Process guide

CNC Metal Processing: An Accurate and Effective Process

This guide walks through CNC metal processing step by step, from raw stock to inspected part. It is written for design engineers and buyers who need to judge whether a part suits milling, turning, or a 5-axis setup. Read it and you can set tolerance bands, pick stock, and spot the steps that cause scrap.

Get Instant Quote Talk to an Engineer

±0.005 mm toleranceRa 0.2–0.8 μm finish12-hour DFM reviewNo minimum order

CNC metal processing of custom auto spare parts on a 5-axis machining center

On this page

1
Key takeaways
2
What CNC metal processing does to a part
3
Which parts suit CNC metal processing
4
Setting tolerance bands that hold in production
5
Why accurate parts still fail inspection
6
How to verify an accurate and effective process
7
Six steps from stock to shipped part
8
Matching the process to the part

Quick read

Key takeaways

Pick the axis count from geometryFlat plates with holes run on 3-axis. Undercuts, angled ports, and deep side pockets need 4 or 5 axes.

Tolerance drives cost more than material±0.005 mm is reachable, but every tight callout adds setup and inspection time.

Stock size sets the processParts up to 4,000 mm fit our largest travel; beyond that, split the design or switch to fabrication.

Thin walls are the main scrap causeBelow 0.8 mm on aluminium, holding flatness and chatter control gets hard.

Inspection is part of the process100% inspection before shipment is standard; ask for reports on critical dimensions.

Basics

What CNC metal processing does to a part

CNC metal processing is subtractive: a computer-controlled spindle or turret removes material from a solid block, bar, or casting until the geometry matches the CAD model. The machine does not decide anything. It follows a toolpath written from your model, so accuracy depends on the model, the setup, and the tooling.

Milling covers pockets, faces, slots, and profiles. Turning covers round parts with grooves, threads, and bores. Mill-turn centers do both in one setup, which removes a re-chuck and the position error that comes with it. GreatLight runs 127 high-precision machines, including 16 simultaneous 5-axis centers.

The process suits metals from 6061 aluminium to 17-4PH stainless and Ti-6Al-4V. It also suits engineering plastics, but the cutting data changes a lot. What stays constant is the sequence: stock preparation, first setup, roughing, semi-finishing, finishing, then inspection.

1
Subtractive, not additiveMaterial is removed, so internal stress in the stock can move the part after cutting.
2
Setup count drives accuracyEvery additional setup adds a datum transfer and a chance for position error.
3
Tool access sets the limitA feature the tool cannot reach is a feature the machine cannot cut.

Fit check

Which parts suit CNC metal processing

CNC metal processing fits low-to-mid volume work where geometry is complex and tolerances matter: brackets, housings, manifolds, medical instrument bodies, EV busbars, and robot end-effector plates. Runs from one prototype to 10,000+ parts are all workable because there is no tooling to amortize.

It is a poor fit for very large flat panels with simple outlines, where sheet metal fabrication is faster and cheaper. It is also a poor fit for hollow shapes with uniform wall thickness across high volumes, where die casting or vacuum casting wins on unit cost.

The practical test is feature density. If a part has pockets, angled faces, cross-holes, and tight bores, machining handles all of them in a few setups. If it is a bent box with a few holes, you are paying for capability you do not need.

Material also steers the choice. Aluminium 6061 and 7075 cut fast and hold tolerance well. Inconel and titanium need slower feeds, more coolant, and sharper tools, which raises cycle time. Tell us the alloy early so the quote reflects real cutting data.

1
Good fitComplex 3D geometry, tight bores, prototypes, bridge volumes.
2
Weak fitSimple large panels, uniform thin-wall shells at high volume.
3
Ask earlyAlloy, heat treatment state, and any post-machining coating.

Tolerances

Setting tolerance bands that hold in production

A tolerance is a cost decision as much as a design decision. ±0.005 mm is achievable on our equipment, but it requires a stable setup, temperature control, and a measuring plan. Applying it to a clearance hole wastes money. Applying ±0.2 mm to a bearing seat will cost you the assembly.

