CNC Model Processing: A Beginner's Guide

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  How a CAD model becomes a cut part
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  Setups, datums and why the second op is harder
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  Tool access and the geometry rules that follow
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  Tolerances, finishes and what you actually pay for
• 5
  When CNC model processing is the wrong answer
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  What a machining quote is really telling you
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  Step by step: from model to first article
• 8
  Which machine type fits which model

CNC model processing starts with geometry, not with a machine. The CAD file holds surfaces, and the CAM step turns those surfaces into toolpaths: a list of points, feed rates and spindle speeds the control can execute. The machine only ever follows that list. If the list is wrong, the part is wrong, no matter how good the machine is.

Every step removes a constraint in order. The model defines shape. CAM defines how the tool reaches that shape. Fixturing defines where the blank sits. The post-processor translates toolpaths into G-code for one specific control. Only then does the spindle turn.

This matters because most beginner problems appear long before the first cut. A wall that is 0.4 mm thick, a pocket 90 mm deep and 6 mm wide, a thread that runs into a fillet: these are model problems. The machine can only report them by breaking a tool or scrapping a blank.

A useful habit is to read your own model as a machinist would. Ask which direction the tool comes from, where the part is held, and what the first op looks like. That single pass catches most of what a DFM review would flag later.

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  ModelSurfaces and tolerances in the CAD file
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  CAMToolpaths, stepover, feeds and speeds
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  FixtureWhere the blank is held and how rigidly
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  ControlG-code executed by the machine

A setup is one clamping of the workpiece. A 3-axis machine needs one setup per accessible face, so a part with features on six sides can need five or six setups. Each new setup adds an alignment error. That error stacks on top of the machine tolerance, so the finished part is worse than the machine's rated accuracy.

The fix is datum discipline. Choose three datums on the first op and machine them clean. Every later op locates against those faces. If the model has no flat, square surface to locate from, the programmer has to invent one, and the invented datum may not match how the part works in the assembly.

5-axis machining removes setups rather than improving them. On a machine with a Ø400 mm rotary table, a part can be reached from many angles in one clamping. Hole-to-hole position then depends on one setup instead of four. For parts with tight true position between angled faces, that is usually the deciding factor.

Not every part needs this. A simple bracket with holes on two faces machines faster on a 3-axis mill with a cheap fixture. Adding axes adds programming and run time. Use them where the geometry actually demands it.

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  Count your setupsEach one adds alignment error to the stack
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  Machine your datums firstFlat, square faces that later ops can trust
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  Check accessibilityCan the tool reach the feature without hitting the fixture?

A cutter is a cylinder with a spinning end. It cannot cut a square internal corner. The radius left in the corner is the tool radius, so a 6 mm end mill leaves a 3 mm corner radius. If the model calls for a sharp internal corner, someone has to either change the design, add an EDM step, or accept the radius.

Pocket depth sets the other limit. A tool needs flute length and shank clearance, and stiffness drops fast as length grows. As a working rule, keep pocket depth under about 4× the tool diameter. A 6 mm cutter handles roughly 24 mm of depth comfortably. Past that, chatter marks and taper appear, and holding Ra 0.8–1.6 μm becomes difficult.

Thin walls bend away from the cutter. Walls under about 0.8 mm deflect under cutting force, so the finished thickness varies along the wall. If a wall must be thin, ask for it to be machined last, after the surrounding material has been removed, so the wall is supported while it is cut.

Undercuts and internal grooves need special tooling. A T-slot cutter or a lollipop cutter can reach them, but the reach is limited and the tool is fragile. When a model has many of these, the honest answer is often that the feature belongs in a casting or a printed part.

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  Internal cornersRadius equals tool radius; sharp corners need EDM
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  Depth to diameterKeep under about 4:1 for stable cutting
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  Thin wallsMachine them last, not first
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  UndercutsPossible, but tooling gets fragile

Tolerance is a cost curve, not a checkbox. A general tolerance of ±0.1 mm on a milled aluminum part is routine. Tightening a single feature to ±0.005 mm means the shop has to control temperature, check the tool for wear, and often inspect on a CMM. Tighten only the features that affect function.

Surface finish works the same way. As-machined finishes sit around Ra 1.6–3.2 μm. A fine finish of Ra 0.2–0.8 μm usually needs a smaller stepover, a sharper tool, or a separate finishing pass, all of which add time. Specify finish only on sealing faces, bearing bores or sliding surfaces.

Material choice changes the numbers. Aluminum 6061 and 7075 cut cleanly and hold ±0.005 mm without drama. Stainless 316 and 17-4PH work-harden, so light passes and sharp tools matter more. Titanium TC4 and Inconel move heat into the tool, so speeds drop and cost rises.

Plastics behave differently again. POM and PEEK hold dimensions well; ABS and PP flex under clamping and expand with heat. For plastic models, avoid tight tolerances on thin sections. The material moves after the cut, so the inspection result may not match the drawing.

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  General tolerance±0.1 mm is normal for milled parts
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  Tight tolerance±0.005 mm on functional features only
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  FinishRa 0.8–1.6 μm covers most sealing faces
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  MaterialAluminum is the easy default; Inconel is not

Machining removes material, so it is best when the part is small enough to fit the work envelope and the material is strong enough to hold its shape. GreatLight runs a 4,000 mm maximum processing size, with travels of 4,000 × 400 × 150 mm on the largest machines, so oversize parts are a real limit.

It is the wrong answer when the geometry has many identical fine features. A heat sink with 200 thin fins, or a lattice with thousands of struts, costs far more to cut than to print. The same applies to internal channels that a cutter cannot reach at all.

It is also the wrong answer at high volume in a soft material. A die casting or an injection-molded part has tooling cost up front but a much lower piece price. For 10,000 identical housings, machining is usually the expensive path, not the fast one.

The middle ground is a machined prototype that proves the design, followed by a cast or molded production part. Machined parts also make good tooling: a CNC-cut mold insert or a jig holds the same geometry that the production part needs.

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  Choose machiningTight tolerance, low volume, hard material
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  Choose 3D printingLattices, internal channels, many fine features
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  Choose castingHigh volume, simple geometry, soft alloy
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  Choose machining firstTo prove the design before tooling spend

A quote is a summary of decisions, not just a number. When one shop is much cheaper than another on the same model, the difference is usually setup count, tolerance, or material stock. Ask which setups the shop planned. If the answer is two and you expected four, the lower price makes sense.

Lead time depends on the same decisions. Simple parts can start production within 24 hours of an approved quote and ship in 3–5 days. Parts that need custom fixturing, anodizing, or a CMM report on every feature take longer, and that is normal rather than a warning sign.

Quality paperwork matters for regulated work. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. For medical or automotive models, say so at the quote stage, because inspection records and traceability are planned at the start, not added at the end.

Keep the model and the drawing in sync. When a CAD revision and a drawing revision disagree, the shop has to stop and ask. A short note listing the revision and the changed features saves a day on most projects.

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  Setup countThe main driver of price on small parts
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  Tolerance scopeTight tolerance on two features, not twenty
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  FinishingAnodizing and plating add their own lead time
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  DocumentsSay early if you need inspection reports