CNC machining basics

How to get started in CNC machining

This guide is for engineers and buyers who have a part on a screen and need a real metal or plastic version of it. We walk through drawing prep, DFM checks, material and tolerance calls, quoting, first-article inspection, and the pilot run. Read it end to end and you will know which decisions to make before you send a file to any shop.

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On this page

1
Key takeaways
2
What you need before you get started in CNC machining
3
Choose the right process and material for the part
4
How to send a job out for the first time
5
Common mistakes on a first CNC order
6
Step by step: from CAD file to first article
7
Which process and tolerance fit your part

Quick answers

Key takeaways

Start with the drawing, not the machineA clean 3D model plus a 2D sheet with datums and critical dimensions removes most quoting delays.

Decide what is critical before you quoteMark the 5 to 10 dimensions that actually affect fit or function; everything else can run looser and cheaper.

Pick a process by geometry, not habit3-axis for prisms, 4-axis for holes around a part, 5-axis for contoured faces and undercuts.

Order a small lot firstOne to five parts proves the setup, the finish and the tolerance before you commit to a full run.

Ask for inspection data on the first articleA dimensional report on the pilot lot tells you whether the process is in control.

Section 1

What you need before you get started in CNC machining

Getting started in CNC machining is mostly a documentation exercise before it is a machining exercise. A CNC shop needs three things: a solid model, a 2D drawing that states which dimensions matter, and a material call with a temper or condition. Miss any of the three and the quote comes back with assumptions written into it. Those assumptions are where cost and lead time go wrong later.

The 3D model defines geometry. STEP and IGES are the safe formats for machined parts. Native files from SolidWorks, NX or Creo are fine too, but check that the exported model is a single solid with no stray surfaces. A model that is 0.02 mm out of square is not a problem, but a model with a self-intersecting face will stop CAM work on the first day.

The 2D sheet carries the information CAM software cannot guess. Put datums on faces that will be fixtured, and call out threads, fits, surface finish and any feature you will measure. If a dimension is not on the sheet, the shop will hold it to the general tolerance block on the border. Most shops default to ISO 2768 medium, which is ±0.1 mm on a 25 mm feature and much looser on larger ones.

Material comes last but decides a lot. Aluminum 6061-T6 cuts fast and holds ±0.005 mm on a rigid setup. Stainless 316L moves more and needs slower feeds. Plastics like POM and PEEK hold tight tolerances but can bow after machining if you take heavy cuts. Say what the part must do in service and let the shop suggest the grade.

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File formatsSTEP or IGES for geometry; PDF for the 2D sheet.
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Drawing must-havesDatums, critical dimensions, threads, finish, material condition.
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AvoidOpen surfaces, duplicate solids, and dimensions that contradict the model.

Section 2

Choose the right process and material for the part

Process choice follows geometry. A flat bracket with holes on two faces runs on a 3-axis mill. A shaft with cross-holes runs on a 4-axis mill or a mill-turn center, because the part indexes instead of being re-fixtured. A part with contoured faces, deep pockets on five sides, or an undercut runs on a 5-axis machine. Each step up adds setup cost but removes re-fixturing error.

GreatLight runs 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 travels from 500 × 310 × 200 mm up to 4,000 × 400 × 150 mm and a Ø400 mm rotary table. That range covers most brackets, housings, manifolds and shafts.

Material sets the cutting parameters. Aluminum 6061, 7075 and 6082 run at high spindle speeds with light chiploads. Stainless 303 and 316L need lower surface speed and more coolant. Titanium TC4 and Inconel 718 cut hot and work-harden, so the shop takes deeper passes at lower speed rather than rubbing the surface. Plastics machine clean but move after the cut, so leave stock and take a finishing pass.

Tolerance should be assigned by function. Most of a part can run at ±0.1 mm and cost less. Reserve ±0.005 mm for bearing bores, mating faces and alignment features. Surface finish follows the same logic: Ra 1.6–3.2 μm as machined for general parts, Ra 0.8–1.6 μm for sealing faces, and Ra 0.2–0.8 μm only where a gasket or a sliding contact needs it.

