked into geometry.
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  Tolerance by functionFits, seals and datums get tight values. Clearance holes do not.
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  Capability is not default±0.005 mm is available. It is not the price baseline.
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  Finish where it mattersRa 0.8–1.6 μm standard; Ra 0.2–0.8 μm on request.

Sharp internal corners cannot be machined. Every end mill has a radius. A 12 mm deep pocket cut with a Ø6 mm tool leaves a 3 mm corner radius. If the model shows a sharp corner, the programmer adds a radius and the part changes. Model the radius you can live with.

Deep pockets are the second cost driver. A pocket deeper than about four times the tool diameter forces a smaller tool, slower feed and more passes. If a pocket is 20 mm deep, plan on a Ø6 mm tool or larger and give corners a 3–4 mm radius.

Threads should be called out, not modeled. A modeled thread with a real helix adds thousands of faces to the file and makes CAM slow. A simple cylindrical hole with a note like M6 × 1.0, 12 mm deep is enough. For a tapped hole, model the minor diameter or the nominal hole and note the thread.

Fillets and chamfers that serve no function are pure cost. A 0.5 mm cosmetic fillet on an inside edge may need a tiny tool and a long finishing pass. Delete it or make it 2–3 mm. Deburring edges get a 0.5 mm × 45° chamfer note on the drawing.

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  Corner radiusAt least one third of the pocket depth, or the tool radius you accept.
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  Pocket depthKeep under 4 × tool diameter. Deeper means smaller tools and higher cost.
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  Threads as notesDo not model helixes. Note the thread and depth.
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  Cosmetic filletsRemove or enlarge. A 0.5 mm internal fillet is expensive.

Export to STEP AP214 for a general solid model. AP203 drops color and some assembly data; AP214 keeps both and imports cleanly in most CAM systems. If the shop runs your native CAD version, send it as a backup, but STEP is the safer handoff.

Set the export resolution high. A low-tessellation export turns a smooth bore into a polygon. A Ø40 mm bore exported as a coarse mesh may measure 39.7 mm across flats. That error is invisible in a viewer and obvious on a CMM.

Send the drawing as a PDF alongside the model. The PDF carries datums, GD&T, thread notes, material and finish. Name both files with the same revision: housing_revB.step and housing_revB.pdf. Version confusion is a real cause of wrong parts.

If the part is confidential, ask for an NDA before sending anything. Our uploads are secure and confidential, and an NDA is available on request. For a first quote we only need enough geometry to estimate cycle time and setup count.

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  FormatSTEP AP214 first, Parasolid or native CAD as a backup.
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  ResolutionFine tessellation so arcs stay round in the exported file.
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  DrawingPDF with datums, tolerances, threads, material and finish.
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  Revision namingSame rev number on model and drawing. Never overwrite a sent file.