Group dimensions by function. Mating faces, bores for press fits, and alignment features get the tight band. Hole positions for fasteners get ±0.1 mm unless the joint is structural. Cosmetic surfaces get a finish callout instead of a size callout.

Finish follows the same logic. Ra 1.6–3.2 μm is the as-machined default and suits most brackets. Ra 0.8–1.6 μm covers seal faces and sliding surfaces. Ra 0.2–0.8 μm is for optical or sealing-critical faces and usually needs a secondary operation.

One more point: call out the datum. If the drawing shows a tolerance without a datum reference, the shop will pick one, and it may not match your assembly. A clear datum scheme removes that ambiguity.

1
Tight band±0.005 mm for fits, bores, and alignment features.
2
Standard band±0.05 to ±0.1 mm for holes, slots, and outlines.
3
Finish calloutRa 1.6–3.2 μm as-machined; tighter needs a plan.

Failures

Why accurate parts still fail inspection

Most rejections trace back to three causes: datum mismatch, thermal movement, and tool deflection. Datum mismatch shows up as a feature that is correct in size but wrong in position. It usually means the setup used a different reference than the drawing.

Thermal movement appears on long cycles and on aluminium. A part that measures correctly on the machine can shrink out of tolerance once it cools. Let parts stabilize before final measurement, and keep the shop temperature steady.

Tool deflection shows as taper in a deep pocket or a bore that is smaller at the bottom. Long, small-diameter tools bend under load. Reduce the axial depth of cut, raise the spindle speed, or use a tool with a shorter flute length.

Distortion from residual stress is the fourth cause and the hardest to see. It shows up after the part is unclamped. Rough, stress-relieve if the alloy allows, then finish. For thin plates, remove equal material from both faces.

1
Position errorUsually a datum mismatch, not a machine error.
2
Size driftCheck part temperature before measuring.
3
TaperReduce tool overhang and cutting load.
4
Post-clamp warpRough, relieve, then finish both faces evenly.

Quality

How to verify an accurate and effective process

Ask for the inspection plan before the first cut. It should name the dimensions being checked, the instrument, and the acceptance band. A shop that cannot describe its plan cannot prove its accuracy.

We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022. Those certificates cover quality systems, automotive, medical devices, and information security. They do not replace a first-article inspection, but they tell you the process is documented.

For repeat orders, keep the same setup and the same datum scheme. Changing a fixture between batches introduces variation that has nothing to do with the machine. If a design change is needed, treat it as a new first article.

Uploads are secure and confidential, and an NDA is available on request. If your drawing is export-controlled or under review, tell us at the quote stage so the workflow matches your requirements.

1
Ask for the planDimensions, instruments, and acceptance bands in writing.
2
Repeat ordersKeep the fixture and datum scheme unchanged.
3
ConfidentialityNDA available; uploads handled as confidential.

How-to

Six steps from stock to shipped part

Follow these in order. Skipping step 1 or step 5 is the most common cause of a failed first article.

1
1. Review the model and set tolerance bandsCheck for missing fillets, zero-thickness walls, and unclear datums before anything is cut. Assign tolerances by function, not by habit: ±0.005 mm on bearing bores and mating faces, ±0.1 mm on clearance holes and non-critical edges. Error to avoid: applying a single tight tolerance to the whole drawing, which inflates cycle time and inspection cost for no functional reason.
2
2. Choose stock and allowancePick the nearest standard stock size and leave 0.5–1.0 mm per side for roughing on small parts, 1.5–3.0 mm on large ones. For castings and forgings, add more because the skin is hard and the surface is uneven. Error to avoid: ordering bar stock that is already at finished size, which leaves no room to correct distortion.
3
3. Plan setups and datumsAim for the fewest setups. Use one primary datum face and locate from it every time. On 5-axis work, a single setup with a Ø400 mm rotary table can reach five faces. On 3-axis work, expect two or three setups with soft jaws or a fixture plate. Error to avoid: re-chucking on a raw surface, which repeats the stock error into the finished part.
4
4. Rough, semi-finish, finishRough with a large tool at high material removal rate, leaving 0.3–0.5 mm for semi-finishing. Semi-finish to even out the load, then finish with the smallest tool that reaches the corner radii. For Ra 1.6–3.2 μm, a standard carbide finisher is enough. For Ra 0.8–1.6 μm, reduce stepover and raise spindle speed. For Ra 0.2–0.8 μm, plan a finishing pass plus polishing or lapping.
5
5. Control heat and stressAluminium moves when it heats up. Use flood coolant or high-pressure through-tool coolant, and keep depth of cut steady. For thin walls under 1.0 mm, take light finishing passes on both sides to balance stress. Error to avoid: a heavy final pass that springs the wall and leaves a taper.
6
6. Inspect and documentCheck the first article against the drawing before running the batch. Measure critical dimensions with the same datum scheme used in setup. We run raw material checks, in-process monitoring, and final inspection on every order, and reports are available on request. Error to avoid: inspecting a part that has not reached room temperature, especially on tight bores.