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3-axisPrismatic parts, one or two setups, flat faces and through-holes.
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4-axisHoles and slots around a cylindrical or indexed part.
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5-axisContoured surfaces, undercuts, five-sided features in one setup.
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Tolerance ruleTight only where the part functions. Loose elsewhere.

Section 3

How to send a job out for the first time

The first quote request sets the tone for the whole project. Send the model, the drawing, the material, the quantity and the finish you expect. If the part has a cosmetic face, say which face and whether tool marks are acceptable. A shop that knows the cosmetic requirement up front will plan the finishing pass instead of discovering the issue at inspection.

Expect a DFM review. At GreatLight, quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours after that. A good DFM note points at a thin wall, a deep pocket with a small corner radius, a thread too close to an edge, or a tolerance that will be expensive to hold. Read the note and answer it. Ignoring it usually means a change order later.

Uploads are handled as confidential, and an NDA is available on request. If your part is under a development agreement, say so in the first email and keep the file transfer inside the agreed channel. Do not post a customer drawing on a public forum to ask whether a feature is machinable; ask the shop directly.

Set the quantity based on what you need to learn. One prototype proves the geometry. Three to five parts prove the setup repeats. Ten to fifty parts reveal tool wear and any drift in the process. There is no minimum order quantity at GreatLight, so a single part and a 10,000+ part run go through the same quoting path.

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IncludeModel, drawing, material, quantity, finish, cosmetic faces.
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AskFor a DFM note with specific feature-level feedback.
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ProtectRequest an NDA before sending sensitive geometry.

Section 4

Common mistakes on a first CNC order

The most expensive mistake is over-tolerancing. A drawing with ±0.01 mm on every dimension forces the shop to slow down, add finishing passes and inspect more. Machining cost tracks tolerance: moving a dimension from ±0.1 mm to ±0.005 mm can add a full setup and a second operation. Put tight tolerance on the features that mate with something else and leave the rest alone.

The second mistake is a deep pocket with a sharp internal corner. A 20 mm deep pocket with a 1 mm corner radius needs a 2 mm cutter with a long reach, which deflects and chatters. Design the corner radius at least one third of the pocket depth, or accept a larger radius and a smaller cutter load. This is the change a DFM review will flag first.

The third is ignoring finish on a mating face. A face that seals against a gasket needs Ra 0.8–1.6 μm or finer, not the as-machined Ra 1.6–3.2 μm that comes off a standard roughing and finishing sequence. If you do not call it out, you will get the default, and the leak shows up during assembly.

The fourth is ordering the full quantity before the process is proven. A pilot lot of one to five parts costs little and reveals clamping marks, tool marks, burrs and drift. Approve the pilot, then release the run.

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Over-tolerancingTight everywhere drives cost with no functional benefit.
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Sharp pocket cornersSmall radius plus deep pocket equals chatter and tool breakage.
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Missing finish calloutDefault as-machined finish may not seal.
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Skipping the pilotFull quantity before process proof multiplies scrap risk.

Step by step

Step by step: from CAD file to first article

Follow the order. Skipping a step is the most common reason a first article fails.

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Step 1 – Clean the modelExport a single solid as STEP. Remove construction surfaces, duplicate bodies and zero-thickness faces. Check that all holes are modeled at the finished size, not as pilot holes.
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Step 2 – Build the 2D drawingAdd datums on faces that will be clamped. Call out threads, fits, surface finish and every dimension you will measure on the incoming inspection. Set the general tolerance block, ISO 2768 medium is a common default.
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Step 3 – Mark the critical dimensionsPick 5 to 10 dimensions that affect fit or function. Flag them on the drawing. Everything else can run at the general tolerance. This single step usually reduces cost more than any other change.
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Step 4 – Choose material and finishName the alloy and temper, for example 6061-T6 or 316L, not just aluminum or stainless. For finish, state the process and the appearance, for example clear anodize, Ra 0.8–1.6 μm on the sealing face.
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Step 5 – Request a quote and DFM noteSend the model, drawing, quantity and lead time you need. Ask for feature-level DFM feedback. Do not accept a price without knowing which tolerances the shop assumed.
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Step 6 – Approve the setup planReview how the part will be held. Thin walls and tall features deflect under clamping force. If the shop plans to hold on a finished face, ask how it will avoid marking it.
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Step 7 – Run a small pilot lotOrder one to five parts. Measure the critical dimensions yourself before the shop ships the rest. Compare the measured values to the drawing, not to the model.
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Step 8 – Review the first-article reportAsk for dimensional results on the critical features plus material and finish certificates if the part is regulated. A 100% inspection before shipment is standard here, with reports on request. Approve the process, then release the production quantity.

Decision table

Which process and tolerance fit your part

Match the row to your geometry first, then set tolerance by function.

Part feature	Process	Typical tolerance	When it does not fit
Flat plate, holes on one face	3-axis mill	±0.05 mm	Undercuts or five-sided pockets
Cross-holes in a shaft	4-axis or mill-turn	±0.025 mm	Very large parts beyond 4,000 mm
Contoured impeller or housing	5-axis	±0.005 mm	Simple prisms, cost not justified
Bearing bore, press fit	Any, finishing pass	±0.005 mm	Soft plastics that creep after cut
Sealing face	Milled + lapped	Ra 0.2–0.8 μm	Cast or rough-machined surfaces
General bracket	3-axis	Ra 1.6–3.2 μm	Cosmetic or sealing surfaces
Thin wall under 0.8 mm	5-axis, light passes	±0.05 mm	Heavy clamping or roughing cuts

Start small, measure, then scale

The fastest way to get started in CNC machining is to send a clean model with a marked-up drawing, run a one-to-five part pilot, and check the first-article data before releasing the full quantity. That sequence catches clamping, finish and tolerance problems while they are still cheap to fix.

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FAQs

Frequently asked questions

What file format should I send for a CNC quote?

Send a STEP or IGES solid model plus a PDF of the 2D drawing. STEP is the safest because it carries solid geometry without translation errors. Native CAD files also work if the shop supports your software version.

If you only have a 2D drawing, say so. A shop can still quote, but it will add a modeling charge and there is a higher risk that the quoted geometry differs from what you intended.

How tight a tolerance can CNC machining hold?

On a rigid setup with the right material, ±0.005 mm is achievable on critical features. That is not a blanket tolerance for the whole part. Apply it to bearing bores, alignment features and mating faces.

General dimensions can run at ±0.05 mm or ±0.1 mm and cost much less. Surface finish follows the same rule: Ra 0.2–0.8 μm is available but should be reserved for sealing and sliding surfaces.

Is there a minimum order quantity?

No. GreatLight has no minimum order quantity. A single prototype and a 10,000+ part run go through the same quoting and inspection process.

For a first order, one to five parts is a practical starting point. The pilot lot proves the setup, the finish and the tolerance before you commit to a larger release.

How long does a first CNC order take?

Quotation and a free DFM analysis come back within 12 hours. Production can start within 24 hours after the quote is approved, and parts typically ship in 3–5 days. Timing depends on material availability, finish and inspection requirements.

If your part needs a special material or a multi-step finish, add time for sourcing and the finishing queue. Tell the shop your target date at the quote stage so it can flag any risk.

What certifications apply to CNC parts?

GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. The relevant one depends on your industry: IATF 16949 for automotive, ISO 13485 for medical devices, ISO 27001 for information security on sensitive files.

100% inspection before shipment is standard, covering raw material check, in-process monitoring and final inspection. Inspection reports are available on request.

How do I protect my design when requesting a quote?

Uploads are secure and confidential, and an NDA is available on request. Ask for the NDA before you send the model if the design is sensitive.

Keep the file transfer inside the agreed channel. Do not post customer drawings on public forums to ask about machinability; send the question to the shop directly.