Selection data

Matching the process to the part

Use this to pick the machine type and tolerance band before you request a quote.

Part feature	Recommended process	Typical tolerance	Watch out for
Flat plate, through holes	3-axis milling	±0.05 mm	Thin plate lifting under clamp pressure
Angled ports, undercuts	5-axis simultaneous	±0.01 mm	Tool reach into deep pockets
Round shaft with cross-holes	Mill-turn center	±0.005 mm	Re-chuck error if split into two ops
Large frame, 4,000 mm	3-axis with long travel	±0.1 mm	Thermal growth over long cycles
Bearing bore, mating face	4-axis or 5-axis	±0.005 mm	Datum transfer between setups
Thin wall under 0.8 mm	5-axis, light finishing	±0.02 mm	Chatter and spring-back
Prototype, one piece	3-axis or 5-axis	±0.05 mm	Stock size limiting design changes

When CNC metal processing is the right call

Choose CNC metal processing when geometry is complex, volume is under 10,000 parts, and tolerances are tighter than ±0.1 mm. Choose sheet metal or casting when the shape is simple and the volume is high. If the part sits between those cases, send the model and we will tell you which route holds tolerance at lower cost.

Get Instant Quote Talk to an Engineer

FAQs

Questions engineers ask

What tolerance can CNC metal processing hold?

We work to ±0.005 mm on critical features when the setup and material allow. Most production parts run at ±0.05 to ±0.1 mm, which covers holes, slots, and outlines.

The tighter band needs a stable datum, temperature control, and a measuring plan. Tell us which dimensions are functional so we apply the tight band only where it matters.

Which metals can be machined?

Aluminium grades 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075, and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C, and 17-4PH; steels 1018, 1045, 4130, 4140, 4340, A36, and tool steel.

We also machine copper and brass, titanium TA1, TA2, TC4, Inconel, magnesium AZ31B and AZ91D, plus plastics such as POM, PEEK, PC, and ABS.

How large can a machined part be?

The largest travel is 4,000 × 400 × 150 mm. Medium travels include 750 × 1,150 × 550 mm and 600 × 600 × 600 mm.

Compact work runs on 500 × 500 × 450 mm and 500 × 310 × 200 mm machines. A Ø400 mm rotary table handles round and angled parts on 5-axis work.

How fast can a quote and a first part be ready?

Quotation and a free DFM analysis come back within 12 hours. Production can start within 24 hours after approval.

Parts ship in 3–5 days for standard work. If a feature needs a custom fixture or a secondary finish, that adds time and we will say so in the quote.

Is there a minimum order quantity?

No. We run from one prototype to 10,000+ part runs. The setup cost is the same either way, so the first part carries the fixture and programming work.

For repeat orders we keep the fixture and the datum scheme unchanged, which is how variation stays low between batches.

Which finishes are available after machining?

Anodizing in clear, colour, hardcoat, and conductive types; electroless nickel, zinc, silver, and gold plating; powder coating and black oxide.

Bead blasting, tumbling, brushing, and polishing are also available, plus laser marking with a minimum character height of 1.5 mm.